Relationships between eutrophication variables: from nutrient loading to transparency

Abstract

Monitoring data obtained from 231 freshwater lakes and ponds in the Netherlands, covering the period 1980–1996, were used to analyse the relationships between (a) transparency and chlorophyll-a, and the effect of system characteristics on this relationship, (b) chlorophyll-a and nutrient concentrations, and the effect of biological variables and (c) nutrient concentrations and nutrient loading. (a) Chlorophyll-a imposes a maximum on water transparency, but deviations from this maximum can be large. Reducing chlorophyll-a, therefore, does not guarantee a sufficient improvement of transparency. Soil type and the average depth of a lake were shown to influence the relationship between chlorophyll-a and transparency. (b) The maximum ratios of both chlorophyll-a: total-P and chlorophyll-a: total-N were higher in systems dominated by filamentous cyanobacteria than in systems dominated by other algae, indicating the efficiency of the former group with respect to nutrients. In systems with an areal coverage with submersed macrophytes above 5%, concentrations of chlorophyll-a and nutrients were lower than in systems with lower coverages. The ratios between chlorophyll-a and nutrients were lower at coverages larger than 10%. This indicates both bottom-up and top-down control of algae by macrophytes. Grazing pressure by zooplankton was also found to lower the chlorophyll-a: nutrient ratios. (c) System specific linear relationships were found between the average concentrations of total-P and total-N in the incoming water and the summer mean concentration in the lake. This allows the assessment of admissible loads for individual lakes, with narrower confidence limits compared to traditional relationships based on combined data from many lakes. From the analysis, it is concluded that the chain of relationships from nutrient loading to transparency is complex, and depends on biological variables as well as system characteristics.