Coastal Eutrophication and Marine Benthic Vegetation
95
P/B ratio of 1.2-1.6. Our estimate, depicted in Fig. 3.10, is based on a
lower maximum standing crop of 86 g C m -2 in the coastal area, and 50
g C m -2 averaged over the total water surface of the lagoon
(TECHNITAL, unpubl. data). The annual P/B ratio resulting from the
simulation was approximately 2.5. The difference in P/B ratio of Ulva
spp. in Lake Veere and in the Venice Lagoon is most probably caused by
the difference in viability. In Lake Veere, the total standing crop remains
viable throughout the growing season, resulting in a high turnover of the
biomass (Nienhuis pers. comm.). In the Venice Lagoon, thick mats of
Ulva spp. are formed during the growing season, with only the upper
layer remaining viable and productive. Recently, the P/B ratio of Ulva
spp. in the Venice Lagoon was found to be inversely related to the
biomass density, ranging from 1.5 to 4.5 (Sfriso et al. 1993).
The carbon budgets of the three lagoons show a remarkable similarity
with differences of less than a factor three between total as well as
individual fluxes, except for phytoplankton and Ulva in the Venice
Lagoon. This similarity contrasts with differences of one order of magnitude in nutrient input, winter nutrient concentrations and spring maximum of chlorophyll. Related to this apparent robustness of the carbon
budget, the internal cycling, i.e. the flux of carbon through the biological
cycle divided by the size of the biological components, is very fast in
Lake Grevelingen, slower in Lake Veere and very slow in the Venice
Lagoon. The consumer foodchain is dominant in Lake Grevelingen, with
more than 50% of the total net primary production being directly
consumed by zooplankton and benthic suspension feeders. This results
in a strong grazing control and rapid turnover of the phytoplankton
biomass. Direct consumption of the total net primary production in Lake
Veere is estimated at 40%. The consumer food chain in the Venice
Lagoon is not important. Phytoplankton, the only primary producer
which would be subject to substantial grazing, only accounts for 15% of
total primary production. The detrital chain is dominant in Venice
Lagoon, resulting in a slow internal cycling, in an accumulation of
organic material, and in consequent anoxia.
The overall characteristics of nitrogen cycling in the three lagoons are
illustrated by the annual nitrogen budgets in Table 3.3, which compares
external balances and internal cycling.
The external input of nitrogen into the three lagoons per unit area or
volume is different by about one order of magnitude. Lake Veere receives
the highest input per unit area, while the Venice Lagoon, due to its
smaller depth, has the highest input per unit volume.
A large amout of the available nitrogen in Lake Grevelingen is removed by denitrification and by retention in refractory detritus. This
95
P/B ratio of 1.2-1.6. Our estimate, depicted in Fig. 3.10, is based on a
lower maximum standing crop of 86 g C m -2 in the coastal area, and 50
g C m -2 averaged over the total water surface of the lagoon
(TECHNITAL, unpubl. data). The annual P/B ratio resulting from the
simulation was approximately 2.5. The difference in P/B ratio of Ulva
spp. in Lake Veere and in the Venice Lagoon is most probably caused by
the difference in viability. In Lake Veere, the total standing crop remains
viable throughout the growing season, resulting in a high turnover of the
biomass (Nienhuis pers. comm.). In the Venice Lagoon, thick mats of
Ulva spp. are formed during the growing season, with only the upper
layer remaining viable and productive. Recently, the P/B ratio of Ulva
spp. in the Venice Lagoon was found to be inversely related to the
biomass density, ranging from 1.5 to 4.5 (Sfriso et al. 1993).
The carbon budgets of the three lagoons show a remarkable similarity
with differences of less than a factor three between total as well as
individual fluxes, except for phytoplankton and Ulva in the Venice
Lagoon. This similarity contrasts with differences of one order of magnitude in nutrient input, winter nutrient concentrations and spring maximum of chlorophyll. Related to this apparent robustness of the carbon
budget, the internal cycling, i.e. the flux of carbon through the biological
cycle divided by the size of the biological components, is very fast in
Lake Grevelingen, slower in Lake Veere and very slow in the Venice
Lagoon. The consumer foodchain is dominant in Lake Grevelingen, with
more than 50% of the total net primary production being directly
consumed by zooplankton and benthic suspension feeders. This results
in a strong grazing control and rapid turnover of the phytoplankton
biomass. Direct consumption of the total net primary production in Lake
Veere is estimated at 40%. The consumer food chain in the Venice
Lagoon is not important. Phytoplankton, the only primary producer
which would be subject to substantial grazing, only accounts for 15% of
total primary production. The detrital chain is dominant in Venice
Lagoon, resulting in a slow internal cycling, in an accumulation of
organic material, and in consequent anoxia.
The overall characteristics of nitrogen cycling in the three lagoons are
illustrated by the annual nitrogen budgets in Table 3.3, which compares
external balances and internal cycling.
The external input of nitrogen into the three lagoons per unit area or
volume is different by about one order of magnitude. Lake Veere receives
the highest input per unit area, while the Venice Lagoon, due to its
smaller depth, has the highest input per unit volume.
A large amout of the available nitrogen in Lake Grevelingen is removed by denitrification and by retention in refractory detritus. This