Nutrient Dynamics in European Water Systems
Synthesis Results

4. Effects of altered nutrient discharges on the functioning of coastal and estuarine food webs (2 of 3)

Monitoring on the progression and fate of phytoplankton blooms in fjords of northern Norway by Archer et al. (2000) suggested that the microzooplankton grazing on single cells and small colonies of Phaeocystis was high enough to decrease the rate of formation of larger colonial phases.

Escaravage & Prins (2002) show from both mesocosm experiments (PHASE project, Oosterschelde) and field operations from the literature (Marsdiep) that there is a rather constant ratio between the heights of consecutive Phaeocystis and ciliates peaks (Figure 4.1(c)). Systematic observation of ciliate's grazing cells from within disrupted colonies of decaying Phaeocystis blooms supported the assumption that a significant fraction of the bloom may be used within the water column.

These examples stress the need for ecological models that explicitly describe the bottom-up and top-down controls of the pelagic food chain for an adequate representation of the fate of the nutrients brought into the water column.

Lancelot et al. (2002) attempted to describe these bottom-up and top-down controls with BIOGEN (EROS-21 project), a high trophic resolution ecological model that was applied to the Black Sea system. The model was used to hindcast the changes in the Black Sea shelf ecosystem over the past decades. It demonstrates in a dramatic way the importance of nutrient ratios in the input waters for the functioning and structure of the ecosystem.

The BIOGEN simulations in the open Black Sea indicate that the surface layer planktonic system is driven by winter phosphate availability, which determines the magnitude and strength of early spring diatom bloom.

BIOGEN predictions clearly illustrate that limiting nutrients determines the structure of the phytoplankton community, which in turn constrains the structure and functioning of the planktonic food web. Well balanced Nitrogen:Phosphorus:Slica enrichment, as observed for example in 1991, has a positive effect on the diatom-copepod food chain, while the regenerated-based microbial food chain remains at its basic level.

Furthermore, model simulations suggest that under conditions of well-balanced nutrient enrichment , a positive link between fishing pressures and gelatinous carnivores can be established: overfishing, in addition to eutrophication, could have played a role in destabilisation of the Black Sea ecosystem reported for years 1989-1991 (Gucu, 2002).

Model scenarios of changing Danube nutrient inputs to the north western Black Sea observed over the 1985-1995 period (Figure 4.1(d)) show the mechanistic BIOGEN model, based on food chain structure and physiological concepts has the required trophic resolution to address the ecological changes evident in the Black Sea since the 1960s.