Loder MGJ, Meunier C, Wiltshire KH, Boersma M, Aberle N
mesocosm, ciliates, helgoland roads, Germany, microzooplankton, 1 m3
Mesocosm experiments coupled with dilutiongrazing experiments were carried out during the phytoplanktonspring bloom 2009. The interactions betweenphytoplankton, microzooplankton and copepods wereinvestigated using natural plankton communities obtainedfrom Helgoland Roads (54 11.30N; 7 54.00E), North Sea.In the absence of mesozooplankton grazers, the microzooplanktonrapidly responded to different prey availabilities;this was most pronounced for ciliates such asstrombidiids and strobilids. The occurrence of ciliates wasstrongly dependent on specific prey and abrupt losses intheir relative importance with the disappearance of theirprey were observed. Thecate and athecate dinoflagellateshad a broader food spectrum and slower reaction timescompared with ciliates. In general, high microzooplanktonpotential grazing impacts with an average consumption of120% of the phytoplankton production (Pp) were measured.Thus, the decline in phytoplankton biomass could bemainly attributed to an intense grazing by microzooplankton.Copepods were less important phytoplanktongrazers consuming on average only 47% of Pp. Microzooplanktonin turn contributed a substantial part to thecopepods’ diets especially with decreasing quality ofphytoplankton food due to nutrient limitation over thecourse of the bloom. Copepod grazing rates exceeded microzooplanktongrowth, suggesting their strong top-downcontrol potential on microzooplankton in the field.Selective grazing by microzooplankton was an importantfactor for stabilising a bloom of less-preferred diatom speciesin our mesocosms with specific species (Thalassiosira spp.,Rhizosolenia spp. and Chaetoceros spp.) dominating thebloom. This study demonstrates the importance of microzooplanktongrazers for structuring and controlling phytoplanktonspring blooms in temperatewaters and the importantrole of copepods as top-downregulators of microzooplankton.