Abstract
Pelagic respiration in a shallow tidal basin of the Northern Wadden Sea was assessed from October 2003 until September 2004. Measurements were carried out as weekly time series at in-situ temperature and parallel at 10°C to exclude temperature effects.
Size fractionated respiration was determined during eight filtering experiments between January until August 2004. The annual mean respiration was ~ 37 g C m-2 y-1. About 50% of the annual respiration was observed during a Phaeocystis globosa bloom in late spring.
Since the study was conducted in an area of high sediment resuspension, it was tested, whether a linear relation exists between suspended matter concentrations and pelagic respiration rates. Both time series were analysed in terms of their relation to temperature, suspended matter concentration, dissolved and particulate organic phosphorus and phytoplankton biomass. No linear relation could be found between suspended matter concentration and pelagic oxygen consumption. Nevertheless suspended matter seemed to influence pelagic respiration as a non-linear background signal: During winter large amounts of suspended matter were observed and respiration rates (measured at 10°C) were higher than summer and autumn values.
Dissolved organic phosphorus explained about 70% of variability of respiration during winter and late spring. Also the experiments showed that dissolved organic matter had a major impact on pelagic respiration: In size fractionated respiration measurements, the fraction < 1µm contributed to 25 - 60% of the total pelagic respiration. Temperature explains one third of the annual variability of respiration rates. It is concluded that dissolved organic material is a major source for bacterial degradation also in the pelagic system of a shallow coastal area with high sediment resuspension.
Introduction
Respiration is an essential part of the marine carbon cycle (DUARTE & AGUSTI 1998, WILLIAMS & DEL GIORGIO 2005). A major part of respiratory processes is bacterial degradation of organic material (WILLIAMS 1981, HOPKINSON ET AL. 1989; GRIFFITH ET AL. 1990). In oceanic waters, the origin of pelagic degraded organic material is basically pelagic primary production and a small fraction that is imported from coastal areas or by atmospheric input (WILLIAMS 1998; DEL GIORGIO & DUARTE 2002). In a shallow coastal ecosystem like the European Wadden Sea, pelagic and benthic processes are closely linked: This ecosystem is strongly influenced by water currents and wave action in combination with tide and wind forcing. Thus, resuspension of benthic material permanently occurs (POSTMA 1982) and for pelagic degradation of organic material also benthic sources should be considered (CADÉE & HEGEMAN 1974; HEIP ET AL. 1995, WAINRIGHT & HOPKINSON 1997, POREMBA ET AL. 1999). Additionally, organic matter is transported from the North Sea into the Wadden Sea and remineralised there (POSTMA
1954, VAN BEUSEKOM ET AL. 1999). In a conceptual model, linking riverine nitrogen input and the annual nitrogen cycle of the Wadden Sea, it is suggested that in years with high riverine nitrogen load more organic matter is produced in the North Sea and subsequently imported and remineralised within the Wadden Sea than in dry years with a low riverine nitrogen load (VAN BEUSEKOM & DE JONGE 2002).
Even in shallow coastal seas, about half of the annual respiration takes place in the water column (HEIP ET AL. 1995, VAN BEUSEKOM ET AL. 1999). Nevertheless, only a few respiration measurements were carried in the European Wadden Sea. ASMUS ET AL. (1998) measured respiration in near shore waters from the tidal flats as two weekly in-situ time series and resulted high annual rates. From these measurements VAN
BEUSEKOM ET AL. (1999) estimated an annual respiration rate of 110 g C m-2 y-1 for the List tidal basin. 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.
This study focuses on the annual cycle of pelagic respiration in the List tidal basin. Two time series of weekly measured respiration rates were conducted from October 2003 until September 2004: (1) respiration at in-situ temperature to quantify the actual respiration rates. (2) Respiration at constant 10°C, to exclude temperature effects on respiration rates. Pelagic respiration rates were analysed in relation to: temperature,
suspended matter, dissolved and particulate organic phosphorus and phytoplankton biomass. DOP was taken as tracer for DOM in the present study. CLARK ET AL. (1998) showed that organic phosphorus is preferentially regenerated from dissolved organic material (DOM), whereby phosphorus in form of esters is rapidly used, and phosphonates are more resistant to bacterial degradation. In filtering experiments size fractionated respiration rates were determined. It will be shown, that despite high suspended matter concentration, dissolved organic material plays a major role in pelagic community respiration. For suspended matter no linear relationship to respiration could be found, but it is assumed to have a non-linear background effect on pelagic respiration.
Methods
Study Site
The study area was the List tidal basin, a 404 km2 semi-enclosed bight in the northern part of the European Wadden Sea, which is connected to the open North Sea by a single tidal inlet (Fig. 1). The water volume is about 845 106 m3 at a mean tidal level. The water depth is on average two meter and up to 40 meter in the main tidal channels. The water column is homogenous mixed. Tides are semidiurnal; the mean tidal range is about two meter. The impact by river runoff is marginal, since only one small creek enters the bight. Sandy sediment covers about 95 % of the area. For detailed descriptions see GÄTJE & REISE (1998).
Water sampling
Over a period of 12 months (from Oct. 2003 to Sept. 2004), seawater samples for respiration measurements were taken weekly from a station (mean depth = 10m) in the List tidal basin. At this station seawater is sampled since two decades twice a week for the Sylt long term time series (e.g. MARTENS & ELBRÄCHTER 1998). Water samples were taken with a 5L Niskin-Bottle and used for all analysis. Water temperature was measured by a thermometer fixed to the Niskin-Bottle. 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. For determination of size fractionated respiration, water samples were taken from this
Figure 1. The study site.
Sampling sites are indicated as black dots.
List tidal basin 55°N / 8.40°E
10 km
List tidal basin 55°N / 8.40°E
10 km
routine station and also from a shallow subtidal station, where high amounts of suspended matter were expected. The positions of the stations are indicated in Figure 1.
Dissolved and particulate phosphorus, phytoplankton biomass and suspended matter
Dissolved phosphorus was determined according to GRASSHOFF ET AL. (1983) Particulate Organic and Inorganic phosphorus was determined after APSILA ET AL. (1976). Suspended matter was determined gravimetrically on 0.47µm pore-size Nucleopore filter. Phytoplankton biomass was determined as chlorophyll a determined according to JEFFREY & HUMPHREY (1976).
Respiration measurements
Seawater was filled from the Niskin-bottle through a silicon tube into 120ml glass bottles. The initial oxygen content was determined in triplicates. Three bottles were incubated in the dark in for about 24 hours at in-situ temperature and parallel three bottles were incubated at 10°C. 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.
Size-fractionated respiration measurements
For determining size fractionated respiration rates, sampled seawater was filtered stepwise through 80µm and 20µm plankton meshes and a 1µm filter. This filter provides a constant pore-size during the filtration process. After filtering, the seawater was filled through a silicon tube into 120 ml glass bottles. The initial oxygen content was determined in triplicate for each filtering treatment, since the treatment may alter the oxygen content of seawater samples. Water samples were incubated in the dark as triplicates from each fraction. As a control, unfiltered seawater was incubated. The bottles were incubated for 24 hours. Oxygen was measured by Winkler-technique (GRASSHOFF ET AL. 1983) using an automatic titration apparatus. Conversion from oxygen to carbon units was made using a conversion factor of 0.89 (DEL GIORGIO &
WILLIAMS 2005) .
Results
The one year period from October 2003 until September 2004 was separated into five seasons, regarding phytoplankton composition and abundance (diatom spring bloom &
Phaeocystis globosa bloom), and annual season as shown in Table 1. and Figure 2.
10-03 11-03 12-03 01-04 02-04 03-04 04-04 05-04 06-04 07-04 08-04 09-04 10-04
date
0 2 4 6 8 10 12 14 16 18 20 22 24
Chl a (µg L-1 )
Phaeocystis bloom Diatom
spring bloom
summer winter
autumn
Figure 2. Annual cycle of chlorophyll a and the five separated seasons.
Linear regressions were analysed for the complete annual cycle as well as for separated periods between respiration and phytoplankton biomass, DOP, POP, and suspended matter concentrations (annual cycles shown in Fig. 5 a-c)
Table 1. Dates of separated season and the respective range of water temperature for each season
Season Date Temperature
-range (°C) Autumn
Winter
Diatom spring bloom Phaeocystis bloom
Summer
06.10.03 - 27.11.04 08.12.03 - 11.03.04 22.03.04 - 15.04.04 26.04.04 – 17.06.04 24.06.04 – 09.09.04
12.7- 7.0 5.7 - 1.9 5.1 - 7.9 10.7 - 15.0 15.4 - 21.5 - 17.6
Annual cycle of pelagic respiration
The mean annual respiration under in-situ conditions was 36.8 g C m-2 y-1. Respiration rates were in a range between 0.1 -12 µg C L-1 h-1 (Fig. 3).
10-03 11-03 12-03 01-04 02-04 03-04 04-04 05-04 06-04 07-04 08-04 09-04 10-04
date
0 2 4 6 8 10 12 14
Respiration [µg C h-1L-1]
Figure 3. Seasonal cycle of respiration at in-situ temperature (black triangles) and at 10°C (white dots)
In autumn and winter, respiration at in-situ temperature (values for measurements at 10°
C in brackets) decreases from 1.5 (1.2) µg C L-1 h-1 to values below 0.5 (0.5 - 2.7) µg C L-1 h-1 until the mid of March. During a diatom spring bloom from the end of March until the middle of April respiration rates were increasing to up to 2.7 (2.5) µg C L-1 h-1. During the Phaeocystis globosa bloom, highest respiration rates of 12.0 (10.6) µg C L-1 h-1 were measured. Respiration during this period contributes to approximately 50% of the total annual pelagic respiration. In summer, respiration reached values between 2 -4 (0-1) µg C L-1 h-1 . Respiration rates at 10°C show higher values during winter than in summer.
Table 2. Mean monthly respiration rates calculated from weekly measurements (mean ± SD;
mg C m-2 d-1 ).
Month Respiration at in-situ temp.
Respiration at 10°C
Oct-03 37.5 ± 26.8 30.4 ± 7.0
Nov-03 35.3 ± 46.6 45.5 ± 51.7
Dec-03 6.5 ± 0.9 26.7 ± 11.3
Jan-04 12.7 ± 2.3 101.9 ± 45.0
Feb-04 29.7 ± 14.4 59.2 ± 3.5
Mar-04 23.6 ± 9.6 60.0 ± 23.9
Apr-04 85.1 ± 39.0 116.1 ± 23.9
May-04 291.0 ± 87.1 228.01 ± 97.4
Jun-04 363.9 ± 166.8 240.2 ± 175.8
Jul-04 116.2 ± 15.8 28.3 ± 9.8
Aug-04 130.4 ± 38.3 24.3 ± 6.2
Sep-04 87.2 ± 26.4 32.5 ± 12.8
The effect of temperature on pelagic respiration
The seawater temperature range ranged between 1.5 °C in March 2004 and 23 °C in August 2004 (Fig. 4). The influence of temperature on pelagic respiration was analysed in two ways: (1) by linear regression between the in-situ respiration time series and temperature and (2) by analysing the correlation between both respiration time series.
10-03 11-03 12-03 01-04 02-04 03-04 04-04 05-04 06-04 07-04 08-04 09-04 10-04
date
0.0 5.0 10.0 15.0 20.0 25.0
Temperature (°C)
Diatom spring bloom
winter autumn
Phaeocystis bloom
summer
Figure 4. The annual dynamics of water temperature in the List tidal basin from Oct. 03-Sept.
04. Boxes show the defined seasons.
The linear regression between temperature and pelagic respiration (r² = 0.34) indicates that about on third of the variability in respiration can be explained by temperature. The comparison of both respiration time series indicates a high correlation (r² = 0.73) between both series. As temperature is the only difference between the two series, this factor is probably responsible for most of the remaining variance. Both analyses show the same result, that temperature is causing approximately one third of variability of annual pelagic respiration.
The influence of organic phosphorus, phytoplankton biomass, and suspended matter concentrations on pelagic respiration
POP showed a significant regression coefficient to respiration in the annual cycle (r2 = 0.43) but not for single periods. Within all seasons, DOP results the highest regression coefficient (r2 = 0.2 – r2 = 0.58) in linear regression to respiration rates.
Table 4 summarises the results of all calculated correlations.
Phytoplankton biomass correlated significantly with respiration rates but the amount of explained variance was low (r2 = 0.29). Suspended matter concentrations did not correlate with respiration rates either for the annual cycle or for separated periods. During winter respiration at in-situ temperature correlated significantly (r2=0.74) with suspended matter.
Figure 5 a-d. Seasonal cycle of a) respiration rates measured at 10°C, b) dissolved organic phosphate. c) particulate organic phosphate d) suspended matter. Separately analysed seasons are indicated.
10-03 11-03 12-03 01-04 02-04 03-04 04-04 05-04 06-04 07-04 08-04 09-04 10-04
date
0 2 4 6 8 10 12 14
Respiration [µg C h-1L-1]
summer winter
autumn
Dia to m
s p ring blo om Phaeocystis
bloom
10-03 11-03 12-03 01-04 02-04 03-04 04-04 05-04 06-04 07-04 08-04 09-04 10-04
date
0.0 0.2 0.4 0.6
DOP (µmol L-1) 10-03 11-03 12-03 01-04 02-04 03-04 04-04 05-04 06-04 07-04 08-04 09-04 10-04
date
0.0 0.4 0.8 1.2 1.6 2.0
POP (µmol L-1) 10-03 11-03 12-03 01-04 02-04 03-04 04-04 05-04 06-04 07-04 08-04 09-04 10-04
date
0.0 10.0 20.0 30.0 40.0
SPM (µg L-1)
Table 4 linear regression coefficients (r2 ) , statistical significance (p), and numbers of data sets (n) were shown for linear regression analysis between pelagic respiration rates and its potential controlling factors.
Respiration at 10° C
Respiration at in-situ temp.
Time interval Parameter r2 p n r2 p n
annual cycle in situ
respiration 0.73 < 0.001 41 --- --- --- annual cycle DOP+POP 0.61 < 0.001 37 0.44 < 0.001 36 annual cycle DOP 0.20 <0.05 37 0.45 < 0.001 39 POP 0.43 < 0.001 41 0.25 <0.001 42 SPM 0.02 n. sign 40 0.01 n. sign 42 Chl a 0.29 < 0.001 41 0.17 <0.05 43
autumn DOP 0.59 0.07 6 0.84 <0.05 6
POP 0.01 n. sign 7 0.01 n. sign 7 SPM 0.02 n. sign 7 0.00 n. sign 7 Chl a 0.22 n. sign 7 0.20 n. sign 7
winter DOP 0.53 <0.05 8 0.04 n. sign 8
POP 0.01 n. sign 9 0.92 < 0.001 9 SPM 0.00 n. sign 8 0.74 < 0.05 8 Chl a 0.01 n. sign 9 0.65 < 0.01 9 Diatom spring DOP 0.58 < 0.05 7 0.38 n. sign 8
bloom POP 0.25 n. sign 7 0.34 n. sign 8
SPM 0.01 n. sign 7 0.00 n. sign 8 Chl a 0.37 n. sign 7 0.33 n. sign 8 Phaeocystis DOP 0.52 < 0.05 9 0.42 n. sign 9
bloom POP 0.33 n. sign 9 0.50 < 0.05 9
SPM 0.18 n. sign 9 0.33 n. sign 9 Chl a 0.10 n. sign 9 0.04 n. sign 9
summer DOP 0.20 n. sign 8 0.05 n. sign 8
POP 0.07 n. sign 10 0.02 n. sign 10
SPM 0.20 n. sign 10 0.02 n. sign 10
Chl a 0.02 n. sign 10 0.00 n. sign 10
The relation between DOP and respiration rates (measured at 10° C) varied more than tenfold between summer and winter, as shown by the slope of the regression line between DOP and respiration rates (Fig 6b):
The effect of DOP on pelagic respiration increases from autumn until the Phaeocystis globosa bloom, where the steepest slope of linear regression is reached during in June.
In summer no significant correlation between respiration and DOP was found and the regression model shows a low respond of respiration to DOP. This indicated the quality of DOP is high in spring and low during summer. Within all seasons –despite for summer- a certain amount of DOP seem to be refractory and not available for respiration, since the intercept of the regression line with the X-axis is within a value of ~0.10-0.15 µmol L-1. Combining the data from winter, the diatom spring bloom and the Phaeocystis bloom, the linear regression is significant (p= 0, r2= 0.73, Fig 6a)
Winter, Diatom bloom, Phaeocystis bloom y = -3.520 + 27.161 * x
r = .85504
0.00 0.05 0.10 0.15 0.20 0.25 0.30 0.35 0.40 0.45 0.50 0.55 0.60 DOP
0 2 4 6 8 10
Respiration µg C h-1L-1
Figure 6a. The linear regressions models of respiration at 10°C and dissolved organic phosphorus (in µmol L-1) for winter, the diatom spring bloom and the Phaeocystis globosa bloom.
0.00 0.05 0.10 0.15 0.20 0.25 0.30 0.35 0.40 DOP
0.0 0.5 1.0 1.5 2.0 2.5 3.0
Respiration µgC h*L-1
Autumn:
r = 0.77, p = 0.07; y = -0.53 + 6.39*x
0.00 0.05 0.10 0.15 0.20 0.25 0.30 0.35 0.40 DOP
0.0 0.5 1.0 1.5 2.0 2.5 3.0
Respiration µgC h*L-1
Winter:
r= 0.73, p = 0.04; y = -1.62 + 17.7*x
0.00 0.05 0.10 0.15 0.20 0.25 0.30 0.35 0.40 DOP
0.0 0.5 1.0 1.5 2.0 2.5 3.0
Respiration µgC h*L-1
Diatom bloom:
r = 0.76, p = 0.046; y = -3.66 + 26.12*x
0.00 0.05 0.10 0.15 0.20 0.25 0.30 0.35 0.40 0.45 0.50 0.55 0.60 DOP
0 2 4 6 8 10 12
Respiration µgC h*L-1
Phaeocystis bloom:
r = 0.72, p = 0.03; y = -5.24 + 32.74*x
0.00 0.05 0.10 0.15 0.20 0.25 0.30 0.35 0.40 0.45 0.50 0.55 0.60 DOP
0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0
Respiration µgC h*L-1
Summer: r = 0.45, p = 0.27; y = 0.07 + 1.52*x
Figure 6b. Linear regressions models of respiration at 10°C and DOP (in µmol L-1) for five defined periods. Correlation coefficient r and the level of significance p are shown.
Regression coefficients r2 were shown in Table 3.
Size fractionated respiration rates
The filter experiments showed, that the amount of respiration within the < 1µm fraction was 25 – 61% of overall community respiration. The percentage of respiration within the > 1 µm fraction was compared to the amount of suspended matter (Fig. 7). No linear relation between both could be found. But even very high amounts of suspended matter (101 mg L-1) did not lead to a higher percentage of respiration within the > 1 µm fraction than 75 %. These results show, dissolved organic material is an important part of the overall community respiration.
Filtering the seawater through 20µm (& 80µm) mesh size did not decrease respiration rates significantly. In some experiments this filtering leads even to an increase of respiration, which could be explained by an artificial crushing of organic particles during the filtering process and a following faster degradation by bacteria. Results of the single experiments are shown in Table 5.
Table 5. Results of the filtering experiment.
date station Temp.
(°C)
mesh- /pore size
respiration (µgC h*L-1)
proportion of overall respiration
(%)
respiration in the >1µm
fraction
suspended matter (mg L-1 )
12.01.04 HS 10 unfiltered 2.8 100
< 80 µm 2.5 90
< 20 µm 2.6 92
< 1 µm 1.7 61 39 % 33.9
15.01.04 1 10 unfiltered 2.8 100
< 80 2.9 103
< 20 2.9 104
< 1 1.2 42 58 % 13.3
02.03.04 HS 10 unfiltered 3.2 100
< 80 2.6 81
< 20 2.5 78
< 1 0.8 25 75 % 50.4
4.3.04 1 10 unfiltered 1.7 100
< 1µm 1.5 61 39 % 16.9
20.4.04 HS 10 unfiltered 5.0 100
< 20µm 4.4 88
< 1µm 1.4 29 71 % 105.6
26.4.04 1 10 unfiltered 2.4 100
< 20µm 2.8 116
< 1µm 1.5 61 39 % 1.0
5.8.04 HS 18 unfiltered 4.6 100
(in-situ) < 20µm 5.3 116
< 1µm 2.6 57 53 % 20.5
20.9.04 1 15 unfiltered 2.6 100
(in-situ) < 20µm 2.6 102
< 1µm 0.7 25 75 % 20.9
Discussion
Temperature was governing one third of the annual variance in pelagic respiration throughout the year, indicating the other two thirds to be controlled mainly by the availability of organic material for bacterial degradation. The hypothesis, that suspended matter concentrations have a proportional impact on pelagic respiration in the List tidal basin could not be verified. Dissolved organic material played an important role for pelagic respiration. The following discussion separates between the impact of dissolved and particulate organic matter, and also the impact of phytoplankton bloom events on pelagic respiration is discussed.
Effect of dissolved organic matter on pelagic respiration
Size fractionating experiments showed, that bacterial respiration of dissolved organic material (the < 1µm fraction) contributed to 25-61% of the overall pelagic community respiration. These results are in a similar range as found by other authors: GRIFFITH ET AL. (1990) reported an increasing percentage of respiration by the < 1µm fraction from 42% in estuarine waters to 65% in near shore waters and 81% in shelf waters. In the Chesapeake Bight SAMPOU & KEMP (1994) found the < 3 µm fraction to account for
~56% of the community respiration.
Results of size fractionated respiration measurements are in line with results of the regression analysis between respiration rates and dissolved organic phosphorus: DOP was taken as tracer for DOM in the present study (CLARK ET AL. 1998, see Introduction) and explained about 50% of respiration (measured at 10 °C) during most seasons and
~70% concerning winter, the diatom bloom and the Phaeocystis globosa bloom. The linear regression between respiration and DOP showed a positive intercept of the regression line with the X-axis. This indicates, that not all DOP was respired and a certain amount might be refractory. Moreover, the slope of the regression line increases five-fold from autumn until the Phaeocystis globosa bloom in June. The slope during summer was again low. DOP seemed to be of higher quality for bacterial degradation between winter and late spring than that in summer and autumn. This might be explained by DOC release from phytoplankton cells: GUILLARD AND WANGERSKY
(1958) found cells in stationary phase sometimes releasing more DOC than exponentially growing cells.
This might explain, that in winter a high amount of the existing DOM derives directly as DOC from phytoplankton cells and is usable for bacterial degradation, whereby in summer zooplankton grazing remains plant cells in an exponential state of growth and the release of fresh and usable DOC by plant cells is low. But also the benthic system may have released degradable DOM during winterly resuspension. During the diatom spring bloom, the chlorophyll a concentration increased, but not the respiration (measured at 10 °C) compared to winterly values. Cell release of DOM might have increased in quality but decreased in quantity. BIDDANDA & BENNER (1997) found DOM to be released by cells during photosynthesis in two forms: high molecular weight (HMW) DOM and low molecular weight (LMW) DOM. These compositional differences result in different microbial reactivity, whereby HMW DOM supports bacterial respiration more effective than LMW DOM (AMON & BENNER 1996).
The effect of phytoplankton blooms on pelagic respiration
During the diatom spring bloom respiration remained at a low level, no zooplankton grazing occurred (Chapter 3) and sloppy feeding effects on respiration did not occur.
More than 50% of the total annual pelagic respiration was observed during the Phaeocystis globosa bloom. Grazing pressure on the Phaeocystis bloom during 2004 in the List tidal basin was high (Chapter 3) and a large amount of organic material that was degraded may result from sloppy feeding (e.g. BANSE 1995, MOELLER 2005) since the predator-prey size ratio causes a high loss of biomass (STELFOX-WIDDICOMBE 2004).
HAMM & ROUSSEAU (2003) suggest for a Phaeocystis bloom in the southern North Sea, that cells of Phaeocystis and their organic matter content are highly susceptible to degradation. High grazing impact on P. globosa (Chapter 3) and high respiration rates indicate that a high amount of P. globosa biomass remains in the pelagic carbon cycle.
Respiration was in low linear relation to phytoplankton biomass in our study This is in contrast to other authors that report on two times higher correlations between pelagic respiration rates and the amount of chlorophyll a in estuarine systems (JENSEN ET AL. 1990, IRIARTE ET AL. 1996). The regression coefficient (r2 = 0.31) found in this study corresponds rather to the correlation (of r = 0.61) reported for the Eastern Atlantic Ocean (ROBINSON ET AL. 2002).
Effect of particulate organic matter on pelagic respiration
In this study pelagic respiration was in no linear relation to the amount of suspended matter and in no linear relation to particulate organic phosphorus. The initial hypothesis, that suspended matter contributes proportional to pelagic respiration rates could not be verified. Moreover, the results of the filtering experiment show, that also respiration within the >1µm fraction is not linear related to the actual amount of suspended matter.
In a simulation model for the Georgia bight WAINRIGHT & HOPKINSON (1997) stressed the importance of sediment resuspension on organic matter remineralisation. In their study they found resuspension to determine the systems (pelagic and benthic) metabolic activity. They estimated, that in the absence of resuspension events water column respiration was 30 % lower than with resuspension events. A comparison of respiration rates measured at 10°C during winter and summer (Dec.-Feb and Jul.-Sep.; Table 1) showed respiration being higher during winter and also the overall amount of suspended matter was higher in winter (Figure 4b). Suspended matter might act as a background signal, increasing pelagic remineralisation in a non-linear way. This is in line with observations of WAINRIGHT & HOPKINSON (1997). Moreover, during persisting winterly wind and resuspension events the organic matter might be degraded within a shorter period than inorganic sediment –contributing mainly to the weight of suspended matter- remains in the water column.
Respiration in the List tidal basin compared to other coastal areas
The measured annual mean respiration of 36.8 g C m-2 y-1 in this study is much lower than a previous value of 110 g C m-2 y-1 estimated for the same area by VAN BEUSEKOM ET AL. (1999). This might be explained by different methods used: The higher value result from water samples taken at a very shallow tidal station at high tide and incubations directly in the field (ASMUS ET AL. 1998). TILLMANN ET AL. (2000) reported an annual mean respiration of 38 g C m-2 y-1 for the Meldorfer Bucht/ Büsum (calculated according to LANGDON 1993) on the basis of primary production measurements). On an aereal scale values for both sites are similar. On volumetric scale, the value for the Meldorfer is lower (12.3 g C m-3 y-1 ; 3 m water depth) than in our study (18.4 g C m-3 y-1 ; 2 m water depth ). Turbidity of the Meldorfer Bucht is much higher (Chapter 1), indicating higher suspended matter concentrations than in the List tidal basin. Since primary production measurements for the List tidal basin yielded
higher values (Chapter 1) than those reported for the Meldorfer Bucht (TILLMANN ET AL. 2000) the ratio of pelagic primary production to respiration equals for both areas.
ROBINSON & WILLIAMS (2005) reported a mean daily respiration in coastal areas of 3.5 O2 m3 d-1 (calculated from 323 observations of a global data base), mean daily respiration on volumetric scale was estimated as 4.25 mmol O2 m3 d-1 in the List tidal basin.
References
AMON MYV, BENNER R (1996) Bacterial utilization of different size classes of dissolved organic matter. Limnol Oceanogr 41: 41-51.
ASMUS RM, JENSEN MH, MURPHY D, DOERFFER R (1998) Primary Production of Microphytobenthos, Phytoplankton and the Annual Yield of Macrophytic Biomass in the Sylt-Roemoe Wadden Sea. In: Gätje C, Reise K (eds) Okösystem Wattenmeer.
Springer Berlin Heidelberg New York.
BAINES B, PACE ML (1991) The production of dissolved organic matter by
phytoplankton and its importance to bacteria: Patterns across marine and freshwater systems Limnol. Oceanogr 36: 1078-1090.
BIDDANDA B, BENNER R (1997) Carbon, nitrogen, and carbohydrate fluxes during the production of particulate and dissolved organic matter by marine phytoplankton. Limnol Oceanogr 42: 506-518.
CADÈE GC, HEGEMAN J (1974) Primary production of phytoplankton in the Dutch Wadden Sea. Neth J Sea Res 8: 240-259.
CLARK LL, INGALL ED, BENNER R (1998) Marine phosphorus is selectively remineralized. Nature 393: 426.
COLE JJ, LIKENS GE STRAYER DL (1982) Photosynthetically produced dissolved organic carbon:An important carbon source for planktonic bacteria. Limnol Oceanogr 27: 1080-l 090.
DEL GIORGIO PA, DUARTE CM (2002) Respiration in the open ocean. Nature 420: 379-384.
DEL GIORGIO PA, WILLIAMS PJ LE B (2005) The global significance of respiration in aquatic ecosystems: from single cells to the biosphere. In: del Giorgio PA, Williams PJ le B (eds) Respiration in Equatic Ecosystems. Oxford University Press: 267-303.
DUARTE CM, AGUSTI S (1998) The CO2 Balance of Unproductive Aquatic Ecosystems.
Science 218: 234-236.
GÄTJE C, REISE K (1998) Ökosystem Wattenmeer. Springer Berlin Heidelberg New York.
GRASSHOFF K, ERHARDT M, KREMLING K (1983) Methods of Seawater analysis. Verlag Chemie Weinheim
GRIFFITH PC, DOUGLAS DJ, WAINRIGHT SC (1990) Metabolic activity of size-fractionated microbial plankton in estuarine, nearshore, and continental shelf waters of Georgia. Mar Ecol Prog Ser 59: 263-270.
GUILLARD RRL, WANGERSKY J (1958) The production of extracellular carbohydrates by some marine flagellates. Limnol Oceanogr 3: 449-454.
HAMM CE, ROUSSEAU V (2003) Composition,assimilation and degradation of Phaeocystis globosa-derived fatty acids in the North Sea. J Sea Res 50: 271-283.
HEIP CHR, GOOSEN NK, HERMAN PMJ, KROMKAMP J, MIDDELBURG JJ, SOETART K (1995) Production and consumption of biological particles in emperate tidal estuaries.
Ocean Mar Biol Annual Review 33: 1-149.
HOPKINSON CS, SHERR B, WIEBE WJ (1989) Size fractionated metabolism of coastal microbial plankton. Mar Ecol Prog Ser 51: 155-166.
IRIARTE A, MADARIAGA I DE, DIEZ-GARAGARZA F, REVILLA M, ORIVE E (1996) Primary plankton production, respiration and nitrification in a ahallow temperate estuarx during summer. J Exp Mar Biol Ecol 208: 127-151.
Jeffrey and Humphrey (1976).
JENSEN LM, SAND-JENSEN K, MARCHER S, HANSEN M (1990) Plankton community respiration along a nutrient gradient in a shallow Danish estuary. Mar Ecol Prog Ser 61:
75-85.
LANGDON C (1993) The significance of respiration in procution measurements based on oxygen. In: Measurements of primary production from the molecular to the global scale.
Li WKW & Maestrini SY (eds) ICES Marine Science Symposium 197: 69-78.
MOELLER EF (2005) Sloppy feeding in marine copepods: prey-size-dependent production of dissolved organic carbon. J Plankt Res 27: 27-53.
POREMBA K, TILLMANN U, HESSE KJ (1999) Tidal impact on planktonic primary and bacterial production in the German Wadden Sea. Helol Mar Res 53: 19-27.
POSTMA H (1954) Hydrography of the Dutch Wadden Sea. Archives neerlandaises de Zoologie 10: 405-511.
POSTMA H (1982) Hydrography of the Wadden Sea: Movements and properties of water and particular matter. Final report of the section ‘Hydrography’ of the Wadden Sea Working Group. Leiden, The Netherlands.
ROBINSON C, SERRET P, TILSTONE G, TEIRA E, ZUBKOV MV, REES AP, WOODWARD
EMS (2002) Plankton Respiration in the Eastern Atlantik Ocean. Deep-Sea Res I 49:
787-813.
ROBINSON C, WILLIAMS PJ LE B (2005) Respiration and ist measurement in surface marine waters. In: del Giorgio PA, Williams PJ le B (eds) Respiration in Equatic Ecosystems. Oxford University Press: 147-180.
TILLMANN U, HESSE KJ, COLIJN F (2001) Planktonic primary production in the German Wadden Sea. J Plankton Res 22: 1277-1298.
VAN BEUSEKOM JEE, BROCKMANN UH, HESSE KJ, HICKEL W, POREMBA K, TILLMANN
U (1999) The importance of sediments in the transformation and turnover of nutrients and organic matter in the Wadden Sea and German bight.
VAN BEUSEKOM JEE, DE JONGE VN (2002) Long-term changes in Wadden Sea nutrient cycles: importance of organic matter import from the North Sea. Hydrobiologia 475/
476: 185-192.
WAINRIGHT SC, HOPKINSON JR CS (1997) Effects of sedimetn resuspension on organic matter prodessing in coastal environments: a simulation model. J Mar Sys 11: 353-368.
WILLIAMS PJ LE B (1981) Microbial contribution to overall marine plankton metabolism: direct measurements of respiration. Oceanologica acta 4: 359-364.
WILLIAMS PJ LE B (1998) The balance of plankton respiration and photosynthesis in the open ocean. Nature 394: 55-57
WILLIAMS PJ LE B, DEL GIORGIO PA (2005) Respiration in aquatic ecosystems: history and background. In: del Giorgio PA, Williams PJ le B (eds) Respiration in Equatic Ecosystems. Oxford University Press: 1-17.
.