A prospectus for periphyton: recent and future ecological research
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J. N. Am. Benthol. Soc., 2010, 29(1):182–206
’ 2010 by The North American Benthological Society
DOI: 10.1899/08-063.1
Published online: 5 February 2010
A prospectus for periphyton: recent and future ecological research
Scott T. Larned1
National Institute of Water and Atmospheric Research, P.O. Box 8602, Riccarton,
Christchurch, New Zealand
Abstract. The presence, abundance, composition, and growth of periphyton are controlled or influenced
by 5 broad classes of environmental variation: disturbances, stressors, resources, hydraulic conditions, and
biotic interactions. In turn, periphyton communities affect water chemistry, hydraulic conditions, habitat
availability, and foodweb dynamics. This review focuses on responses of periphyton communities to
environmental variation. A specific objective of the review is to identify robust periphyton–environment
relationships and insightful concepts. Contributors to J-NABS have led the field in testing and expanding
concepts in periphyton ecology. J-NABS papers about periphyton patch dynamics, light- and nutrient-
limited periphyton growth, and the effects of disturbances on periphyton structure and function have been
particularly influential. However, many topics in periphyton ecology remain unexplored and under-
explored. These topics include resource colimitation, physiological responses to stressors, allelopathy,
competitive inhibition and exclusion, and the effects of drag forces and turbulence. Periphyton ecology
studies in J-NABS tend to be multivariable, phenomenological, and nonmechanistic. Such studies provide
information about temporal and spatial patterns, but rarely provide evidence for the causes of those
patterns. These studies are often impaired by low statistical power and insufficient experimental control.
Periphyton ecology needs more rigorous manipulative experiments, particularly experiments that generate
clear relationships between environmental drivers and ecological responses.
Key words: periphyton, physical disturbance, stressors, resource limitation, competitive interactions,
hydraulic ecology.
Periphyton communities are solar-powered biogeo- Further contributions from periphyton ecology will
chemical reactors, biogenic habitats, hydraulic rough- require a more diverse set of periphyton–environment
ness elements, early warning systems for environ- relationships.
mental degradation, and troves of biodiversity. This The term periphyton refers to assemblages of
abridged list gives some indication of the ecological freshwater benthic photoautotrophic algae and pro-
and cultural importance of periphyton. The roles of karyotes. Heterotrophic and chemoautotrophic bacte-
periphyton in freshwater ecosystems and society have ria, fungi, protozoans, metazoans, and viruses inhabit
warranted several book-length reviews (Stevenson periphytic communities, but are not included in this
et al. 1996 [Fig. 1], Wehr and Sheath 2003, Azim et al. review. Symbiotic and endophytic algae and cyano-
2005). My review focuses on the responses of bacteria also are excluded because they are partially
periphyton communities to variation in the chemical, isolated from the external environment by their hosts.
solar, thermal, and hydraulic environment, and to The taxa that are included comprise a diverse group,
competitive interactions. By advancing our knowl- represented by thousands of taxa, with a size range
edge of these responses, freshwater ecologists can spanning 6 orders of magnitude (mm–m).
contribute to the growth of ecological theory and to About 150 papers with periphyton ecology as a
improvements in ecosystem management. Contribu- primary focus have been published in J-NABS (based
tions of periphyton ecology to theory and manage- on searches of Web of Science, Google Scholar,
ment are partly based on the availability of robust Biological Abstracts, and J-NABS bibliographies).
mechanistic relationships, such as those linking Most of these papers can be classified into 7 broad
diversity to flood frequency (Biggs and Smith 2002), topics: 1) effects of physical disturbances, 2) effects of
and photosynthesis to light (Graham et al. 1995). exposure to stressors, 3) limiting abiotic factors, 4)
competitive interactions, 5) effects of herbivores, 6)
1
E-mail address: s.larned@niwa.co.nz periphytic algae as environmental indicators, and 7)
1822010] PERIPHYTON ECOLOGY 183
FIG. 1. Selected publications in periphyton ecology from the early 20th century to the present. This timeline illustrates the
diversity of research topics, not the original, most important, or most highly-cited publications. Bold font indicates paper was
published in J-NABS.
the roles of periphyton in nutrient cycling in food A Brief History
webs and between abiotic pools. The last 3 topics are
My review is focused on research in periphyton
covered in companion reviews on interspecific inter-
ecology over the last 25 y (since the start of J-NABS in
actions (Holomuzki et al. 20101), biomonitoring
1986), but biologists have studied periphyton for far
(Dolédec and Statzner 2010), and nutrient dynamics
longer. The origins of periphyton ecology are difficult
(Mulholland and Webster 2010), and are not dis-
to identify precisely because benthic algae have been
cussed here. An additional topic in this review is the
used for centuries as model systems for studies of
influence of hydraulic conditions (e.g., drag forces
phototropism, light limitation, and other problems in
and turbulence) on periphyton. Despite an encourag-
ecophysiology (e.g., Blackman and Smith 1910, Alli-
ing review in J-NABS (Statzner et al. 1988), studies of
son and Morris 1930; Fig. 1). Two of the earliest
periphyton–hydraulic interactions are uncommon.
reports from periphyton field studies (Fritsch 1906,
Because J-NABS is one of the principal journals in
Brown 1908 [Fig. 1]) concern seasonal variation in the
periphyton ecology, the specific contributions of J-
growth and composition of stream and pond algae.
NABS articles to the topics in this review are
Similar descriptive studies dominated the field for
highlighted at the end of each topic section.
several decades (e.g., Transeau 1916, Eddy 1925
[Fig. 1], Butcher 1932). In the 1940s, 2 new research
areas emerged: the effects of abiotic factors on
1
Boldface indicates paper was published in J-NABS periphyton composition and abundance, and the use184 S. T. LARNED [Volume 29 of periphyton communities as indicators of stream and taxonomic turnover, can also be applied to health. Ruth Patrick (Academy of Natural Sciences, periphyton landscapes at mm- or mm-scales (Battin Philadelphia) and R. W. Butcher (British Ministry of et al. 2007). Agriculture and Fisheries) were pioneers of both areas Many of the advances in periphyton ecology in (Butcher 1940 [Fig. 1], 1947, Patrick 1948 [Fig. 1], the last decade were made possible by parallel 1949). Periphyton ecology in the modern sense (e.g., advances in technology. Microelectrodes and op- manipulative experiments and multiple response and todes allow researchers to make detailed biogeo- explanatory variables) began in the 1950s and chemical profiles within periphyton mats (Kühl and expanded rapidly. This period coincided with the Polerecky 2008). Acoustic and optical velocimeters development of radiotracers and other technologies, a are used to characterize hydraulic conditions near growing awareness of human impacts on the envi- and within periphyton communities, and boundary- ronment, and a focus on freshwater ecosystems by layer transport to and from periphyton (Hart and systems ecologists (e.g., Odum 1956; Fig. 1). The new Finelli 1999 [Fig. 1], Larned et al. 2004). Confocal technologies facilitated research in stream metabolism laser scanning microscopy has influenced the pe- and organic matter budgets (Odum 1956, 1957, Teal riphyton–landscape perspective by making fine- 1957, McIntire 1966 [Fig. 1]), light-limited primary scaled studies of intact periphyton structures possi- production (McConnell and Sigler 1959; Fig. 1), and ble (Larson and Passy 2005). The development of nutrient uptake by periphyton (Whitford and Schu- pulse-amplitude modulated (PAM) fluorometers in macher 1961; Fig. 1). Some of the seminal field studies the 1980s allowed ecologists to measure photosyn- of lotic periphyton and grazing, succession, and thetic performance in periphyton communities in nutrient cycling were published between the late situ and without instrument interference. Applica- 1950s and the early 1970s (e.g., Douglas 1958, Tippett tions of PAM fluorometry in periphyton ecology 1970, Elwood and Nelson 1972 [Fig. 1], Horne and quickly diversified and now range from toxicology Carmiggelt 1975). During this period, the Hubbard to light and nutrient limitation (Vopel and Hawes Brook Ecosystem Study, the Walker Branch Water- 2006, Muller et al. 2008). shed Project, and other ecosystems research programs provided logistical support for a large cohort of Physical Disturbances stream ecologists, including periphyton specialists. The mid-1970s to late 1990s were characterized by Ecologists generally define physical disturbances as rapid growth in studies of natural disturbances and episodic events that remove organisms at rates faster light and nutrient limitation. A remarkable number of than rates of accrual or recruitment (Peterson 1996, these studies were published in J-NABS; among the Biggs et al. 1999a, Stanley et al. 2010). In earlier most frequently cited are Lowe et al. (1986), Pringle et studies, including a widely cited J-NABS review al. (1988; Fig. 1), Grimm and Fisher (1989), Mulhol- (Resh et al. 1988), disturbances were defined as land et al. (1991), McCormick et al. (1996), and Biggs unpredictable events, and were distinguished from et al. (1999a). Since the late 1990s, the number of predictable events, such as spring snowmelt. The disturbance and resource-limitation studies in J-NABS rationale for this restricted definition was that motile and other aquatic ecology journals has declined, aquatic organisms might be genetically programmed presumably because of shifts in research agendas to avoid deleterious predictable events (Townsend and funding. and Hildrew 1994). For immobile periphyton, no clear The last decade has seen continued growth in thresholds separate predictable and unpredictable periphyton ecology and new conceptual models about disturbances, and the restricted definition is unnec- the development of periphyton communities and essary. their responses to the external environment. One of In broad terms, physical disturbances that affect the most intriguing new models is the microbial (or periphyton include desiccation, anoxia, freezing, periphyton) landscape, which views periphyton com- rapid changes in osmotic potential, acute contaminant munities as dynamic structures with spatially distrib- exposure, substrate movement, and rapid increases in uted populations, shifting zones of biogeochemical hydraulic forces, heat, and light. Hydraulic forces, activity, and continual community–environment feed- substrate movement, and desiccation are frequently back (Battin et al. 2003, 2007, Arnon et al. 2007, studied by periphyton ecologists, and their effects are Besemer et al. 2007, Kühl and Polerecky 2008). A the focus of this section. Periphyton responses to long- corollary of the periphyton landscape perspective is term, sublethal exposure to contaminants and other that processes traditionally associated with large-scale stressors are discussed in the following section. landscape ecology, such as metapopulation dynamics Separating stresses and disturbances on the basis of
2010] PERIPHYTON ECOLOGY 185
duration is artificial, but it highlights the need to In addition to the direct effects of increased shear
consider disturbances in the context of periphyton stress, elevated flows affect periphyton by increasing
generation times and rates of community succession. sediment mobility, which leads to abrasion by
Taxa with generation times shorter than disturbance suspended sediment and substrate tumbling (Grimm
recurrence intervals are likely to be more resilient to and Fisher 1989, Uehlinger 1991, Biggs et al. 1999a).
those disturbances than taxa with longer generation In field studies, the direct effects of shear stress are
times (Steinman and McIntire 1990). Similarly, pe- rarely distinguished from the effects of sediment
riphyton communities characterized by rapid succes- movement. However, Biggs et al. (1999a) identified
sion are likely to be more resilient than those natural conditions in which these factors vary
characterized by slow succession or many seral stages somewhat independently. In armored stream reaches,
(Fisher et al. 1982, Poff and Ward 1990). sediment mobility at a given bed shear level is lower
Periphyton responses to physical disturbances are than in depositional reaches, and both reach types
typically assessed in 2 phases, responses to the onset occur over a wide range of flood regimes. Biggs et al.
and duration of disturbance, and responses to the (1999a) used 12 streams to create a natural factorial
cessation of disturbance. Responses to the onset of experiment with sites with high or low sediment
disturbance are related to susceptibility or resistance mobility and high or low flood frequencies (a proxy
(e.g., periphyton biomass loss and changes in metab- for bed shear stress). Sediment mobility strongly
olism and taxonomic composition). Responses to the affected periphyton biomass in their study, but flood
cessation of disturbance are related to recovery and frequency had no detectable effect. Subsequent field
resilience (e.g., regrowth, recolonization, and the and flume studies with varied sediment and velocity
degree to which similar communities develop during levels support the view that sediment abrasion and
each recovery phase). tumbling dominate the negative effects of floods on
periphyton (Francoeur and Biggs 2006, Lepori and
Responses to the onset of disturbances Malmqvist 2007).
Periphyton responses to the onset of desiccation
Field and laboratory investigations indicate that vary with mat or biofilm thickness, physiognomy,
susceptibility to hydraulic disturbances (e.g., elevated taxonomic composition, and production of extracel-
shear stress) is influenced by periphyton growth form lular mucilage, among other factors (Hawes et al.
and age, magnitude of disturbance, rate of onset, and 1992, Blinn et al. 1995, Stanley et al. 2004, McKnight et
the hydraulic conditions under which periphyton al. 2007, Ledger et al. 2008). In some taxa, the onset of
communities develop (Peterson and Stevenson 1992, desiccation triggers protective physiological respons-
Biggs and Thomsen 1995). For a given community, a es, such as encystment, cell-wall thickening, and
hydraulic threshold exists beyond which structural synthesis of osmolytes that increase resistance to
failure occurs. Biomass loss is rarely a linear function changes in osmotic potential. Broad taxonomic differ-
of disturbance intensity or duration; most biomass ences in desiccation resistance are clearly seen in dam
loss occurs immediately after the threshold is exceed- tailwaters, which are alternately exposed and sub-
ed (Biggs and Thomsen 1995, Francoeur and Biggs merged by operating flows. Shallow tailwaters with
2006). Highly susceptible growth forms include chain- frequent exposure often are dominated by sheathed
forming diatoms, uniseriate filaments, and loosely- cyanobacteria and deep, rarely exposed areas by
attached cyanobacterial mats, and highly resistant susceptible taxa, such as Cladophora (Blinn et al.
forms include prostrate diatoms, and chlorophyte 1995, Benenati et al. 1998). In general, desiccation
basal cells and rhizoids (Grimm and Fisher 1989, resistance is high in mucilaginous cyanobacteria and
Biggs et al. 1998a, Benenati et al. 2000, Passy 2007). diatoms, and low in chlorophytes, rhodophytes, and
Susceptibility of periphyton to hydraulic disturbance nonmucilaginous diatoms. Exceptions include desic-
generally increases as intervals between disturbances cation-tolerant chlorophytes that occur in both terres-
increase. When disturbances are infrequent, thick trial and aquatic habitats, such as Chlorella, Klebsormi-
mats develop with weak attachment to substrata dium, and Trebouxia (Morison and Sheath 1985, Gray
because of basal cell senescence; these mats can be et al. 2007).
removed by small increases in shear stress (Power
1990, Peterson 1996). Information about thresholds for Responses to the cessation of disturbances
periphyton removal is important for designing envi-
ronmental flows intended to remove periphyton Periphyton communities recover from hydrody-
proliferations in regulated rivers (Biggs 2000 [Fig. 1], namic disturbances by recolonization or by regrowth
Osmundson et al. 2002). from persistent cells. The relative importance of these186 S. T. LARNED [Volume 29
pathways has been partially evaluated by comparing disturbances and nutrient levels have been confound-
periphyton biomass accrual on newly exposed sub- ed by variation in bed armoring and herbivory (Biggs
strata (colonization only) with that on substrata and Smith 2002, Riseng et al. 2004). In laboratory
containing persistent tissue (colonization plus re- experiments, the direct effects of increased flow or
growth) (Hoagland et al. 1986, Dodds et al. 1996, scour were significant, but the effects of nutrient
Downes and Street 2005). Colonization dominates concentrations on biomass loss during disturbances
recovery when substrata are overturned during and on subsequent recovery were generally small or
floods, exposing previously buried surfaces. Re- undetectable, particularly when concentration differ-
growth might dominate when substrata are abraded, ences among treatments were relatively small (Mul-
but not overturned. Community succession on newly holland et al. 1991, Humphrey and Stevenson 1992,
exposed substrata depends on the composition of the Peterson et al. 1994). The study by Biggs et al. (1999b)
propagule pool (Peterson 1996). Succession on sub- was an exception; a 10-fold increase in NO32
strata with persistent cells depends on the rate of concentration and a 4-fold increase in dissolved
regrowth and interactions between resident taxa and reactive P (DRP) concentration 10 d before and 2 to
propagules. In the latter case, macroalgae with 9 d after a scouring flood significantly reduced
persistent basal crusts (e.g., Stigeoclonium) often periphyton biomass loss in experimental streams
dominate early successional stages (Power and and reduced the time required for biomass to return
Stewart 1987). to predisturbance levels. Most disturbance 3 resource
Recolonization and regrowth are also the pathways studies have addressed periphyton responses to
by which periphyton recover from emersion and floods and nutrient availability, but Wellnitz and
desiccation. Stanley et al. (2004) predicted that Rader (2003) combined floods with variation in light
recolonization is the main recovery pathway follow- level and grazing intensity and reported several 3-
ing rapid drying (e.g., downstream of hydroelectric way (flood 3 light 3 grazing) interactions.
dams), because resident periphyton have insufficient
time for protective physiological responses. Recovery Contributions of J-NABS
by regrowth might be more important at sites with
gradual water loss caused by seepage or evaporation. The roles of physical disturbances in periphyton
community structure and succession have been major
Physiological recovery from desiccation has been
themes in J-NABS for most of its history. This research
measured in many environments and periphyton
began before J-NABS (e.g., Douglas 1958), but many of
taxa. The premier examples are cyanobacterial mats
the subsequent experiments and syntheses were
in Antarctic and Mediterranean streams. Upon rewet-
reported in J-NABS (e.g., Pringle et al. 1988, Grimm
ting, these mats reach pre-emersion photosynthetic
and Fisher 1989, Townsend 1989, Sinsabaugh et al.
rates in minutes (Vincent and Howard-Williams 1986,
1991, Cooper et al. 1997, Stevenson 1997a, Lake 2000).
Hawes et al. 1992, Romanı́ and Sabater 1997,
The patch dynamics concept was used in some of
McKnight et al. 2007). Mechanisms that promote such
rapid recovery include osmotic changes that reduce these papers to help explain complex community-
level responses to disturbances (Winemiller et al.
membrane and organelle damage and antioxidant
production (Potts 1999, Gray et al. 2007). 2010). This concept has been the basis of several
influential hypotheses in periphyton ecology, includ-
ing: 1) spatial patchiness increases diversity at
Interactions between disturbances and resource availability
interpatch scales (Lake 2000, Hagerthey and Kerfoot
Several field surveys and laboratory stream studies 2005); 2) fine-scaled patchiness increases variability in
have been motivated by observations of elevated larger-scale processes, such as nutrient spiraling
nutrient concentrations during floods (Grimm and (Pringle et al. 1988); 3) sizes and configurations of
Fisher 1989, Mulholland et al. 1991, Humphrey and periphyton patches are determined by processes
Stevenson 1992, Peterson et al. 1994, Biggs et al. operating at multiple scales, including turbulent flow
1999b, Biggs and Smith 2002, Riseng et al. 2004). In and sediment movement (Sinsabaugh et al. 1991).
these cases, periphyton experiences both the negative One limitation of the patch dynamics concept is that it
effects of sediment movement and the positive effects is viewed from a planar perspective. Stream channels
of nutrient enrichment. Two general hypotheses have and lake basins are concave, and vertical gradients
been tested: 1) that nutrient limitation increases exist in the durations and frequencies of emersion and
periphyton susceptibility to disturbances, and 2) that other disturbances. These gradients should produce
nutrient enrichment hastens recovery following vertical zonation in periphyton communities along
floods. In field surveys, the direct effects of flood channel and basin cross-sections. Vertical zonation2010] PERIPHYTON ECOLOGY 187
has been reported in dammed rivers, where operating shading, water depth, and dissolved and particulate
flows for power generation cause frequent depth suspended matter (Vinebrooke and Leavitt 1998,
changes (Benetati et al. 1998, Burns and Walker 2000), Kelly et al. 2001, 2003, Frost et al. 2005, Weidman
and in lakes subject to wave disturbances (Hoagland et al. 2005 [Fig. 1]). UVR exposure also varies during
and Peterson 1990). No comparable cases of flow- succession in periphyton communities because of self-
driven vertical zonation have been reported for shading (Kelly et al. 2001, Tank and Schindler 2004).
natural streams and rivers. Variable exposure makes measuring UVR at realistic
Among the notable contributions of J-NABS papers scales challenging. In most field studies, UVR
to disturbance ecology are studies of simultaneous measurements are made above periphyton surfaces
negative and positive effects of hydrological distur- with large (.50-cm tall) spectrophotometers. More
bances on periphyton (e.g., Humphrey and Steven- realistic measurements at periphyton surfaces and
son 1992, Biggs et al. 1999a, Riseng et al. 2004). These within periphyton matrices will require microprobes
disturbances remove organisms, but they also gener- (Garcia-Pichel 1995).
ate resources (e.g., nutrients and bare substrate), Molecular and physiological studies of the effects
alleviate competition, and transport propagules. The of UVR on aquatic photoautotrophs have focused on
responses of periphyton populations and communi- damage to deoxyribonucleic acid (DNA) and D1, the
ties to disturbances reflect the balance between main structural protein of photosystem-2. DNA
increased mortality and emigration, and increased damage delays mitosis, and damage to D1 reduces
growth, reproduction, and immigration (Stevenson electron transport and slows the generation of
1990). The J-NABS papers have contributed to the reducing power for C fixation (Grzymski et al.
modern view of hydrological disturbances as complex 2001, Helbling and Zagarese 2003). Collectively,
phenomena with pervasive ecological effects in all but these changes can reduce growth rates, but studies
the most stable streams. of UVR-effects on periphyton growth have had
The intermediate disturbance hypothesis has also mixed results. Growth in UVR-exposed periphyton
been used as a working model in periphyton studies was suppressed in some laboratory and field studies
published in J-NABS (McCormick and Stevenson (Bothwell et al. 1993, Kiffney et al. 1997, Frost et al.
1989, Suren and Duncan 1999) and other journals 2007 [Fig. 1]), but no effects were detected in other
(Fayolle et al. 1998, Biggs and Smith 2002). These field studies (DeNicola and Hoagland 1996, Hill et
studies focused on patterns of diversity-disturbance al. 1997, Hodoki 2005, Weidman et al. 2005). The
relationships, and whether those patterns confirmed generally weak effects of UVR on periphyton
the prediction of peak diversity at intermediate growth have been attributed to protective UVR-
disturbance levels. Most of the studies were correla- absorbing compounds, rapid repair of UVR-induced
tive, and the proximate causes of diversity–distur- damage, self-shading, attenuation by tree canopies
bance patterns could not be confirmed. As Lake (2000) and water, and solar trophic cascades (Bothwell et
noted, biodiversity is not controlled by disturbance al. 1994, Francoeur and Lowe 1998, Frost et al. 2007).
alone, but by combinations of disturbance, resource Solar trophic cascades occur when the negative
supply, reproduction, and other processes. How effects of UVR on herbivores reduce grazing losses
disturbances interact with other biological and eco- in periphyton, compensating for direct negative
logical processes, and how these interactions control effects of UVR on periphyton (Bothwell et al.
species coexistence are central questions in commu- 1994). As with periphyton growth, UVR exposure
nity ecology (Agrawal et al. 2007). The high diversity at natural levels appears to have moderate to
and ease of manipulation of periphyton communities undetectable effects on periphyton taxonomic com-
make them ideal systems for addressing those position (DeNicola and Hoagland 1996, Vinebrooke
questions experimentally (Steinman 1993). and Leavitt 1996, Tank and Schindler 2004). Long-
term exposure to elevated UVR leads to shifts in
Exposure to Stressors periphyton communities to a predominance of UVR-
tolerant taxa (Vinebrooke and Leavitt 1996, Navarro
Ultraviolet radiation
et al. 2008). A comparison of the responses of UVR-
Ultraviolet radiation (UVR) is a frequently studied tolerant and UVR-sensitive periphyton to Cd expo-
stressor in phytoplankton ecology, but is less fre- sure indicated that UVR-tolerant periphyton were
quently studied in periphyton ecology. Current also relatively Cd-tolerant (Navarro et al. 2008).
knowledge of UVR effects on aquatic autotrophs UVR exposure and metal exposure both induce
comes primarily from phytoplankton studies. For reactive O2 in algal cells, and UVR–Cd cotolerance
periphyton, UVR exposure varies with riparian might be caused by similar physiological responses188 S. T. LARNED [Volume 29
to both stressors, including enhanced antioxidant C uptake (to cope with low inorganic C concentra-
activity (Prasad and Zeeshan 2005). tions), and tolerance or resistance to proton influx
through cell membranes—a necessity in H+-rich
Thermal stress environments (Gross 2000). At sites of recent, anthro-
pogenic acidification, the original periphyton com-
Optimal temperatures for growth of hot-spring
munities are often replaced by depauperate commu-
algae can be .50uC (Ciniglia et al. 2004, Liao et al.
nities of acidophilic chlorophytes and diatoms
2006), but optimal temperatures for most periphyton
(Turner et al. 1991, Verb and Vis 2000, Greenwood
range from 10 to 30uC, and higher temperatures
and Lowe 2006). Recovery of algal diversity and
induce heat stress and reduce growth (DeNicola
reduction in acidophile dominance are indicators of
1996). Direct physiological effects of high tempera-
success in remediation of acidified ecosystems (Vine-
tures include protein and nucleic acid denaturation,
brook 1996, Vinebrooke et al. 2003).
photosystem degradation, and respiration in excess of
The direct effects of acidification on algal cells are
C fixation (Davison 1991, Wahid et al. 2007).
not well known, but might include osmotic stress and
Optimal growth temperatures and maximum tem-
disruption of cell division (Gross 2000, Visviki and
peratures for survival bracket the high-temperature
Santikul 2000). In addition to direct toxic effects,
tolerance ranges of periphyton. Some evidence indi-
acidification increases exposure to other stressors.
cates that heat tolerance varies among major taxa.
Stream acidification often is accompanied by metal
Cyanobacteria generally tolerate higher temperatures
(.30uC) than do chlorophytes, which tolerate higher dissolution and subsequent deposition of metal
temperatures than do diatoms and rhodophytes hydroxides downstream of the acid source. Acidic
(DeNicola 1996). Variation in tolerance suggests that freshwater typically is enriched in dissolved Fe, Zn,
thermal regimes influence spatial patterns in periph- Ni, Hg, Al, and other metals. Potential negative effects
yton communities. Surveys along thermal gradients of dissolved metal exposure include alterations in
created by coolant water outfalls and geothermal membrane permeability, inhibition of photosynthetic
springs provide some support for this proposition, electron transport, and metal–phosphate coprecipita-
with cyanobacteria dominant near thermal sources tion, which can reduce P availability (Pettersson et al.
and diatoms dominant in cooler distal reaches 1985, Genter 1996, Kinross et al. 2000). Deposition of
(Squires et al. 1979, Bonny and Jones 2003). Additional oxidized metals in moderately acidic streams elimi-
support comes from heated artificial streams, in nates many periphyton taxa and favors a small
which rhodophytes were eliminated at temperatures number of tolerant taxa (e.g., Ulothrix, Mougeotia,
12.5uC above ambient, and cyanobacteria dominated and Zygogonium) (Niyogi et al. 1999, 2002, Kleeberg et
at temperatures .30uC (Wilde and Tilly 1981). Last, al. 2006). The properties of these algae that confer
the effect of water temperature is often significant in tolerance to oxidized metal deposition are unknown,
multivariate analyses of geographic or seasonal but might include growth rates that exceed the rate of
variation in periphyton composition (e.g., Wehr deposition, or mucilaginous sheaths that prevent
1981, Griffith et al. 2002). However, other environ- deposition.
mental factors covary with temperature, and con-
trolled experiments are the best means of assessing Contributions of J-NABS
direct thermal effects.
Studies of abiotic stressors published in J-NABS
tend to focus on multivariable problems, rather than
pH stress
direct effects of single stressors. For example, Fair-
Studies of stream and lake acidification generally child and Sherman (1993) used nutrient-diffusing
focus on the effects of anthropogenic acidification substrata in 12 lakes representing a broad acidity
caused by mine drainage and fallout from smelters gradient (pH 4.4–8.8) to explore the combined effects
and power plants. Many sources of natural acidifica- of acidification and nutrient limitation on periphyton
tion exist (e.g., wetland outflows, volcanic ashfall, growth and composition. Their results suggested that
pyrite weathering), but these have received less C enrichment enhanced periphyton growth, which
attention as stressors (for exceptions, see Sheath et lends support to the hypothesis that acidification
al. 1982, Sabater et al. 2003, Baffico et al. 2004). causes C limitation. Several J-NABS papers that
Naturally acidic freshwater environments are inhab- addressed periphyton responses to multiple stressors
ited by acidophilic periphyton (DeNicola 2000, Gross (e.g., acidity, herbivory, and nutrient limitation) used
2000). The taxa that make up these communities tend exploratory multivariate analyses (Vinebrook 1996,
to share some physiological traits, including efficient Naymik et al. 2005, Cao et al. 2007, Stevenson et al.2010] PERIPHYTON ECOLOGY 189
2008). Responses to individual stressors were not (e.g., Wellnitz and Rinne 1999). Responses to long-
identified or quantified in these studies because of term variation, such as month-long nutrient additions,
covariance or because ordination axis scores were the are dominated by biomass and taxonomic changes
only dependent variables reported. Vinebrook (1996) (e.g., Harvey et al. 1998). Responses to very long-term
used reciprocal periphyton transplants and herbivore (mo–y) variation include secondary effects, such as
exclosures to study periphyton responses to a lake species turnover and changes in foodweb structure
acidity gradient. This approach controlled for effects (Blumenshine et al. 1997, Slavik et al. 2004). The
of herbivory and colonization sources, but the direct following discussion considers the main effects of
effect of acidity could not be separated from other light-, temperature- and nutrient-limitation on pe-
abiotic factors that varied among lakes or from riphyton communities, and pairwise interactions
enclosure artifacts. Complex periphyton responses to between these factors.
multiple stressor gradients are important in environ-
mental monitoring programs and indicator analyses, Light limitation
but it is also important to identify and quantify
univariate periphyton–stressor relationships. The lat- Most periphytic organisms are obligate photoauto-
ter are needed to predict the effects of stressor trophs that use sunlight to generate reducing power
exposure, detect trends, and identify mechanisms by and dissolved inorganic C to produce carbohydrates.
which stressors affect periphyton. Some periphyton taxa are facultative photohetero-
trophs (sunlight + organic C substrates) or chemoor-
ganoheterotrophs (organic compounds for reducing
Limiting Abiotic Factors
power + organic C substrates). Nutrient assimilation
Periphytic organisms vary in their minimum in photoautotrophs is catalyzed by chemical energy in
resource requirements for survival because of differ- C compounds, and light energy is required to produce
ences in physiology and morphology, e.g., diatoms those compounds. Consequently, nutrient limitation
have high Si requirements and N2-fixing cyanobacte- is detected frequently under high-light conditions and
ria have high Mo requirements compared with other rarely under low-light conditions (Hill and Knight
major taxonomic groups (Stal 1995). These differences 1988, Bourassa and Cattaneo 2000, Larned and Santos
are well established for many planktonic algae, for 2000, Greenwood and Rosemond 2005).
which minimum cell quotas for nutrients and com- Facultative heterotrophy has been proposed as a
pensation light intensities have been defined in mechanism that allows periphyton communities to
culture (Reynolds 2006). Minimum resource require- persist under severe light limitation (Tuchman 1996,
ments are unknown for most periphyton taxa, but Tuchman et al. 2006). Heterotrophy is energetically
these requirements are certain to vary as they do for favored only at high dissolved organic C (DOC)
phytoplankton (Hill 1996). concentrations, so facultative heterotrophy should be
Transient changes in physiological condition and restricted to dark, DOC-rich environments, such as lake
growth form lead to short-term variation in periph- sediments and the interiors of dense periphyton mats.
yton resource requirements (Graham et al. 1995, The influence of riparian tree canopies on light
Hillebrand and Sommer 1999). For example, nutri- transmission to stream periphyton has been a prom-
ent-replete periphyton might have higher light-satu- inent research topic since the 1950s (e.g., McConnell
rated photosynthetic rates and higher photosynthetic and Sigler 1959, Hansmann and Phinney 1973, Hill
efficiencies than nutrient-deficient periphyton (Taul- and Knight 1988, DeNicola et al. 1992, Larned and
bee et al. 2005, Hill and Fanta 2008). However, the Santos 2000, Hill and Dimick 2002, Ambrose et al.
scarcity of autecological studies of periphyton makes 2004). Most of these studies were categorical compar-
predicting changes in resource requirements difficult. isons of periphyton under canopy gaps (high light) or
Resource-limitation studies generally address re- dense canopies (low light). Studies that use continu-
sponses of whole periphyton communities, not ous gradients in riparian light transmission provide
individual taxa. It is implicit in community studies more information about complex responses to light
that aggregate responses of multiple taxa are being input. For example, Hill and Dimick (2002) measured
measured, and these responses might include changes in situ periphyton photosynthesis over the wide
in taxonomic composition and relative abundance irradiance range created by a heterogeneous forest
(e.g., Lohman et al. 1991, Hill and Fanta 2008). canopy and reported nonlinear effects of irradiance
Specific responses depend on the time scale of on photosynthetic efficiency, photosaturation, light
resource variation. Responses to short-term (,1 d) utilization efficiency, and pigment concentrations.
variation in resources are primarily physiological Heavily shaded photoautotrophs undergo a physio-190 S. T. LARNED [Volume 29
logical process of photoacclimation that increases replete conditions (DeNicola 1996), but these condi-
photosynthetic efficiency at low light levels and tions are rare in natural aquatic systems.
reduces the irradiance required for maximum photo-
synthesis (Hill 1996). Nutrient limitation
At finer spatial scales, the 3-dimensional structure
Nutrient limitation is one of the best-studied topics
of periphyton communities causes variation in light
in periphyton ecology. The high level of interest in
transmission to cells within the community matrix.
nutrient limitation reflects concern about eutrophica-
Vertical light attenuation increases with increasing
tion and recognition of the role of nutrient limitation
periphyton density and thickness (Meulemans 1987,
in community and ecosystem processes (Rosemond
Dodds 1992, Johnson et al. 1997). Presumably, algae
1993, Smith et al. 1999, Hillebrand 2002 [Fig. 1],
at the bases of periphyton mats are shade-acclimated.
Holomuzki et al. 2010). Studies of nutrient limitation
Supporting evidence for this assumption includes
in periphyton generally focus on macronutrients (e.g.,
reports of increased photopigment concentrations at
N, P, Fe, Si), and rarely on micronutrients (e.g., Mo, B,
mat bases (Tuchman 1996) and a positive relationship
Zn) (but see Pringle et al. 1986). Nutrients that are in
between photosynthetic efficiency and depth in a
low demand relative to availability (e.g., K, Mg) are
filamentous periphyton mat (Dodds 1992).
rarely limiting. Other nutrients (e.g., N, P, Si) are
frequently limiting because demand is high relative to
Temperature limitation
availability. For a 3rd class of nutrients (e.g., Fe, Ca),
Thermal energy is not a resource in the strict great taxonomic and geographical variation in de-
sense; it is not consumed and is not an object of mand and availability suggests that limitation ranges
competition. However, periphyton (like all organ- from negligible to severe.
isms) requires thermal energy for enzyme-catalyzed A common goal in nutrient limitation studies is to
reactions, and thermal energy deficits can limit identify a single nutrient that controls periphyton
growth and other physiological processes. Algal growth because its availability relative to demand is
photosynthesis and growth respond unimodally to lower than any other nutrient. The concept of single-
temperature variation, as they do to light variation nutrient limitation is enshrined in Liebig’s Law of the
(Graham et al. 1995, O’Neal and Lembi 1995). Minimum, which states that plant yield declines as
Temperatures corresponding to maximum photosyn- the scarcest nutrient is depleted (De Baar 1994). The
thesis and growth in freshwater algae (excluding expectation that periphyton growth will be limited by
extremophiles such as hot-spring algae) range from a single nutrient might have originated with influen-
10 to 30uC (Butterwick et al. 2005). This range tial studies of single-nutrient limitation in phyto-
suggests that thermal energy could be a limiting or plankton cultures (Droop 1974). Comparable results
colimiting factor in cold climates. Empirical models are not always observed in periphyton studies, in
for periphyton indicate that photosynthetic rates which N, P, or other nutrients can be colimiting
increase with temperature over a 5 to 25uC range (Francoeur et al. 1999, Dodds and Welch 2000,
(Morin et al. 1999), and experimental results Francoeur 2001 [Fig. 1]).
indicate positive relationships between light-saturat- Discrepancies between Liebig’s Law of the Mini-
ed photosynthesis and temperature, with a rate of mum and experimental results for periphyton have
change caused by a 10uC increase in temperature several causes. In some studies, multiple-nutrient
(Q10) § 2 (DeNicola 1996). Temperature acclimation enrichment causes switching between single limiting
can reduce the severity of temperature limitation. nutrients. A more fundamental discrepancy is
Algae that are acclimated to low temperature have community colimitation, which occurs when periphy-
higher maximum photosynthetic rates, lower tem- ton communities contain taxa that are limited by
perature optima for photosynthesis, and higher different nutrients, and multiple-nutrient enrichment
concentrations of C-fixing enzymes than nonaccli- enhances growth in more taxa than single-nutrient
mated algae (Davison 1991). enrichment. Community colimitation often occurs in
Thermal energy is unlikely to be the sole limiting natural phytoplankton communities, as indicated by
factor for periphyton under natural light and nutrient differential nutrient limitation assays (Arrigo 2005,
levels. Temperature acclimation incurs nutrient costs Danger et al. 2008, Saito et al. 2008). Community
for increased enzyme synthesis, and low tempera- colimitation appears to be common in periphyton
tures might lead to nutrient limitation rather than (Francoeur 2001), but differential nutrient limitation
temperature limitation per se. Thermal energy could in periphyton is rarely tested (Fairchild and Sher-
be the sole limiting factor under light- and nutrient- man 1993).2010] PERIPHYTON ECOLOGY 191 Nutrient limitation has been assessed in many these effects comes from a comparison of NO32- periphyton studies, using several different methods. enriched and unenriched substrata in Sycamore Differences in methods can affect the outcomes of Creek, Arizona (Peterson and Grimm 1992). In that these studies, and obscure general patterns (Fran- study, unenriched substrata were dominated by coeur 2001). Examination of ambient nutrient concen- diatoms with N2-fixing endosymbiontic cyanobacteria trations is the most economical method for assessing during early successional stages, and by N2-fixing nutrient limitation, but it has some short-comings. cyanobacteria during later stages. NO32-enriched Periphyton growth rates are not always positively substrata were initially colonized by non-N2-fixing correlated with ambient nutrient concentrations. In diatoms and filamentous chlorophytes and later, by fact, growth–nutrient relationships can be inverse cyanobacteria. Seral stages were more apparent, and because of ambient nutrient depletion by fast-growing diversity higher, on NO32-enriched substrata than on periphyton (Welch et al. 1988, Stevenson et al. 2006). unenriched substrata. General relationships between Different molecular forms of a given nutrient vary in periphyton diversity and the availability of limiting bioavailability, which increases uncertainty in con- nutrients are still elusive; enrichment experiments centration–growth relationships (Saito et al. 2008). have resulted in increased, decreased, and unchanged Ratios of 2 or 3 ambient nutrient concentrations (e.g., diversity (Carrick et al. 1988, Pringle 1990, Miller et al. N:P:Si) provide information about their relative 1992, Peterson and Grimm 1992, Greenwood and availability, but not about the nutrient requirements Rosemond 2005). for periphyton growth (Francoeur et al. 1999, Stelzer To sustain long-term growth, periphyton commu- and Lamberti 2001). Cellular nutrient concentrations nities require nutrients from external sources to offset and ratios may be used in lieu of ambient nutrients to losses. The major pathways for external nutrient assess nutrient limitation (e.g., Hillebrand and Som- supplies are advection from the overlying water mer 1999), but these measures can also be unreliable. column and diffusion or upwelling from underlying Periphyton taxa and communities vary in nutrient substrata (Pringle 1987, Henry and Fisher 2003). storage capacity, and it is rarely clear whether cellular Nutrient recycling within dense periphyton mats nutrient levels reflect nutrient storage capacities or and biofilms can temporarily uncouple periphyton nutrient requirements for growth. from external nutrient sources (Mulholland 1996, Experimental nutrient addition is generally a more Mulholland and Webster 2010). Internal recycling reliable method for assessing nutrient limitation than cannot sustain periphyton indefinitely, but it can be examinations of nutrient concentrations. Nutrient surprisingly efficient over short time scales. In additions have been used in periphyton studies for laboratory periphyton communities, recycling ac- 60 y, starting with Huntsman (1948; Fig. 1). Most of counted for 10 to 70% of the P uptake, and daily P these studies used nutrient diffusing substrata (NDS) turnover was ,15%/d (Mulholland et al. 1995, that release dissolved nutrients from enriched media Steinman et al. 1995). Periphyton communities are into porous substrata on which periphyton attaches likely to acquire nutrients from multiple sources and grows (e.g., Coleman and Dahm 1990, Francoeur simultaneously. Observations of vertical gradients in et al. 1999). Limiting nutrients are then identified by nutrient concentrations and enzyme activities within comparing the biomass on substrata enriched with periphyton communities suggest that surface cells different nutrients. Other studies have used whole- rely on water-column nutrient sources, whereas cells stream nutrient enrichment (Peterson et al. 1993, in the matrix rely on sediment-derived nutrients and Greenwood and Rosemond 2005). Two general issues recycling (Wetzel 1993, Mulholland 1996). are addressed in nutrient addition studies: whether Calcareous periphyton mats have a unique mech- periphyton growth is nutrient-limited (and by which anism for P recycling (Noe et al. 2003). Daytime nutrients), and the effects of nutrient limitation on photosynthesis raises pH levels within mats, which periphyton community composition and succession. promotes rapid P precipitation with and adsorption to The 1st issue is usually addressed with NDS deploy- calcium carbonate. Respiration at night reduces pH ments that are brief enough to prevent switching levels within mats, which promotes calcium carbonate between limiting nutrients, generally ,1 mo. dissolution and P desorption. In a P tracer study, Noe The 2nd issue, concerning nutrient-limited commu- et al. (2003) estimated that 80% of P uptake by nity development, is addressed with long NDS periphyton was initially bound to Ca, and this deployments or whole-stream enrichment. The results fraction dropped to 15% after 1 d because of biotic of these studies indicate that nutrient availability uptake. P enrichment can lead to the replacement of influences periphyton succession and mediates inter- calcareous cyanobacterial mats by filamentous chlor- specific interactions. One of the clearest examples of ophytes, which lack the P adsorption–desorption
192 S. T. LARNED [Volume 29
mechanism, but have higher growth potential cellular P concentrations as predicted, although P-
(McCormick and O’Dell 1996). enrichment clearly increased productivity. However,
Hill et al. (2009) reported a negative relationship
Light 3 nutrient interactions between cellular N and light intensity, a result that
suggested that N–light colimitation was in effect, not
Tests of nutrient 3 light interactions have been P–light colimitation. Support for the light:nutrient
made in natural streams with nutrient diffusing hypothesis as it applies to periphyton is equivocal at
substrata and water-column enrichment beneath present, but further testing could be done by varying
riparian canopy gaps and closed canopies (Lowe et the concentrations of nutrients other than P.
al. 1986, Hill and Knight 1988, Larned and Santos
2000, Von Schiller et al. 2007) and in artificial streams Temperature interactions with light and nutrients
with water-column enrichment and shade screens
(Rosemond 1993, Bourassa and Cattaneo 2000, Hill Temperature 3 light interactions in periphyton
and Fanta 2008). In the field studies, increased have been tested in several experiments (Graham et
irradiance generally enhanced periphyton growth, al. 1985, 1995, O’Neal and Lembi 1995). Graham et al.
nutrient enrichment had little or no detectable effect, (1995) used a high-resolution design (58 temperature
and the nutrient 3 light interaction terms (when 3 light combinations) and reported that optimal and
tested) were rarely significant. Possible explanations compensation irradiances for photosynthesis in-
for the lack of detectable effects of nutrient enrich- creased with temperature up to ,30uC. At higher
ment included consumption of periphyton in en- temperatures, net photosynthesis decreased across the
riched treatments by grazers and competitive inhibi- light gradient. Reduced photosynthesis at high tem-
tion by benthic heterotrophs. In contrast, Rosemond peratures is partly caused by photorespiration, as
(1993), Hill and Fanta (2008), and Hill et al. (2009) cellular CO2 concentrations decrease with increasing
reported significant main effects of both light and temperature more rapidly than O2 concentrations
nutrient levels in artificial streams. Hill et al. (2009) (Davison 1991).
used simultaneous gradients in DRP concentration The effects of temperature 3 nutrient interactions
and irradiance to quantify nutrient 3 light interac- on periphyton communities have not been studied
tions. In that study, the positive effects of DRP under controlled conditions. However, seasonal NDS
enrichment on periphyton growth increased with deployments indicate that the frequency and severity
increasing irradiance. Photoinhibition was apparent of nutrient limitation increase during warm seasons
at high irradiance and low DRP concentrations, and (Allen and Hershey 1996, Francoeur et al. 1999). The
DRP enrichment alleviated photoinhibition, presum- most likely physiological basis for temperature-
ably through increased synthesis of protective pig- dependent nutrient limitation is that respiration and
ments. Colimitation at low light and DRP levels in photosynthesis rates increase with temperature,
these studies was less apparent, although a residual which increases nutrient demand for C fixation,
analysis in the study by Hill and Fanta (2008) biomolecule synthesis, and other growth-related
indicated that P-enrichment enhanced growth at low processes (Falkowski and Raven 2007).
light levels.
Contributions of J-NABS
One of the predicted outcomes of nutrient-light
colimitation is an increase in cellular nutrient levels Resource-limited periphyton metabolism and
with decreasing irradiance caused by the nutrient growth have been the topics of many J-NABS papers.
requirements of shade acclimation (Sterner et al. 1997, Several of the papers are from multivariable studies
Hessen et al. 2002). Sterner et al. (1997) developed the that tested for interactions between resource avail-
light:nutrient hypothesis to explain how the balance ability and physical disturbances or herbivory (Mul-
of light energy and nutrient supplies controls algal holland et al. 1991, Humphrey and Stevenson 1992,
productivity and stoichiometry in lakes. In its original Gafner and Robinson 2007). These latter papers
formulation, the hypothesis was based on qualitative contributed to an emerging picture of the relative
comparisons of low and high light:P ratios. High importance of processes that control periphyton
ratios were predicted to cause P limitation and low growth and composition. Resource limitation is
ratios to cause C limitation. The light:nutrient unlikely to have a strong influence if disturbances or
hypothesis was subsequently tested with periphyton grazing pressure maintain periphyton at low-biomass
by Frost and Elser (2002), Hill and Fanta (2008), and levels and early successional stages (Biggs 1996,
Hill et al. (2009). In these latter studies, variation in Steinman 1996). In turn, light limitation is often more
light intensity did not affect periphyton C:P ratios or severe than nutrient limitation, as indicated by the2010] PERIPHYTON ECOLOGY 193
general lack of response to nutrient enrichment at flow directly affected periphyton or whether the
very low light levels (e.g., Lowe et al. 1986, Ambrose researchers were measuring the most relevant vari-
et al. 2004). However, it should be noted that the ables. Two explanatory variables, volumetric dis-
primacy of light limitation in periphyton field studies charge and velocity in the downstream direction, are
might reflect the limited range of environments used in most flow–biota studies. Velocity is usually
studied, not a strict physiological hierarchy of limiting measured at the water surface or at a depth thought to
factors. The most common environments used for correspond to vertically-averaged velocity (e.g., 60%
comparisons of light and nutrient limitation in J- of total depth). In many cases, the measurement depth
NABS papers were densely shaded, mesotrophic is in the outer or logarithmic portion of the benthic
streams. Comparisons made over broader gradients boundary layer (the region of water column influ-
(from ultraoligotrophic to hypereutrophic and from enced by friction at the benthos). These velocity
densely-shaded to full sun) might provide a more measurements are referred to as free-stream velocities
accurate view of resource limitation. (e.g., Poff et al. 1990, Francoeur and Biggs 2006).
Nutrient limitation studies in J-NABS have focused Discharge and free-stream velocity might be correlat-
on N, P, or both. In species-rich periphyton commu- ed with hydraulic processes that affect benthic
nities, community colimitation might involve .2 organisms, but they are rarely direct causes of
nutrients, simultaneously or sequentially (Passy biological changes (Biggs et al. 1998a, Hart and Finelli
2008). Enrichment studies with multiple nutrients 1999). For example, periphyton detachment during
and tests of differential nutrient limitation among the periods of elevated discharge is not caused by free-
populations in mixed-species communities have stream velocity per se (e.g., Horner et al. 1990), but by
advanced our understanding of phytoplankton co- drag and lift acting on periphyton. Spatially and
limitation (Arrigo 2005). Similar studies are needed to temporally averaged metrics, such as discharge and
advance periphyton ecology, although these studies free-stream velocity, do not accurately describe the
could be challenging to design and interpret because hydraulic conditions that benthic organisms experi-
of spatial variation in the forms and severity of ence. Metrics that describe near-bed flow at spatial
nutrient limitation within periphyton communities and temporal scales relevant to periphyton include
(Burkholder et al. 1990, Johnson et al. 1997). bed shear stress and near-bed turbulence intensity
The NDS periphyton assay has been a popular tool (Statzner et al. 1988, Nikora et al. 1998a, Stone and
for J-NABS contributors; NDS results appear in 15 J- Hotchkiss 2007).
NABS papers, and the results of 237 NDS assays were The focus of this section is the influence of
compared in a meta-analysis published in J-NABS hydraulic conditions on periphyton communities.
(Francoeur 2001). In J-NABS papers, NDS assays were The term hydraulic conditions is used here to refer to
combined with a range of environmental variables, the near-bed water movements that impinge on
including riparian canopy removal (Lowe et al. 1986), periphyton communities and is distinguished from
lake acidification (Fairchild and Sherman 1993), hydrological conditions, which refer to stream flow
salmon carcass addition (Ambrose et al. 2004), and regimes (Stevenson 1997a).
zebra mussel invasion (Pillsbury et al. 2002). The
NDS approach has been productive, but it has many Solute transport
limitations, including uncertainty about nutrient
diffusion rates and whether the nutrient source Studies of hydraulic effects on solute transport to or
simulated by NDS is the natural substrate or the from periphyton have focused on N, P, and C uptake
overlying water (Pringle 1987, Coleman and Dahm and O2 release (Riber and Wetzel 1987, Borchardt
1990, Hillebrand 2002, Rugenski et al. 2008). 1994, Larned et al. 2004). These solutes move between
the water column and periphyton in several steps:
Hydraulic Conditions rapid, turbulent transport to and from the near-bed
region, slower transport near and within periphyton
The pervasive effects of flow on benthic organisms matrices, mass-transfer through the viscous sublayers
are evident to most aquatic ecologists. The paper that (VSLs) of benthic boundary layers, and transport
popularized the idea of flow as the master variable in through cell membranes. These steps can be viewed
lotic ecosystems has been cited .300 times (Poff et al. as resistors in series; the slowest step poses the
1997). Experiments and field surveys have generated greatest transport resistance and will control nutrient
relationships between stream flow and periphyton for uptake rates (Larned et al. 2004). Transport through
,100 y (e.g., Brown 1908). However, it is not always outer benthic boundary layers is mediated by turbu-
clear in these studies which components of stream lent diffusion and is rarely rate-limiting; either mass-You can also read
