Phytoplankton, light and nutrients along a gradient of mixing depth: a field test of producer-resource theory

1. Variation in depth of the mixed surface layer of temperate lakes should affect phytoplankton dynamics because, with increasing mixing depth, average light intensity in and specific sedimentation losses out of the mixed layer both decrease. 2. Our aim was to test a recent dynamic model which relates phytoplankton biomass and the availability of production-limiting resources (light and dissolved mineral nutrients) to mixing depth and nutrient supply from external sources. 3. During summer stratification we sampled the mixed layers of 30 dimictic, phosphorus-limited, oligo- to mesotrophic, mostly non-humic lakes north of the Alps. 4. The results agree well qualitatively with model expectations. Algal concentration in the mixed layer was negatively related to mixing depth or its surrogate log-transformed lake area. Light intensity at the bottom of the mixed layer decreased whereas the concentration of available, inorganic phosphorus increased with increasing mixing depth. Across all depths, higher total phosphorus content was accompanied by higher phytoplankton biomass, lower light availability, and higher inorganic phosphorus concentration. 5. Our data match the predicted shift with increasing mixing depth from predominantly nutrient limitation towards increased light limitation of algal biomass.