PREDICTING CHLOROPHYLL VERTICAL-DISTRIBUTION IN RESPONSE TO EPILIMNETIC NUTRIENT ENRICHMENT IN SMALL STRATIFIED LAKES

Light-limited metalimnetic phytoplankton communities are thought to be negatively impacted by epilimnetic nutrient enrichment because of shading by increased epilimnetic phytoplankton biomass. We tested this expectation with a dynamic simulation model that was calibrated to three lakes undergoing whole-lake nutrient and food web manipulations. Total areal chlorophyll increased due to nutrient enrichment in each lake, but the magnitude of the response varied between lakes. Modeling experiments, which allowed analysis of separate components of each lake's response to nutrient enrichment, indicated that the response to enrichment depended on lake water color and food web structure. In weakly stained lakes (similar to 10 mg Pt I-1, k(d) = 0.4 m (-1)), metalimnetic chlorophyll was stimulated by nutrient enrichment up to moderate levels (1 mu g P I-1 day(-1)). In more strongly colored lakes (25 mg Pt 1(-1), k(d) 1.0), metalimnetic chlorophyll responded negatively to nutrient enrichment at all P loading rates. Food web structure, as expressed by rates of zooplanktivory, interacted with water color in two ways. One impact was through direct grazing losses on metalimnetic chlorophyll. The other process involved was indirect impact from grazing on epilimnetic phytoplankton, which reduced shading on metalimnetic chlorophyll. Vertical redistribution of chlorophyll between the epilimnion and the metalimnion led to little accumulation of areal chlorophyll with increased P loading over limited ranges of water color and nutrient input rates. Model predictions may be most effectively tested with whole-lake experiments contrasting food web structure, water color and nutrient loading.