Eutrophication -- 11.4 Autotrophic biomass as an indicator of eutrophication

In predominantly bottom-up regulated ecosystems, autotrophic biomass concentration directly reflects primary production (de facto new production). Eutrophication can thus be directly reflected in increased chlorophyll a concentrations and decreased visibility (Figure 7). However, in all ecosystems both bottom-up and top-down regulation exists and are closely entangled (see Sedimentatino chapter), but the ratio between these regulation modes varies. The angle of the dose-response relationship is 1:1 when only bottom-up regulation controls phytoplankton growth while top-down regulation (e.g. grazing, Figure 3); leakage (Figure 4) etc. lowers the slope of the relationship. The effect of eutrophication is thus reflected in the autotrophic biomass concentration as new production minus the effect of top-down regulation. Chlorophyll a, a standard parameter in environmental monitoring thus reflects the effects of eutrophication only if top-down regulation is low. In the case of the north-western Mediterranean Sea the increase in suspended autotrophic biomass due to increased nutrient input is so small that the impression arises that eutrophication is negligible (Figure 1). Autotrophic biomass is thus a convenient, but unreliable, universal measure of eutrophication (Figure 8).

carFig8

Figure: Mean spring-summer concentrations of chlorophyll-like pigments in European seas as determined from SeaWiFS satellite observations in 1999. Deep blue = 0.2 µg L^-1. Yellow = 7-12 µg L^-1; Red = 20-25 µg L^-1. Important note: The concentrations are only valid for oceanic waters and overestimate to a large and variable degree the chlorophyll concentrations in coastal seas (e.g. the central and eastern Baltic Sea). Humic substances from rivers play an important role in estuaries and brackish waters. Examples: the Baltic Sea and river mouths such as the Don, Danube, Po, Rhone and Rhine. High chlorophyll concentrations are found in the northern Adriatic Sea, around the western Iberia, the Biskaya, British Isles, the Channel, southern North Sea, Kattegat and Skagerrak.

Eutrophication is best measured as productivity (e.g. `new production' (based on limiting nutrients such as N or P) or zooplankton or benthos biomass). Currently such measurements are rarely carried out on a routine basis in eutrophication programmes.

carFig7

Figure: Long-term observation from the Himmerfj�rden estuary south of Stockholm, Sweden over a period of years in which nitrogen and phosphorous additions were experimentally altered through changes in sewage treatment. The relationship between the mean concentration of total nitrogen and the chlorophyll a in the surface layer (A) and the water clarity (B). Modified after Elmgren and Larsson [149] and Anonymous [].

What eutrophication really implies has to be discussed in a system ecological context. It may well be that the DIN input -- primary production dose-response relationship is universal (Figure 1) while top-down regulation influences the DIN input -- autotrophic biomass relationship in variable degrees in different ecosystems (Figure 2). In order to predict the effect of eutrophication we have to ensure that conventional eutrophication monitoring programmes adequately detect and quantify eutrophication.