Spring isothermal mixing in the Great Lakes: evidence of nutrient limitation and nutrient-light interactions in a suboptimal light environment

During the spring isothermal mixing period (April-May) in 1993-1995, photos ynthesis-irradiance and growth-irradiance experiments were conducted in Lak es Erie, Huron, Michigan, and Ontario to assess light limitation. Additiona lly, nutrient enrichment experiments were conducted in Lake Ontario. Result s from the photosynthesis-irradiance experiments suggested that phytoplankt on communities in all the lakes can be either light limited or light satura ted, as the threshold parameter (I-k) was similar to mean water column irra diances (mean (I) over bar(wc), ratio = 1.0). Growth-irradiance experiments also suggested the potential for light saturation; mean daily irradiance e xceeded the threshold growth irradiance (I-k,I-g) in 95% of cases. Growth r ates became light saturated at lower irradiances than photosynthetic rates. Evidence for a nutrient-light interaction in controlling in situ growth ra tes was also found in the nutrient enrichment experiments at incubation irr adiances greater than or equal to (I) over bar(wc). Our results suggest tha t an interaction between nutrients and light is often controlling phytoplan kton growth during spring mixing in the Great Lakes. The role of these nutr ient-light interactions has increased in the past decade due to increased l ight availability in the lower lakes caused by phosphorus load reductions a nd the filtering activities of nonindigenous mussels.