Relationships between eutrophication variables: from nutrient loading to transparency

Monitoring data obtained from 231 freshwater lakes and ponds in the Netherl ands, covering the period 1980-1996, were used to analyse the relationships between (a) transparency and chlorophyll-a, and the effect of system chara cteristics on this relationship, (b) chlorophyll-a and nutrient concentrati ons, and the effect of biological variables and (c) nutrient concentrations and nutrient loading. (a) Chlorophyll-a imposes a maximum on water transpa rency, 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 relat ionship between chlorophyll-a and transparency. (b) The maximum ratios of b oth chlorophyll-a: total-P and chlorophyll-a: total-N were higher in system s dominated by filamentous cyanobacteria than in systems dominated by other algae, indicating the efficiency of the former group with respect to nutri ents. 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 l ower at coverages larger than 10%. This indicates both bottom-up and top-do wn control of algae by macrophytes. Grazing pressure by zooplankton was als o found to lower the chlorophyll-a: nutrient ratios. (c) System specific li near 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 base d on combined data from many lakes. From the analysis, it is concluded that the chain of relationships from nutrient loading to transparency is comple x, and depends on biological variables as well as system characteristics.