Cladoceran body length and Secchi disk transparency in northeastern U.S. lakes

Abstract

Mean cladoceran body length of 59 northeastern U.S. lakes was estimated from functional groups that broadly define taxonomic, body size, and grazing potential. Multiple regression of body length, color, and chlorophyll a or total phosphorus against Secchi disk transparency explained 72% and 83% of the variation across lakes, respectively. Analysis that included body length, color, and particulate carbon, a proxy for light backscattering and absorption by suspended organic particles, explained 85% of the variance in transparency. Body length was as important a predictor of water clarity as chemical factors. Furthermore, body length was significantly correlated to temporal variation in transparency within lakes. Because cladocerans primarily filter organic particles in size ranges having high light attenuation efficiencies, body length was consistently more strongly correlated to transparency than to chlorophyll a. Monitoring cladoceran body length should help to distinguish changes in lake transparency due to nutrient loading from changes that reflect fish population size structure and predation intensity on zooplankton. This simple size index can greatly increase the interpretative value of Secchi transparency observations to lake managers.