Eutrophication -- 11.2 Comparative analysis of dose-response relationships

Comparative analyses, both of existing data sources and concerted experiments, can be used to establish threshold nutrient levels, test for possible regional differences in these thresholds, and identify the processes responsible for such differences. Coordinated mesocosm experiments in the Norwegian Sea, the Baltic Sea and the North West Mediterranean Sea by the EU project COMWEB (Comparative analysis of food webs based on flow networks) revealed major differences in dose-response patterns (see also Mariculture chapter). Mediterranean communities demonstrate significantly lower biomass yield per nutrient input compared to those from the Baltic and Norwegian Sea (Figure 2). These results suggest much lower critical nutrient loadings to develop high algal biomass and possible eutrophication problems in Atlantic/Baltic compared to Mediterranean waters. The results suggest that local nutrient emissions to the Mediterranean Sea could possibly be far higher compared to Norwegian or Baltic Sea before unfavourable chlorophyll a concentrations would occur. The results appear surprising and contra-intuitive.

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Figure: The relationship between the average phytoplankton biomass developed in coordinated mesocosm experiments conducted in different regions in relation to nitrogen inputs []. Filled circles: Mediterranean Sea. Open circles: Baltic Sea. Open squares: Norwegian Sea. The arrow indicates the effect of increased top-down regulation on autotrophic biomass.

One of the explanations that are offered to account for the difference is the functioning of the food webs in the three investigated regions. Grazing rates by metazoans (Figure 3) and DOC release rates by autotrophs (Figure 4) were, on average, 3-fold higher in the Mediterranean compared to the Baltic and Norwegian communities for a similar primary production. Hence, the accumulation of autotrophic biomass is far smaller, for a given nutrient input, in the Mediterranean than in the Atlantic, while metazoan production is higher. Also, more autotrophic biomass per primary production is available for other process es then grazing in the Baltic and Norwegian Sea, e.g. vertical export. This implies that in the latter seas eutrophication can result in higher supply of organic matter to the benthos, adding to low oxinc and anoxic bottom water conditions.

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Figure: The relationship between the average ingestion rate by mesozooplankton grazers and the primary production in coordinated mesocosm experiments conducted in different regions []. The 1:1 line indicates that all primary production is grazed by zooplankton. The arrow indicates the effect of decreasing top-down regulation by mesozooplankton.

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Figure: The relationship between the average dissolved organic carbon (DOC) release by the food web and the primary production in coordinated mesocosm experiments conducted in different regions [].

Evaluating the production of phytoplankton biomass from a strictly bottom-up perspective is obvious inadequate to predict the effect of eutrophication (Figure 2). As discussed in the chapters by T. Smayda,P. Wassmann and O. Ranguenau, top-down regulation is an indispensable to evaluate the effects of variable phytoplankton growth. Food-web analysis is most often not part of eutrophication and HABs studies. Considerable gaps in our knowledge result in that scientific advice regarding the eutrophication of coastal areas cannot be based on sufficiently solid and robust knowledge at present.