Trophic State of Clear and Colored, Soft- and Hardwater Lakes with Special Consideration of Nutrients, Anoxia, Phytoplankton and Fish

Abstract

Idiosyncrasies, erroneous assumptions and gaps are still hampering lake evaluations, despite several decades of defining the trophic state of lakes. I reevaluated nutrient and algal biomass limits that group lakes into oligo-, meso-, eu- and hyper eutrophic lakes at transition concentrations of 10, 30 and 100 μg · L−1 average total phosphorus concentration of the summer epilimnion. I investigated the direct and indirect effects of general lake characteristics like morphometry and geochemistry (water hardness and color) on lake trophic state and formulated models to deal with their significant influences. Nutrient and biomass concentrations as well as quantitative measures of anoxia were used to determine the trophic state. I demonstrated that the number of coldwater fish species may be used to classify the lower trophic levels in geographically constraint areas. Limits and models were established with data from well-studied lakes and verified with literature data from worldwide lakes. Trophic state limits were set for oligo-, meso-, eu- and hypereutrophic lakes at transition concentrations for average total nitrogen of 350, 650 and 1,200 μg·L−1, for summer chlorophyll of 3.5, 9 and 25 μg·L−1, for summer transparency (Secchi disk depth) of 4, 2 and 1 m, for die anoxic factor of 20, 40 and 60 d·summer−1, and for the areal oxygen depletion rate of 250, 400 and 550 mg·m−2·d−1.