Coastal Eutrophication and Marine Benthic Vegetation
99
3.4 Discussion
Comparison of the three lagoons reveals the following main findings:
1. Rubustness of Overall Carbon Budgets and Nitrogen Cycling. Differences of one order of magnitude in nitrogen input by area and by
volume leads to or is accompanied by, differences of less than a factor
three between total as well as individual fluxes of carbon and nitrogen
through the biological components, with only a few exceptions. The
cycling index or ratio of old versus new production is inversely
related to the nutrient input.
2. Differences in Dominance Between Primary Producers at Comparable
High Levels of Nitrogen Input. The high input of nitrogen into Lake
Veere coincides with a sustained dominance of phytoplankton over
macroalgae, whereas a comparable input of nitrogen into the Venice
Lagoon (a smaller input by area but a larger input by volume)
coincides with a dominance of Ulva spp. over phytoplankton in terms
of production and nitrogen uptake.
3. Differences in Biomass Accumulation and P*B Ratio of Ulva spp. in
Different Lagoons. The storage of nitrogen in accumulating Ulva spp.
biomass in the Venice Lagoon is a significant fraction (> 25%) of the
annual nitrogen input, and is more than five times larger than the
accumulated DIN in the water column in winter. The storage of
nitrogen in Ulva spp. biomass in Lake Veere is insignificant when
compared to the input (about 2%) as well as to the DIN pool in the
water column during winter (less than 5%). Concomitantly, the P/B
ratio of Ulva in the Venice Lagoon is much lower (at least a factor 4)
than the potential P/B ratio of a viable population.
These findings illustrate that eutrophication symptoms such as dominance and mass accumulation of macro algae, specifically Ulva spp. are
not unequivocally related to the causes of eutrophication viz. high
nutrient loadings and resulting concentrations of nutrients. Other factors
must play a decisive role in determining the specific eutrophication
symptoms which emerge under generally eutrophic conditions. The following factors are identified and discussed: (1) light climate, (2) storage
capacity for nutrients, (3) nutrient supply to macroalgae by flushing and
(4) control of phytoplankton by grazing and flushing.
99
3.4 Discussion
Comparison of the three lagoons reveals the following main findings:
1. Rubustness of Overall Carbon Budgets and Nitrogen Cycling. Differences of one order of magnitude in nitrogen input by area and by
volume leads to or is accompanied by, differences of less than a factor
three between total as well as individual fluxes of carbon and nitrogen
through the biological components, with only a few exceptions. The
cycling index or ratio of old versus new production is inversely
related to the nutrient input.
2. Differences in Dominance Between Primary Producers at Comparable
High Levels of Nitrogen Input. The high input of nitrogen into Lake
Veere coincides with a sustained dominance of phytoplankton over
macroalgae, whereas a comparable input of nitrogen into the Venice
Lagoon (a smaller input by area but a larger input by volume)
coincides with a dominance of Ulva spp. over phytoplankton in terms
of production and nitrogen uptake.
3. Differences in Biomass Accumulation and P*B Ratio of Ulva spp. in
Different Lagoons. The storage of nitrogen in accumulating Ulva spp.
biomass in the Venice Lagoon is a significant fraction (> 25%) of the
annual nitrogen input, and is more than five times larger than the
accumulated DIN in the water column in winter. The storage of
nitrogen in Ulva spp. biomass in Lake Veere is insignificant when
compared to the input (about 2%) as well as to the DIN pool in the
water column during winter (less than 5%). Concomitantly, the P/B
ratio of Ulva in the Venice Lagoon is much lower (at least a factor 4)
than the potential P/B ratio of a viable population.
These findings illustrate that eutrophication symptoms such as dominance and mass accumulation of macro algae, specifically Ulva spp. are
not unequivocally related to the causes of eutrophication viz. high
nutrient loadings and resulting concentrations of nutrients. Other factors
must play a decisive role in determining the specific eutrophication
symptoms which emerge under generally eutrophic conditions. The following factors are identified and discussed: (1) light climate, (2) storage
capacity for nutrients, (3) nutrient supply to macroalgae by flushing and
(4) control of phytoplankton by grazing and flushing.