Transfer of diazotrophic nitrogen to mesozooplankton in Kiel Fjord, Western Baltic Sea: a mesocosm study

In a mesocosm experiment in Kiel Fjord, the plankton community < 250 m was exposed to a mesozooplankton density gradient (5 to 80 individuals l -1 ) dominated by the calanoid copepod Acartia clausi. Over the experimental period (9 d), the diazotrophic cyanobacterium Nodularia spumigena increased exponentially, irrespective of mesozooplankton densities, attaining maximum concentrations of 1200 cells ml -1 (3700 filaments l -1 ). At the end of the experiment, the 15 N of particulate organic matter was negatively correlated with N. spumigena concentrations, indicating the fixation of isotopically 'light', diazotrophic nitrogen. In all treatments, final copepod 15 N were lower (-0.7 to -2.7 ) than initial copepod 15 N, indicating the transfer of diazotrophic nitrogen to mesozooplankton. Based on a simple isotopic mixing model, diazotrophic nitrogen was calculated to contribute 45 to 6% to final copepod 15 N along the mesozooplankton gradient. This translates to a transfer of 2 to 24% of net nitrogen fixation to the mesozooplankton standing stock. The absence of any mesozooplankton impact on N. spumigena, and the negative impact found for other microplankton, including diatoms and ciliates, suggest that diazotrophically fixed nitrogen reached mesozooplankton indirectly through trophic vectors. This is consistent with the fact that copepod 15 N decreased with decreasing mesozooplankton densities, since only a quantitatively limited dietary source may be expected to result in density-dependent changes in copepod 15 N. Considering that natural mesozooplankton densities in the Baltic Sea rarely exceed 10 ind. l -1 , the contribution of diazotrophically fixed nitrogen to mesozooplankton may be substantial (23 to 45%) during summer blooms of diazotrophic cyanobacteria.