212
P.R. Nienhuis
values for Nand Si, but depletion occurs during the growing season (de
Vries et al. 1988b). Westerschelde and Ems-Dollard have the highest
nutrient concentrations, never approaching zero during spring and
summer and reaching high values during winter (8mgl- I N, 4mgl- I p,
and 9 mgl- l Si, Nienhuis 1992).
The total nitrogen loadings to Dutch coastal waters vary by two orders
of magnitude (4-235gNm- 2 year-I). Mean N concentrations show a
much smaller range of 0.5 - 4.6 mg I-I. Obviously nitrogen loads of 40200 g N m -2 year- I do not give rise to extremely high chlorophyll
concentrations during summer in the turbid Dutch estuaries and in the
coastal waters. In such places light availability rather than nutrient
supply may limit net primary production. In the clear waters of Veerse
Meer, a load of 34gNm- 2 year- I results in high chlorophyll concentrations (100 mg chI a m -3 or even higher; according to de Vries et al.
1988b). Production of phytoplankton biomass in Veerse Meer, and
consequently deposition of particulate organic carbon on the bottom
sediments, was large enough during 1980-1983 to increase the area of
anaerobic sediment from 4 to 25% of the bottom surface. Veerse Meer
Lagoon is vulnerable to eutrophication because of the long residence
time of the water mass, the low extinction coefficient, and the almost
permanent stratification (Nienhuis 1992).
In Fig.7.11 a tentative carbon budget of the main categories of
primary producers in all Dutch estuaries is given. It has to be realized
that the "pies" depict average, annual, integrated data, useful for reasons
of comparison, but do not reflect temporal and spatial variability.
Phytoplankton are the dominant primary producers in Dutch estuaries,
contributing 45-7l% to the overall annual budget of organic material.
Notwithstanding the large differences in nutrient loadings of the separate
waters, primary production of phytoplankton only shows a 2A-fold
difference between the lightlimited, turbid Ems-Dollard (100 gCm- 2
yeac l and Westerschelde (125 g C m -2 year-I), and the clear,
presumably not nutrient-limited Veerse Meer (240gCm- 2 year-I).
Production levels in Wadden Sea, Oosterschelde, and Grevelingen
are intermediate. Production of microphytobenthos (benthic diatoms,
green algae, etc.) is roughly 30-70% of the production of phytoplankton.
High turbidity and exposure to waves and tides prevent the potential
sediment habitats in Westerschelde and Ems-Dollard from being invaded
by macrophytes. Oosterschelde and Western Wadden Sea have only local
growth of macrophytes on sediment substrates in sheltered regions
(Zostera marina, Z. noltii, Enteromorpha prolifera, Ulva species). In the
brackish lagoons Grevelingen and Veerse Meer, macro algae (mainly
P.R. Nienhuis
values for Nand Si, but depletion occurs during the growing season (de
Vries et al. 1988b). Westerschelde and Ems-Dollard have the highest
nutrient concentrations, never approaching zero during spring and
summer and reaching high values during winter (8mgl- I N, 4mgl- I p,
and 9 mgl- l Si, Nienhuis 1992).
The total nitrogen loadings to Dutch coastal waters vary by two orders
of magnitude (4-235gNm- 2 year-I). Mean N concentrations show a
much smaller range of 0.5 - 4.6 mg I-I. Obviously nitrogen loads of 40200 g N m -2 year- I do not give rise to extremely high chlorophyll
concentrations during summer in the turbid Dutch estuaries and in the
coastal waters. In such places light availability rather than nutrient
supply may limit net primary production. In the clear waters of Veerse
Meer, a load of 34gNm- 2 year- I results in high chlorophyll concentrations (100 mg chI a m -3 or even higher; according to de Vries et al.
1988b). Production of phytoplankton biomass in Veerse Meer, and
consequently deposition of particulate organic carbon on the bottom
sediments, was large enough during 1980-1983 to increase the area of
anaerobic sediment from 4 to 25% of the bottom surface. Veerse Meer
Lagoon is vulnerable to eutrophication because of the long residence
time of the water mass, the low extinction coefficient, and the almost
permanent stratification (Nienhuis 1992).
In Fig.7.11 a tentative carbon budget of the main categories of
primary producers in all Dutch estuaries is given. It has to be realized
that the "pies" depict average, annual, integrated data, useful for reasons
of comparison, but do not reflect temporal and spatial variability.
Phytoplankton are the dominant primary producers in Dutch estuaries,
contributing 45-7l% to the overall annual budget of organic material.
Notwithstanding the large differences in nutrient loadings of the separate
waters, primary production of phytoplankton only shows a 2A-fold
difference between the lightlimited, turbid Ems-Dollard (100 gCm- 2
yeac l and Westerschelde (125 g C m -2 year-I), and the clear,
presumably not nutrient-limited Veerse Meer (240gCm- 2 year-I).
Production levels in Wadden Sea, Oosterschelde, and Grevelingen
are intermediate. Production of microphytobenthos (benthic diatoms,
green algae, etc.) is roughly 30-70% of the production of phytoplankton.
High turbidity and exposure to waves and tides prevent the potential
sediment habitats in Westerschelde and Ems-Dollard from being invaded
by macrophytes. Oosterschelde and Western Wadden Sea have only local
growth of macrophytes on sediment substrates in sheltered regions
(Zostera marina, Z. noltii, Enteromorpha prolifera, Ulva species). In the
brackish lagoons Grevelingen and Veerse Meer, macro algae (mainly