Size‐fractionated NH4+ regeneration in the pelagic environments of two mesotrophic lakes

Abstract

Seasonal variation in NH 4 + regeneration rates was examined by means of 15 N‐isotope dilution for different size fractions of organisms (<1 µ m, <20 µ m, <100 µ m, and >100 µ 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 NH 4 + regeneration (L. Kizaki, 0.011–0.261 µ mol liter −1 h −1 ; L. Biwa, 0.040–0.163 µ mol liter −1 h −1 ), suggesting that microbial food webs were primarily important in NH 4 + regeneration. Contributions of crustacean mesozooplankton to NH 4 + regeneration were generally minor (<16%), except on one occasion in early summer when crustaceans accounted for 62% of total NH 4 + regeneration in Lake Kizaki. The weight‐specific NH 4 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 NH 4 + 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 4 + regeneration may vary greatly depending on the size structure of planktonic food webs.