0 5 10 15 20
1
0 250 500 750 1000
8
0 10 20 30
12 0
100 200 300 400
3
0 500 1000 1500
4
0 10 20 30 40 50
2 0
500 1000 1500 2000
5
0 25 50
11 0
100 200
10
0 1000 2000 3000
6
0 250 500 750 1000
7
0 100 200 300 400
9
SPRING SUMMER
AUTUMN/ WINTER
Figure 11. Comparison of primary production (mg C m-2 d-1) in the two periods. Numbers on the x-axis indicate the month (e.g. 3= March).
BLUE BAR (LEFT): monthly means of primary production from 1984, 85, 87, 90 (n=5)
RED BAR (RIGHT): monthly means from 2000-2004 (n= 5). Standard deviations are indicated. No significant differences were obtained.
primary production (mg C m-2 d-1 )
Discussion
Primary production in the List tidal basin
The annual primary production of 212 g C m-2 y-1 in 2004 was in the same range of primary production estimates in other areas of the Wadden Sea and in the North Sea:
TILLMANN ET AL. (2000) estimated primary production in the Meldorfer Bucht by the
14C-technique in 1995/ 96 to be 124 and 176 g C m-2 y-1, respectively. The light attenuation coefficient in the Meldorfer Bucht ranged between > 7- 0.8 m-2, indicating the water column of the Meldorfer Bucht to be much more turbid than the List tidal basin, where the light attenuation coefficient was between 0.3 – 3.7 m-2 (this study). For the Marsdiep area in the western Wadden Sea primary productivity decreased from >
400 g C m-2 y-1 in 1994 to 250 g C m-2 y-1 in 2000. The Marsdiep turbidity increased from a Secchi –depth of 1.2 m in 1970 to a Secchi-depth of 1.0 m in 2000 (assumed to be caused by an increased cockle fishery) (CADÉE & HEGEMAN 2002). This corresponds to an increase of the attenuation coefficient (a) from 5 m-2 to 6 m-2. Both, the decrease of productivity and the increase of turbidity in the Marsdiep are in contrast to the findings of our study as discussed below. Moreover, the relation between suspended matter and light attenuation found within this study was similar to that found by COLIJN
(1982) in the Ems-Dollard-Estuary. In the Westerschelde KROMKAMP & PEENE (1995) estimated the pelagic primary production in 1991 in a range between 100 - 300 g C m-2 y-1 dependent on local variations between the western and the eastern part of the Westerschelde. For the German Bight JOINT & POMROY (1993) estimated a primary production of 261 g C m-2 y-1 in 1988-1989, using the 14C-technique.
Primary Production from 1984 until 2004
Results of this study indicate that the annual pelagic primary production has not significantly changed since the mid 1980’s: annual production between 1984-1989 was 167-301 g C m-2 y-1 and between 2000 -2004 the annual production was 181- 291 g C m
-2 y-1. The observed interannual variability of annual primary production reached 50%.
This range of interannual variability has also been observed in other coastal area (e.g.
Chesapeake Bay & Delaware Estuary, HEIP ET AL. 1995). Primary production remaining on a high level since the mid 1980’s is likely to be a result of increasing water column irradiance due to decreasing suspended matter concentrations and lower nutrient loads
by the river Rhine and Elbe. On an annual scale, less nitrogen could be counterbalanced by a better water column light field: Declining eutrophication would show at least two effects on productivity in the Wadden Sea as also stated by COLIJN & VAN BEUSEKOM
(2005): (1) Less nitrogen reduces primary production mainly during summer and (2) reduced suspended matter concentrations enhance primary productivity within the period from September until February.
Light is a limiting factor (Liebig’s law) of primary production in the Wadden Sea, except for summer, when nitrogen limitation prevails (COLIJN & CADÉE 2003). Mainly in spring and autumn high suspended matter concentrations and low surface irradiance keep water column irradiance below the saturation irradiance Ek. At irradiances below Ek productivity responds linear to increasing irradiance (SAKSHAUG ET AL. 1997).
Approximately 14% higher productivity could be reached in the period 2000-2004 due to lower suspended matter concentrations. This potential increase of annual productivity could statistically not be assessed during our study, since the interannual variability reached up to 50%. Decreased suspended matter concentrations since the mid 1980’s could on long term scale be a result of reduced organic matter import from the North Sea, since riverine nitrogen loads into the Wadden Sea slowly decreased since the mid 1980 (VAN BEUSEKOM ET AL. 2005). VAN BEUSEKOM & DE JONGE (2002) suggest that the import of organic matter from the North Sea depends on riverine nitrogen loads, - on short term scale suspended matter concentrations are strongly affected by wind and wave forcing.
A comparison of monthly production in the 1980’s with those observed in 2000-2004 (Fig. 11) supports these considerations: During summer primary production was slightly higher in the period 1985 until 1990 than between 2000 and 2004. In January and February and from September until December primary production was higher in the period from 2000 until 2004. Moreover, high summer productivities in 1987, 1988 and 1990 are in accordance with high nutrient loads by the river Rhine in these years as shown in Table 2.
Together, these results indicate decreased summer productivity due to decreased riverine nutrient loads and an increased productivity from September until February since the mid 1980’s, which was caused by better light conditions due to reduced suspended matter concentrations.
In spring the pelagic productivity of the List tidal basin was highly variable, March and April showed the highest standard deviations. MARTENS (2001) showed, the formation of spring blooms in the List tidal basin was negatively related to the respective winter temperature.
Table 2. Comparison of annual primary production in the Sylt- Bight and total nitrogen load of the river Rhine. Data on nitrogen loads from van Beusekom et al (2005), after LENHART &
PÄTSCH (2001). *) data from ASMUS ET AL. 1998.
Year Annual PP (g C m2 y-1
Total nitrogen load of the river Rhine
(103 tons y-1)
1980 50* 580
1984 167 590
1985 198 470
1987 220 670
1988 301 610
1990 167 350
1996 160* 240
2000 291 350
2001 181 370
2002 187 400
2003 204 -
2004 212 -
Critical evaluations on calculations of primary production in former years.
Only little information on P/I-parameters (Pmax, alpha, Ek) is available for the northern Wadden Sea. Primary production measurements by ASMUS ET AL. (1998) in the List tidal basin by in-situ incubations, which integrate changing irradiance over time as well as changing irradiance over water depth due to tidal impact and thus do not allow for assessing irradiance specific primary production rates. In a study on pelagic primary production in the Meldorfer Bucht, TILLMANN ET AL. (2000) showed P/I-parameters for a two-year period in 1995/ 96. In their study, the 14C-technique was used, but nevertheless P/I-parameters were in a similar range as found in this study for the List tidal basin using O2-technique. In summer, some measurements of the present study show higher values of Pmax, and alpha than found in the Meldorfer Bucht. Different species composition between years as well as different photo acclimations of the
phytoplankton community on a modified light field and the ambient temperature result in short-term modified P/I-parameters and productivity (COTÉ & PLATT 1983, MACEDO ET AL. 2002). Especially Phaeocystis globosa is known to have higher Pmax values than diatoms (VERITY ET AL. 1991). Particularly in terms of the large Phaeocystis bloom during six weeks in 2004 and the application of its P/I-parameters to former years, this proceedure may have caused rather high production rates. Ek was in the same range throughout the year in our study as found for the Meldorfer Bucht.
This similar range of P/I-parameters in the List tidal basin in 2004 and in the Meldorfer Bucht ten years ago gives evidence, that combining chlorophyll a - and suspended matter concentrations of former years with P/I-parameters of 2004 may be a feasible way to estimate the productivity in former years, if no further information is available.
Integrating primary production measurements into monitoring programmes would improve studies on the effect of de-eutrophication in the Wadden Sea.
References
AARUP T (2002). Transparency of the North Sea and Balsic Sea – a Secchi depth data mining study. Oceanologia 44: 323-337.
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) Ökosystem Wattenmeer.
Springer Berlin Heidelberg New York
BEUKEMA JJ, CADÉE GC, DEKKER R (2002) Zoobenthic biomass limited by phytoplankton abundance: evidence from parallel changes in two long-term data series in the Wadden Sea. J Sea Res 48: 111-125.
CADÉE GC, HEGEMAN J (1974) Primary production of phytoplankton in the Dutch Wadden Sea. Neth J Sea Res 8: 240-259.
CADÉE GC, HEGEMAN J (1993) Phytoplankton in the Marsdiep at the end of the 20th century; 30 years monitoring biomass, primary production, and Phaeocystis blooms. J Sea Res 48: 97 -110.
COLIJN F (1982) Light absorption in the waters of the Ems-Dollard Estuary and its consequences for the growth of phytoplankton and microphytobenthos. Neth J Sea Res 15: 196-216.
COLIJN F, CADÉE GC (2003) Is phytoplankton growth in the Wadden Sea light or nitrogen limited? J Sea Res 49: 83-93.
COLIJN F, VAN BEUSEKOM JEE (2005, in prep) Effect of eutrophication on phytoplankton productivity and growth in the Wadden Sea. In: Wilson JG (ed) The Intertidaal Ecosystem: The Value of Ireland’s Shores: 55-66. Dublin: Royal Irish Academy.
COTÈ B, PLATT T (1983) Day-to-day variations in the spring-summer photosynthetic parameters of coastal marine phytoplankton. Limnol Oceanogr 28: 320-344.
DE JONGE VN VAN BEUSEKOM JEE (1995) Wind- and tide-induced resuspension of sediment and microphytobenthos from tidal flats in the Ems Estuary. Limnol Oceanog 766-778.
DE JONGE VN, BAKKER JF, VAN STRALEN MR (1996) Possible change in the contribution of the river Rhine and the North Sea to the eutrophic status of the western Dutch Wadden Sea. Neth J Aquat Ecol 30: 27-39.
DE VRIES L, DUIN RNM, PEETERS JCH, LOS FJ, BOKHORST M, LAANE WPM (1998)