Effects of microzooplankton on the planktonic food web

Planktonic food web analyses have long focused on interactions between phytoplankton, copepods and fish larvae. However, heterotrophic dinoflagellates and ciliates, here defined as microzooplankton, are known to be efficient consumers of primary production. Moreover, they have been shown to be good quality prey items for copepods and small, first-feeding fish larvae. Known to feed also on pico- and nanoplankton, microzooplankton has a crucial role in linking the microbial loop to higher trophic levels, thus enhancing energy transfer. However, neither have the involvements of microzooplankton in trophic interactions been analyzed in a comprehensive trophic food web, nor have its effects as good quality prey been examined in the context of grazers' condition or growth in the field. In this study, fluorimetric measurements of the RNA content of female adult copepods of the species Eurytemora affinis and of RNA/DNA ratios of small larval herring (Clupea harengus L.) were performed on samples from Kiel Canal taken during spring season in 2014. The RNA/DNA ratio of larval herring served as basis for temperature-dependent larval growth rates. Besides biochemical measurements, abundance of larval herring and copepods, as well as chlorophyll a concentration and microzooplankton biomass were weekly assessed. Here it is shown that microzooplankton biomass affects the condition of female adult copepods of Eurytemora affinis. Conversely, copepod grazing on microzooplankton seems to have reduced microzooplankton biomass, indicating a top-down control. Larval herring growth does not seem to have been directly affected by the occurrence of microzooplankton, but rather by copepod nauplii and copepodite abundance. Microzooplankton seems to have been an important grazer of phytoplankton, as bloom formation was observed during lowest microzooplankton biomass. However, additional grazing on phytoplankton by copepods as well as the influence of abiotic factors might be possible. This field study was able to elucidate the importance of microzooplankton to copepod condition using a biochemical approach. Even though a direct trophic link between microzooplankton and first-feeding larval herring was not observed, an indirect connection between the two trophic levels with copepods as intermediate step is possible.