Seasonality, substrate perference and attachment sites of epizoic diatoms on cladoceran zooplankton

Abstract

Cladocerans from three lakes in northwest Iowa were examined for attached diatoms over a period of 1 year to assess the seasonality, substrate preference and attachment site specificities of epizoic diatoms. Over 90% of the cladocerans had attached diatoms immediately following ice thaw, when the lakes were not yet thermally stratified, surface water temperatures were low(10–15°C)and water transparency was at its annual high. Mean maximum density of diatoms on cladocerans averaged among lakes at this time was 3000 cells animal −1 . The dominant epizoic diatom was Synedra cyclopum Brutschy, which was found most often on large, pelagic Daphnia pulicaria and Daphnia galeata mendotae . Heavily infested animals had diatoms concentrated on the second antennae and anterior ventral margin of the carapace. Animals with <5 epizoic diatoms were more likely to have cells attached to the postabdomen or tail spine. In addition to living epizoically, S.cyclopum was found free living in the plankton, exhibiting temporal fluctuations in densities similar to epizoic counterparts However, the densities of the free-living cells seldom exceeded epizoic densities, suggesting that S.cyclopum is adapted for growth on motile cladoceran substrates.