The ecology and taxonomy of epizoic diatoms on Cladocera
We examined cladocerans from littoral and pelagic zones of 62 north temperate marshes and lakes during four sampling periods to collect and identify epizoic diatoms and correlate their presence and prevalence to limnological characteristics. Forty-nine diatom taxa were found on 20 species of Cladocera. Epizoic diatom assemblages on littoral cladocerans were diverse and contained typical epiphytic taxa. However, a single diatom taxon, Synedra cyclopum, comprised 63\% of all epizoic diatoms and was most common on large pelagic Daphnia spp. Thirty-five percent of the study lakes had epizoic diatoms on cladocerans. These spanned a large range of trophic states and geographic regions and differed from lakes without epizoic diatoms by having cladoceran assemblages dominated by Daphnia spp. Prevalences and densities of epizoic diatoms were highest shortly after ice-out when the lakes were not stratified and temperatures were between 10 and 15°C. The seasonality of epizoic diatoms on Cladocera may result from a combination of temperature and nutrient preference of the diatoms and the temperature-dependent cladoceran molt rate. Densities averaged 119 cells per carapace in lakes with epizoic diatoms, with patterns of arrangement on the carapace and the relationship of density with prevalence indicating rapid growth of S. cyclopum on cladocerans. Such growth may represent a burden affecting cladoceran life history and behavior.