Submarine groundwater discharge impacts on coastal nutrient biogeochemistry
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REvIEWS
Submarine groundwater discharge
impacts on coastal nutrient
biogeochemistry
Isaac R. Santos 1,2 ✉, Xiaogang Chen 3, Alanna L. Lecher4, Audrey H. Sawyer5,
Nils Moosdorf 6,7, Valentí Rodellas 8, Joseph Tamborski9, Hyung-Mi Cho10,
Natasha Dimova11, Ryo Sugimoto12, Stefano Bonaglia1, Hailong Li13,
Mithra-Christin Hajati6 and Ling Li3
Abstract | Submarine groundwater discharge (SGD) links terrestrial and marine systems, but
has often been overlooked in coastal nutrient budgets because it is difficult to quantify. In this
Review, we examine SGD nutrient fluxes in over 200 locations globally, explain their impact on
biogeochemistry and discuss broader management implications. SGD nutrient fluxes exceed
river inputs in ~60% of study sites, with median total SGD fluxes of 6.0 mmol m−2 per day for
dissolved inorganic nitrogen, 0.1 mmol m−2 per day for dissolved inorganic phosphorus and
6.5 mmol m−2 per day for dissolved silicate. SGD nitrogen input (mostly in the form of ammonium
and dissolved organic nitrogen) often mitigates nitrogen limitation in coastal waters, since SGD
tends to have high nitrogen concentrations relative to phosphorus (76% of studies showed N:P
values above the Redfield ratio). It is notable that most investigations do not distinguish saline
and fresh SGD, although they have different properties. Saline SGD is a ubiquitous, diffuse pathway
releasing mostly recycled nutrients to global coastal waters, whereas fresh SGD is occasionally
a local, point source of new nutrients. SGD-derived nutrient fluxes must be considered in water
quality management plans, as these inputs can promote eutrophication if not properly managed.
Submarine groundwater
Excessive anthropogenic nutrient inputs drive wide- SGD occurs on timescales of hours to millennia, spa-
discharge spread eutrophication in global coastal waters 1,2. tial scales of metres to kilometres and as a low flux over
The flow of water through Despite large investments to reduce nutrient inputs large areas, making it challenging to quantify28 and, thus,
continental margins from the from wastewater and urban and agricultural runoff3,4, sometimes misinterpreted. As a result, SGD has often
seabed to the coastal ocean,
coastal eutrophication and hypoxia continue intensify- been considered a nutrient source to coastal waters
with length scales of metres to
kilometres, regardless of fluid ing worldwide, even where these conventional nutrient only after the ‘standard’ pathways, such as atmospheric
composition or driving force. sources have decreased5–7. Alternative nutrient sources deposition, rivers and sewage, are ruled out.
and pathways such as submarine groundwater discharge SGD is ubiquitous in sandy, muddy and rocky shore-
(SGD) also contribute to persistent water quality issues lines and represents a combination of fresh and saline
in the coastal ocean2. Pioneering local-scale research in groundwater interacting with coastal surface waters29,30
the 1980s revealed extremely high nitrate concentra- (Fig. 1). Fresh SGD is driven by a positive terrestrial hydrau-
tions in fresh coastal groundwater in Western Australia8, lic gradient and emerges from shallow or deep aquifers
where fresh SGD fluxes exceeded river nitrate loads and intersecting the shoreline31,32 carrying natural and anthro-
explained ~50% of local primary productivity9. pogenic nutrients from land. Saline SGD (sometimes also
Quantitative investigations have since revealed that referred to as seawater circulation in sediments) is defined
SGD delivers nutrients and affects water quality in as the advection of saline groundwater through inter-
diverse coastal ecosystems, such as estuaries10,11, coral tidal zone sediments and/or across the coastal seafloor,
reefs12–14, coastal embayments and lagoons15–17, intertidal and/or advective porewater exchange on scales larger
wetlands such as mangroves18,19 and saltmarshes20–22, than one metre28,30,33. Saline groundwater also mixes with
✉e-mail: isaac.santos@gu.se the continental shelf23–25 and even the global ocean26. fresh SGD owing to the interactions of tides and waves,
https://doi.org/10.1038/ Nevertheless, nutrient fluxes via SGD remain overlooked in density-driven flow and dispersion processes34, with the
s43017-021-00152-0 most coastal nutrient budgets and water quality models27. resulting brackish SGD transporting both land-derived
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new water and dissolved species from the marine per-
Key points
spective. In contrast, saline SGD often flushes out
• Submarine groundwater discharge (SGD) is an essential component of biogeochemical recycled nutrients generated during the degradation
budgets. Fresh SGD is a source of new nutrients, whereas saline SGD often releases of sediment organic matter, as well as external nutrient
recycled nutrients from sediments. sources entrained from the mixing of fresh and saline
• SGD-derived nitrogen fluxes exceeded river inputs in ~60% of the reviewed cases and waters35,52. Saline SGD has a net zero water volume
usually counteracted nitrogen limitation in coastal waters due to high N:P exceeding exchange over timescales longer than the cyclic pressure
the Redfield ratio.
oscillations driving it. Seawater that infiltrates coastal
• Positive impacts of SGD on coastal ecosystems include enhanced coral calcification, sediments eventually returns to the ocean with a differ-
primary productivity, fisheries, denitrification and pollutant attenuation.
ent chemical composition53 on timescales ranging from
• Negative impacts of SGD include eutrophication, algal blooms, deoxygenation and days to weeks when driven by tides or storms35,54,55, and
localized ocean acidification, depending on site-specific conditions. from seasons to years when driven by convection or
• Considering SGD is crucial to reach the United Nations Sustainable Development sea-level oscillations56–59. Much emphasis has been given
Goals pollution targets. The US Supreme Court decision to consider SGD under the to ubiquitous nearshore tidally driven saline SGD with
Clean Water Act represents a positive policy change, signalling broader appreciation
semi-diurnal, diurnal or fortnightly variations34,60. Fewer
of SGD impacts.
studies have addressed irregular forcing, such as varying
wave conditions61, storms62, estuarine density inversions63
Permeability and marine-derived nutrients30,35. Brackish SGD occurs or sea-level anomalies64, that can flush the upper few
A measure of the ability of further offshore, where confined aquifers intersect embay- metres of coastal permeable sediments and produce
unconsolidated rocks and ments and on the continental shelf31,36. These deeper aqui- large episodic pulses or seasonal offshore saline SGD65
sediments to allow fers are less vulnerable to nutrient contamination from and deliver both new and recycled nutrients.
groundwater flow.
onshore activities because of geological isolation. Where The volume of fresh SGD entering the global ocean
Hydraulic heads land-derived nutrients are present in confined aquifers, is relatively small compared with rivers66,67, accounting
Vertical and horizontal pressure travel times offshore can reach centuries or longer37. for ~1% of total freshwater inputs to the ocean andReviews
a Sandy coasts: permeable sediments
Anthropogenic sources N2 fixation
N2
POM infiltration High tide
NO3– Mineralization Brackish SGD
NO3–
Denitrification
NH4+ NH4+ ~ DON > NO3– Low tide
Nitrification
DNRA
Soil OM NH4+
Mineralization
DON
Leaching
b Muddy coasts: secondary permeability created by burrows
NO3– N2 fixation
High tide
N2
POM
Denitrification
Plant uptake Aerobic mineralization NO3–
High DNRA Suboxic burrows
Nitrification Saline SGD >> fresh SGD
soil Anaerobic NH4+
OM mineralization Low tide
DON
Leaching
DON + NH4+
c Rocky coasts: fractures and/or conduits in karstified carbonate or volcanic rocks
NO3– sources
Oxic conditions
Low OM
Minor NH4+ and DON production
Ineffective NO3– attenuation High tide
Low tide
Fresh SGD > saline SGD
Submarine springs NO3– > DON ~ NH4+
Fresh SGD Saline SGD
Fig. 1 | the nitrogen cycle in sandy, muddy and rocky coastal aquifers. The sizes of the background arrows qualitatively
indicate the relative magnitude of fresh and saline submarine groundwater discharge (SGD). a | Sandy coasts are often
characterized as having brackish SGD with higher concentrations of ammonium (NH4+) and dissolved organic nitrogen
(DON) than nitrate (NO3−). b | Muddy coasts often host burrowing fauna, which create secondary sediment permeability
and promote aerobic mineralization, nitrate reduction and saline SGD. c | Rocky coast SGD tends to be dominated by
freshwater, with relatively high concentrations of NO3− relative to DON and NH4+. DNRA, dissimilatory nitrate reduction
to ammonium; OM, organic matter; POM, particulate organic matter.
saline groundwater concentrations, then saline SGD can SGD inputs of dissolved inorganic nitrogen (DIN), dis-
enhance microbial denitrification by consuming nitrate solved inorganic phosphorus (DIP) and DSi seem to be
Denitrification and, thus, attenuate nitrogen pollution90. For example, derived from saline SGD26, with fresh SGD represent-
Microbial process in the
nitrogen cycle that converts
saline groundwater flow through intertidal sediments ing a minor contribution66. However, at sites where fresh
nitrate to nitrogen gas that removes nitrogen from surface waters in coastal wet- SGD is volumetrically important (usually karst or vol-
flows to the atmosphere. lands receiving high nitrogen loads91. Most of the global canic landscapes with high permeability) (Fig. 1), nutrient
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Karst fluxes supplied by fresh SGD dominate the local nutrient Global distribution of SGD studies
Landscape formed by sources to coastal waters49,92,93. Here, we compiled fresh and/or saline SGD-derived
carbonate rocks often Fresh and saline SGD pathways vary between sandy, fluxes of at N, P and/or Si reported by 239 study cases
weathered by dissolution muddy and rocky coastlines, owing to the unique hydro- from 31 countries (Fig. 2, Supplementary Table 1). Most
and with abundant conduits
for fast groundwater flow.
geological characteristics of coastal aquifers. Sandy of the flux data relied on radon (27%) and radium (45%)
coasts generally consist of highly permeable sediments isotope measurement of SGD rates. These methods
Unconfined aquifers that effectively connect aquifers to the coastal ocean result in SGD rates that are, on average, a factor of two
Surficial aquifers situated (Fig. 1a). A typical unconfined surficial sandy aquifer greater than estimates based on modelling approaches
above a low-permeability layer
stores fresh groundwater from upland regions, dis- (Supplementary Table 2), likely reflecting the large num-
of sediment or rock, and with
the upper water layer at
charging to the sea within or below the intertidal zone. ber of marine processes driving (mostly saline) SGD that
atmospheric pressure. Tidal or wave dynamics can create seawater circulation are captured by radon and radium isotopes34,118, whereas
cells nearshore within beach sediments94,95, while var- hydrological models quantify specific driving forces and
Oxidation-reduction ious forcing mechanisms can drive saline SGD farther components of fresh and saline SGD33,119,120.
potential
Measure of the tendency of
offshore52,65,96–98. In contrast, muddy coasts dominated by From a climatic zone perspective, SGD nutrient
a chemical species to acquire mangroves and saltmarshes (Fig. 1b) are characterized investigations are similarly split between the tropics
electrons, to be reduced or by lower permeability sediments that facilitate saline (27%), subtropics (30%) and temperate (32%) regions
to lose electrons, or to SGD once the secondary permeability has been enhanced (Fig. 2). Polar regions remain severely understudied, with
be oxidized.
by burrows, root structures or buried vegetation99–102. only two studies quantifying SGD-derived nitrogen
Rocky coasts (Fig. 1c) contain fractures and/or conduits fluxes in Alaska121. Of all studies in the tropics, 50% are
that allow direct fresh SGD flows to the sea with no or located in Asia and 25% are from the Hawaiian Islands.
minor biogeochemical transformations14,69,103,104. The In the subtropics, 37% of the studies are from the USA
fresh SGD component is usually expected to exceed alone, and only 19% of the study sites are located in the
saline SGD in karst and volcanic coastal aquifers, with Southern Hemisphere (primarily Australia). Temperate
fresh groundwater flows susceptible to regulation by regions between 35° and 60° are mainly represented by
tidal forcing mechanisms46,104. Europe (38%) and the east coast of the USA (26%), and
Topography and geomorphology can also influence are highly skewed to the Northern Hemisphere (93%).
SGD, but the effects remain largely unquantified. For In total, 38% (n = 79) of the compiled studies were from
example, the regional topography of the coastal zone dic- Asia, followed by North America (33%), Europe (16%)
tates the slope of the water table and the inland hydraulic and Australia/Oceania (11%). Only two investigations
gradient in coastal unconfined aquifers, which, in turn, quantified SGD-d erived nitrogen inputs in South
influences fresh SGD105,106. Nearshore morphological America (bay and lagoon ecosystems in Brazil25,122)
features, such as beach slope breaks, tidal creeks and and three in Africa (estuary and lagoon ecosystems
heterogeneous stratigraphy, affect seawater circulation in Egypt123 and South Africa124). Thus, there is a clear
in beaches and saline SGD, as observed and modelled in need to conduct SGD investigations in poorly repre-
a coarse carbonate sand aquifer on the Cook Islands107,108 sented areas in Africa, South America and high latitudes
and in saltmarshes in China100,109. across all ecosystem types. The limited existing datasets
Fresh SGD carries land-derived nutrients that are and large uncertainties in individual estimates prevent
an external nutrient source to coastal waters, with con- inferring any specific pattern across different climates
siderable variability between sandy, muddy and rocky (Supplementary Table 3).
coastlines. For example, seagrass, mangrove and salt- Several interesting inferences emerge comparing
marsh vegetation assimilate nutrients directly from measurements between ocean basins. Median (and inter-
groundwater (Fig. 1b). Sediment properties like organic quartile range) SGD rates and inorganic nutrient fluxes
matter content control oxidation-reduction potential are greatest for the Indian Ocean (SGD = 17, 5–48 cm
and the energetic favourability of denitrification. per day; DIN = 11, 3–29 mmol m−2 per day), where there
Phosphorus or silicate-bearing minerals in rocks can was the smallest number of study cases (Supplementary
act as a natural source of DIP and DSi, whereas iron Table 4). For the Pacific Ocean, median SGD rates
oxides immobilize DIP through sorption87. Phosphorus (9, 2–22 cm per day) and DIN (8, 2–27 mmol m−2 per
can be released back to porewater when iron oxides day) and DSi (9, 2–60 mmol m−2 per day) fluxes exceed
are reduced, as observed in saltmarshes110,111 and sandy those of the Atlantic Ocean (SGD = 4, 1–10 cm per day;
aquifers87 exposed to both fresh and saline SGD. SGD DIN = 2, 2–60 mmol m−2 per day; DSi = 2, 0–12 mmol m−2
nutrient inputs are also conditioned by the discharge per day), in spite of a large natural variability. The dif-
type. Slow, diffusive fresh and saline SGD through ferences in DSi fluxes are likely driven by differences in
sandy permeable sediments allow for greater nutrient continental lithology and the presence of active (Pacific)
transformations in subterranean estuaries82,112,113, but and passive (Atlantic) margins125. The median DIP flux
rapid fresh groundwater discharges through conduits for the Mediterranean Sea (0.03, 0.01–0.10 mmol m−2
(for example, karstic or volcanic aquifers) prevent sub- per day; n = 24) is approximately three times lower
stantial nutrient attenuation114,115. Fresh and saline SGD than that of the Atlantic and Pacific oceans (0.10, 0.02–
ultimately deliver regenerated nutrients associated with 0.48 mmol m−2 per day), because the Mediterranean
the decomposition of organic matter in soils and sedi- coastline hosts many karstified aquifers that retain
ments, and these natural and internal nutrient sources phosphate126.
are also a component of nutrient budgets in coastal From the synthesis here, sites with high SGD-
marine waters116,117. derived DIN fluxes are often located in regions with
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a
d c e
b
f
Ecosystem type SGD rate (cm per day)
Estuary, bay and lagoon 0–1
Mangrove and marsh 1–5
Karst aquifer 5–15
Coral reef
Sandy beach 15–40
Marginal bay and shelf 40–280
Others
b c
200 km 1,000 km
d e f
500 km 1,000 km 1,000 km
Fig. 2 | SGD rates from study cases reviewed here. a | Submarine groundwater discharge (SGD) fluxes globally, colour-
coded by ecosystem type, where the size of the circle represents the reported SGD rate. Similar maps for each nutrient are
shown in the supplementary material. Investigations where SGD rates are reported without any nutrient fluxes were not
included in the compilation. b | SGD in Hawaii, USA, with ecosystems coloured and rates scaled as above. c | SGD in the
Mediterranean. d | SGD on the east coast of the USA. e | SGD in East Asia. f | SGD on the eastern coast of Australia.
contaminated coastal aquifers. These sites include macroalgal growth and eutrophication have been linked
groundwater flowing across septic systems in Hawaii127, to SGD from multiple perspectives and methods41,130,131.
heavily fertilized catchments in the northeast USA128, However, fresh and saline SGD can sustain relatively
urban embayments in China129 and coastal aquifers with high nitrogen fluxes, even at sites with no apparent
naturally high nitrate due to large bird populations13. anthropogenic contamination sources, such as protected
High DIN fluxes in coral reefs and estuaries (Fig. 3) might saltmarshes on the USA east coast22,110,132.
be due to measurement bias towards ecosystems that are The study sites considered ranged from small near-
already known to be impacted by nutrient enrichment. shore sites that spanned ~100 m2 along beaches133,134 to
For example, in Waquoit Bay (MA, USA), excessive large regions that spanned marginal seas such as the
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Subterranean estuaries Mediterranean Sea 126 and the Yellow Sea 135, or the Nutrient ratios and speciation
The locations in coastal global ocean26. Although there was no direct corre- Biogeochemical transformations within coastal aqui-
aquifers where there is mixing lation between the area covered by individual study fers and subterranean estuaries (where fresh and saline
between fresh groundwater cases and SGD rates or related nutrient fluxes, group- groundwater mix136–138) dramatically modify nutrient
and seawater, and chemical
reactions modify the
ing the available data into three major classes revealed concentrations and chemical speciation along SGD flow
composition of submarine greater nutrient fluxes on the small (Reviews
a Nutrient limitation b Nitrogen speciation 0
10,000 100 SGD
N:Si = 16:15
Ten largest rivers
Rivers sampled
with SGD
25
1,000 75
5 NH4+
DSi rich =1 dominated
:P
Si
DON
DIN:DIP
50 NH4+
100 N rich 50
N:P = 16
75
10 25
DON NOx
dominated dominated
P rich
100
1 0
0.001 0.01 0.1 1 10 100 0 25 50 75 100
DIN:DSi NOx
Fig. 4 | nutrient limitation and speciation in SGD versus rivers. a | Dissolved inorganic nitrogen (DIN):dissolved
inorganic phosphorus (DIP) versus DIN:dissolved silicate (DSi) ratios in submarine groundwater discharge (SGD) from
our global compilation, with the same ratios in the ten largest rivers globally included for comparison. b | The relative
contribution of the three main nitrogen species in SGD and rivers, showing that SGD is often dominated by ammonium
(NH4+) and dissolved organic nitrogen (DON), whereas rivers are often dominated by nitrate (represented as NOx)
and DON.
Soil organic matter is remineralized by microorganisms primary producer uptake, microbial mineralization and
in oxic or anoxic conditions149, resulting in ammonium sediment denitrification162,163. As a result, groundwater
release. Ammonium is readily oxidized to nitrate through inputs with a high N:P or N:Si ratio can encourage the
nitrification in the presence of oxygen (Fig. 1a). Because growth of certain phytoplankton groups163. For example,
of oxygen paucity in many organic-rich coastal aquifers, diatom blooms often occur at N:Si ratios lower than 1,
nitrification is generally constrained to the sediment whereas harmful species (usually dinoflagellates) usu-
surface, but can become very important in the presence ally bloom at higher ratios164. The DIN:DIP ratios in
of burrowing animals in muddy sediments150 (Fig. 1b). SGD were above the Redfield ratio of 16:1 in 75% of
In sandy and muddy coastal areas, nitrogen fixation the study sites, demonstrating that SGD often atten-
related to abundant sulfate-reducing bacteria in inter- uates nitrogen limitation and stimulates primary pro-
tidal sediments can eventually turn atmospheric N2 into ductivity in coastal waters (Fig. 4a). The DIN:DIP ratios
ammonium151,152, which can be easily incorporated in SGD study cases ranged from 1 to 12,100 (aver-
into organic matter and infiltrate subterranean estuaries, age ± standard deviation = 259 ± 1,090; n = 169) and
Nitrogen fixation owing to waves and tides (Fig. 1a,b). the DIN:DSi ratios ranged from 0.1 to 47.5 (2.0 ± 5.4;
Microbial process that leads to
the conversion of nitrogen gas
Nitrate is removed by the microbial conversion to n = 96). Based on those ratios, SGD in 58% of the com-
into ammonia/ammonium. N2 through denitrification in the absence of oxygen piled study sites had Si-enriched conditions, 36% were
and the presence of organic carbon in muds and sand N-enriched and 6% were P-enriched relative to the
Dissimilatory nitrate aquifers90,140,153. Nitrate can be converted back to ammo- Redfield ratio (Fig. 4a). DIN:DIP ratios were usually
reduction to ammonium
nium by the dissimilatory nitrate reduction to ammonium >16, even at sites classified as Si-enriched, demonstrat-
Microbial process in the
nitrogen cycle that converts (DNRA)154, both of which can be enhanced by tidally ing that SGD counters N-limited conditions in most
fixed nitrogen from nitrate driven SGD in muddy intertidal marshes155 or permea- coastal waters.
to ammonium. ble sands156–158. Moreover, both ammonium and nitrate High DIN:DIP ratios in SGD are expected, as phos-
are also temporarily removed by microbial and plant phorus is often immobilized through adsorption to min-
Diatom
Microscopic algae (unicellular
uptake (Fig. 1b). In contrast to muddy and sandy coasts, eral surface sites of Fe/Mn oxides87,89,165 or scavenged by
and non-flagellate) with a however, high nitrate loading and oxygen presence in co-precipitation with calcium carbonate166. Hence, in
characteristic wall made up of volcanic and karst coasts (as in Hawaii, Yucatan and hypoxic and anoxic aquifers, including saltmarshes and
silica and are one of the most in the Mediterranean) lead to a simplified nitrogen mangroves, DIN:DIP ratios in SGD can be controlled
important groups of planktonic
cycle, with little nitrate attenuation and high export to by the seasonal reduction and oxidation cycling of Fe
marine microalgae.
the sea72,115,159,160 (Fig. 1c). oxides driving DIP88,167,168. Particularly high DIN:DIP
Dinoflagellates The ratio of nutrients supplied to coastal waters ratios are observed in coastal aquifers contaminated by
Group of microscopic algae (Fig. 4a) can limit primary production and influence sewage and fertilizers because the phosphorus source is
(mostly unicellular and biological communities if the source differs sub- often attenuated faster than nitrogen along groundwa-
flagellate) representing one of
the most important groups
stantially from the Redfield ratio161. In the absence ter flow paths145. Moreover, groundwater nitrogen from
of both marine and freshwater of anthropogenic sources, the coastal ocean is often fertilizers applied in the last century can still be found in
phytoplankton. nitrogen-limited, owing to efficient coupling between coastal aquifers37,169. Despite substantial improvements
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in fertilizer management in some European countries, Comparing SGD and river fluxes
nitrate concentrations in groundwater have not shown Rivers are often assumed to be the primary nutrient
any immediate decreasing trend following reductions in source to coastal waters, so riverine nutrient fluxes pro-
fertilizer application170,171. vide a valuable reference frame for contextualizing SGD
Our data compilation supports earlier model pre- (Fig. 5). Global estimates of nutrient fluxes supplied by riv-
dictions145 that the discharge of legacy N-contaminated erine discharge to the coastal ocean184–186 are on the order
groundwater will eventually change the coastal ocean of ~40 Tg N per year, ~9 Tg P per year and ~140 Tg Si per
from the current N-limited to a P-limited state. Such a year, although these estimates vary widely depending on
pattern has been observed in a SGD-dominated urban the model used187,188. River nutrient fluxes vary greatly
embayment in China, where surface water DIN:DIP among the continents, reflecting the regional differences
ratios have increased from 25 to 96 between the 1980s in population, the associated anthropogenic nutrient
and the mid-2010s, owing to seepage of contaminated inputs and the hydrological cycle189,190. For instance,
SGD172. In the Po river estuary in Italy, a notable increase natural sources are the main contributor to N fluxes sup-
of DIN:DIP ratios from 47 to 100 between 1970 and 2016 plied by rivers in Africa, Oceania and South America,
was linked to the discharge of nitrogen-polluted ground- whereas most of the N is supplied by anthropogenic
water173. Increasing anthropogenic nitrogen inputs in sources in Asia, North America and Europe188.
coastal regions could lead to an increasing N:Si ratio, Basin-w ide or global-s cale assessments of SGD
which provides an unfavourable environment for dia- have suggested that total SGD-derived nutrient inputs
toms, while enhancing the likelihood of dinoflagellates are comparable to or higher than river-derived nutri-
and cyanobacteria blooms174,175. ent fluxes in the Mediterranean Sea126, the coast of
Although nitrogen is often the nutrient of greatest China172 and in the global ocean26. For example, total
concern in SGD, few studies have reported detailed SGD-derived (19 × 1010 mol per year) nitrogen fluxes into
nitrogen speciation data. Only 31 studies reported the Mediterranean Sea exceed river fluxes (5 × 1010 mol
the three major nitrogen species, and 13 studies also per year)126 by a factor of ~4. Fresh SGD from karstic
reported N speciation in nearby rivers (Fig. 4b). Previous springs in the Mediterranean, a dominant regional fea-
studies often focused on DIN145 (such as nitrate and ture, account for 8–31% of these river-derived nitrogen
ammonium, which are more readily available to pri- fluxes72. In China, an upscaling of local case studies to
mary producers) and overlooked SGD-derived DON the entire coastal zone revealed that total SGD-derived
(which is assimilated at slower rates176) because the con- fluxes of nitrogen, phosphorus and silicate account for
tribution of DON to primary production is unknown. >50% of all known sources, including rivers, atmospheric
Additionally, many SGD studies emphasize nitrate deposition and diffusion from sediments172.
because anthropogenic activities often contribute large At a local scale, SGD-derived nutrient fluxes exceeded
nitrate loads115,177,178, yet, only six of the 31 SGD studies river fluxes in >48% of the compiled study cases, and
reporting ammonium, nitrate and DON found nitrate SGD-derived nutrient fluxes were at least 10% of the
to be the dominant form of nitrogen. All of those sites river fluxes in >90% of the study sites (Fig. 5). Note
were heavily influenced by local contamination sources. that several SGD studies did not report riverine fluxes
Groundwater and seawater DON is often derived of nutrients, perhaps because they were conducted in
from soil leachates, zooplankton excretion and leach- areas with no or minor surface runoff114,191. Furthermore,
ing from microbial and algal biomass that infiltrate we highlight that any comparison between rivers and
subterranean estuaries112,176,179,180. DON increases along SGD at a local scale can be biased, owing to a poten-
the coastal ocean and in surface estuaries, where it tial selection of sites where fresh SGD is expected to be
often constitutes the largest fraction (73 ± 23%) of high and groundwater pollution is known or expected.
the total dissolved nitrogen pool180. Only 40 out of the Direct comparisons of SGD fluxes across hydrolog-
239 study sites included here reported DON data, and ical or land-use gradients using the same method are
no study revealed the composition and bioavailability of uncommon, despite observations in Hawaii 160 and
DON in SGD. On average, DIN accounted for 57 ± 28% northeast USA176,192 showing a clear impact of land use
(median 61%) and DON accounted for 43 ± 27% on SGD-derived nitrogen fluxes.
(median 39%) of total dissolved nitrogen fluxes via SGD. Global patterns of SGD and river distributions show
DON and ammonium are relatively more abundant in a similar dependency on land use, with higher nutri-
non-contaminated groundwater181, but DON may also ent concentrations and N:P ratios in densely populated
originate from anthropogenic sources176. Refractory and agricultural areas145,172,193,194. However, nutrient
DON uptake is often attributed to heterotrophic bacte- fluxes supplied by SGD and rivers might be consid-
ria over timescales of millennia, but the less abundant erably different, depending on the magnitude of dis-
labile DON compounds such as amino acids and urea charge. For instance, about 70% of global SGD occurs
are used up by autotrophic microbes and phytoplank- in the Indo-Pacific Oceans, while less than half of the
ton on timescales of hours to days180. Because of high river waters are discharged in the Indo-Pacific30. River
DON contributions via SGD (Fig. 4b), even a small and SGD fluxes are also considerably different at a local
labile portion could make a difference to the amount or regional scale. In contrast to river discharge that is
of N ultimately available to primary producers. Overall, restricted to specific point sources along the coast such
our compilation supports earlier suggestions that as river mouths, SGD (particularly the saline compo-
DON represents a significant portion of nitrogen in nent) is ubiquitous along permeable sediment and
SGD141,176,179,182,183. muddy shorelines, and is relatively diffuse. Therefore,
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a Water flow DIN DIP DSi
m3 per year Tmol per year
River 3.8 × 1013 1.35 0.045 0.045
Fresh SGD 3.3 × 1011 0.03 0.001 0.001
Total SGD 1.2 × 1014 2.30 0.060 0.060
River inputs
Fresh SGD Saline SGD
b
SGD < river SGD > river
DIN
DIN 59%
25 DIP 53% DIP
DSi 48% DSi
20
Number of study cases
15
10
5
0
100
SGD:river nutrient flux ratios
Fig. 5 | river and SGD-derived nutrient inputs to the ocean. a | A summary of global-scale fluxes compiled from
river163,187,188, fresh submarine groundwater discharge (SGD)63 and total (mostly saline) SGD27,65 estimates. b | Histogram
of ratios between SGD and river-derived dissolved inorganic nitrogen (DIN), dissolved inorganic phosphorus (DIP) and
dissolved silicate (DSi) fluxes summarized from the global study cases reviewed here. In >48% of the global study cases,
SGD-derived nutrient fluxes exceeded river fluxes. In ~90% of the study cases, SGD nutrient fluxes were >10% of river
fluxes, making SGD a non-negligible nutrient pathway in nearly all study sites.
SGD is likely to affect larger coastal areas than river in anthropogenic nitrogen loads in rivers in recent
discharges195. decades198,199. Although rivers are usually dominated by
Both SGD-derived and river-derived fluxes of water a mixture of nitrate and DON, nitrogen in SGD (par-
and dissolved nutrients to the coastal ocean are affected ticularly saline and brackish) is mostly composed of
by seasonal patterns in the hydrological cycle. Seasonal DON and ammonium, owing to reducing conditions in
changes in recharge, evapotranspiration and groundwater organic-rich shallow coastal sediments and mineraliza-
extraction drive water-level changes onshore that pro tion of organic matter (Fig. 1). The contrasting nitrogen
pagate offshore by pressure diffusion. As a result, SGD speciation in SGD and rivers highlights the need for
typically experiences a delayed response to seasonal including the three major dissolved nitrogen species in
fluctuations relative to river fluxes66. Fresh and saline future investigations.
Evapotranspiration SGD rates and associated nutrient fluxes can lag peak The river nutrient transport to the ocean has more
The quantity of water that recharge periods by several months, depending on flow than doubled during the twentieth century184,186,187,200, as
moves to the atmosphere path lengths, aquifer transmissivity, storage properties a result of increases in population and fertilizer use201.
from the plants and soil; and recharge volume59,196,197. Although no similar datasets exist for long-term changes
describes the joint effect of
transpiration, through the
Rivers and SGD are characterized by unique sto- in total SGD, modelled fresh SGD-d erived nitrate
plants, and evaporation, ichiometric ratios and nutrient speciation (Fig. 4) . fluxes increased by about 40% over the second half
directly from the soil. Nitrate accounts for much of the global increase of the twentieth century193. Given the slower response of
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Microphytobenthos groundwater to anthropogenic nutrient inputs, ground- change, depending on the specific location or time of
Living organisms, such as water polluted several decades ago can continue to the year.
unicellular eukaryotic algae discharge, releasing legacy nutrients that impact water The most documented response to SGD-derived
(mainly diatoms) and quality in rivers and the coast even after pollution nutrient loading is related to increasing primary pro-
cyanobacteria, growing in the
upper layers of illuminated
sources cease to exist2,202. For instance, recent investiga- ductivity of phytoplankton or microphytobenthos205.
aquatic sediments. tions at the mouth of the Mississippi River revealed that Chlorophyll is often measured as a proxy for primary
most of the N in surface water had been in the water- productivity derived from SGD206 and most attempts
Cyanobacteria shed for >30 years, as a consequence of the time spent to link SGD and chlorophyll have revealed a positive
Ubiquitous phylum of
both in the soils and travelling along slow groundwater response207 (Fig. 6). The increase in primary productivity
single-celled bacteria that
carry out photosynthesis.
transport pathways2,145,193. Therefore, despite the poten- by SGD inputs from uncontaminated aquifers has been
tial mitigation measures aimed at decreasing terrestrial linked to diatom abundance that effectively use up the
Macrophytes nutrient loads in polluted areas, it can take decades to nitrogen, particularly in areas where SGD can alleviate
Large aquatic plants and achieve the desired reduction of SGD-derived nutrient co-limitation of N and Si (ref.208). A trend towards larger
multicellular algae widespread
in marine, brackish and
loads2,145,193. phytoplankton cell sizes, such as diatoms, in response
freshwater environments, The contribution of groundwater-borne nutrients to to SGD was noted in Hawaiian coastal waters receiving
which are referred to as coastal ocean budgets will likely increase as human activity fresh SGD209. However, it is clear that increased primary
macrophytes to distinguish in coastal watersheds increases181. Climate-change- production resulting from SGD nutrient supply does not
from unicellular algae
derived alterations of precipitation and evapotranspi- always exert a positive response in the ecosystem (Fig. 6).
(phytoplankton).
ration regimes, as well as land-use change, are known Dinoflagellate and cyanobacteria blooms can occur when
to modify the quantity, the quality and the availability ammonium is present in SGD or when inorganic nitro-
of groundwater resources203. Climate-driven sea-level gen is transformed by diatoms into organic nitrogen210.
rise is also known to modify SGD and biogeochemical As observed in Korea211,212 and Florida (USA)213, SGD
cycling within coastal aquifers, and will likely affect the can trigger, fuel and sustain harmful algal blooms, with
magnitude of SGD-driven nutrient inputs56,64 and its devastating consequences to coastal ecosystems. In some
impact on coastal biological communities. However, cases, however, no response was found near sites receiv-
long-term quantitative predictions about the effects of ing fresh groundwater springs, indicating that SGD
climate change on SGD are unavailable. loading does not always induce an increase in primary
productivity214.
Biological impacts of SGD nutrients Macrophyte cover can increase or decrease in response
Research on how SGD nutrients impact marine biota has to SGD. The most studied macrophyte in a SGD con-
increased in recent years204, with nearly 90% of all arti- text are Ulva spp., a leafy alga commonly known as
cles on this topic having been published in the last dec- sea lettuce, which grows faster and increases in abun-
ade (see the supplementary material). The documented dance in response to SGD-derived nitrogen inputs191,215.
response of marine organisms to SGD is quite variable Moreover, nitrogen-rich SGD can also increase the N:P
and site-specific, and can be positive or negative from ratio in macrophyte tissues, which can reduce herbivory
species, community or ecosystem perspectives (Fig. 6). because fish prefer macrophytes with lower N:P ratios216.
The response to SGD is sometimes unclear and could However, macrophytes can also reduce reproduction to
Response to SGD nutrients
Biological response Increase Decrease Unclear/mixed
Species scale Organism abundance 17 2 9
Growth and biomass 9 1 2
Tissue N:P ratio 5 1 2
Disease 1 – –
Community scale Richness 1 1 1
Chlorophyll a 20 – 6
Diversity 3 5 2
N sourcing 21 – 1
Benthic density – 1 3
Ecosystem scale Productivity/photosynthesis 11 – 2
Respiration 2 1 1
Anoxia/deoxygenation 1 – –
Calcification 3 1 –
Fig. 6 | the biological impacts of SGD. The table counts the number of studies demonstrating responses at the species,
community and ecosystem scales to submarine groundwater discharge (SGD). SGD can drive multiple biological
responses, depending on local conditions. The original references are summarized in the supplementary online material.
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0123456789();:Reviews Acid sulfate soils prioritize growth and take advantage of a nitrogen-rich which was observed off a Korean volcanic island with Naturally occurring soils environment created by SGD217. In Hawaii, for instance, large, fresh SGD inputs244. Alternatively, high CO2 from usually found in coastal oligotrophic waters receiving N-enriched SGD had sediment organic matter decomposition54,245,246 or H2SO4 wetlands with a high content of increased macroalgae coverage from
Reviews
Table 1 | a summary of key research topics that require further investigation in the field of submarine groundwater discharge
topic research question major obstacles and challenges research priorities Key
references
Fresh vs saline What are the local and Geochemical tracer investigations often Combine tracers and other approaches 51,79,112
SGD global contributions quantify total SGD. Multiple techniques are to quantify both fresh and saline
of fresh vs saline SGD required to separate fresh from saline SGD. SGD. Integrate marine and terrestrial
and new vs recycled investigations. Adopt a nomenclature that
nutrients? better represents the different processes.
Spatial and What are the temporal Models quantify specific driving forces, Understand the role of spatio-temporal 31,34,56,120
temporal scales and spatial scales whereas geochemical tracers integrate heterogeneity in regional-scale estimates
represented by specific multiple processes on timescales to allow predictions in space and time.
SGD estimates? comparable to the tracer residence time.
Nutrient What biogeochemical Defining the nutrient endmember in SGD Identify how microbial communities drive 145,148,266
transformations processes control requires understanding of sources and nutrient cycling. Quantify the effect of
nutrient transformation pathways. Transformations are governed by subterranean estuaries in regional-scale
in the subsurface? dynamic hydrological and biogeochemical land–ocean nutrient budgets.
processes at multiple scales.
Long-term How will ongoing Poor quantitative understanding of drivers Make long-term observations. Enhance 193,203,267
observations climate change, of SGD. No straightforward typological collaborations with climate change experts
and predictions sea-level rise and classification is available for both fresh and and modellers to estimate uncertainty and
land-use intensification saline SGD. Case studies often represent improve the compatibility between
modify SGD? snapshot estimates. observations and predictions.
Spatial bias Is our current Ongoing focus on areas of known SGD, Quantify SGD in poorly represented regions. 66,67
knowledge of SGD such as visible springs or locations with Representative regional-scale quantification
biased owing to polluted groundwater. Poorly represented of SGD to understand occurrence,
spatial gaps and areas (such as South America, Africa and heterogeneity and/or patchiness.
site selection? the poles).
Management How can SGD Groundwater and surface water often seem Promote outreach activities and exchange 138,170,232
be incorporated disconnected. SGD perceived to be a highly knowledge on SGD with society and local/
into water quality specialized research niche. regional managers. Develop best-practice
management plans? recommendations for management.
Biological Is supply of nutrients SGD effects are complex and site-specific. Include biota assessments in SGD studies. 204,215,216
effects via SGD beneficial Most investigations focus on individual Explore effects of SGD from the base of
or harmful to marine species or small-scale organisms. food webs through the entire ecosystem.
ecosystems? Use manipulative experiments to explore
biological effects.
Uncertainties What are the Uncertainties of methods used to derive Report real uncertainties in SGD estimates, 70,75
uncertainties SGD are difficult to constrain and often not including errors in model conceptualization.
associated with local reported. Uncertainties linked to spatial or Apply mathematical methods to express
and global SGD temporal integrations are unknown. uncertainties based on unavertable
estimates? limitations in the representation of SGD.
For example, the flow of groundwater from a large been shown to be particularly high in urbanized areas
septic system in California (USA) has been managed in developing countries such as Indonesia 194, the
to prevent pollution of popular swimming beaches256 Philippines261 and China172. Because SGD can enhance
affected by groundwater-borne faecal contamination257. primary productivity and fish abundance229,230, it would
Engineering solutions have been attempted to reduce also connect to Goal 2 ‘Zero Hunger’ (Target 2.3), par-
fresh SGD and secure onshore groundwater use. In par- ticularly in the context of regional-scale fisheries that
ticular, attempts to close karstic caves or tap subma- are sometimes sustained by SGD-d erived nutrient
rine springs were made in the French Mediterranean inputs231. SGD affects artisanal fisheries in small-island,
coast 258. In China’s Bohai Sea, underground con- tropical developing countries262, where fresh SGD is
crete dams were constructed to prevent connections also especially relevant66,69. Interventions like China’s
between seawater and fresh groundwater, reducing SGD underground dams that are intended to increase drink-
and seawater intrusion, and improving local freshwater ing water availability also link SGD management to
availability259. Goal 6’s Target 6.4 to “ensure sustainable (water) with-
SGD is relevant to a wide range of the United Nations drawals” and Target 6.6 to “protect and restore (fresh-)
Sustainable Development Goals. For example, SGD con- water-related ecosystems” that could exist around
nects clearly to Goal 14 ‘Life Below Water; and Target submarine springs 263. Through sustaining marine
14.1 to reduce pollution in marine ecosystems. Hence, ecosystems as well as releasing alkalinity and carbon
SGD-derived nutrient fluxes should be considered par- dioxide to surface waters264, SGD is relevant to Goal 13
ticularly when sensitive coastal ecosystems degrade194 or ‘Climate Action’.
during coastal development modifying groundwater– The cultural value of places is traditionally recog
surface water connectivity, such as the construction nized in planning and legislation. In addition to
of drains and canals260. Nutrient fluxes via SGD have apparent links to the Sustainable Development Goals,
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SGD also has local cultural relevance232. Many subma- how it contributes to coastal nutrient budgets, a number
rine springs have significant spiritual value and relate of major research questions remain open (Table 1).
to local legends. For example, the magical Hawaiian sea Our growing knowledge in the last decade shows
turtle Kauila has been told to have dug local springs for that considering SGD is clearly essential for developing
its offspring. The Kaurna Aboriginal people in Australia coastal and marine nutrient budgets on local and global
tell of Tjilbruke, a magical spirit who wept at the beach scales. About 60% of the reviewed investigations revealed
and made the springs flow. In Bali, the Tanah Lot tem- that total SGD-derived nutrient fluxes exceed rivers on
ple, which was built on a submarine spring to worship local, regional or global scales. However, SGD studies are
a magical being (Dang Hyang Nirartha) that moved the generally site-specific and fixed in time, without predic-
spring from land to the sea, attracts around 2 million tive power. Climate and land-use change are expected
visitors annually232. We do not know the abundance of to modify patterns of global water use, drive sea-level
such cases, since the cultural significance of SGD has not rise, push or pull seawater into coastal aquifers and
been documented in detail. modify the chemical composition of groundwater93,203.
The connections to multiple Sustainable Development Combined, these changes are expected to modify fresh
Goals and their cultural relevance illustrate the complex- and saline SGD. A better understanding of SGD fluxes,
ity with which SGD can be intertwined to livelihoods. drivers and pathways is essential for determining the
These connections should justify the assimilation of carrying capacity of coastal seas and their response to
SGD into coastal management plans, but assimilation increased anthropogenic pressures (Table 1). Nutrient
has seldom occurred. A more integrated approach con- budgets considering SGD are required for the effective
sidering SGD, not only rivers, is needed to maximize interpretation of natural and anthropogenic sources,
coastal water quality management outcomes250. The slow as well as creating management solutions in highly
movement of SGD relative to rivers implies that current modified coastal systems.
contaminant and nutrient flows reflect past inputs, and Large investments have been made on the mitigation
management approaches must prepare for increasing of coastal eutrophication and the protection of marine
loads in the decades to come93. biodiversity. However, recent reductions in river and
atmospheric nutrient inputs in developed countries
Summary and outlook have not been enough to reduce coastal eutrophication
Quantifying SGD-derived nutrient fluxes is challeng- and related hypoxic events in key areas such as the Baltic
ing and involves nuanced assumptions and interpre- Sea7, the shelf off the Mississippi River265 and the China
tations, and a wide range of skills in oceanography, coast172. As SGD fluxes, pathways and drivers are better
hydrology and biogeochemistry. A disciplinary frag- understood, it will be possible to detect how changes in
mentation, time lags in groundwater flows and slow SGD relate to disturbances such as land-use change,
management responses have created barriers to scientific habitat clearing and climate change.
progress and incorporation of SGD in coastal nutrient
budgets. To further build the SGD field and understand Published online xx xx xxxx
1. Howarth, R. W. Coastal nitrogen pollution: A review Chinese estuaries: the Pearl River Estuary and the 19. Tait, D. R., Maher, D. T., Sanders, C. J. & Santos, I. R.
of sources and trends globally and regionally. Changjiang River Estuary. Estuar. Coast. Shelf Sci. Radium-derived porewater exchange and dissolved
Harmful Algae 8, 14–20 (2008). 203, 17–28 (2018). N and P fluxes in mangroves. Geochim. Cosmochim.
2. Van Meter, K. J., Van Cappellen, P. & Basu, N. B. 11. Charette, M. A. & Buesseler, K. O. Submarine Acta 200, 295–309 (2017).
Legacy nitrogen may prevent achievement of water groundwater discharge of nutrients and copper to an 20. Wilson, A. & Morris, J. The influence of tidal forcing
quality goals in the Gulf of Mexico. Science 360, urban subestuary of Chesapeake Bay (Elizabeth River). on groundwater flow and nutrient exchange in a salt
427–430 (2018). Limnol. Oceanogr. 49, 376–385 (2004). marsh-dominated estuary. Biogeochemistry 108,
Revealed how the legacy of nitrogen pollution 12. Paytan, A. et al. Submarine groundwater discharge: 27–38 (2012).
in soils will eventually reach the coastal ocean, An important source of new inorganic nitrogen to coral 21. Tobias, C. R., Macko, S. A., Anderson, I. C.,
even after sources are controlled. reef ecosystems. Limnol. Oceanogr. 51, 343–348 Canuel, E. A. & Harvey, J. W. Tracking the fate of a
3. HELCOM. First version of the ‘State of the (2006). high concentration groundwater nitrate plume through
Baltic Sea’ report–June 2017. HELCOM http:// 13. McMahon, A. & Santos, I. R. Nitrogen enrichment a fringing marsh: A combined groundwater tracer and
stateofthebalticsea.helcom.fi (2017). and speciation in a coral reef lagoon driven by in situ isotope enrichment study. Limnol. Oceanogr.
4. Ilnicki, P. Emissions of nitrogen and phosphorus into groundwater inputs of bird guano. J. Geophys. 46, 1977–1989 (2001).
rivers from agricultural land‒selected controversial Res. Oceans 122, 7218–7236 (2017). 22. Moore, W. S., Blanton, J. O. & Joye, S. B. Estimates
issues. J. Water Land Dev. 23, 31–39 (2014). 14. Oehler, T. et al. Nutrient dynamics in submarine of flushing times, submarine groundwater discharge,
5. Breitburg, D. et al. Declining oxygen in the global groundwater discharge through a coral reef and nutrient fluxes to Okatee Estuary, South Carolina.
ocean and coastal waters. Science 359, eaam7240 (western Lombok, Indonesia). Limnol. Oceanogr. 64, J. Geophys. Res. 111, C09006 (2006).
(2018). 2646–2661 (2019). 23. Smith, C. G. & Swarzenski, P. W. An investigation of
6. Conley, D. J. et al. Hypoxia is increasing in the coastal 15. Lee, Y.-W., Hwang, D.-W., Kim, G., Lee, W.-C. & submarine groundwater—borne nutrient fluxes to the
zone of the Baltic Sea. Environ. Sci. Technol. 45, Oh, H.-T. Nutrient inputs from submarine groundwater west Florida shelf and recurrent harmful algal blooms.
6777–6783 (2011). discharge (SGD) in Masan Bay, an embayment Limnol. Oceanogr. 57, 471–485 (2012).
7. Carstensen, J., Andersen, J. H., Gustafsson, B. G. & surrounded by heavily industrialized cities, Korea. 24. Wang, X., Baskaran, M., Su, K. & Du, J. The important
Conley, D. J. Deoxygenation of the Baltic Sea during Sci. Total Environ. 407, 3181–3188 (2009). role of submarine groundwater discharge (SGD) to
the last century. Proc. Natl Acad. Sci. USA 111, 16. Rodellas, V. et al. Submarine groundwater discharge derive nutrient fluxes into River dominated Ocean
5628–5633 (2014). as a source of nutrients and trace metals in a Margins – The East China Sea. Mar. Chem. 204,
8. Johannes, R. E. The ecological significance of the Mediterranean bay (Palma Beach, Balearic Islands). 121–132 (2018).
submarine groundwater discharge. Mar. Ecol. Mar. Chem. 160, 56–66 (2014). 25. Niencheski, L. F. H., Windom, H. L., Moore, W. S. &
Prog. Ser. 3, 365–373 (1980). 17. Liefer, J. D., MacIntyre, H. L., Su, N. & Burnett, W. C. Jahnke, R. A. Submarine groundwater discharge of
Pioneering regional-scale demonstration of Seasonal alternation between groundwater discharge nutrients to the ocean along a coastal lagoon barrier,
the importance of SGD-derived nutrient fluxes and benthic coupling as nutrient sources in a shallow Southern Brazil. Mar. Chem. 106, 546–561 (2007).
in Australia. coastal lagoon. Estuar. Coasts 37, 925–940 (2014). 26. Cho, H. M. et al. Radium tracing nutrient inputs
9. Johannes, R. E. & Hearn, C. J. The effect of submarine 18. Gleeson, J., Santos, I. R., Maher, D. T. & through submarine groundwater discharge in the
groundwater discharge on nutrient and salinity regimes Golsby-Smith, L. Groundwater–surface water global ocean. Sci. Rep. 8, 2439 (2018).
in a coastal lagoon off Perth, Western Australia. exchange in a mangrove tidal creek: Evidence A global estimate of total SGD-derived nutrient
Estuar. Coast. Shelf Sci. 21, 789–800 (1985). from natural geochemical tracers and implications fluxes into the ocean.
10. Liu, J., Du, J., Wu, Y. & Liu, S. Nutrient input through for nutrient budgets. Mar. Chem. 156, 27–37 27. Sawyer, A. H., David, C. H. & Famiglietti, J. S.
submarine groundwater discharge in two major (2013). Continental patterns of submarine groundwater
Nature Reviews | Earth & Environment
0123456789();:Reviews
discharge reveal coastal vulnerabilities. Science 353, 47. Johnson, A. G., Glenn, C. R., Burnett, W. C., tropical islands: a review. Grundwasser 20, 53–67
705–707 (2016). Peterson, R. N. & Lucey, P. G. Aerial infrared imaging (2015).
Continental-scale modelling revealing hotspots of reveals large nutrient-rich groundwater inputs to 70. Zhou, Y., Befus, K. M., Sawyer, A. H. & David, C. H.
fresh SGD and nutrient inputs in the United States. the ocean. Geophys. Res. Lett. 35, L15606 (2008). Opportunities and challenges in computing fresh
28. Taniguchi, M. et al. Submarine groundwater discharge: 48. Wilson, J. & Rocha, C. Regional scale assessment groundwater discharge to continental coastlines: a
updates on its measurement techniques, geophysical of submarine groundwater discharge in Ireland multimodel comparison for the United States Gulf and
drivers, magnitudes, and effects. Front. Environ. Sci. 7, combining medium resolution satellite imagery Atlantic Coasts. Water Resour. Res. 54, 8363–8380
141 (2019). and geochemical tracing techniques. Remote Sens. (2018).
29. Burnett, W., Bokuniewicz, H., Huettel, M., Moore, W. S. Environ. 119, 21–34 (2012). 71. Kim, G., Lee, K. K., Park, K. S., Hwang, D. W. &
& Taniguchi, M. Groundwater and pore water inputs to 49. Weinstein, Y. et al. What is the role of fresh Yang, H. S. Large submarine groundwater discharge
the coastal zone. Biogeochemistry 66, 3–33 (2003). groundwater and recirculated seawater in conveying (SGD) from a volcanic island. Geophys. Res. Lett. 30,
Established the modern definition of SGD and nutrients to the coastal ocean? Environ. Sci. Technol. 2098 (2003).
reviewed progress in the field. 45, 5195–5200 (2011). 72. Chen, X. et al. Karstic submarine groundwater
30. Moore, W. S. The effect of submarine groundwater 50. Sadat-Noori, M., Santos, I. R., Tait, D. R. & Maher, D. T. discharge into the Mediterranean: Radon-based
discharge on the ocean. Annu. Rev. Mar. Sci. 2, Fresh meteoric versus recirculated saline groundwater nutrient fluxes in an anchialine cave and a basin-wide
59–88 (2010). nutrient inputs into a subtropical estuary. Sci. Total upscaling. Geochim. Cosmochim. Acta 268, 467–484
31. Bratton, J. F. The three scales of submarine Environ. 566–567, 1440–1453 (2016). (2020).
groundwater flow and discharge across passive 51. Rodellas, V. et al. Groundwater-driven nutrient inputs 73. Garcia-Solsona, E. et al. An assessment of karstic
continental margins. J. Geol. 118, 565–575 (2010). to coastal lagoons: The relevance of lagoon water submarine groundwater and associated nutrient
Discussed the scales of SGD and implications of recirculation as a conveyor of dissolved nutrients. discharge to a Mediterranean coastal area
sea-level rise. Sci. Total Environ. 642, 764–780 (2018). (Balearic Islands, Spain) using radium isotopes.
32. Taniguchi, M., Burnett, W., Cable, J. E. & Turner, J. V. 52. Wilson, A. M. Fresh and saline groundwater discharge Biogeochemistry 97, 211–229 (2010).
Investigation of submarine groundwater discharge. to the ocean: A regional perspective. Water Resour. 74. Abbott, B. W. et al. Human domination of the global
Hydrol. Process. 16, 2115–2129 (2002). Res. 41, W0216 (2005). water cycle absent from depictions and perceptions.
Early compilation of study cases estimating 53. Correa, R. E. et al. Submarine groundwater discharge Nat. Geosci. 12, 533–540 (2019).
SGD rates. and associated nutrient and carbon inputs into Sydney 75. Rodellas, V. et al. Conceptual uncertainties in
33. Burnett, W. C. et al. Quantifying submarine Harbour (Australia). J. Hydrol. 580, 124262 (2020). groundwater and porewater fluxes estimated by
groundwater discharge in the coastal zone via multiple 54. Call, M. et al. High pore-water derived CO2 and CH4 radon and radium mass balances. Limnol. Oceanogr.
methods. Sci. Total Environ. 367, 498–543 (2006). emissions from a macro-tidal mangrove creek in the https://doi.org/10.1002/lno.11678 (2021).
34. Robinson, C. E. et al. Groundwater dynamics in Amazon region. Geochim. Cosmochim. Acta 247, 76. Sadat-Noori, M., Santos, I. R., Sanders, C. J.,
subterranean estuaries of coastal unconfined aquifers: 106–120 (2019). Sanders, L. M. & Maher, D. T. Groundwater discharge
Controls on submarine groundwater discharge and 55. Seidel, M. et al. Biogeochemistry of dissolved organic into an estuary using spatially distributed radon time
chemical inputs to the ocean. Adv. Water Resour. 115, matter in an anoxic intertidal creek bank. Geochim. series and radium isotopes. J. Hydrol. 528, 703–719
315–331 (2018). Cosmochim. Acta 140, 418–434 (2014). (2015).
A summary of global experiments comparing 56. Wilson, A. M., Evans, T. B., Moore, W. S., Schutte, C. A. 77. Sadat-Noori, M., Santos, I. R., Tait, D. R., Reading, M. J.
methods that have become the essential tools & Joye, S. B. What time scales are important for & Sanders, C. J. High porewater exchange in a
to quantify SGD. monitoring tidally influenced submarine groundwater mangrove-dominated estuary revealed from short-lived
35. Santos, I. R., Burnett, W. C., Chanton, J., Dimova, N. discharge? Insights from a salt marsh. Water Resour. radium isotopes. J. Hydrol. 553, 188–198 (2017).
& Peterson, R. Land or ocean?: Assessing the driving Res. 51, 4198–4207 (2015). 78. Weinstein, Y. et al. Role of aquifer heterogeneity in
forces of submarine groundwater discharge at a 57. Wilson, A. M. The occurrence and chemical fresh groundwater discharge and seawater recycling:
coastal site in the Gulf of Mexico. J. Geophys. Res. implications of geothermal convection of seawater in An example from the Carmel coast, Israel. J. Geophys.
114, C04012 (2009). continental shelves. Geophys. Res. Lett. 30, 2127 Res. 112, C12016 (2007).
36. Post, V. E. A. et al. Offshore fresh groundwater (2003). 79. Tamborski, J. et al. A comparison between water
reserves as a global phenomenon. Nature 504, 58. Moore, W. S. A reevaluation of submarine groundwater circulation and terrestrially-driven dissolved silica
71–78 (2013). discharge along the southeastern coast of North fluxes to the Mediterranean Sea traced using radium
Described SGD beyond the continental shelf with America. Glob. Biogeochem. Cycles 24, GB4005 isotopes. Geochim. Cosmochim. Acta 238, 496–515
implications for global offshore water resource use (2010). (2018).
and management. 59. Gonneea, M. E. & Charette, M. A. Hydrologic controls 80. Li, L., Barry, D. A., Stagnitti, F. & Parlange, J. Y.
37. Sanford, W. E. & Pope, J. P. Quantifying groundwater’s on nutrient cycling in an unconfined coastal aquifer. Submarine groundwater discharge and associated
role in delaying improvements to Chesapeake Environ. Sci. Technol. 48, 14178–14185 (2014). chemical input to a coastal sea. Water Resour. Res.
Bay water quality. Environ. Sci. Technol. 47, 60. Santos, I. R. et al. Extended time series measurements 35, 3253–3259 (1999).
13330–13338 (2013). of submarine groundwater discharge tracers (222Rn 81. Lopez, C. V., Murgulet, D. & Santos, I. R. Radioactive
38. Moore, W. S. Large groundwater inputs to coastal and CH4) at a coastal site in Florida. Mar. Chem. 113, and stable isotope measurements reveal saline
environments revealed by 226Ra enrichments. Nature 137–147 (2009). submarine groundwater discharge in a semiarid
380, 612–614 (1996). 61. Rodellas, V. et al. Temporal variations in porewater estuary. J. Hydrol. 590, 125395 (2020).
First large-scale quantification of SGD in the fluxes to a coastal lagoon driven by wind waves and 82. Beck, M. et al. The drivers of biogeochemistry in
coastal ocean using radium-226, inspiring a changes in lagoon water depths. J. Hydrol. 581, beach ecosystems: A cross-shore transect from the
generation of SGD researchers. 124363 (2020). dunes to the low-water line. Mar. Chem. 190, 35–50
39. Burnett, W. C., Kim, G. & Lane-Smith, D. A continuous 62. Moore, W. S. & Wilson, A. M. Advective flow through (2017).
monitor for assessment of 222Rn in the coastal ocean. the upper continental shelf driven by storms, 83. Billerbeck, M. et al. Surficial and deep pore water
J. Radioanal. Nucl. Chem. 249, 167–172 (2001). buoyancy, and submarine groundwater discharge. circulation governs spatial and temporal scales of
40. Cable, J. E., Bugna, G. C., Burnett, W. & Chanton, J. P. Earth Planet. Sci. Lett. 235, 564–576 (2005). nutrient recycling in intertidal sand flat sediment.
Application of 222Rn and CH4 for assessment of 63. Santos, I. R., Cook, P. L. M., Rogers, L., de Weys, J. & Mar. Ecol. Prog. Ser. 326, 61–76 (2006).
groundwater discharge to the coastal ocean. Eyre, B. D. The “salt wedge pump”: Convection-driven 84. Santos, I. R. et al. Porewater exchange as a driver of
Limnol. Oceanogr. 41, 1347–1353 (1996). pore-water exchange as a source of dissolved organic carbon dynamics across a terrestrial-marine transect:
41. Michael, H. A., Lubetsky, J. S. & Harvey, C. F. and inorganic carbon and nitrogen to an estuary. Insights from coupled 222Rn and pCO2 observations
Characterizing submarine groundwater discharge: Limnol. Oceanogr. 57, 1415–1426 (2012). in the German Wadden Sea. Mar. Chem. 171, 10–20
A seepage meter study in Waquoit Bay, Massachusetts. 64. Gonneea, M. E., Mulligan, A. E. & Charette, M. A. (2015).
Geophys. Res. Lett. 30, 1297 (2003). Climate-driven sea level anomalies modulate coastal 85. Charbonnier, C., Anschutz, P., Poirier, D., Bujan, S. &
42. Oberdorfer, J. A. Hydrogeologic modeling of groundwater dynamics and discharge. Geophys. Res. Lecroart, P. Aerobic respiration in a high-energy sandy
submarine groundwater discharge: Comparison to Lett. 40, 2701–2706 (2013). beach. Mar. Chem. 155, 10–21 (2013).
other quantitative methods. Biogeochemistry 66, 65. George, C. et al. A new mechanism for submarine 86. Andrisoa, A., Stieglitz, T. C., Rodellas, V. &
159–169 (2003). groundwater discharge from continental shelves. Raimbault, P. Primary production in coastal lagoons
43. Evans, T. B., White, S. M. & Wilson, A. M. Coastal Water Resour. Res. 56, e2019WR026866 (2020). supported by groundwater discharge and porewater
groundwater flow at the nearshore and embayment 66. Luijendijk, E., Gleeson, T. & Moosdorf, N. Fresh fluxes inferred from nitrogen and carbon isotope
scales: a field and modeling study. Water Resour. Res. groundwater discharge insignificant for the world’s signatures. Mar. Chem. 210, 48–60 (2019).
56, e2019WR026445 (2020). oceans but important for coastal ecosystems. 87. Charette, M. A. & Sholkovitz, E. R. Oxidative
44. Russoniello, C. J., Heiss, J. W. & Michael, H. A. Nat. Commun. 11, 1260 (2020). precipitation of groundwater-derived ferrous iron in
Variability in benthic exchange rate, depth, and A global modelling effort to estimate fresh SGD, the subterranean estuary of a coastal bay. Geophys.
residence time beneath a shallow coastal estuary. identifying regional hotspots. Res. Lett. 29, 85-1–85-4 (2002).
J. Geophys. Res. Oceans 123, 1860–1876 (2018). 67. Zhou, Y., Sawyer, A. H., David, C. H. & Famiglietti, J. S. 88. Spiteri, C., Regnier, P., Slomp, C. P. & Charette, M. A.
45. Swarzenski, P. W. et al. Combined time-series Fresh submarine groundwater discharge to the near- pH-Dependent iron oxide precipitation in a
resistivity and geochemical tracer techniques to global coast. Geophys. Res. Lett. 46, 5855–5863 subterranean estuary. J. Geochem. Explor. 88,
examine submarine groundwater discharge at (2019). 399–403 (2006).
Dor Beach, Israel. Geophys. Res. Lett. 33, L24405 68. Kwon, E. Y. et al. Global estimate of submarine 89. Spiteri, C., Slomp, C. P., Tuncay, K. & Meile, C.
(2006). groundwater discharge based on an observationally Modeling biogeochemical processes in subterranean
46. Dimova, N. T., Swarzenski, P. W., Dulaiova, H. & constrained radium isotope model. Geophys. Res. estuaries: Effect of flow dynamics and redox conditions
Glenn, C. R. Utilizing multichannel electrical resistivity Lett. 41, 8438–8444 (2014). on submarine groundwater discharge of nutrients.
methods to examine the dynamics of the fresh water– Estimates global SGD rates based on radium Water Resour. Res. 44, W02430 (2008).
seawater interface in two Hawaiian groundwater isotopes. 90. Erler, D. V. et al. Nitrogen transformations within
systems. J. Geophys. Res. Oceans 117, C02012 69. Moosdorf, N., Stieglitz, T., Waska, H., Dürr, H. H. & a tropical subterranean estuary. Mar. Chem. 164,
(2012). Hartmann, J. Submarine groundwater discharge from 38–47 (2014).
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