1.1. Diatom spring bloom (end of March –middle of April)
The diatom spring bloom started in the end of March and ended in the middle of April.
A phytoplankton production maximum of ~1 g C m-2 d-1 was reached. Silicate was depleted by the middle of April from ~ 40 µmol L-1 to values below 1 µmol L-1. Phosphorus depleted by the middle of April from ~ 1 to 0.1 µmol L-1. Nitrate decreased from ~60 to ~40 µmol L-1.
No impact of zooplankton grazing could be detected in the three experiments during this period.
Respiration reached maximum values of 130 mg C m-2 d-1, but was well below production rates. Suspended matter varied between ~ 15 and 45 mg L-1 until the beginning of April and then decreased to 1 mg L-1 in the mid of April. High sediment resuspension until the beginning of April reduced the water column light field, and may have delayed the spring bloom and enhanced pelagic respiration. Most of the spring bloom biomass may have entered the benthic food web by sediment percolation and zoobenthos filtering activity, since no zooplankton grazing occurred. Also the decaying diatom bloom was likely to sink to the benthic layer (e.g. SMETACEK 1985).
PELAGIC BENTHIC phyto
plankton biomass
water column light
field
nutrients (P,N,SI detritus
Diatom spring bloom
Figure 1. Schematic diagram of pelagic processes and the interaction between pelagic and benthic compartments in spring. Grey arrows depict the transfer of detritus between benthic and pelagic system, the green arrows show the loss of phytoplankton biomass (sinking&
benthic filtering and decay). The blue arrow depicts nutrient uptake by phytoplankton. The brown arrow depicts the reducing of water column irradiance by sediment resuspension.
1.2 Phaeocystis bloom (end of April – middle of June)
The Phaeocystis globosa bloom followed directly after the diatom bloom in the period from the end of April until the middle of June. Daily primary production was high and reached maximum values of 6 g C m-2 d-1. Nitrate decreased from ~40 µmol L-1 to values below 1 µmol L-1 in the middle of May. Silicate and phosphorus remained at values below 1 µmol L-1 and 0.2 µmol L-1.
Respiration was high, maximum values of 600 mg C m-2 d-1 were reached, presumably enhanced by pelagic processes, such as sloppy feeding and the release of carbohydrates during photosynthesis.
The pelagic grazing impact on Phaeocystis was high: the daily grazing rate was ~ 50-60% of the standing stock tested in two experiments in the middle of April and the beginning of June. More phytoplankton biomass was presumably grazed by zooplankton than assimilated by the benthic system.
Suspended matter varied between ~1 and 25 mg L-1. Benthic filtering was the major benthic impact on pelagic processes, since sediment resuspension was low compared to the foregoing season.
PELAGIC BENTHIC phyto
plankton biomass
water column light
field
nutrients (P,N,SI
Phaeocystis globosa bloom
zooplankton biomass
detritus
Figure 2. Schematic diagram of pelagic processes and the interaction between pelagic and benthic compartment during the Phaeocystis globosa bloom. Grey arrows depict the transfer of detritus between benthic and pelagic system, the green arrows show the loss of phytoplankton biomass (sinking & benthic filtering, grazing, sloppy feeding, decay). The red arrow depicts detritus increase by zooplankton biomass and faeces. The blue arrow depicts nutrient uptake by phytoplankton. The brown arrow depicts the reducing of water column irradiance by sediment resuspension.
1. 3 Summer (end of June –end of September)
During this period primary production varied between 0.2 -0.9 g C m-2 d-1. Highest values were reached in the middle of August. Nitrate and silicate remained at values below 1 µmol L-1 until the middle of September (09. Sept) and then started to increase.
Phosphorus increased during summer from 0.3 to 0.6 µmol L-1 in the end of September.
The impact of zooplankton grazing was high. In one experiment in the beginning of August daily zooplankton grazing consumed up to 100% of the standing stock of phytoplankton biomass, indicating high carbon turnover rates.
Respiration varied between 100 -200 mg C m-2 d-1 and was below production.
From the middle of July until the beginning of September suspended matter concentrations showed lowest values within the year between 2-12 mg L-1. Within the second half of September suspended matter concentration increased from 2 until 30 mg L-1. Sediment resuspension had little impact on the water column light field or pelagic respiration. Benthic filtering was the only benthic impact on pelagic processes. Since large amounts of pelagic biomass were grazed by zooplankton, phytoplankton biomass remained at low concentrations.
PELAGIC BENTHIC phyto
plankton biomass
water column light
field
nutrients (P,N,SI
Summer
zooplankton biomass
detritus
Figure 3. Schematic diagram of pelagic processes and the interaction between pelagic and benthic compartment during summer. Grey arrows depict the transfer of detritus between benthic and pelagic system, the green arrows show the loss of phytoplankton biomass (sinking
& benthic filtering, grazing, sloppy feeding, decay). The red arrow depicts detritus increase by zooplankton biomass and faeces. The blue arrow depicts nutrient uptake by phytoplankton.
The brown arrow depicts the reducing of water column irradiance by sediment resuspension.
1.4 Autumn (end of September –end of December)
Primary production during autumn decreased from 200 mg C m-2 d-1 in September to 10 mg C m-2 d-1 in December. Silicate, nitrate and phosphorus increased from the end of September until the end of December to values of Silicate: ~25 µmol L-1 , nitrate: ~30 µmol L-1 and phosphorus ~1 µmol L-1. Grazing was determined in one experiment in the beginning of October to be ~15% of the standing stock per day.
Suspended matter varied between ~5 and 60 mg L-1. High sediment resuspension decreased the low water column light field of autumn.
Respiration during this time was between 25-75 mg C m-2 d-1 (autumn data in Chapter 3 were from 2003. unpublished data from 2004 show values in a similar range).
Respiration was presumably enhanced by resuspended benthic organic material.
Presumbly nutrient release from pore water (only little benthic primary production) (see HEDTKAMP 2005) increased pelagic nutrient concentrations. Benthic filtering processes presumably consumed similar or slightly smaller amounts of pelagic biomass than pelagic grazing activity.
PELAGIC BENTHIC phyto
plankton biomass
water column light field nutrients
(P,N,SI
Autumn
zooplankton biomass
detritus
Figure 4. Schematic diagram of pelagic processes and the interaction between pelagic and benthic compartment during autumn. Grey arrows depict the transfer of detritus between benthic and pelagic system, the green arrows show the loss of phytoplankton biomass (benthic filtering & sinking, grazing, sloppy feeding, decay). The red arrow depicts detritus increase by zooplankton biomass and faeces. The blue arrow depicts nutrient uptake by phytoplankton.
The brown arrow depicts the reducing of water column irradiance by sediment resuspension.
1.5 Winter (January – end of March)
During winter primary production remained at values below 50 mg C m-2 d-1 until the end of February. In March primary production started to increase and was between 100-200 mg C m-2 d-1.
Respiration remained at values between 25-50 mg C m-2 d-1. However, respiration measured at 10°C showed higher values than measured at 10 °C in summer. Suspended matter varied between ~15 and 45 mg L-1. High sediment resuspension decreased the low water column light field of autumn/ winter. Respiration was enhanced by resuspended benthic organic material.
Nutrient release of pore water (only little benthic primary production) increased pelagic nutrient concentrations of silicate to ~40 µmol L-1, nitrate to ~60 µmol L-1 and phosphorus to 1 µmol L-1.
Grazing was not regularly determined during winter. One experiments in the mid of March revealed no zooplankton grazing impact. Presumably phytoplankton biomass entered the benthic layer rather than it was grazed by zooplankton. MARTENS (1980) showed low abundances of mesozooplankton during winter in the List tidal basin.
PELAGIC BENTHIC phyto
plankton biomass
water column light field nutrients
(P,N,SI
Winter
detritus
Figure 5. Schematic diagram of pelagic processes and the interaction between pelagic and benthic compartment during winter. Grey arrows depict the transfer of detritus between benthic and pelagic system, the green arrows show the loss of phytoplankton biomass (sinking &
benthic filtering, decay). The blue arrow depicts nutrient release of the benthic and uptake by phytoplankton. The brown arrow depicts the reducing of water column irradiance by sediment resuspension.
Summarizing and comparing carbon dynamics of the different seasons shows, that the benthopelagic coupling varied throughout the year (Figure 6). The benthopelagic coupling affects the pelagic carbon dynamics presumably more during autumn and winter than during late spring (the Phaeocystis globosa bloom) and summer.
Winter Autumn
Diatom spring bloom
Phaeocystis bloom
Summer
PELAGIC
BENTHIC
PELAGIC
BENTHIC
Figure 6. The assumed dynamics of benthopelagic coupling summarized and compared between all seasons. Grey arrows depict the exchange of detritus, blue arrows depict the exchange of nutrients, green arrows show the loss of phytoplankton biomass and brown arrows depict the reducing of water column irradiance by sediment resuspension.
2. Interrelationships among autotrophic and heterotrophic processes
2.1 The ratio of pelagic primary production and respiration in the List tidal basin
The ratio of primary production to respiration (P/R) indicates to which extend an ecosystem is a sink or a source of CO2. If P/R >1, the ecosystem is autotrophic and acts as sink of CO2, if P/R < 1 the ecosystem is heterotrophic and a source for CO2 (DEL
GIORGIO & WILLIAMS 2005). The difference between production and respiration is termed as net ecosystem production (NEP). Like many coastal ecosystems, also the Wadden Sea was estimated as a heterotrophic system (SCHOLTEN ET AL. 1990, HOPPEMA 1991). For the pelagic system, POREMBA (1999) suggests the deeper tidal channels within the Wadden Sea to be heterotrophic, whereas the shallow parts are autotrophic because of favourable light conditions.
Pelagic primary production in the present study exceeds pelagic respiration 5 fold. The annual mean P/R ratio is 210/37 = ~ 5.7 ranging between 0.7 in December and 11.2 during the Phaeocystis globosa bloom (Figure 7 a & b).
0 100 200 300 400 500 600 700 800
Jan-04 Feb-04
Mar-04 Apr-04
May-04 Jun-04
Jul-04 Aug-04
Sep-04 Oct-04
Nov-04 Dec-04 mg C m2*d-1
1.21.6 7.87.1
11.2
3.5 3.8 4.4 3.83.1
2.2 0.7 0.0
2.0 4.0 6.0 8.0 10.0 12.0
01-04 02-04
03-04 04-04
05-04 06-04
07-04 08-04
09-04 10-04
11-04 12-04
PP / R
Figure 7 a & b. a) Comparison of monthly means of pelagic primary production (grey) and pelagic respiration (white). b) The ratio of pelagic primary production / pelagic respiration in 2004
a b
a
The P/R ratio of 5.7 is high compared to data reported from other coastal areas (HOPKINSON & SMITH 2005) but in a similar range as reported for the Meldorfer Bucht (~4) in the Northern Wadden Sea (TILLMANN ET AL. 2001). In their study Tillmann et al.
calculated respiration rates from primary production measurements following the model of LANDON ET AL. 1993.
Calculations of the P/R ratio strongly depend on water depth. Especially at sites were the mean water depth is greater than the euphotic depth, the P/R-ratio decrease with increasing water depth: respiration is linear to water depth and production decreases according to irradiance until the critical depth (SVERDRUP 1953, HEIPP ET AL.1995) is reached. Whether to estimate the P/R ratio on a volumetric or depth integrated scale is still under debate for the oceans global carbon cycle (WILLIAMS 1998): volumetric data analysis lead to an unbalanced system (DEL GIORGIO ET AL. 1997), whereas depth integrated calculations lead to a more balanced (and slightly autotrophic) open ocean (WILLIAMS 1998, DEL GIORGIO &WILLIAMS 2005)
Calculations in the present study were made on a depth integrated scale for a mean water depth of 2 m. The assumed mean water depth of the List tidal basin is 2m (GÄTJE
& REISE 1998), resulting from the ratio of water volume at mean tidal level/ total area = 840 106 m3 / 402 km2. (BACKHAUS ET AL. 1998, BAYERL & KÖSTER 1998). The mean photic depth in the List tidal basin is ~ 4 m and within the mean water depth of 2 m approximately 75% of the overall water column production takes place.
It has to be taken into account that data on primary production and on respiration resulted from a sampling site of 10m depth. If data were extrapolated from 2m to greater water depth (e.g. of the sampling site) calculations result in a linear increase of respiration rates and only a slight increase of primary production (Table 1).
Table 1. Depth integrated annual primary production (P) and respiration (R ) calculated for different mean water depths.
assumed depth of the water column
(m)
P (g C m2 y-1)
R (g C m2 y-1)
P/R
2 211 37 5.7
4 256 74 3.5
5 262 93 2.8
8 266 148 1.8
10 266 185 1.4
15 266 278 0.96
20 266 370 0.72
If measured data were extrapolated to a lower water depth than 2 m following assumptions have to be made: approximately 33% of the total area are tidal flats and emerged during low tide, the water column is more turbid and decreases primary production in the more shallow parts of the bight. In contrast, respiration might have been higher since more resuspension of benthic material was possible. Both, extrapolation of data for increased or decreased water depth decreases the P/R-ratio.
Nevertheless, this study indicates an autotrophic water column of the List tidal basin.
The Wadden Sea is characterised as a heterotrophic systems, where respiration exceeds primary production (HEIP ET AL. 1995, VAN BEUSEKOM ET AL. 1999). From the North Sea organic matter is mainly imported rather than exported (VAN BEUSEKOM & DE
JONGE 2002). For the List tidal basin VAN BEUSEKOM ET AL. (1999) estimated an annual system (= benthic + pelagic) production of ~300 g C m-2 y-1 and a system remineralisation of ~450 g C m-2 y-1. Presumably high amounts of organic carbon were exported from the water column to benthic remineralisation between spring and autumn.
In winter, the ratio of P/R is nearly 1 (Figure 7) and together with the mentioned uncertainties from integrating production and respiration over water depth, it seems possible, that the water column may shift to heterotrophy (P/R <1) during short periods in winter.
2.2 Zooplankton grazing - a link between primary production and respiration
Zooplankton grazing transfers phytoplankton biomass into the pelagic food web (BANSE
1992). According to the availability of nitrogen and carbon and the size structure of plankton communities, pelagic trophic systems range between herbivorous and microbial pathways (LEGENDRE & RASSOULZADEGAN 1995, VARELAI ET AL. 2003). In the presence of large zooplankton, grazing simultaneously provides top-down control of biomass and bottom-up nutrient supply by urea excretion and sloppy feeding (GLIBERT
1998). The production of DOC by sloppy feeding depends on apparent gross growth efficiency and copepod-to-prey size ratio (LAMPERT 1978, MOELLER 2005). Moreover large zooplankton is grazing on protozoa and thus decreasing nitrogen regeneration rates (CARON & GOLDMAN 1990). If no zooplankton grazing occurs, the biomass is entering the benthic food web by benthic ingestion or decaying and sinking algal cells (SMETACEK 1985).
In the List tidal basin, grazing on phytoplankton biomass occurred from the end of April until October, with a peak in August. During summer 50-100% of
phytoplankton standing stock was transferred by grazing into the pelagic food web.
During this period nitrogen limitation is likely to occur. But beside nutrient (nitrogen) limitation, also a control of zooplankton grazers on phytoplankton productivity might have occurred during summer: Under low supply of new nitrogen, zooplankton grazing by large herbivores (e.g. copepods) or by microzooplankton (e.g. protozoa), controls primary production by direct consumption of cells and at the same time enhance nutrient uptake through the regeneration of dissolved nitrogen forms (Gaul et al. 1999).
In the List tidal basin zooplankton is a food supply for fish, especially herring (Clupea harengus) (HERMANN ET AL. 1998). Data on abundances of planktivorous fish species is scarce. In a network analysis for the List tidal basin (BAIRD ET AL. 2004) herring was estimated to consume 0.0002 mg C m-2 d-1 of zooplankton. But until now no statement on the impact of fish predation on the zooplankton community development is possible.
3. This study in the framework of COSA and the Sylt long term time series
Investigations of this study were spatially and temporally linked with the Sylt long term time series (e.g. MARTENS & ELBRÄCHTER 1998). This time series is monitoring e.g.
temperature, salinity, inorganic and organic nutrients, chlorophyll a and suspended matter concentrations. The linking allowed to investigate pelagic processes on the background of nutrient dynamics. Therefore a comprehensive view on annual pelagic carbon/nutrient dynamics was possible during this study.
In terms of the EU-project COSA, this study gives background information on annual pelagic carbon/ nutrient dynamic. It was shown, that seasonal variability in pelagic carbon/nutrient pathways and their ‘loss’ of biomass to the benthic system is high and this seasonal variability has to be considered also for the understanding of benthic processes (and vice versa) and the coupling and functioning of the overall system.
Conclusions
The Wadden Sea has markedly changed by anthropogenic impact (LOTZE ET AL. 2005) and eutrophication was one important aspect of changes within the past decades (see Introduction). Since nutrient loads into the Wadden Sea decreased since the mid 1980’s (e.g. VAN BEUSEKOM 2005) a decrease in primary production since the mid 1980’s was likely to occur, but could not be confirmed during this study. Primary production was still on a high level and investigations and results of this study reflect carbon dynamics of a eutrophic system.
In terms of the benthopelagic coupling in the List tidal basin, this study showed that mainly during a Phaeocystis globosa bloom (late spring) and in summer the pelagic system has a high activity and the share of benthic influence is presumably low compared to other seasons. A benthic filtering of 5-20% d-1 of the water volume was assumed for all seasons, (Chapter 3) since no seasonal specific data were available.
High amounts of pelagic biomass production during late spring and summer were consumed by zooplankton and thereby transferred into the pelagic food web as indicated by high grazing rates (Chapter 3). Moreover, large amounts of produced biomass in late spring and summer were degraded in the pelagic system as indicated by high pelagic respiration rates (Chapter 4). Since a ‘bottom-up’ control is assumed for the Wadden Sea (LOTZE ET AL 2005), the high variability of primary production in different years especially during spring and summer (Chapter 2, Figure 11, March- August), indicates a high interannual variability of zooplankton grazing and pelagic respiration. However, during autumn, winter, and early spring, benthic and pelagic compartments were closely coupled. Sediment resuspension reduces the water column light field and thereby primary production. A doubling of suspended matter concentrations would reduce production mainly from autumn until spring in a range between 20-50% (Chapter 2, Figure 7b). It was shown, that produced phytoplankton biomass is merely or not consumed by zooplankton and major parts may have entered the benthic system by benthic assimilation.
No statistical difference for monthly rates of primary production between 1984-1990 and 2000-2004 could be found, but nevertheless during autumn and winter means of primary production were higher in the latter period. Assuming an increase in productivity within the past twenty years, primarily the benthic system would have benefit from that: Since zooplankton grazing during these seasons had a low impact,
higher plankton productivity during autumn and winter fuels the benthic system rather than the pelagic.
The mathematical model on the basis of experimental and measured data could not in detail simulate natural processes, even though the general course of nitrogen and of chlorophyll a was well reflected. This indicates that in the model presumably mainly the simulation of benthic processes and exchange rates between pelagic and benthic compartments has to be improved. Both systems –benthic and pelagic- have to be considered in detail to simulate the carbon/nutrient fluxes in the List tidal basin.
Improving the understanding of this coupling would be a scope for future research, since information on exchange rates between pelagic and benthic compartments is scarce. A further scope for future research are detailed information on short term variability (days) of the processes primary production, benthic and pelagic grazing and respiration, especially during bloom situations: Benthic filtering presumably has a high impact on phytoplankton biomass development at the starting of a spring bloom. Until now only little information is available on food competition between benthic suspension feeders and zooplankton and particularly on spatial and seasonal variability of the benthic filtering process. Moreover, predation on zooplankton by fish is poorly investigated, but an important link of the food web and provides information on the transfer and export of biomass (fishery) from the Wadden Sea. The tidal impact on the mentioned processes was not an objective of this study, but should also be considered in future research.
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