RELATIONSHIPS BETWEEN CLADOCERAN SIZE STRUCTURE AND HYDROGEOMORPHIC AND TROPHIC FEATURES OF MAINE LAKES

Due to their position in the middle of aquatic food chains, cladocerans are influenced by both top-down (predation) and bottom-up (nutrient) pressures. These two trophic pressures shape the size structure of the cladoceran community. In addition, hydrogeomorphic (HGM) features such as depth have also been found to influence size structure. In order to study the relationships between cladoceran size structure and lake trophic and HGM features, we looked at the size structure of cladoceran communities in 76 Maine lakes. These lakes were chosen to represent a range of depth, surface area, and location in the state. The cladoceran samples and water chemistry data came from the Maine Department of Environmental Protection. We measured the body length of adult cladocerans and calculated size spectrum metrics such as quartiles, size frequency and the Stemberger Index. The cladoceran body size spectra for our lakes varied both in size (median from 0.25 to 1.30mm and 75^th quartile from 0.25 to 1.63mm) and in inter-quartile range (0.02 to 0.51mm). Size metrics were produced using non-metric multidimensional scaling (NMDS) on three sets of measures from each lake: (1) the 10th, 25th, 50th, 75th and 90th quartiles of adult cladocerans, (2) the quartiles of Daphnia and bosminid body length, and (3) size frequency within 0.2mm interval bins. We calculated correlations between the resultant NMDS axes and HGM features (depth, elevation, and pH) and trophic status (TP and chlorophyll). The only significant correlations between body size NMDS axes and the lake features were for elevation and depth (p<0.05). We will further investigate the effects of HGM and trophic features (including invertebrate predator density and fish community characteristics) through multiple linear regression. Our results will be used to develop a tool that allows lake managers to use cladocerans as an indicator of how food web configuration influences lake trophic status of Maine lakes.