Coastal Eutrophication and Marine Benthic Vegetation
107
Flushing may also reduce phytoplankton biomass by exporting it out
of the water system. The same comparison of time constants can be
applied for flushing. If the water residence time is shorter or equal to the
time constant of phytoplankton growth, flushing will control phytoplankton biomass and prevent high chlorophyll concentrations.
If phytoplankton biomass is controlled by grazing andlor flushing, the
ability of phytoplankton to compete with other primary producers not
subject to grazing or flushing is affected. Grazing control andlor flushing
may thus enhance the dominance of Uiva over phytoplankton.
An effective grazing control exists in Lake Grevelingen. The calculated
annual average time constant for phytoplankton growth (the BIP ratio)
is approximately 3 days, and the benthic suspension feeders are able to
filter the whole volume of the lake in several days (Verhagen 1983). The
phytoplankton biomass remains very low due to grazing, notwithstanding the considerable DIN concentration during winter. A large
part of the winter pool of DIN is transferred and reversibly stored in the
detritus pool on the sediment surface during summer. This is done
mainly by biodeposition, thus increasing nitrogen limitation.
An East-West gradient in grazing control of phytoplankton exists in
lake Veere, resulting in high phytoplankton concentrations in the eastern
part of the lake and low concentrations in the western part. However, the
highest Uiva coverage is also found in the eastern part of the lake.
Grazing control of phytoplankton in the western part obviously does not
lead to an increase of macro algal production. This is probably due to a
restricted habitat suitability for macroalgae. The western part of Lake
Veere is more wind exposed and Ulva thalli may be torn away by
shearing forces of waves and exported into the gullies.
Because of limited information on macrozoobenthic standing stock
and species composition, i.e. the share of suspension feeders, no conclusive evidence exists with respect to grazing control in the Venice Lagoon.
A recent survey over six transects each containing an inner, central and
outer station, revealed significant macrobenthic biomass with an average
value of 200 g m- 2 wet weight (Consorzio Venezia Nuova unpub.data),
which equals approximately 8 g C m- 2 (dry wt/wet wt = 0.1 and C/DW =
0.4, approximately). A bivalve standing stock of 7 g m -2 ash-free dry
weight, averaged over four stations in the central lagoon and over five
sampling periods was measured in June 1991-September 1992 (C.J.M.
Philipp art, unpubl. data). Both data sets indicate a substantial standing
stock of benthic suspension feeders, of approximately 3 g C m -2.
Assuming an effective clearance rate of 0.10 m 3 g C- 1 day-l (Jorgensen
1990), the water recycling time by suspension feeders in the Venice
Lagoon is 5-6 days, and 3-4 days in the central area of the lagoon. The
107
Flushing may also reduce phytoplankton biomass by exporting it out
of the water system. The same comparison of time constants can be
applied for flushing. If the water residence time is shorter or equal to the
time constant of phytoplankton growth, flushing will control phytoplankton biomass and prevent high chlorophyll concentrations.
If phytoplankton biomass is controlled by grazing andlor flushing, the
ability of phytoplankton to compete with other primary producers not
subject to grazing or flushing is affected. Grazing control andlor flushing
may thus enhance the dominance of Uiva over phytoplankton.
An effective grazing control exists in Lake Grevelingen. The calculated
annual average time constant for phytoplankton growth (the BIP ratio)
is approximately 3 days, and the benthic suspension feeders are able to
filter the whole volume of the lake in several days (Verhagen 1983). The
phytoplankton biomass remains very low due to grazing, notwithstanding the considerable DIN concentration during winter. A large
part of the winter pool of DIN is transferred and reversibly stored in the
detritus pool on the sediment surface during summer. This is done
mainly by biodeposition, thus increasing nitrogen limitation.
An East-West gradient in grazing control of phytoplankton exists in
lake Veere, resulting in high phytoplankton concentrations in the eastern
part of the lake and low concentrations in the western part. However, the
highest Uiva coverage is also found in the eastern part of the lake.
Grazing control of phytoplankton in the western part obviously does not
lead to an increase of macro algal production. This is probably due to a
restricted habitat suitability for macroalgae. The western part of Lake
Veere is more wind exposed and Ulva thalli may be torn away by
shearing forces of waves and exported into the gullies.
Because of limited information on macrozoobenthic standing stock
and species composition, i.e. the share of suspension feeders, no conclusive evidence exists with respect to grazing control in the Venice Lagoon.
A recent survey over six transects each containing an inner, central and
outer station, revealed significant macrobenthic biomass with an average
value of 200 g m- 2 wet weight (Consorzio Venezia Nuova unpub.data),
which equals approximately 8 g C m- 2 (dry wt/wet wt = 0.1 and C/DW =
0.4, approximately). A bivalve standing stock of 7 g m -2 ash-free dry
weight, averaged over four stations in the central lagoon and over five
sampling periods was measured in June 1991-September 1992 (C.J.M.
Philipp art, unpubl. data). Both data sets indicate a substantial standing
stock of benthic suspension feeders, of approximately 3 g C m -2.
Assuming an effective clearance rate of 0.10 m 3 g C- 1 day-l (Jorgensen
1990), the water recycling time by suspension feeders in the Venice
Lagoon is 5-6 days, and 3-4 days in the central area of the lagoon. The