Interannual variability of phytoplankton productivity and related parameters in Lake Constance: no response to decreased phosphorus loading?

In meso-eutrophic Lake Constance (Germany-Austria-Switzerland), phytoplankton bioraass, pigments and water transparency, as well as primary productivity, have been followed between 1980 and 1989. During this period, municipal phosphorus loading declined significantly. Since 1981, soluble reactive phosphorus (SRP) concentrations during deep lake mixing have decreased from ∼3.0 to currently 1 6 mmol m −3 at a rate of 7% year −1 . Nitrate concentrations, by contrast, continued to rise. During the period of maximum phosphorus loading, flushing through the outlet and sedimentation were about equally important sinks of phosphorus from the euphotic zone. Recently, however, sedimentation and subsequent burial of P in the bottom deposits contributed about three-quarters to the overall P-losses from the system Main reasons for this shift are unchanged settling fluxes of phosphorus out of the euphotic zone and decreasing concentrations of total phosphorus in the water. Only during spring, do concentrations of soluble reactive phosphorus within the euphotic zone decrease in proportion to the formation of particulate organic matter. Later during the season, euphotic SRP concentrations continue to be low but are no longer matched by high plankton biomass because phosphorus is efficiently removed by settling of particles In spite of the observed dramatic decrease in phosphorus loading since 1980, chlorophyll concentrations and water transparency, as well as annual phytoplankton productivity (∼300 g C m −2 ), have not shown a consistent downward trend. However, the intensity of phosphorus regeneration within the euphoric zone, which can be used as a measure of the degree of nutrient limitation, is likely to have increased significantly The most probable explanation for the insensitivity of important trophic state indicators to reduced nutrient loading is that, in Lake Constance, biomass accumulation to a greater extent is controlled by losses, mainly grazing by zooplankton and sedimentation, than by primary resources. This is concluded from the observation that phytoplankton biomass always falls far short of the nutrient-dependent carrying capacity of the system.