MEPS 324:105-112 (2006)  -  doi:10.3354/meps324105

ABSTRACT: In a mesocosm experiment in Kiel Fjord, the plankton community <250 µm 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 δ¹⁵N of particulate organic matter was negatively correlated with N. spumigena concentrations, indicating the fixation of isotopically ‘light’, diazotrophic nitrogen. In all treatments, final copepod δ¹⁵N were lower (–0.7 to –2.7‰) than initial copepod δ¹⁵N, 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 δ¹⁵N 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 δ¹⁵N decreased with decreasing mesozooplankton densities, since only a quantitatively limited dietary source may be expected to result in density-dependent changes in copepod δ¹⁵N. 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.

KEY WORDS: Diazotrophic cyanobacteria · Nitrogen · Nodularia spumigena · Stable isotopes