SIZE-FRACTIONATED NH4+ REGENERATION IN THE PELAGIC ENVIRONMENTS OF 2 MESOTROPHIC LAKES

Seasonal variation in NH4+ regeneration rates was examined by means of N-15-isotope dilution for different size fractions of organisms (< 1 mu m, < 20 mu m, < 100 mu m, and > 100 mu m) in samples from mesotrophic Lakes Kizaki and Biwa, Japan. In both lakes, nanoflagellates and microzooplankton (mainly rotifers) accounted for a large fraction (17-55 and 0-67%, respectively) of total NH4+ regeneration (L. Kizaki, 0.011-0.261 mu mol liter(-1) h(-1); L. Biwa, 0.040-0.163 mu mol liter(-1) h(-1)), suggesting that microbial food webs were primarily important In NH,+ regeneration. Contributions of crustacean mesozooplankton to NH,I regeneration were generally minor (<16%), except on one occasion in early summer when crustaceans accounted for 62% of total NH4+ regeneration in Lake Kizaki. The weight-specific NH4+ regeneration rates (sR) of nanoflagellates and rotifers-ciliates were 20-30-fold greater than the corresponding rates for crustacean mesozooplankton, consistent with the general relationship between sR and body size of grazers. Bacterial contributions to NH4+ regeneration were high (26-51%) in summer but low (<10%) in spring and winter, which was explained in part by the strong dependency of bacterial sR on temperature. Our data demonstrate that rates and major pathways of NH,I regeneration may vary greatly depending on the size structure of planktonic food webs.