Coastal Eutrophication and Marine Benthic Vegetation
97
of the benthic subsystem, due to the tight and continuous
sediment-water interaction. We therefore propose the following definition: (1) new production: production based on the external input of
nitrogen; (2) old (regenerated) production: production based on pelagic
and benthic regeneration of nitrogen, i.e. internal input or loading.
The cycling index given in Table 3.3 is based on this definition, and is
defined as the ratio of uptake by primary producers and external input.
When comparing the three water systems, this cycling index shows a
strong inverse relation with the external input per unit volume.
3.3.3 Contribution of Macrophytes to Nitrogen Cycling
Figure 3.11 illustrates the differences between the three lagoons with
respect to the dominance of macrophytes and phytoplankton, by
comparing the contribution of primary producers to nitrogen cycling as
well as to the accumulation of nitrogen in their biomass.
The difference of one order of magnitude in nutrient input between
Lake Grevelingen and Lake Veere only causes quantitative and gradual
differences in nitrogen cycling and storage. Phytoplankton is the
dominant producer in both lagoons. The production by macrophytes is
even smaller than the calculated production by benthic diatoms on the
shallow sediment surface. Due to the high PIB ratio of phytoplankton
and microphyto-benthos, the accumulation of nitrogen in their biomass
is small. Only 15-20% of the winter accumulated DIN is stored in the
biomass of these producers during spring and summer. Eelgrass (Zostera
marina), the dominant macrophyte in Lake Grevelingen, is replaced by
approximately the same amount of Ulva spp. in Lake Veere, which
however represents a significantly larger nitrogen uptake due to its
nitrophily and higher PIB ratio.
The Venice Lagoon shows qualitative differences with the other two
lagoons. Ulva spp. are the dominant producers in the Venice Lagoon due
to their higher production compared to the other lagoons but also due to
the low production of phytoplankton and microphytobenthos. The
calculated total production and nitrogen uptake in the Venice Lagoon is
even smaller than in Lake Grevelingen, due to this shift in dominance
from phytoplankton to Ulva.
The most significant difference between the Venice Lagoon and the
other two lagoons is the accumulation of Ulva biomass and coinciding
storage of nitrogen. The maximum biomass accumulating in the Venice
Lagoon is almost ten times the maximum macrophyte biomass estimated
97
of the benthic subsystem, due to the tight and continuous
sediment-water interaction. We therefore propose the following definition: (1) new production: production based on the external input of
nitrogen; (2) old (regenerated) production: production based on pelagic
and benthic regeneration of nitrogen, i.e. internal input or loading.
The cycling index given in Table 3.3 is based on this definition, and is
defined as the ratio of uptake by primary producers and external input.
When comparing the three water systems, this cycling index shows a
strong inverse relation with the external input per unit volume.
3.3.3 Contribution of Macrophytes to Nitrogen Cycling
Figure 3.11 illustrates the differences between the three lagoons with
respect to the dominance of macrophytes and phytoplankton, by
comparing the contribution of primary producers to nitrogen cycling as
well as to the accumulation of nitrogen in their biomass.
The difference of one order of magnitude in nutrient input between
Lake Grevelingen and Lake Veere only causes quantitative and gradual
differences in nitrogen cycling and storage. Phytoplankton is the
dominant producer in both lagoons. The production by macrophytes is
even smaller than the calculated production by benthic diatoms on the
shallow sediment surface. Due to the high PIB ratio of phytoplankton
and microphyto-benthos, the accumulation of nitrogen in their biomass
is small. Only 15-20% of the winter accumulated DIN is stored in the
biomass of these producers during spring and summer. Eelgrass (Zostera
marina), the dominant macrophyte in Lake Grevelingen, is replaced by
approximately the same amount of Ulva spp. in Lake Veere, which
however represents a significantly larger nitrogen uptake due to its
nitrophily and higher PIB ratio.
The Venice Lagoon shows qualitative differences with the other two
lagoons. Ulva spp. are the dominant producers in the Venice Lagoon due
to their higher production compared to the other lagoons but also due to
the low production of phytoplankton and microphytobenthos. The
calculated total production and nitrogen uptake in the Venice Lagoon is
even smaller than in Lake Grevelingen, due to this shift in dominance
from phytoplankton to Ulva.
The most significant difference between the Venice Lagoon and the
other two lagoons is the accumulation of Ulva biomass and coinciding
storage of nitrogen. The maximum biomass accumulating in the Venice
Lagoon is almost ten times the maximum macrophyte biomass estimated