Annual Dynamics of Pelagic Carbon
Fluxes in a Tidal Lagoon
Dissertation zur Erlangung eines
Doktorgrades in den Naturwissenschaften
-Dr. rer. nat.-
dem Fachbereich Biologie/Chemie
der Universität Bremen
vorgelegt von
Martina Löbl
im Dezember 2005
1. Gutachter: Herr Prof. Dr. Victor Smetacek
2. Gutachter: Herr Prof. Dr. Karsten Reise
CONTENT
SUMMARY... 3
ZUSAMMENFASSUNG... 7
CHAPTER 1 INTRODUCTION... 11
CHAPTER 2 PELAGIC PRIMARY PRODUCTION IN THE NORTHERN WADDEN SEA - NO QUANTITATIVE CHANGES SINCE THE MID 1980’S? ... 29
CHAPTER 3 IMPACT OF ZOOPLANKTON GRAZING ON PHYTOPLANKTON DYNAMICS IN SHALLOW COASTAL WATERS... 57
CHAPTER 4 SEASONAL DYNAMICS OF PELAGIC RESPIRATION IN A SHALLOW TIDAL LAGOON... 79
CHAPTER 5 MODELLING ATTEMPT ON CARBON/ NUTRIENT PROCESSES IN A TIDAL LAGOON TO DIRECT FUTURE RESEARCH... 97
CHAPTER 6 GENERAL DISCUSSION... 111
DANKSAGUNG...127
The Wadden Sea is a coastal ecosystem under high anthropogenic impact. Evaluation of man-made changes and impacts -e.g. eutrophication and fishery- require a detailed understanding of the ecosystem functioning; especially in terms of carbon fluxes through the ecosystem. Only little is known on annual dynamics of pelagic carbon turnover in the Wadden Sea and its linking to the benthic system.
In aquatic ecosystems, phytoplankton primary production, zooplankton grazing, and pelagic respiration are important processes of carbon dynamics. The aim of this study is to quantify the annual dynamics of pelagic primary production, zooplankton grazing, and respiration in a shallow coastal system and to investigate possible benthic impacts. Pelagic primary production, zooplankton grazing, and respiration were investigated as weekly/monthly time series over a one year period in the northern Wadden Sea. Studies were related to the Sylt long term time series, providing data on temperature, salinity, inorganic and organic nutrients, chlorophyll a and suspended matter concentrations (e.g. MARTENS & ELBRÄCHTER 1998). This study was conducted in the framework of the European Research Project COSA (Coastal Sands as Biocatalytical Filters) to relate the temporal dynamics of pelagic carbon dynamics with benthic processes.
Primary production measurements showed that:
• Annual pelagic primary production was 210 g C m-2 y-1 .
• Highest production rates occurred during a Phaeocystis globosa bloom.
The impact of suspended matter (SPM) on the water column light field and primary production was estimated:
• A doubling of SPM reduces ~20-50 % of primary production, with highest impact in autumn, winter and spring.
Suspended matter concentrations decreased significantly within the past twenty years. The impact of this decrease on primary production was tested. Primary production was calculated for the years 1984-1990 and for 2000-2004 on the basis of measured P/I-curves and hourly irradiance of 2004 and chlorophyll a and suspended matter concentrations of the respective years.
m y ; No change in primary production since the mid 1980’s could be observed.
Zooplankton grazing impact on phytoplankton biomass was measured during a sequence of nine dilution experiments from March until October.
• During a diatom spring bloom no grazing occurred.
• The grazing impact during a Phaeocystis globosa bloom was ~60 % d-1 of the standing stock; during the subseqent summer ~ 100% d-1 of the standing stock was grazed;
• during these periods presumably more phytoplankton biomass was maintained within in the pelagic than lost to the benthic food web.
The applicability of data from dilution experiments on natural processes is still under debate in literature and has not been shown before. Phytoplankton growth rates of dilution experiments were compared to in-situ measured growth rates. For this comparison a conversion factor regarding decreasing light intensities throughout the water column was introduced.
• Experimental growth rates were in a good fit with in-situ measured growth rates; • The overestimation of biomass by experimental results is in a good fit to benthic
assimilation of phytoplankton biomass reported by other studies.
Pelagic respiration measurements were carried out in two parallel time series, one at
in-situ temperature and one at 10° C to exclude temperature effects. The possible impact of
suspended matter concentrations, dissolved and particulate organic phosphorus and temperature was tested. Size fractionated measurements were conducted to estimate the share of the < 1µm fraction to pelagic respiration
• Annual pelagic respiration was 38 g C m-2 y-1;
• Approximately 50% of the annual respiration occurred during a Phaeocystis bloom;
• Dissolved organic material seemed to have a high share (20-70%) in bacterial degradation: The <1µm fraction contributes to 20-60% of the overall respiration; A significant linear regression (r2 = 0.7) between respiration and dissolved organic phosphorus was observed during winter and late spring.
respiration rates could be found.
• High respiration rates in winter (measured in the 10°C time series) together with high sediment suspension indicates that resuspension of organic matter increases pelagic respiration as a non-linear background signal during winter.
• The water column is predominantly autotrophic during most time of the year.
The development of a mathematical NPZD-box model on the basis of experimental results from this study showed, that pelagic processes can be simulated only if benthic processes were included in detail.
In terms of the benthopelagic coupling in the List tidal basin, this study shows that during autumn, winter, and early spring, pelagic carbon dynamics were closely coupled to the benthic system, due to an assumed biomass loss to the benthic compartment and due to high sediment resuspension, transferring organic matter as well as nutrients into the pelagic system. In late spring and summer large amounts of the pelagic carbon/nutrient turnover proceed under low benthic influences.
Das Wattenmeer ist ein Küstenökosystem unter hohem anthropogenem Einfluß. Um die Effekte anthropogener Veränderungen –z.B. Eutrophierung und Fischerei- einschätzen zu können, ist ein detailliertes Wissen über die Funktionsweise des Ökosystems notwendig; besonders im Hinblick auf den Transport von Nährstoffen und Biomasse in Form von Kohlenstoff durch das System.
Phytoplankton Primärproduktion, Grazing durch Zooplankton und pelagische Respiration sind wichtige Prozesse der Kohlenstoffdynamik aquatischer Ökosysteme. Über die Jahresdynamik der pelagischen Kohlenstoffumsätze im Wattenmeer und seiner Vernetzung mit dem benthischen System ist bisher wenig bekannt. Ziel dieser Studie war es, die jährliche Dynamik von pelagischer Primärproduktion, Zooplankton Grazing und Respiration im Lister Tidebecken zu quantifizieren. Weiterhin sollten die Beziehungen zwischen diesen Prozessen sowie der mögliche Einfluß des Benthos auf das Pelagial u.a. durch Schwebstoffaufwirbelungen untersucht werden. Phytoplankton Primärproduktion, Grazing durch Zooplankton und Respiration wurden über den Zeitraum von einem Jahr als wöchentlich/monatliche Zeitreihe in einem flachen Tidebecken des nördlichen Wattenmeeres untersucht. Diese Untersuchungen waren an Messungen der Sylter Langzeitreihe gekoppelt, die Daten über Wassertemperatur, Salzgehalt, organische und anorganische Nährstoffe, sowie Chlorophyll a und Schwebstoffmengen liefert. Diese Arbeit wurde im Rahmen des europäischen Forschungsprojektes COSA (Coastal Sands as Biocatalytical Filters) durchgeführt, um pelagische Hintergrundinformationen für Untersuchungen über benthische Prozesse zur Verfügung zu stellen.
Pelagische Primärproduktionsmessungen ergaben:
• Die jährliche pelagische Primärproduktion betrug ~ 210 g C m-2 y-1;
• Die höchsten Produktionsraten wurden während einer Phaeocystis globosa gemessen;
Der Einfluß von Schwebstoffen auf das Lichtklima der Wassersäule und dadurch auf die Primärproduktion wurde errechnet:
• Eine Verdopplung der Schwebstoffmengen reduziert die Primärproduktion um ~20-50 %, wobei sich der größte Einfluß in Herbst, Winter und Frühling zeigt;
signifikant zurückgegangen. Der Einfluß dieses Rückganges auf die Primärproduktion wurde getestet. Die pelagische Primärproduktion wurde für die Jahre 1984-1990 und für 2000-2004 auf der Basis von in 2004 gemessenen P/I-Kurven, stündlichen Lichtwerten in 2004 und anhand von Chlorophyll a - und Schwebstoffmengen der jeweiligen Jahre errechnet.
• Die jährliche pelagische Primärproduktion von 1984-2004 schwankte zwischen ~160-300 g C m-2 y-1 ;
• Es konnten seit 1984 keine Veränderungen in der jährlichen Gesamtmenge der Produktion festgestellt werden;
Der Einfluß des Zooplankton Grazings auf die Phytoplanktonbiomasse wurde in einer Sequenz von 9 Verdünnungsexperimenten zwischen März und Oktober gemessen.
• während der Diatomeen Frühjahrsblüte konnte kein Grazing festgestellt werden; • der Grazing Einfluß während einer Phaeocystis globosa war ~60 % d-1 des
Bestandes, im Sommer etwa 100% d-1 ; zu dieser Zeit wurde vermutlich mehr Biomasse in das pelagische als in das benthische Nahrungsnetz transportiert; In Verdünnungsexperimenten gemessene Phytoplankton Wachstumsraten wurden mit
in-situ gemessenen Wachstumsraten des Phytoplanktons verglichen, da diese
Übertragbarkeit von Daten aus Verdünnungsexperimenten auf natürliche Prozesse in der Literatur noch immer zur Diskussion steht. Für diesen Vergleich wird ein Umrechnungsfaktor bezüglich der abnehmenden Lichtmenge in der Wassersäule vorgestellt.
• Experimentelle Wachstumsraten waren in der gleiche Größenordnung wie
in-situ gemessenen Wachstumsraten;
• Die daraus resultierende Überschätzung der Phytoplanktonbiomasse ist in der Größenordnung von benthischer Assimilation wie sie in anderen Arbeiten berichtet wird;
Pelagische Respirationsmessungen wurden parallel in zwei Zeitreihen durchgeführt. Eine Zeitreihe bei in-situ Temperaturen und eine bei 10°C um Temperatureffekte auf die Respirationsraten auszuschließen. Der mögliche Einfluß von Schwebstoffmengen, gelösten und partikulärem Phosphat sowie von Temperatur auf die pelagischen Respirationsraten wurde untersucht. In 8 Experimenten zur Größenfraktionierung wurde der Anteil der <1µm Fraktion auf die pelagische Respiration untersucht.
• die jährliche pelagische Respirationsmenge betrug 38 g C m-2 y-1; etwa 50% der jährlichen Veratmung fanden während der Phaeocystis Blüte statt; • Gelöstes organisches Material hatte einen großen Anteil (20-70%) am bakteriellen Abbau: Zwischen Respiration und gelöstem organischen Phosphat konnte eine signifikante lineare Regression (r2 = 0.7) festgestellt werden. Die <1µm Fraktion trug 20-60% zur gesamten pelagischen Veratmung bei;
• Zwischen Schwebstoffmenge und Respiration konnte keine lineare Beziehung festgestellt werden;
• Hohe Veratmungsraten im Winter (gemessen bei 10°C) und hohe Schwebstoffmengen im Winter deuten darauf hin, daß Resuspension von organischen Material aus dem Benthos die pelagischen Veratmungsraten erhöht; • Die Wassersäule ist autotroph;
Die Entwicklung eines mathematischen NPZD-box Modells auf der Basis von experimentellen Daten zeigte, daß pelagische Prozesse nur simuliert werden können, wenn benthische Prozesse detailliert miteinbezogen werden.
Im Hinblick auf die bethopelagische Kopplung im Lister Tidebecken zeigt diese Studie, daß von Herbst bis zum frühen Frühjahr die pelagischen Kohlenstoffprozesse in einer engen Kopplung zu benthischen Prozessen stehen. Diese Kopplung ergibt sich vor allem durch einen Verlust an Biomasse an das Benthos, sowie durch hohe Resuspension und den dadurch bedingten Transport von organischem Material sowie von Nährstoffen in die Wassersäule. Auch werden die Lichtbedingungen zur Primärproduktion durch die Resuspension verschlechtert. Im späten Frühjahr (Phaeocystis Blüte) sowie im Sommer finden große Mengen des pelagischen Kohlenstoffumsatzes unter vermutlich geringem benthischen Einfluß statt.
C
HAPTER
1
I
NTRODUCTION
1. THE ROLE OF COASTAL ECOSYSTEMS IN A GLOBAL CONTEXT
2. THE EUROPEAN WADDEN SEA - A CHANGING COASTAL ECOSYSTEM
3. PELAGIC CARBON CYCLING IN THE WADDEN SEA
4. SINKS AND SOURCES OF THE PELAGIC CARBON/NUTRIENT CYCLE
4. THESIS OUTLINE
1. The role of coastal ecosystems in a global context
Aquatic ecosystems play an important role in the global carbon cycle. Approximately 45 gigatons organic carbon are generated by marine phytoplankton per year, corresponding to half of the overall global plant production (WILLIAMS 1998, FALKOWSKI ET AL. 1998). About half of marine production is contributed by coastal waters and in contrast to the autotrophic open ocean, which is a sink for CO2 from the atmosphere (SABINE ET AL. 2004, TAKAHASHI 2004, ROBINSON & WILLIAMS 2005), coastal areas are mainly heterotrophic and a source of CO2 for the atmosphere (SMITH & HOLLIBAUGH 1993, HEIP ET AL. 1995, WILLIAMS 1998, HOPKINSON & SMITH 2005). Despite their role in the global carbon cycle, coastal systems are fundamental from an anthropogenic perspective: About half of the world population lives within 60 km of the coast. CONSTANZA ET AL. (1997) estimated the monetary value of natural coastal
systems to be 10.6 trillion US $, approximately on third of the monetary value of the overall global ecosystem. This - theoretical - price is a result of the importance in ’purification function, food production, disturbance regulation, recreation, biological control and cultural value’. Coastal systems are highly vulnerable habitats, and high nutrient loads enter these systems by riverine, groundwater and atmospheric input (JICKELLS 1998).
2. The European Wadden Sea - a changing coastal ecosystem
The European Wadden Sea is a shallow coastal sea along the North Sea coast of The Netherlands, Germany and Denmark. This transition zone between land and sea is of approximately 470 km in length and -except for the central part- protected by barrier islands. The Wadden Sea is connected to the North Sea by tidal channels and characterized by large areas of sandy or muddy tidal flats, emerging during low tide. Tides are semidiurnal in a range between 1.3 to 4 m (DIJKEMA ET AL. 1980). Because of
a tidal asymmetry- the duration between maximum high tide and maximum low tide is longer than the duration between maximum low tide and maximum high tide- , the settling lag (once the current speed is allows settling of particles, these are carried further inland until they reach the bottom), the scour effect (a higher current speed is needed to suspend a particle than at which a particle can settle) and because of a high filtering and storage capacity of particles of the benthos, the Wadden Sea is a particle trap and thus an accumulative area (POSTMA 1981, POSTMA 1982). Resuspension of benthic particles is a permanent process due to tidal currents and wind forcing (DIJKEMA ET AL. 1980, HEIP ET AL. 1995). The water column is highly turbid, the euphotic depth is lower than in the adjacent North Sea (AARUP 2002).
Large rivers entering the Wadden Sea are IJssel, Ems, Weser, and Elbe. The direct annual discharge is ~60 km3 of freshwater (VAN BEUSEKOM ET AL. 2001). The river Meuse and Rhine are also influencing the Wadden Sea via the continental coastal current of the North Sea. Salinity of the Wadden Sea ranges between 27 -32 psu.
The anthropogenic impact on the Wadden Sea is high. Due to a loss in functional diversity within the Wadden Sea during the past 1000 years, a reduced efficiency in energy use and transfer might have led to lower biomass production in the Wadden Sea (LOTZE ET AL. 2005, VAN BEUSEKOM 2005). In their study LOTZE ET AL. (2005) stated
that food-web control probably shifted from a former top-down control dominated by consumers to a bottom-up control dominated by resource availability. Within the past decades, increased riverine nutrient discharge, fishery and diking (ESSINK ET AL. 2005,
HOFSTEDE ET AL. 2005, VAN BEUSEKOM ET AL. 2005, VORBERG ET AL. 2005) are the
main direct influences on this ecosystem in addition to global changes like sea level rise, atmospheric warming and increasing CO2 concentrations (FÜHRBÖTER 1989, KEELING ET AL. 1995, REISE ET AL. 2005).
LIST
TIDAL
BASIN
NORTH
SEA
LIST
TIDAL
BASIN
NORTH
SEA
Figure 1. Map of the North Sea, the Barrier Isalnds, and the Wadden Sea. The study site ‘List
tidal basin’ is indicated.
Riverine nutrient loads increased during the last 50 years by anthropogenic impact and as a result of this eutrophication, primary production has increased and the ratio of nutrients (N:P:Si) was changed. Since the mid 1980’s riverine phosphorus input decreased, while the input of nitrogen decreased much slower (PHILLIPPART & CADÉE
Diking, land reclamation and rising sea level has decreased the overall area of the Wadden Sea, and especially the transition zone between land and sea and increased the tidal current velocity (LOTZE ET AL. 2005). Less mud is deposited near the mainland and a loss of fine-grained sand and particulate organic matter has occurred in the long term (ESSINK 2005).
Resuspension and in reverse sedimentation and the advective particle transport into the sediment are important factors linking benthic and pelagic ecosystems (POSTMA 1981,
RUSCH & HUETTEL 2000, RUSCH ET AL. 2000). Since sediment composition and current
velocities in the Wadden Sea have changed, both processes of particle transport (from and into the benthos) may also be altered, affecting the overall water column light field. Since the Wadden Sea is considered to be light limited rather than nutrient limited (COLIJN & CADÉE 2003), changes in suspended particle concentration might affect
phytoplankton productivity.
3. Pelagic carbon cycling in the Wadden Sea
The pelagic carbon/ nutrient cycle in a coastal system differs from oceanic processes. Pelagic and benthic processes are closely coupled. In the open oceans one third of fixed biomass is lost to deeper layers and removed for long time periods from the atmosphere (FALKOWSKI ET AL. 1998, ROBINSON & WILLIAMS 2005). In a coastal system, pelagic biomass is passed to the benthic layer by particle trapping, advective pore water transport of permeable sands and by suspension feeders, but is remineralised within short time periods and supplied as inorganic nutrients for benthic and pelagic primary production (HEIP ET AL. 1995, HUETTEL ET AL. 1998 CROSSLAND ET AL. 2005).
Phytoplankton primary production, zooplankton grazing on phytoplankton, and bacterial degradation of organic material are major processes of the pelagic carbon cycle and were the main focus of this study. In the past research on these processes has mainly focused on primary production, only a few studies report on pelagic community respiration and pelagic community grazing activity:
a) Primary production
An increase of primary production due to eutrophication was observed in the Wadden Sea since the 1970’s (COLIJN & VAN BEUSEKOM 2005). Primary production was
investigated most frequently in the western Wadden Sea. There, in the Marsdiep area, phytoplankton productivity increased from 150 g C m-2 y-1 during the 1970’s up to 440 g C m-2 y-1 in the 1980’s and 1990’s and decreased to 250 g C m-2 y-1 until 2000 (CADÉE
& HEGEMAN 1974, 2002). Investigations on limiting factors showed that during most
time of the year, light is the limiting resource (Liebig’s law). During a few weeks in summer, nitrogen limits phytoplankton productivity, but also phosphorus is under discussion as co-limiting nutrient (DE VRIES ET AL. 1998, PHILLIPPART & CADÉE 1999,
COLIJN & CADÉE 2003, COLIJN & VAN BEUSEKOM IN PREP.). In the northern Wadden
Sea less research on primary production was done. Studies exist for the Meldorfer Bucht/ Büsum (TILLMANN ET AL. 2001) and the List tidal basin (ASMUS ET AL. 1998). In
the Sylt- RømøBight (List Tidal Basin) phytoplankton primary production was 50 g C m-2 y-1 in 1980 and 160 g C m-2 y-1 in 1995 (ASMUS ET AL. 1998). In the Meldorfer Bucht primary production ranges between ~130-170 g C m-2 y-1 in 1995/96 (TILLMANN ET AL. 2001).
b) Pelagic respiration
Research on respiration in the Wadden Sea was mainly done with focus on the benthic ecosystem, on benthic community level as well as on species level (e.g. HEIP ET AL. 1995, KRISTENSEN ET AL. 1997, ASMUS &ASMUS 1998, RUSCH & HUETTEL 2000). Only a few studies are available on pelagic respiration and degradation of organic matter. MANUELS & POSTMA stated 1974 for the western Wadden Sea organic carbon to be in a range between 3% (winter) to 15% (summer) of the suspended matter concentration. For the Meldorfer Bucht near Büsum, TILLMANN ET AL. (2001) calculated respiration within the annual cycle on the basis of primary production measurements, according to a model of Langdon 1993. They estimated the water column to be autotrophic with a P/R ratio of ~ 4. POREMBA ET AL. (1999) stated that production processes are
dominating in the shallow parts of the tidal flats, whereas decomposition processes dominate in the deep tidal channels. For the List tidal basin VAN BEUSEKOM ET AL.
(1999) estimated an annual respiration rate of 110 g C m-2 y-1 . VAN BEUSEKOM & DE
JONGE (2002) presented of a conceptual model, linking riverine nitrogen input and the
annual nitrogen cycle of the Wadden Sea. They showed that in years with high riverine nitrogen load more organic matter is produced in the North Sea, and imported and remineralised within the Wadden Sea than in dry years.
c) Zooplankton grazing
On a plankton community level, the grazing impact of zooplankton was poorly investigated within the Wadden Sea. TILLMANN & HESSE (1998) investigated the seasonal distribution and biomass of heterotrophic microplankton in the central Wadden Sea. On the basis of phytoplankton biomass and microplankton carbon data, they estimated microplankton to graze between 12% and 50 % of phytoplankton standing stock in spring and 16- 32% in summer. In several studies the grazing impact of single species was tested: Field studies on grazing impact of the copepod Temora longicornis on Phaeocystis globosa were conducted in the Marsdiep by HANSEN (1991, 1995). He found T. longicornis to have a minor grazing impact on Phaeocystis globosa. In laboratory experiments grazing of copepods (Temora longicornis), heterotrophic dinoflagellates (Oxyrrhis marina) and ciliates (Strombidinopsis acuminatum) on
enhanced the growth of Phaeocystis (HANSEN ET AL. 1993). KLEIN BRETELER & KOSKI
(2003) observed a similarly low impact of Temora nauplii on Phaeocystis globosa in mesocosm experiments. LANDRY & HASSETT (1982) developed a simple method to test the grazing effect on phytoplankton biomass by incubating seawater in a sequence of dilution steps. This method allows the separation of apparent growth rates into net growth rates and zooplankton grazing rates. For the Wadden Sea no such studies are yet available that test the grazing impact of natural zooplankton communities on the phytoplankton biomass standing stock, using this dilution technique. Also no regular observations are available that describe the seasonal development of the grazing impact on phytoplankton. phytoplankton biomass primary production zooplankton biomass nutrients (P,N,SI respiration grazing
Figure 2. Diagram of pelagic processes investigated in this study. The interrelation of this
cycling to further processes of the ecosystem is shown in Figure 3.
Primary production, phytoplankton growth rates, zooplankton grazing and respiration are closely interrelated in several ways (Figure 2): From inorganic nutrients and CO2, phytoplankton cells produce algal biomass and oxygen via photosynthesis (e.g. FALKOWSKI 1992). Zooplankton grazing transfers algal biomass into the next higher trophic level and parallel, the loss of biomass during grazing (‘sloppy feeding’) fuels bacterial mineralisation (BANSE 1992). The term respiration includes two sub-processes:
bacterial degradation of organic material (WILLIAMS 1981). The latter represents the main fraction of pelagic respiration and is fuelled by primary production (release of extracellular organic carbon during photosynthesis) as well as by sloppy feeding (WILLIAMS & DEL GIORGIO 2005). During respiration organic material is degraded into CO2 and inorganic nutrients.
4. Sinks and sources of the pelagic carbon/nutrient cycle
The pelagic carbon/ nutrient cycle of the List tidal basin is linked to several sinks and sources (Table 1, Figure 3): A major sink or sources of carbon/ nutrients is the exchange of 8-12% of water volume with the adjacent North Sea per tidal cycle (FAST ET AL. 1999). A source is the riverine runoff into the Wadden Sea area. The close coupling between pelagic and benthic ecosystem may reverse between source and sink.
PELAGIC BENTHIC phytoplankton biomass primary production water column light field surface irradiance zooplankton biomass nutrients (P,N,SI respiration filter feeders respiration primary production North Sea exchange sediment perculation resuspension predatory heterotrophics grazing
Figure 3. A schematic diagram of pelagic carbon/nutrient cycling and it’s linking to benthic
Table 1. Potential sinks and sources of nutrients and organic matter for the pelagic ecosystem
of the List tidal basin.
source sink
organic matter nutrients organic matter nutrients
atmospheric
nutrient input transfer to higher trophic levels import of POM
from the North Sea
riverine nutrient loads into the Wadden Sea benthic resuspension pore water exchange and animal excretion sedimentation and benthic suspension feeding benthic primary production North Sea exchange North Sea exchange North Sea exchange North Sea exchange
For studies of carbon/nutrient cycling it has to be considered that in the List tidal basin the pelagic and the benthic parts of the ecosystem are closely coupled by sediment percolation, suspension feeders and resuspension:
Sandy sediment prevails at ~95% of the overall area (GÄTJE & REISE 1998). The
sediment is assumed to filter the whole water volume of the bight within 1-2 weeks. This filtering activity contributes to major parts to the exchange of nutrients and organic matter between both systems.
Moreover benthic suspension feeders (e.g. mussels, oysters, and lugworms) have a high share in this exchange. The lugworm Arenicola marina is an ecosystem engineer on tidal flats (REISE & VOLKENBORN 2004) and pumps approximately 3 L seawater h-1 m-2
into the sediment, assuming a density of 30 ind. m-2 (RIJSGAARD 1996). Since the respiratory activity within sandy sediments and primary production by microphytobenthos is high, high carbon and nutrient turnover rates may prevail in the benthos. Nutrient release from the sediment in times of low microphytobenthic productivity refuels nitrogen in the water column and in turn organic matter import from the water column leads to high remineralisation rates in the benthos (HEDTKAMP 2005). The linking of the carbon/ nutrient flow between both compartments by resuspension is poorly investigated.
The European Research Project COSA (Coastal Sands as Biocatalytical Filters, [EVK-CT2002-0076], www.eu-cosa.org ) investigated the role of sandy habitats in coastal ecosystems from November 2002 until November 2005. Study sites were the Wadden Sea and the Baltic Sea. COSA focussed on sandy permeable sediments, their biogeochemical processes and their role as filters for coastal waters. Since benthic and pelagic systems are closely linked, also pelagic processes were included. In the framework of COSA, the present study investigated the pelagic carbon/ nutrient cycle in a shallow tidal basin of the Northern Wadden Sea. A parallel sediment study (HEDTKAMP 2005) investigated the seasonal dynamics of nutrient pore water
concentrations, permeability and organic matter.
Chapter 5 of this study was enabled by a fellowship of the Marie-Curie Foundation for training of phD-students at the Biological Station/SINTEF ‘Plankton-Site’, Trondheim, Norway. North Sea North Sea Rømø Sylt Sylt-Rømø-Bight 5 km List tidal basin Sampling site
5. Thesis outline
The primary objective of this study was a parallel assessment of annual cycles of primary production, respiration, and zooplankton grazing to estimate seasonal dynamics of pelagic carbon/nutrient fluxes in the List tidal basin. To get a comprehensive view on these dynamics, all investigations were linked to the Sylt long term time series that is conducted twice a week since twenty years and provides data on temperature, salinity, phytoplankton biomass, dissolved and particulate inorganic and organic nutrients for the List tidal basin (Figure 4).
In Chapter 2 the annual dynamics of primary production of the List tidal basin are presented. Photosynthesis-versus-irradiance (P/I) parameters and light attenuation coefficients were measured weekly in 2004. The influence of suspended matter concentrations of pelagic primary production was calculated. The hypothesis is tested, whether annual primary production has significantly changed since the mid 1980’s as a result of decreasing eutrophication and decreased suspended matter concentrations. The annual primary production was calculated on an hourly scale for the years 1985- 1990 and 2000- 2004. The annual production of these years was between 160- 300 g C m-2 y -1
, and no significant trend in terms of decreasing eutrophication since the mid 1980’s could be found.
In Chapter 3 the annual dynamics of pelagic respiration were investigated. The impact of suspended matter and further potential affecting parameters on respiration was tested. Two time series were conducted weekly: One at in-situ temperature and one at a constant temperature of 10°C, to exclude seasonal temperature effects. In filtration experiments, respiration rates were measured for different size classes. Results showed, that dissolved organic material impacted 20-70 % of the overall community respiration. No linear relation between respiration and suspended matter concentrations could be found. However, respiration at 10°C showed higher values during winter than in summer. In combination with higher suspended matter concentrations during winter, this indicates benthic resuspension to enhance pelagic respiration as a non-linear background effect.
In Chapter 4 the impact of zooplankton grazing on phytoplankton was investigated from March until October. During a diatom spring bloom no grazing occurred, but during a
Phaeocystis globosa bloom approximately 50-60% of the phytoplankton standing stock
was grazed. Highest grazing impact on the phytoplankton community was found in August. Presumably more phytoplankton biomass was transported in the pelagic than into the benthic food web during these periods. Results on growth rates in the grazing experiments were compared to in-situ measured data, to test the applicability of experimental results on natural processes. Therefor an irradiance conversion factor was introduced.
In Chapter 5 the actual progress of a mathematical model based on experimental data for the List tidal basin is shown. The model is a NPZD (nitrogen –phytoplankton-zooplankton detritus)-box-model with the benthic layer included as an additional state variable. The model is coupling experimental data on pelagic processes (P/I parameters, irradiance, light attenuation, grazing rates) of the present study and data of the Sylt long term time series. A simplified benthic ecosystem is included, and can be extended in more detail in a later phase, when experimental data on benthic processes (e.g. primary production, respiration and filtering) are available on a detailed temporal and/ or spatial scale.
In Chapter 6, the general discussion of this study, the aim is to synthesize the results of foregoing chapters. An overview on the investigated pelagic processes is given and the presumed linking between pelagic and benthic compartments of the List tidal is discussed.
Table 2: List of abbreviations used in this study in alphabetical order, their explanation and
units.
Abbrev. Explanation Unit
µ Apparent growth rate (estimated by dilution experiments) d-1
µf Apparent growth rate calculated from (k55*f)-g d-1
µis Apparent in-situ phytoplankton growth rate d-1
a attenuation coefficient m-1
B0is Phytoplankton biomass, in-situ at time t0 µg L
-1
Btis Phytoplankton biomass, in-situ at time t µg L-1
Btsim Phytoplankton biomass at time tsimulated without using f for
depth correction (=Eq. [2], Chapter 3) µg L
-1
Btsim(f) Phytoplankton biomass at time tsimulated by including f for
depth correction (=Eq. [4], Chapter 3) µg L
-1
Chl a Chlorophyll a µg L-1
DOP dissolved organic phosphorus µmol L-1
Ek Irradiance at which Pmax is reached
µmol photons m-2 s-1
fd conversion factor between k55 and k200 ---
g grazing rate of heterotrophic plankton d-1
I Irradiance µmol photons
m-2 s-1
k200 theoretical growth rate as calculated for the mean water depth
of 200 cm d
-1
k55 theoretical growth rate of phytoplankton estimated by dilution
experiments given a mean water depth of 55 cm d
-1
P P variable units, basic unit: µmol C h-1 L-1
P Bm Phytoplankton biomass µg L-1
Pmax Parameter describing the maximum photosyntetic activity
POP particulate organic phosphorus µmol L-1
PP200 In-situ primary production estimated for the mean water depth
of the experimental basin (200 cm) µmol C h
-1 L-1
PP55 In-situ primary production estimated for experimental water
depthof 55 cm µmol C h
-1 L-1
R Respiration variable units, basic unit: µmol C h-1 L-1
SPM suspended matter µg L-1
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C
HAPTER
2
P
ELAGIC PRIMARY PRODUCTION IN THE NORTHERN
W
ADDEN
S
EA
-
NO QUANTITATIVE CHANGES SINCE THE MID
1980’
S
?
Abstract
Pelagic primary production was quantified from January until December 2004 in the List tidal basin, a shallow bight of the northern Wadden Sea. The annual mean was 211 g C m-2 y-1. Since primary production in the Wadden Sea is light limited during most time of the year, the impact of suspended matter on the underwater light field and phytoplankton productivity in the List tidal basin was investigated: Light attenuation shows a seasonal trend with highest values in winter (3.4 m-2) and lowest values in summer (0.3 m-2). Light attenuation was significantly correlated with suspended matter concentrations. To explore the impact of suspended matter concentrations on phytoplankton productivity, primary production was calculated on the basis of modified suspended matter concentrations: A doubling of suspended matter concentrations would decrease annual primary production in a range of ~20 %.
Suspended matter concentrations between 1985 and 1990 were significantly higher than between 2000 and 2004. The hypothesis was tested, whether altered suspended matter concentration within the past twenty years could have altered the seasonal primary productivity within the water column. Therefore, pelagic primary production from 1984 until 2003 was estimated by combining the irradiance and P/I- parameters of 2004 with chlorophyll a data and suspended matter concentrations of the respective years. Annual primary production ranged from 161- 301 g C m-2 y-1. No change in productivity over the past 20 years could be observed. It is suggested that primary production has increased within the period from September until February as a result of lower suspended matter concentrations. Summer production is suggested to have decreased due to nitrogen limitation as a result of reduced riverine nitrogen loads supplied to coastal waters of the North Sea.
Introduction
The Wadden Sea is a shallow tidal sea along the Dutch, German and Danish North Sea coast. It is characterized by high riverine nutrient discharges and high turbidity. From the 1950’s until the mid 1980’s riverine nutrient loads into the Wadden Sea increased by anthropogenic impact (VAN BEUSEKOM 2005). Since the mid 1980’s phosphorus
loads decreased again, but nitrogen loads remained on a high level until the end of the 1990’s and thus, the N/ P ratio changed within the past decades (DE JONGE ET AL. 1996, DE VRIES ET AL. 1998, VAN BEUSEKOM ET AL. 2005). Phytoplankton productivity of this eutrophic ecosystem is limited by low light intensities during most time of the year (COLIJN & CADÉE 2003) and nutrient limitation likely occurs within a few months in summer (COLIJN & CADÉE 2003, TILLMANN ET AL. 2001). Both nitrogen and phosphorus are discussed as the main limiting nutrients (DE JONGE ET AL. 1996, COLIJN
& CADÉE 2003, PHILLIPPART & CADÉE 2000).
Light limitation in the Wadden Sea is a result of high suspended matter concentrations (COLIJN 1983), whereby suspended matter concentrations in the Wadden Sea are in general higher than in the adjacent North Sea (POSTMA 1954, VAN STRAATEN & KUENEN 1957). Organic matter and inorganic particles originate mainly from the North Sea (VAN STRAATEN & KUENEN 1957, POSTMA 1980, VAN BEUSEKOM & DE JONGE
2002). Accumulation mechanisms (POSTMA 1954, VAN STRAATEN & KUENEN 1957)
include the asymmetry of the tides (low tide is shorter than high tide), the settling lag (once the current speed is allows settling of particles, these are carried further inland until they reach the bottom), the scour effect (a higher current speed is needed to resuspend a particle than at which a particle can settle) and filter feeders. In periods with strong winds, fine particles were resuspended into the water column by increasing wave forcing and tidal currents (POSTMA 1982, HEIP ET AL. 1995, DE JONGE & VAN
BEUSEKOM 1995).
Research on phytoplankton productivity in the Wadden Sea was mainly done for the western Wadden Sea: In the Marsdiep area, phytoplankton productivity increased from 150 g C m-2 y-1 during the 1970’s up to 440 g C m-2 y-1 in the 1980’s and 1990’s and decreased to 250 g C m-2 y-1 until 2000 (CADÉE & HEGEMAN 1974, CADÉE & HEGEMAN
2002). For the northern part only a few studies (ASMUS ET AL. 1998, TILLMANN ET AL. 2001) exist. For the Sylt- RømøBight (List Tidal Basin) an increase from 50 to 160 g C m-2 y-1 was observed between 1980 and 1995 (ASMUS ET AL. 1998).
In the Wadden Sea phytoplankton is an important food supply for the pelagic and benthic food web (reference): In the Marsdiep, pelagic algal biomass had a significant positive effect on zoobenthos biomass (BEUKEMA ET AL. 2002).
This study focuses on the annual cycle of pelagic primary production and the impact of suspended matter concentrations on light availability. From January to December 2004 primary production was measured weekly to monthly (n = 33) at a routine sampling station in the List tidal basin. Depth integrated annual primary production was calculated on hourly scale on the basis of hourly surface irradiance and hourly interpolated P/I-parameters, chlorophyll a and suspended matter concentration. Recent primary production levels were compared with levels from the mid 1980’s until 2003, by combining suspended matter and chlorophyll a data from these years with P/I-parameters and irradiance measured in 2004.
Methods
Study Site
Primary production was determined in the List tidal basin, a 404 km2 semi-enclosed bight in the northern part of the Wadden Sea, which is connected to the open North Sea by a single tidal inlet (Figure 1). Salinity ranges between 27.5 -32 psu. The water volume at mean tidal level is about 845 mio m3. The mean water depth is 2 m but reaches up to 40 meter in the main tidal channel. The water column is homogenously mixed. Tides are semidiurnal; the mean tidal range is about two meter. The direct impact by river runoff is marginal; two small rivers enter the tidal basin. Detailed
descriptions for the area are given in GÄTJE & REISE (1998). Eutrophication of the area
is mainly determined by import of organic matter from the North Sea (VAN BEUSEKOM ET AL. 1999).
Seawater Sampling
Seawater was sampled for all measurements in 1m depth at a routine station in the main channel (Figure 1), water depth at this station was 10 m. Sampling time was between 8 a.m. and 9 a.m. Water samples were taken from shipboard, using a 5L Niskin-Bottle.
List tidal basin 55°N / 8.40°E 10 km List tidal basin 55°N / 8.40°E 10 km
Figure 1. The study site.
The sampling site is indicated as black dot.
Oxygen was determined with the Winkler method (GRASSHOFF ET AL. 1983). For oxygen and respiration measurements, water was filled into incubation bottles directly after sampling. The initial oxygen content was immediately fixed by adding Winkler chemicals.
Nutrients, suspended matter and phytoplankton biomass
Standard methods according to GRASSHOFF ET AL. (1983) were used for determining dissolved NO3, NO2, NH4, PO4, and Silicate. Volumetric suspended matter concentrations were determined gravimetrically on a 0.47µm pore-size Nucleopore filter. Phytoplankton biomass was determined as chlorophyll a after JEFFREY AND
HUMPHREY (1978). These measurements were conducted twice a week as parts of the
Sylt long term time series.
Light attenuation
Light attenuation in the water column was measured weekly in 2003 and 2004 with a LI-COR-Sensor (LI-COR; LI-193) attached to a CTD measuring 4 data sets s-1 and a lowering speed of approximately 10 cm s-1. The vertical attenuation coefficient (a) in µmol photons m-2 s-1 is expressed by
) exp(
0 ad
I
I
d = − [1]where Id is irradiance at a given depth (d) and I0 reflects surface irradiance. The relation between light attenuation and suspended matter was determined by linear regression:
a (SPM) = y + x SPM. [2]
Primary production versus irradiance curves
P/I –curves were determined weekly (during winter monthly) with the light- and dark bottle oxygen method under controlled laboratory conditions. The initial seawater oxygen content was measured in triplicate. Ten incubation bottles were stored on a rotating wheel. Before, eight bottles were wrapped with different neutral density light filters and two bottles with aluminium foil. Incubation irradiance was provided by a cool light fluorescent emitter (Norka). Irradiance was measured and re-adjusted for each experiment in pre-experimental studies inside the incubations bottles using a mini-light
sensor (LIC-COR). Incubation irradiance was between 70 and 780 µmol photons s-1 m-2. Incubation time was between 4 and 7 hours depending on the expected increase of oxygen concentration. Oxygen was measured with the Winkler-technique (GRASSHOFF ET AL. 1983) using an automatic titration apparatus (Metrohm Multi-Dosimat 645). With this method an accuracy of ± 0.45 µmol O2 L-1 can be reached. Respiration rates were determined from dark bottles and used to calculate gross primary production. Results were converted from mg O2 into mg C assuming a conversion factor of 1.3 from oxygen to carbon (ASMUS ET AL. 1998).
Primary production versus irradiance (P/I) curves were fitted according to equation [3] (PLATT ET AL. 1980) using Statistica 6.0 (StatSoft).
PN =Pmax ((1-exp (-α Id/Pmax)) (exp (β Id/Pmax))) [3]
From that fit, the maximum rate of photosynthesis Pmax (mg C mg Chl-1 h-1), the initial slope of the curve α (mg C mg Chl-1 h-1 (µmol photons m-2 s-1) -1) and the saturation parameter Ek (Pmax/ α) were calculated. Primary Production expressed by PN is standardised to chlorophyll a units (µg L-1).
Depth integrated primary production
Depth integrated primary production was calculated on an hourly scale, since the responding of primary production to irradiance is not linear and mean daily irradiance values do not necessarily produce mean primary production values (SAKSHAUG ET AL.
1997). P/I parameters, chlorophyll a and suspended matter concentrations were interpolated to hourly values. Light attenuation was calculated from suspended matter values with the linear regression established during this study. Surface irradiance was based on global radiation measurements by the Deutsche Wetterdienst Station List/ Sylt, assuming a conversion factor of 1 W m-2 = 4.14 µM photons m-2 s-1 (TILLMANN ET AL.
2001). The annual primary production was calculated following equation [4]:
∫
∫
d s N Y P chla * 1 [4]where hourly primary production is integrated over water depth (s = surface waterdepth,
Impact of suspended matter concentrations on primary production
Suspended matter concentrations strongly influence the water column light field. The effect of suspended matter concentrations on annual primary production was analysed by multiplying suspended matter data of 2004 with a fixed factor (e.g. 0.5; 2) and including them into equation [1] to yield a modified water column light field. On the basis of this modified water column light field, the potential primary production was calculated following equation [4].
Primary Production between 1984 and 2004
Suspended matter values from the 1980’s were combined with P/I-parameters and chlorophyll a data of 2004 to calculate a hypothetic depth integrated primary production for these years. The calculations were carried out with the light data from 2004 and the actual light data to evaluate the influence of interannual variation in global radiation.
Pelagic respiration
Respiration was measured in 24 h incubations in dark bottles parallel to primary production measurements. The initial oxygen content was determined in triplicate. Three bottles were incubated in the dark in for about 24 hours at in-situ temperature. Oxygen concentrations were measured by the Winkler-technique. The conversion factor from oxygen to carbon units was 0.89 (WILLIAMS & DEL GIORGIO 2005).
Results
Annual cycles of phytoplankton biomass, nutrients and suspended matter
Phytoplankton biomass (Figure 2 a) followed an annual pattern: From January until March chlorophyll a concentrations remained between 2 and 8 µg L-1. During a diatom spring bloom in April chlorophyll a increased up to 18 µg L-1. After the diatom spring bloom decreased, a Phaeocystis globosa bloom followed from May to June, with chlorophyll a concentrations values up to 24 µg L-1. In summer and autumn chlorophyll
a values were again between 2 and 8 µg L-1.
0 5 10 15 20 25 30 01-0 4 02-0 4 03-0 4 04-0 4 05-0 4 06-0 4 07-0 4 08-0 4 09-0 4 10-0 4 11-0 4 12-0 4 month C h lo ro p h y ll a ( µ g L -1 ) 0 10 20 30 40 50 60 70 80 01-0 4 02-0 4 03-0 4 04-0 4 05-0 4 06-0 4 07-0 4 08-0 4 09-0 4 10-0 4 11-0 4 12-0 4 month S P M ( m g L -1 ) 0 10 20 30 40 50 60 70 80 01-0 4 02-0 4 03-0 4 04-0 4 05-0 4 06-0 4 07-0 4 08-0 4 09-0 4 10-0 4 11-0 4 12-0 4 month N it ra te ( µ m o l L -1 ) 0 10 20 30 40 50 60 01-0 402-0403-0404-0405-0406-0407-0408-0409-0410-0411-0412-04 month S il ic a te ( µ m o l L -1) 0.0 0.5 1.0 1.5 2.0 01-0 402-0403-0404-0405-0406-0407-0408-0409-0410-0411-0412-04 month P h o s p h a te ( µ m o l L -1 )
Figure 2 a-e. Annual cycle of a) chlorophyll a, b) suspended matter (SPM), c) nitrate, d) silicate,
and e) phosphate in the List tidal basin in 2004.
Phaeocystis bloom Diatom spring bloom a) c) d) e) b) c)
Suspended matter concentrations (Figure 2b) ranged between 1- 60 mg L-1. Between April and August concentrations were lower (1- 27 mg L-1) than in winter and autumn (6 - 60 mg L-1). The annual mean ± SD was 18.4 ± 14.3 mg L-1 .
Nutrients (Figure 2 c-e) decreased from high winter levels (NO3 ~ 60 µmol L-1, PO4 ~ 1 µmol L-1 and Silicate ~ 40 µmol L-1 to values below 1 µmol L-1 in spring. The threshold of 1µmol L-1 was reached for silicate and phosphorus in the middle of April, for nitrate one month later in the middle of May. Phosphorus slightly increased directly after reaching its minimum in April and reached its winter maximum until November. Silicate and Nitrate remained low until the beginning of September and then increased again and reached their winter maximum in January of the following year.
Light attenuation in relation to suspended matter and chlorophyll a concentrations
The light attenuation coefficient in the List tidal basin ranged between 0.3 and 3.7 m-1. This corresponds to a Secchi depth (ds) of 20 – 1.6 m (Aarup 2002). The annual mean light attenuation coefficient was 1.05 m-2 (ds = 5.7 m) in 2003 and 1.44 m-1 (ds = 4.2 m) in 2004. In both years light attenuation showed a similar annual pattern, but values in spring and in autumn 2004 were higher than in 2003 (Fig. 3). In winter (November until March) the attenuation coefficient was in most observations during both years higher than 1 m-2. From April until September most observations showed values below 1 m-1.
0 1 -0 3 0 1 -0 3 0 3 -0 3 0 4 -0 3 0 5 -0 3 0 6 -0 3 0 7 -0 3 0 8 -0 3 0 9 -0 3 1 0 -0 3 1 1 -0 3 1 2 -0 3 0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 a tt e n u a ti o n c o e ff 0 1 -0 4 0 2 -0 4 0 3 -0 4 0 4 -0 4 0 5 -0 4 0 6 -0 4 0 7 -0 4 0 8 -0 4 0 9 -0 4 1 0 -0 4 1 1 -0 4 1 2 -0 4 0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 a tt e n u a ti o n c o e ff ( m -1 )
Figure 3. Light attenuation coefficients in 2003 and in 2004. All measurements were conducted
Light attenuation correlated linearly with suspended matter (r2 = 0.76, p = 0.0) and also with chlorophyll a (r2 = 0.076, p = 0.02) shown in Figure 4 a & b. Multiple regression between light attenuation and suspended matter plus chlorophyll a concentrations results in r2 = 0.80 (beta chlorophyll a = 0.17 and beta suspended matter = 0.86, p = 0.0). y = 0.0431x + 0.5973 R2 = 0.7601 0 0.5 1 1.5 2 2.5 3 3.5 4 0 20 40 60 80 100 suspended matter (mg L-1) a tt e n u a ti o n c o e ff ic ie n t (m -2) y = 0.037x + 0.9518 R2 = 0.0764 0 0.5 1 1.5 2 2.5 3 3.5 4 0 10 20 30 40 Chlorophyll a (µg L-1) a tt e n u a ti o n c o e ff ic ie n t (m -2)
Figure 4 a & b. Linear regression a) between light attenuation and suspended
matter concentrations, b) between light attenuation and chlorophyll a concentrations.
Annual dynamics of Photosynthesis versus Irradiance (P/I) – parameters Parameters describing the P/I-curves (Pmax, Ek, alpha) represent physiological light adaptations of the actual phytoplankton community. The chl a –specific maximum photosynthetic rate Pmax ranged from 1.8- 14.5 mg C mg Chl-1 h-1. The highest values were observed during a Phaeocystis globosa bloom in May and June, lowest values were observed in December and in January. Ek ranged between 107 and 521 µmol photons m-2 s-1. The initial slope of P/I-curves represented by alpha ranged between 0.014 and 0.033 mg C mg Chl-1 h-1 (µmol photons m-2 s-1). Annual dynamics of all mentioned parameters are shown in Figure 5.
1 0 -0 3 1 1 -0 3 1 2 -0 3 0 1 -0 4 0 2 -0 4 0 3 -0 4 0 4 -0 4 0 5 -0 4 0 6 -0 4 0 7 -0 4 0 8 -0 4 0 9 -0 4 1 0 -0 4 1 1 -0 4 0.00 0.02 0.04 0.06 0.08 0.10 0.12 0.14 0.16 0.18 0.20 a lp h a (m g C m g -1 C h l h -1 (µ m o l p h o to n s m -2 s -1 ) -1 ) 1 0 -0 3 1 1 -0 3 1 2 -0 3 0 1 -0 4 0 2 -0 4 0 3 -0 4 0 4 -0 4 0 5 -0 4 0 6 -0 4 0 7 -0 4 0 8 -0 4 0 9 -0 4 1 0 -0 4 1 1 -0 4 0 50 100 150 200 250 300 350 400 Ek ( µ m o l p h o to n s m -2 s -1) 1 0 -0 3 1 1 -0 3 1 2 -0 3 0 1 -0 4 0 2 -0 4 0 3 -0 4 0 4 -0 4 0 5 -0 4 0 6 -0 4 0 7 -0 4 0 8 -0 4 0 9 -0 4 1 0 -0 4 1 1 -0 4 0 2 4 6 8 10 12 14 Pm a x (m g C m g -1 C h l h -1 )
Figure 5 a-c. Parameters describing P/I-curves according to Platt et al. (1982). a) Alpha reflects
the initial slope of the linear part of the P/I-curve. b) Ek is the light intensity at which
Photosynthesis becomes saturated; c) Pmax is the maximum of production.
a)
b)
Depth integrated annual primary production in 2004
Depth integrated primary production rates were from 2.2 to 6739 mg C m-2 d-1 (Figure 6). The annual mean production was 212 g C m-2 d-1. More than 50% was produced during a Phaeocystis globosa bloom. During this six weeks lasting bloom, highest daily production values were observed (1090 - 6739 mg C m-2 d-1). During the diatom spring bloom the highest observed value was 1115 mg C m-2 d-1. The mean euphotic zone was ~4 m. Within the mean water depth of 2 m approximately 75% of the overall production occurred. 0 1000 2000 3000 4000 5000 6000 7000 01 -04 02-04 03-04 04-04 05-04 06-04 07-04 08-04 09-04 10-04 11-04 12-04 date pr im ar y pr od uc tio n (m g C m -2 d -1 ) 0 250 500 750 1000 01 -04 02-04 03-04 04-04 05-04 06-04 07-04 08-04 09-04 10-04 11-04 12-04 date pr im ar y pr od uc tio n (m g C m -2 d -1 )
Figure 6. Annual dynamics of primary production in 2004 on different scales (Y-axis) to
demonstrate the variability between seasons. Both plots show daily values (mg C m-2 d-1), resulting from integrated hourly calculations.
Primary Production in 2004 in relation to suspended matter concentrations
To test a general response of primary production on suspended matter concentrations, SPM-data of 2004 were altered by multiplying a constant factor. Annual primary production was calculated on the basis of this suspended matter values. The annual primary production of 2004 served as reference value for changed production rates (Figure 7 a & b). A doubling of SPM-concentrations as measured in 2004 reduced 20 % of the annual primary production (Figure 7 a). The daily primary production decrease in a range up to 50 % (Figure 7 b). Reducing SPM-concentrations by factor 0.5 increases annual primary production to 20% (Figure 7 a). The daily primary production increases in a range up to 75 % (Figure 7 b). The highest impact of suspended matter on primary production was observed from October until April.
0 100 200 01 02 03 04 05 06 07 08 09 10 11 12 month % p ri m ar y pr od uc ti on SPM*0.5 SPM*2
Figure 7 a- b. Annual dynamics of relative daily primary production (a) and total annual primary
production (b) as a function of altered suspended matter concentrations. Reference (100%) is daily primary production values in 2004. A feedback between primary production and phytoplankton biomass growth was not included.
0 50 100 150 200 250 300 0 50 10 0 150 200 250 300 350 400 450 500 % of suspended matter to ta l a n n u a l p ri m . p ro d . 0 20 40 60 80 100 120 140 % o f a n n u a l p ri m . p ro d . a b
