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 sha ding by increased epilimnetic phytoplankton biomass. We tested this ex pectation with a dynamic simulation model that was calibrated to three lakes undergoing whole-lake nutrient and food web manipulations. Tota l areal chlorophyll increased due to nutrient enrichment in each lake, but the magnitude of the response varied between lakes. Modeling expe riments, which allowed analysis of separate components of each lake's response to nutrient enrichment, indicated that the response to enrich ment depended on lake water color and food web structure. In weakly st ained lakes (similar to 10 mg Pt I-1, k(d) = 0.4 m (-1)), metalimnetic chlorophyll was stimulated by nutrient enrichment up to moderate leve ls (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 nutri ent enrichment at all P loading rates. Food web structure, as expresse d by rates of zooplanktivory, interacted with water color in two ways. One impact was through direct grazing losses on metalimnetic chloroph yll. The other process involved was indirect impact from grazing on ep ilimnetic phytoplankton, which reduced shading on metalimnetic chlorop hyll. Vertical redistribution of chlorophyll between the epilimnion an d the metalimnion led to little accumulation of areal chlorophyll with increased P loading over limited ranges of water color and nutrient i nput rates. Model predictions may be most effectively tested with whol e-lake experiments contrasting food web structure, water color and nut rient loading.