Controls of Oxygen Consumption of Sediments in the Upper Elbe Estuary
←
→
Page content transcription
If your browser does not render page correctly, please read the page content below
2021
The Author
Mathias Spieckermann
Mathias J. Spieckermann was born in Eutin.
He studied geoecology at the Technical
University of Braunschweig with a focus on Controls of Oxygen Consumption of
soil science and environmental modeling.
In his master thesis, he reproduced a new Sediments in the Upper Elbe Estuary
system for the decentralized treatment of
wastewater in the laboratory and developed
a 2-D model for the simulation of solute
fluxes in this system. During his PhD thesis
he focused on the oxygen consumption
potential (OCP) of sediments from the
upper Elbe estuary. He investigated the
spatial and seasonal variability of the OCP
and developed a predictive model to
calculate the oxygen consumption of
resuspended sediments.
Verein zur Förderung der Bodenkunde Hamburg M. Spieckermann
Band 101 Band 101
2021
c/o Institut für Bodenkunde - Universität Hamburg
https://www.geo.uni-hamburg.de/de/bodenkunde.html ISSN: 0724-6382
Hamburger Bodenkundliche Arbeiten Hamburger Bodenkundliche Arbeiten
HBA
Controls of Oxygen Consumption
of Sediments in the
Upper Elbe Estuary
Dissertation
with the aim of achieving a doctoral degree
at the Faculty of Mathematics,
Informatics and Natural Sciences
Department of Earth Sciences
at Universität Hamburg
Submitted by
Mathias Johannes Spieckermann
From Eutin, Germany
Hamburg, 2021
Accepted as Dissertation at the Department of Earth Sciences.
Reviewers: Prof. Dr. Annette Eschenbach
Dr. Alexander Gröngröft
Date of Disputation: 15 April 2021
Chair of the subject doctoral committee: Prof. Dr. Dirk Gajewski
Dean of Faculty of MIN: Prof. Dr. Heinrich Graener
Published as
Hamburger Bodenkundliche Arbeiten, Volume 101
Editor: Verein zur Förderung der Bodenkunde Hamburg
Allende‐Platz 2, 20146 Hamburg
Editorship: Dr. Klaus Berger
https://www.geo.uni‐hamburg.de/en/bodenkunde/ueber‐das‐institut/hba.html
Preface
I would like to thank all those who stood by me during my doctoral thesis, supported me and
helped me to focus on the essentials. Without your help, I would not have made it this far.
Special thanks go to my supervisors Annette Eschenbach and Alexander Gröngröft, who
made my work possible. From the very beginning, they supported me with words and deeds.
I especially want to thank Monika Voß, Deborah Harms, Birgit Grabellus, Birgit Schwinge
and Sumita Rui for their great support in the laboratory and their friendly and helpful manner.
The many analyses and experiments performed in the laboratory would not have been
possible without your help.
Also, special thanks to Volker Kleinschmidt, who helped me with the design and setup of
my experiments and always had a solution to my technical problems.
I would also like to thank Kay‐Christian Emeis, who accompanied my work in the course
of the School of Integrated Climate System Sciences, for the interesting and stimulating
discussions.
Further, I would like to thank Stephan Schwank and Rolf Lüschow, who supported me
during the numerous samplings on the Elbe River.
I would like to thank Maja Karrasch and Julia Gebert for the nice collaboration and
stimulating discussions.
I also want to thank Liz, Miriam, Adrian, Alex, Jona and Lars for the many fruitful
discussions and their moral support and the whole Institute of Soil Science for the nice working
atmosphere. I felt comfortable with you guys from the first day and I will miss you.
To all my family, Marten and Timo I would like to thank you for always believing in me
and always finding encouraging words for me. I would also like to give a very special thank you
to my wife Christin, whom I met and fell in love with here, as she supported and cheered me
up throughout the entire time.
I would also like to thank my wife Christin, whom I met and fell in love with here at the
institute, as she supported and cheered me up throughout the entire time.
This study was funded by the Hamburg Port Authority.
3
Publications related to this Dissertation
Appendix Publication A
Oxygen consumption of resuspended sediments of the upper Elbe estuary: Process
identification and Prognosis.
M.J. Spieckermann, A. Gröngröft, M. Karrasch, A. Neumann, A. Eschenbach
Submitted to Aquatic Geochemistry on 17 January 2021.
Appendix Publication B
Oxygen consumption of resuspended sediments of the upper Elbe estuary: Spatial and
temporal dynamics.
M.J. Spieckermann, A. Gröngröft, M. Karrasch, A. Eschenbach
Submitted to the Journal of Soils and Sediments on 11 September 2020.
Appendix Publication C
Temperature dependent oxygen consumption of sediments of the upper Elbe estuary.
M.J. Spieckermann, A. Gröngröft, A. Eschenbach.
Submitted to Estuaries and Coasts on 14 April 2021.
4
Contents
Preface........................................................................................................................................ 3
Publications related to this Dissertation .................................................................................... 4
List of Abbreviations ................................................................................................................... 8
Abstract ...................................................................................................................................... 9
Zusammenfassung.................................................................................................................... 11
1 Introduction ...................................................................................................................... 13
1.1 Sediments as Sinks of Oxygen .................................................................................... 13
1.2 Spatial and Seasonal Variability ................................................................................. 15
1.3 The Upper Elbe Estuary .............................................................................................. 16
1.4 Oxygen Concentration in the Elbe Estuary ................................................................ 17
2 Objectives of the Study ..................................................................................................... 19
3 Material and Methods ...................................................................................................... 21
3.1 Sampling Approaches ................................................................................................. 21
3.2 Development of an Oxygen Consumption Model ..................................................... 23
4 Summary of Key Results .................................................................................................... 25
4.1 Sediment Characteristics ........................................................................................... 25
4.2 OCP of the Sediments ................................................................................................ 25
4.3 Seasonal Dynamic ...................................................................................................... 27
4.4 Spatial Variability ....................................................................................................... 28
4.5 Prognosis Model......................................................................................................... 28
4.6 Oxygen Consumption under Stable Conditions ......................................................... 29
5 Outlook and Implications .................................................................................................. 31
Appendix publication A ........................................................................................................... 35
Oxygen Consumption of Resuspended Sediments of the Upper Elbe Estuary: Process
Identification and Prognosis..................................................................................................... 35
A.1 Introduction ............................................................................................................... 36
A.2 Material and Methods ............................................................................................... 38
A.2.1 Study Site and Sampling ................................................................................. 38
A.2.2 Sediment Characterisation ............................................................................. 39
5
A.2.3 Oxygen Consumption ..................................................................................... 39
A.2.4 Calculation of Oxidation Reactions ................................................................ 40
A.2.5 Development of an Oxygen Consumption Model.......................................... 41
A.3 Results ........................................................................................................................ 41
A.3.1 Characterisation of the Sediments ................................................................. 41
A.3.2 OCP ................................................................................................................. 43
A.3.3 Stoichiometric Analysis and Correlations....................................................... 45
A.3.4 Development of an Oxygen Consumption Model.......................................... 47
A.3.5 Validation of the Oxygen Consumption Model .............................................. 51
A.4 Discussion ................................................................................................................... 54
A.4.1 Sediment Composition ................................................................................... 55
A.4.2 OCP and Stoichiometric Analysis.................................................................... 55
A.5 Conclusion .................................................................................................................. 57
Appendix publication B ........................................................................................................... 59
Oxygen Consumption of Resuspended Sediments of the Upper Elbe Estuary: Spatial and
Temporal Dynamics .................................................................................................................. 59
B.1 Introduction ............................................................................................................... 60
B.2 Material and Methods ............................................................................................... 62
B.2.1 Study Site and Sampling ................................................................................. 62
B.2.2 Sediment Characterisation ............................................................................. 64
B.2.3 OCP ................................................................................................................. 64
B.3 Results ........................................................................................................................ 65
B.3.1 Oxygen Dynamics in the Water Phase ........................................................... 65
B.3.2 Temporal Dynamics of Sediment Characteristics .......................................... 66
B.3.3 Temporal Dynamics of OCP ............................................................................ 68
B.3.4 Spatial Variability of OCP ................................................................................ 70
B.4 Discussion ................................................................................................................... 72
B.4.1 Temporal Dynamic ......................................................................................... 72
B.4.2 Spatial Variability ............................................................................................ 74
B.5 Conclusion .................................................................................................................. 75
6
Appendix publication C ........................................................................................................... 77
Temperature‐Dependent Oxygen Consumption of Sediments of the Upper Elbe Estuary ..... 77
C.1 Introduction ............................................................................................................... 78
C.2 Material and Methods ............................................................................................... 80
C.2.1 Sampling and Basic Analyses .......................................................................... 80
C.2.2 High Resolutions Oxygen Concentration Profiles .......................................... 81
C.3 Results ........................................................................................................................ 82
C.3.1 Sediment Characteristics ................................................................................ 82
C.3.2 Oxygen Concentration Profiles ...................................................................... 82
C.3.3 Oxygen Consumption Rates ........................................................................... 84
C.4 Discussion ................................................................................................................... 85
C.5 Conclusion .................................................................................................................. 87
References ................................................................................................................................ 89
7
List of Abbreviations
Ctotal Total Carbon
MSL Mean Sea Level
NRMSE Normalized Root Mean Squared Error
Ntotal Total Nitrogen
OCP Oxygen Consumption Potential
OCP168 Oxygen Consumption Potential after 168 h of Incubation
RMSE Root Mean Squared Error
SOD Sediment Oxygen Demand
TOC Total Organic Carbon
8
Abstract
This thesis deals with sediments and their influence on the oxygen balance of the tidal Elbe
River. The study area is the upper Elbe estuary, including the Port of Hamburg. In this area,
minimal oxygen content zones are frequently formed during the summer months, where the
oxygen content falls below the critical value of 3 mg O2 l‐1 for fish. The biogeochemical
processes that lead to this oxygen depletion are well known in the water phase. However, the
influence of sediments with their spatially and temporally variable composition on the oxygen
balance is largely unknown. The way sediments consume oxygen varies. Under resuspension,
large amounts of oxygen‐consuming substances are quickly released into the water phase and
can negatively affect the oxygen balance. Under stable conditions, oxygen diffusely penetrates
the sediment surface and is slowly but continuously consumed in the oxic sediment layer.
The aim of this thesis is to clarify how strong the sediment‐induced influence on oxygen
consumption is, which processes play a major role in this and how much the sediments and
their oxygen consumption potential (OCP) differ on a spatial and seasonal level during
resuspension. In two approaches, the spatial and seasonal variability of sediment composition
and OCP during sediment resuspension was determined. In a third approach, the oxygen
consumption of sediments under stable conditions and its temperature dependence was
determined.
The OCP was quantified by seven‐day incubation experiments, in which the sediment
samples were kept in resuspension. To identify the oxygen‐consuming sub‐processes, the CO2
formed, and the pore water concentration of relevant anions and cations were determined
before and after the experiments. From this information, the most important oxygen‐
consuming sub‐processes could be distinguished, and their respective shares of the total
oxygen consumption were calculated stoichiometrically. Based on the analysed sediment
properties and the quantified sub‐processes, we developed a prognosis model that predicts
the OCP of sediments by using a single key parameter. In order to assess the oxygen
consumption of sediments under stable conditions, high‐resolution oxygen depth profiles
were recorded for the first time on sediment cores from three sites in the upper Elbe estuary.
Within the upper Elbe estuary, the sediments showed a high spatial and seasonal
variability in their composition and OCP. Towards the North Sea, the OCP of the sediments
decreased, which can be attributed to a reduced input of fresh biomass in combination with
a decrease in the degradability of the organic matter. The OCP of the sediments varied
spatially between 0.005 and 0.967 mmol O2 g d.wt.‐1 and was up to 5.5 times higher in
summer than in winter. This seasonality was also evident in the pore water composition with
an enrichment of ammonium and a decrease in sulphate concentration in summer, due to
more reduced conditions. Spatially, the highest OCP and chlorophyll concentrations were
recorded in the transition zone between the shallow and the deeper and navigable
downstream area of the harbour. Looking at the OCP of the sediments during a seven‐day
resuspension event, the spatial variability was greater than the seasonal variability. Oxygen
consumption under stable conditions showed a clear increase with temperature, with one of
9the three samples characterised by a higher temperature effect due to a lower TOC/Ntotal ratio
of the sample, indicating a more easily degradable biomass. The input and degradability of
fresh biomass in conjunction with seasonal changes such as sediment and pore water
composition along with bacterial productivity/biomass control the oxygen consumption under
stable conditions.
The results from the spatial and seasonal analysis showed that the OCP of the sediments
during a seven‐day resuspension event is controlled by the concentration of fresh organic
matter. As a proxy for the fresh organic matter, we used the concentration of chlorophyll in
the sediment. The total chlorophyll concentration showed the highest correlation with the
OCP, followed by Ntotal and TOC. The most important biogeochemical processes involved in
oxygen consumption during a resuspension event were identified as the rapid biochemical
oxidation of reduced compounds (Fe2+, Mn2+, and sulphur compounds like H2S, FeS, FeS2), the
nitrification (mean velocity) and the slower mineralisation of organic matter. Based on these
findings, a model was developed that predicts the oxygen consumption of the sediments of
the upper Elbe estuary with only one sediment parameter (Ntotal). The developed prediction
model is well suited to calculate the oxygen consumption of resuspended sediments in the
Hamburg harbour area during the relevant warmer months (NRMSEZusammenfassung
Diese Arbeit beschäftigt sich mit Sedimenten und deren Einfluss auf den Sauerstoffhaushalt
der Tideelbe. Im Untersuchungsgebiet, dem oberen Elbeästuar einschließlich des Hamburger
Hafens, bilden sich in den Sommermonaten häufig Sauerstoffminimumzonen aus, in denen
der Sauerstoffgehalt unter den für Fische kritischen Wert von 3 mg O2 l‐1 fällt. Die
biogeochemischen Prozesse in der Wasserphase, die zu diesem Sauerstoffdefizit führen, sind
bereits erforscht. Welchen Einfluss aber die Sedimente mit ihrer räumlich und zeitlich
variablen Zusammensetzung auf den Sauerstoffhaushalt haben, ist weitestgehend unbekannt.
Sedimente können bei Resuspension schnell große Mengen sauerstoffzehrender Stoffe in die
Wasserphase abgegeben und somit die Sauerstoffbilanz unmittelbar negativ beeinflussen.
Unter stabilen Bedingungen hingegen, dringt der Sauerstoff diffus in die Sedimentoberfläche
ein und wird in der oxischen Sedimentschicht langsam, aber kontinuierlich verbraucht.
Ziel dieser Arbeit ist es, exemplarisch für das obere Elbeästuar zu erfassen, wie groß der
sedimentbedingte Einfluss auf den Sauerstoffverbrauch ist, welche sauerstoffzehrenden
Prozesse eine wesentliche Rolle spielen und wie stark sich die Sedimente und ihr
Sauerstoffzehrungspotential (OCP) während einer Resuspension auf räumlicher und
saisonaler Ebene unterscheiden. In zwei Messkampagnen wurde die räumliche und saisonale
Variabilität der Sedimentzusammensetzung und des OCP während der Resuspension von
Sedimenten erfasst. In einem dritten Untersuchungsansatz wurde die Temperatur‐
abhängigkeit der Sauerstoffzehrung von Sedimenten unter stabilen Bedingungen bestimmt.
Zur Quantifizierung des OCP wurden siebentägige Inkubationsversuche, bei denen
Sedimentproben in Resuspension gehalten wurden, durchgeführt. Um die sauerstoff‐
zehrenden Teilprozesse zu identifizieren, wurden das gebildete CO2 und die Porenwasser‐
konzentration relevanter An‐ und Kationen vor und nach den Experimenten bestimmt.
Anhand dieser Informationen konnten die wichtigsten sauerstoffzehrenden Teilprozesse
unterschieden und ihre jeweiligen Anteile am gesamten Sauerstoffverbrauch stöchiometrisch
berechnet werden. Auf dieser Grundlage wurde ein Prognosemodell zur Berechnung des OCP
entwickelt. Zur Erfassung des Sauerstoffverbrauches von Sedimenten unter stabilen
Lagebedingungen, wurden erstmalig hochauflösende Sauerstofftiefenprofile an Sediment‐
kernen von drei Standorten aus dem oberen Elbästuar erfasst.
Die Sedimente wiesen innerhalb des oberen Elbeästuars eine hohe räumliche und
saisonale Variabilität in ihrer Zusammensetzung und im OCP auf. In Richtung Nordsee nahm
das OCP der Sedimente ab, was auf einen verringerten Eintrag von frischer Biomasse in
Kombination mit einer Abnahme der Abbaubarkeit der organischen Substanz zurückgeführt
werden kann. Das OCP der Sedimente variierte räumlich zwischen 0,005 und 0,967 mmol O2 g
TS‐1 und war im Sommer bis zu 5,5‐mal höher als im Winter. Diese Saisonalität zeigte sich auch
in der Porenwasserzusammensetzung mit einer Anreicherung von Ammonium und einer
Abnahme der Sulfatkonzentration im Sommer, aufgrund stärker reduzierter Bedingungen.
Räumlich wurden die höchsten OCP und Chlorophyllkonzentrationen in der Übergangszone
zwischen dem flachen und dem tieferen und schiffbaren stromabwärts gelegenen Bereich des
11Hafens erfasst. Betrachtet man das OCP der Sedimente während eines siebentägigen
Resuspensionsereignisses, so war die räumliche Variabilität größer als die saisonale. Die
Sauerstoffzehrung unter stabilen Bedingungen zeigte einen deutlichen Anstieg mit der
Temperatur, wobei eine der drei Proben durch einen höheren Temperatureffekt aufgrund
eines niedrigeren TOC/Ntotal‐Verhältnisses der Probe gekennzeichnet war, was auf eine
leichter abbaubare Biomasse hinweist. Der Eintrag und die Abbaubarkeit von frischer
Biomasse in Verbindung mit den saisonalen Veränderungen, wie der Sediment‐ und
Porenwasserzusammensetzung und der bakteriellen Produktivität/Biomasse, steuern die
Sauerstoffzehrung unter stabilen Bedingungen.
Die Ergebnisse aus der räumlichen und saisonalen Analyse zeigten, dass das OCP der
Sedimente, während eines siebentägigen Resuspensionsereignisses, durch die Konzentration
frischer organischer Substanz gesteuert wird. Die Chlorophyllkonzentration im Sediment dient
dabei als Indikator für die frische organische Substanz und zeigte die höchste Korrelation mit
dem OCP der Sedimente, gefolgt vom Ntotal und TOC Gehalt. Als wichtigste biogeochemische
Prozesse, die an der Sauerstoffzehrung während eines Resuspensionsereignisses beteiligt
sind, wurden die schnelle biochemische Oxidation von reduzierten Verbindungen (Fe2+, Mn2+
und Schwefelverbindungen wie H2S, FeS, FeS2), die Nitrifikation (mittlere Geschwindigkeit)
und die langsamere Mineralisierung von organischem Material identifiziert. Basierend auf
diesen Befunden wurde ein Modell entwickelt, dass mit nur einem Sedimentparameter (Ntotal)
den Sauerstoffverbrauch der Sedimente des oberen Elbeästuars prognostiziert. Das
entwickelte Prognosemodell ist gut geeignet, den Sauerstoffverbrauch von resuspendierten
Sedimenten im Hamburger Hafengebiet während der relevanten wärmeren Monate zu
berechnen (NRMSE“The saddest aspect of life right now is that
science gathers knowledge faster than society gathers wisdom”
–Isaac Asimov–
1 Introduction
The availability of dissolved oxygen is a prerequisite for an intact aquatic ecosystem. It directly
affects the biological health of river systems (Williams and Boorman 2012) and is used as an
indicator for the quality of surface water bodies (Cude 2001, Bayram et al. 2015). As the most
abundant electron acceptor, oxygen has a major influence on the sulphur, nitrogen,
phosphorus, and carbon cycles and influences the metabolic processes taking place in the
sediment (Glud 2008). The water body’s oxygen content is the result of oxygen‐consuming
and oxygen‐supplying processes. Atmospheric inputs and the oxygen‐supplying
photosynthesis of algae enrich the water with oxygen. Oxygen‐consuming processes, such as
the mineralisation of organic matter, respiration, or the oxidation of reduced compounds lead
to a reduction of oxygen. An imbalance between oxygen‐supplying and oxygen‐consuming
processes leads to oxygen depletion, which occurs in coastal waters and estuaries worldwide
(Morris et al. 1982; Sarma et al. 2013; Su et al. 2017). In the worst case, the oxygen
concentration falls below critical values and leads to aquatic life's mortality. The appearance
of this imbalance is a problem that seems to be growing globally (Díaz and Rosenberg 2008,
Gilbert et al. 2010) and in relation to climate change it is unclear how this will affect the
different types of estuaries (Bruce et al. 2014).
The oxygen consumption in estuaries takes place both in the water phase and in the
sediment. Depending on their composition, sediments can have a strong influence on the
oxygen balance of water bodies. They can act as an oxygen sink by removing oxygen from the
water phase through oxygen‐consuming processes within the sediment or as a source of
oxygen‐consuming substances that enter the water phase through diffuse fluxes or by
resuspension. The upper Elbe estuary, as a highly dynamic system, serves as the study area to
answer the open questions regarding the oxygen consumption properties of sediments.
1.1 Sediments as Sinks of Oxygen
Sediments can be regarded as a kind of archive that by their composition reflect the prevailing
environmental conditions under which they were formed. Their composition is influenced by
several factors, such as their location, sedimentation rates, flow velocities, temperature, and
the input of organic biomass. Due to the high number of different factors, sediments can differ
immensely from one another, leading to a strong spatial and temporal variability in oxygen
consumption from sediments. According to Veenstra and Nolen (1991), sediment oxygen
demand (SOD) is defined as the rate of oxygen consumption, biologically or chemically, on or
in the sediment at the bottom of a water body. The SOD combines two main oxygen sinks:
(i) the sediment oxygen uptake by aerobic mineralisation of organic matter as well as
oxidation of reduced substances and (ii) the flux of reduced substances out of the sediment
13(Steinsberger et al. 2019). Barcelona (1983) and Rong and Shan (2016) divided the SOD into
chemical oxidation of dissolved iron, manganese, hydrogen sulphide/sulphur, and into
biochemical oxidation of ammonium and nitrite to nitrate, in addition to the mineralisation of
organic matter and respiration.
In the case of SOD, a distinction must be made between oxygen consumption at stable
conditions and consumption at resuspension caused by a disturbance of the sediment surface.
Under stable conditions, SOD is diffusion limited. Oxygen diffusely penetrates the sediment.
The thickness of the oxic zone can range from a few mm in organic rich sediments (Sweerts et
al. 1989) to several cm (Wenzhöfer et al. 2001) in sandy sediments. The penetration depth is
regulated by the organic carbon degradation, the oxygen concentration of the water, and the
transport of oxygen from the water phase into the sediment, whereby the sediment porosity
and the diffusion coefficient of oxygen plays a role (Cai and Sayles 1996). If oxygen is no longer
available for the energy metabolism, other electron acceptors take over its part (suboxic and
reduced zone). As a result of the diagenetic processes, the sediment and pore water
accumulates with reduced compounds from the anaerobic metabolism (Figure 1).
Figure 1: A schematic illustration of some significant diagenetic processes in sediments (Adapted
according to Glud 2008).
The SOD during resuspension is the sum of multiple processes driven by the oxygen supply,
the biochemical processes taking place, and the sediment properties. The oxygen
consumption potential (OCP) of sediments becomes apparent during their resuspension, as
there is a sudden release of all solid sediment components and the pore water. The release of
reduced compounds such as iron, manganese, or hydrogen sulphide leads to rapid
biochemical oxygen consumption. Ammonium, as the end product of the anaerobic
degradation of organic matter, accumulates in the pore water and is released during
resuspension. The nitrification that then occurs consumes 4.57 gram of oxygen per gram of
ammonium (Wezernak and Gannon 1967). Sedimented organic matter is released and is
available for aerobic mineralisation. In organic rich sediments, oxygen is the only limiting
factor and the OCP can be seen as a function of the electron acceptors (Bryant et al. 2010).
14Almroth et al. (2009) and Sloth et al. (1996) showed in their experiments that a resuspension
of sediments causes a local and temporary increase of reduced inorganic compounds and
organic matter within the water, which enhances the oxygen consumption.
Resuspension of sediments takes place in both limnic and marine areas worldwide. It
occurs when the shear stress is high enough to transport sediment particles into the water
column (Almroth et al. 2009). This critical shear stress can be exceeded by natural forces such
as tidal currents, wind, and biological activities (Sanford et al. 1991; Graf and Rosenberg 1997),
or by human activities such as ship induced waves (Schoellhamer 1996) and dredging
operations (Cappuyns et al. 2006).
For an assessment of human activities on a water body's oxygen balance, models that
combine physical and biochemical processes can be used. However, a shortcoming of many
numerical models that include biochemical processes at the sediment‐water interface is that
they are unable to model the resuspension of sediments and its effect on oxygen and nutrient
dynamics (Moriarty et al. 2018). Therefore, it is of high interest to quantify the impact of
resuspended sediments to improve water quality modelling.
1.2 Spatial and Seasonal Variability
The influence of sediments on the oxygen balance of water bodies is mainly controlled by their
composition. As already mentioned, many different factors control the composition of the
sediments. If one wants to know more about the ability of sediments to consume oxygen, the
prevailing hydrographic conditions and the distances to potential sources of input play an
important role. Sedimentation rates, flow velocities, and accumulation of organic material
depend on these factors. The particle size distribution is influenced by the flow velocity and
thus the input of organic matter. A reduction in flow velocity leads to an increased
sedimentation of finer particles and thus to an accumulation of organic material that is
associated with the fine‐grained fraction (de Haas et al. 2002; Giles et al. 2007). This leads to
a change in the composition of the sediments and consequently to a variability in the ability
to consume oxygen. In addition to this rigid spatial structure, the sediments also differ on the
temporal scale. Due to seasonal variations in temperature, discharge rates, sedimentation
rates, and algal biomass development, the sediment is subject to a constant change. Besides,
precipitation and the resulting runoff lead to an input of terrestrial organic material. A change
in water temperature has a direct influence on the oxygen saturation and controls the activity
of the micro‐ and macrofauna. Many sediment properties have been found to correspond with
SOD, such as sediment organic matter, nitrogen, pigments (chlorophyll a, phaeopigment a,
total carotenoids), as well as the water depth and spatial location of the sediments (Vidal et
al. 1992; Duineveld et al. 1997; Grant et al. 2002; Giles et al. 2007; Mügler et al. 2012). In
addition, water temperature shows a positive correlation with SOD (Hopkinsion et al. 2001;
Fulweiler et al. 2010). Many authors have reported a seasonal variability in SOD, with an
increase in summer and a decrease in winter in rivers and coastal sediments (Rasmussen and
Jørgensen 1992; Rysgaard et al. 1995; Hopkinsion et al. 2001; Akomeah and Lindenschmidt
2017). This summer‐winter difference is explained by changes in the biological activity and a
15variation in the input of labile organic matter such as dead settled algae. These facts make it
obvious that sediments are subject to spatial and seasonal dynamics. However, little is known
about the spatial and seasonal variability of the SOD and sediment parameters that control
the sediment oxygen consumption.
As this study deals with the variability of sediments, the lower Elbe estuary serves as the
selected study region. Due to the Port of Hamburg, the tidal influence, and its location in a
climate zone with distinct seasons, the upper Elbe estuary is a spatially and temporally variable
system. This results in a system of differently composed sediments, each with different oxygen
consumption properties, which is necessary to answer the existing questions.
1.3 The Upper Elbe Estuary
The upper Elbe estuary (Germany), characterized by fresh water from the Middle Elbe,
stretches from the weir Geesthacht (stream‐km 586) 46 km downstream to Wedel (stream‐
km 541) and includes the large area of the Port of Hamburg (Figure 2). Between stream‐km
609 and 626, the Elbe forms an inland delta with the Northern Elbe and the Southern Elbe as
its major branches. With the start of the trafficability for ocean vessels at about stream‐km
619 (Southern Elbe) and stream‐km 624 (Northern Elbe), the fairway's depth and the harbour
basins have been increased from 2.7 m to 15 m below MSL.
The hydrology of the upper Elbe estuary is spatially and temporally dynamic. Flow
velocities vary strongly between the fairway and the rear parts of harbour basins. Due to low‐
flow areas or differences in cross‐flow sections, hot spots of sedimentation occur. Particulate
organic matter originates from the Middle Elbe and the North Sea and is mixed in varying
proportions within the upper estuary depending on the headwater discharge (Gröngröft et al.
1998; Kleisinger et al. 2015; Reese et al. 2019). Additionally, seasonal changes in temperature
and phytoplankton concentration prevail. Therefore, it is assumed that this variability
influences the oxygen demand of the Hamburg port sediments.
Figure 2: Port of Hamburg with the Northern and Southern Elbe, which form the inland delta (source:
www.bing.com/maps/).
16The Port of Hamburg is the largest seaport in Germany and the third‐largest in Europe. Seen
from the mouth of the Elbe, it lies 110 km east in the country's interior. With 136.6 million
tonnes of cargo handled in 2019 (Port of Hamburg 2020), the port's primary use is cargo
handling. An essential aspect of the management of the harbour is to ensure the navigable
depth of the Elbe. This falls under the task of the Hamburg Port Authority (HPA).
Due to high sedimentation rates in the fairways and adjacent harbour basins in the
Hamburg port area each year an amount of 1.7 to 6 million tons (as dry substance) must be
dredged to keep the fairway navigable (2010–2018; Hamburg Port Authority unpublished
data). For this purpose, different systems such as Hopper dredging, bed levelling, and water
injection are used in the Elbe estuary. During hopper dredging operations, sediments are
sucked up from the riverbed and dumped back into the water column at the respective
disposal site. Bed levellers use a plough to relocate the sediment without raising it into the
water column. Water injection application uses nozzles to pump water directly onto the
sediment. The remobilised sediments are then transported away from the site of operation
by the current. These procedures are subject to various regulations. At specific oxygen
concentrations in the river water, they are prohibited in order to prevent the oxygen content
from falling to critical levels. Due to the critical oxygen concentration in summer in the upper
estuary, the disposal of sediments is restricted to the winter months. In other not oxygen‐
depleted areas, such as the outer estuary and the North Sea, disposal of Elbe sediments is also
carried out in summer. In the tidal Elbe, it is restricted below 6 mg O2 l‐1 and prohibited below
5 mg O2 l‐1 in order to avoid oxygen depletion due to sediment disposal (BfG 2008). Water
injection applications are used with restrictions in summer and are prohibited at an oxygen
concentration of below 4 mg O2 l‐1 (Hamburg Port Authority 2018).
1.4 Oxygen Concentration in the Elbe Estuary
In the upper part of the Elbe estuary with the Port of Hamburg and all its harbour basins, a
strong decline in dissolved oxygen concentrations is frequently observed during the summer
months (Figure 3) (Bergemann et al. 1996; Schroeder 1997; Schöl et al. 2014) regularly
reaching critical values of less than 3 mg O2 l‐1.
Before the fall of the Iron Curtain, large quantities of oxygen‐consuming and toxic
substances were discharged into the Elbe. This resulted in the fact that the minimum oxygen
concentration of 3 mg O2 l‐1 required for fish in the tidal Elbe was frequently not reached
(Figure 3). Such oxygen minimum zones resulted in regularly occurring fish mortality. After
reunification, the closure of factories and the installation of wastewater treatment plants led
to an improvement in the tidal Elbe's oxygen content (Bergemann et al. 1996; Sanders et al.
2017). Despite an improvement, there is still the formation of oxygen minimum zones within
the tidal Elbe. Due to the high nutrient content of the relatively shallow Middle Elbe, strong
algae masses are formed in spring and summer. These algae are transported to the navigable
depth of the tidal Elbe. Due to turbulent transport, they reach the lightless, deeper zone, and
oxygen is first consumed by respiration and then by their decomposition. Therefore, these
oxygen minimum zones are mainly attributed to algal respiration and carbon degradation
17(Schroeder 1997) as well as zooplankton grazing (Schöl et al. 2014). Kerner (2000) also came
to the conclusion that microbial oxygen consumption, coupled with the degradation of freshly
transported organic material from the upper stream, controls the oxygen concentration in the
warmer seasons. Schroeder (1997) postulated that nitrification and sediment processes are of
minor relevance, while Sanders et al. (2017) found a substantial increase in nitrification in the
Hamburg area in spring and summer. Therefore, mineralisation and respiration have a strong
influence on oxygen consumption in the water phase. Studies in other rivers have shown that
the oxygen consumption of sediments can be the major source of water column oxygen
depletion (Boynton and Kemp 1985; Rutherford et al. 1991; Matlock et al. 2003; MacPherson
et al. 2007). So far, sediment driven oxygen consumption is less investigated in the Hamburg
harbour area. It is not possible to make any quantitative statements about the influence of
the Hamburg harbour sediments on the oxygen budget of the Elbe. Especially with regard to
the regularly occurring oxygen minimum zones in the upper Elbe estuary, it is not yet clear
what role the sediments have in this.
Figure 3: Daily mean values of oxygen concentration at the Hamburg‐Seemannshöft measuring station
1982 – 2006 (Adapted according to ARGE ELBE / FGG ELBE 2007).
As a result of the global climate change, the influence of the processes mentioned above is
likely to change. In order to better understand the influence of different dredging activities
and the possible effects of climate change on sediment composition and thus on the SOD, it
is essential to identify the individual parameters and processes that influence the oxygen
consumption by sediments.
182 Objectives of the Study
As described in the previous sections, the effective and potential oxygen consumption of
sediments is composed of different sub‐processes and is controlled by the properties of the
water body and the sediment composition. With regard to the sediments of the upper Elbe
estuary, the following questions arise:
‐ Which biochemical processes control the OCP of sediments?
‐ What is the share of the main biochemical processes?
‐ Which sediment properties control the involved biochemical processes?
Due to the economic importance of the Port of Hamburg, its waterways must be maintained
regularly. This can lead to a resuspension of the sediments, resulting in a decrease in the water
phase's oxygen concentration. Being able to estimate the influence of resuspended sediments
on the basis of individual sediment parameters would help to optimize maintenance measures
and to reduce the risk to the aquatic environment. Therefore, next question is:
‐ Is it possible to predict the OCP of sediments from known sediment parameters?
As the Port of Hamburg is located in a climate zone with distinct seasons, there are strong
seasonal dynamics in terms of temperature, precipitation, solar radiation, and the runoff
between summer and winter. As a result, there is a seasonal change in sediment composition
and SOD. In order to determine the influence of this seasonality on the sediments and their
OCP, the following question has to be answered:
‐ What seasonal changes occur with regard to the composition and the OCP of
sediments?
Due to the structure of the Port of Hamburg, with its different harbour basins, water depths,
tidal influences, cross‐sections, distance to the Middle Elbe, and the resulting flow velocities
and sedimentation rates, was a further question:
‐ How does the OCP vary spatially?
Previous research questions have dealt with resuspended sediments and the processes and
sediment parameters that control the potential oxygen consumption. Besides the effect of
resuspension, where the shear resistance must be overcome so that the sediment
components pass into the water phase, sediments also consume oxygen when they are under
stable conditions. The oxygen consumption of sediments at stable conditions takes place
independently of external disturbances and can therefore be regarded as a continuous
process. For the upper Elbe estuary, no studies are available so far that use high‐resolution
oxygen depth profiles to investigate the oxygen consumption of sediments at stable
conditions. The temperature dependence and sediment composition and its influence on the
19oxygen consumption of sediments at stable conditions was, therefore, the focus of the next
questions.
‐ How much oxygen is consumed from the sediments under stable conditions?
‐ How much does temperature control the SOD?
203 Material and Methods
In this thesis, the oxygen consumption of sediments in the upper Elbe estuary was determined
and investigated by three approaches (Figure 4). The first approach focused on the spatial
variability of sediment composition and oxygen consumption during resuspension (OCP). The
second approach aimed to determine the seasonal dynamics of sediment composition and
oxygen consumption during resuspension (OCP), and the third approach investigated the
oxygen consumption of sediments at stable conditions (SOD).
3.1 Sampling Approaches
During the first approach to study the spatial variability, sediments from 21 sites in the upper
Elbe estuary were investigated (Figure 5, P1‐P21). The sampled area extended from the
easternmost point Stover Strand (stream‐km 589) to the westernmost point at the sediment
trap Wedel (stream‐km 643). Samples in the area of the Hamburg harbour were taken both in
the fairway and at the entrances of the harbour basins and their ends. The sampling was
carried out from 2017‐06‐30 to 2017‐07‐04. To characterise the influence of sediment age,
these samples were divided into two age categories: 2 years. For an estimation
of the age of the sediments, the data of the last maintenance dredging at the sites were taken
into account. In addition, the load of trace elements was taken into account, which has been
decreasing over time in the Elbe since the 1990s (HPA, unpublished document 2019).
In order to answer the research questions on seasonal dynamics, monthly samples were
taken at a single location for a period of two years from December 2016 to November 2018
(2nd approach). To ensure that the sediment composition represents the current
environmental conditions, we selected a location with a known high sedimentation rate. This
condition applies to the entrance area of the Hansahafen (Figure 5, P10). This area is
characterized by a widening of the cross section of the Northern Elbe, where increased
sedimentation rates occur with the formation of a mud lens. At the edge of the mud lens
sedimentation rates are reported to be up to 2 cm day‐1 and up to 9 cm day‐1 in the centre of
the lens. Consequently, dredging activities in this area are quite frequent.
Samples were taken from a ship with a core sampler (Frahm‐Lot, length 80 cm, inner
diameter 10 cm). In both approaches, the layers of 0‐20 cm and 40‐60 cm were analysed.
Seven‐day incubation experiments served to determine the OCP of the sediments. Typical
sediment parameters such as organic carbon content (TOC), total nitrogen content (Ntotal) or
grain size distribution were determined. A more detailed overview of the methodological
design is given in Figure 4.
21Figure 4: Schematic overview of the study design including sampling, preparation of samples in the
laboratory, and the analysis performed.
22Figure 5: Sampling locations within the upper Elbe estuary and the Port of Hamburg. Black dots
represent the sampling locations between river km 589 (P1) and 643 (P21) to analyse the spatial
variability. Point P10 refers to the access area of the Hansahafen for the analysis of the seasonal
dynamic. The circles represent the sampling locations for the analysis of oxygen consumption of
sediments under stable conditions. From the left: Köhlfleet, Hansahafen, and the Fairway Norderelbe
(source of map: unpublished and adapted data from Hamburg Port Authority).
To quantify the individual processes (biochemical oxidation of reduced compounds and
nitrification) involved in oxygen consumption, the concentrations of sulphate, nitrate, nitrite,
iron, and manganese were determined before and after the incubation experiments. By the
change of concentration, the respective proportions of the individual processes in the oxygen
consumption was calculated stoichiometrically. The proportion of mineralisation in the
oxygen consumption was deduced from the measured amount of CO2, that has been formed
during the incubation experiments.
In a 3rd approach, samples from three locations were taken (Figure 5 circles). These
samples were used to investigate the oxygen consumption of sediments under stable
conditions with high resolution oxygen depth profiles (Berg et al. 1998). The measurements
were carried out at 5 °C, 15 °C, and 25 °C to determine the temperature effect on SOD under
stable conditions. The sample at the location Hansahafen was taken on 2018‐04‐20, the
sample at the location Köhlfleet on 2018‐06‐23, and the sample at the location Fairway
Norderelbe on 2018‐08‐28.
3.2 Development of an Oxygen Consumption Model
To develop a model for OCP calculation, we used the following approach: (i) The cumulative
oxygen consumption curves of the samples from the 1st approach were divided into three
23phases that characterize the biogeochemical chain of processes. The respective curves of the
individual phases were fitted. (ii) The resulting parameters of the fitting functions of all
samples were correlated with the sediment properties. This resulted in six linear or nonlinear
regression equations between sediment properties and fitting parameters. Using the
sediment properties, we were able to calculate the six parameters per sample, which allowed
us to calculate the oxygen consumption and their involved processes along the timeline. For
all samples, the normalized root mean squared error (NRMSE) between measured and
modelled oxygen consumption was calculated. (iii) To validate the model, the samples from
the 2nd approach were used. Using the sediment properties, the six parameters per sample
were determined, and the oxygen consumption was calculated and compared with the
measured oxygen consumption. Thus, the accuracy of the model could be quantified.
244 Summary of Key Results 4.1 Sediment Characteristics The study of Elbe estuary sediments reveals that the properties of the analysed samples from all three approaches varied strongly. For instance, the sand content of the samples from the spatial analysis (1st approach) ranged between 1.1 and 99.2%‐d.wt. and the amount of fine grained particles (
first hours (Phase 1), the consumption rate is highest and is dominated by the oxidation of reduced compounds. After consuming the corresponding educts, the rate decreases, and the nitrification (Phase 2) controls the oxygen consumption. If ammonium is no longer available, the oxygen consumption rate decreases a second time substantially, and the consumption is controlled by the mineralisation of organic matter (Phase 3). That the assumptions about the processes are correct showed a Spearman correlation between the measured oxygen consumption and the stoichiometrically calculated oxygen consumption, which gave a Spearman correlation coefficient (rsp) of 0.960 (p
proportions of the total consumption. This can be attributed to the age of the organic material.
During degradation, labile organic compounds are preferentially degraded, which leads to a
selective decrease in the reactivity of the organic material over time (Arndt et al. 2013).
Additionally, with ongoing anaerobic degradation of organic matter sulphate respiration leads
to an accumulation of reduced sulphur compounds (FeS or FeS2), resulting in a higher
proportion in the total oxygen consumption.
Figure 7: Schematic illustration of the sediment age‐related changes in sediment composition and the
resulting changes in the percentages of the sub‐processes involved in OCP during an incubation period
of 168 hours.
4.3 Seasonal Dynamic
In the laboratory, samples from the summer months showed an OCP up to 5.5 times higher
than samples from the winter (Figure B 6). Seasonal dynamics in water temperature, in the
water's oxygen content, or in the formation of algal blooms influence the sediment and pore
water composition and the OCP of the sediments. Increased water temperatures lead to a
reduction of the solubility of oxygen in the water and to increased microbiological activity. As
a result, this causes a change in the pore water composition due to more reduced conditions.
The porewater is therefore enriched with ammonium (decomposition of organic matter) and
reduced sulphur compounds such as H2S, FeS and FeS2 (sulphate reduction). A resuspension
leads to an increased oxygen consumption by releasing these substances. A reduction in
headwater discharge results in an increased sedimentation of finer particles and thus to an
accumulation of organic material associated with the fine‐grained fraction (de Haas et al.
2002; Giles et al. 2007). The increased algae masses also lead to an enrichment of the
sediment with fresh, easily degradable algae biomass. The chlorophyll concentrations varied
from 50 to 250 µg g d.wt.‐1 between winter and summer (Figure B 7). As a result, the
27sediments are enriched with Ntotal, which leads to a tighter TOC/Ntotal ratio in summer. What
fits well into the overall picture is that the quotient between the measured oxygen
consumption and the TOC content of the sediments was subject to a seasonal dynamic (Figure
B 7). The results showed that more oxygen per gram TOC is consumed in summer, indicating
a change in the organic matter's degradability.
4.4 Spatial Variability
The upper Elbe estuary had a strong spatial variability regarding the OCP (Figure B 9) and
sediment parameters. The lowest OCP was determined at site 7 with 0.005 mmol O2 g d.wt.‐1
and an almost 200‐fold higher value was determined at site 4 with 0.967 mmol O2 g d.wt.‐1.
Both sites were strongly contrasting in terms of TOC, chlorophyll, and sand content. Clear
statements about the spatial behaviour of the OCP of sediments between the different sites
(fairway, entrances, and the end of different harbour basins) cannot be made since the
number of samples is too small. However, certain spatial statements can be made. The OCP
of the sediments showed a decrease with increasing stream‐km (Figure B 9). This results from
a reduced input of fresh biomass (distance to the Middle Elbe increases) combined with a
decrease in the degradability of the organic mass, as described by Zander et al. (2020). The
transition zone from the shallower (2.7 m below MSL) to the deeper area (15 m below MSL)
of the Port of Hamburg leads to an increased accumulation of dead algae. The increased water
depth creates a larger lightless area in the water, which causes the death of the algae
(Schöl et al. 2014). Accordingly, the sediments are enriched with fresh algal biomass, which
leads to an increase in OCP. The distance from possible sources of oxygen‐consuming
substances and labile organic matter input controls the OCP of sediments on a spatial scale.
4.5 Prognosis Model
To improve predictions and water quality models, we developed a model that calculates the
OCP of sediments based on the Ntotal content. Together with the chlorophyll content, the Ntotal
content showed the highest correlations with the OCP of the sediments for both
measurement approaches. For the model Ntotal was selected because it is one of the standard
parameters in sediment analysis and this parameter is also measurable in the lower sediment
layers. The prediction model describes the three processes involved in oxygen consumption.
The biochemical oxidation of reduced compounds and the mineralisation of organic matter
are described by first order degradation functions and the nitrification by a constant rate with
a certain duration. The respective function parameters are calculated by means of established
correlations and the Ntotal content of the sediments. The prognosis model was developed using
the data from the 1st approach and validated using the data from the 2nd approach. A linear
regression between the modelled and measured oxygen consumption showed a good fit with
an R2 of 0.840 (1st approach) and an R2 of 0.818 (2nd approach) for an incubation period of
168 h. Figure 8 shows an example of a comparison between the measured oxygen
consumption and the oxygen consumption derived from the prediction model with its
28individual components. The total oxygen consumption results from the sum of the individual
processes and can be calculated for any time of resuspension.
Figure 8: Comparison between measured and predicted oxygen consumption calculated from the
prognosis model based on the Ntotal content of the sample. The model calculates the oxygen
consumption based on the biochemical oxidation of reduced compounds (SO42‐‐formation), the
nitrification of ammonium, and the mineralisation of organic matter. The sum results in the total
oxygen consumption of the sample.
For shorter resuspension times of 3 h and 24 h the model also provided good adjustments
(Figure A 5). For both approaches, the R2 was greater than 0.784. The slope of the regression
line ranged between 0.609 and 1.152, which can lead to an overestimation or underestimation
of the total oxygen consumption. The results from the 2nd approach showed a clear difference
in the adjustment between the winter and summer samples. The summer samples showed a
lower scatter and smaller NRMSE than the winter samples. This difference can be attributed
to seasonal processes that are not covered by our model. The activity and composition of the
microbial community, the quality of the organic matter or a changed TOC/Ntotal ratio can lead
to the differences. Nevertheless, the model provides a good basis to calculate the OCP of
sediments in the relevant warm season and thus to determine the influence of increased
quantities of resuspended sediment.
4.6 Oxygen Consumption under Stable Conditions
A clear temperature dependence can be seen in the oxygen consumption of sediments under
stable conditions (Figure C 3). With increasing temperature, the oxygen penetration depth
decreased, and the oxygen consumption rates increased, which is consistent with other
studies (Seiki et al. 1994; Hancke and Glud 2004; de Klein et al. 2017). The rising temperature
also increases the microbial activity (McDonnell 1969), which leads to higher metabolism
rates. However, the temperature effect was smaller than expected (10‐degree rule), since the
29You can also read
