| Summary: | Zooplankton species are key players in pelagic marine ecosystems and link primary production to higher trophic levels. During a six-week mesocosm study conducted in the Raunefjord (Bergen, Norway) in May/June 2011 we investigated how the zooplankton community responds to ocean acidification. Nine mesocosms of 25 m length enclosing approx. 80 m3 of fjord water were enriched with eight different CO2 concentrations (ca 280 (x2), 390, 560, 840, 1120, 1400, 2000, 3000 atm). Temperature and chlorophyll a content in the mesocosms were measured daily. On day 14 nutrients were added to induce a phytoplankton bloom. Once a week zooplankton samples from each mesocosm were taken with an Apstein net (mesh size: 55 µm) and fixed in buffered formalin for abundance analyses. The zooplankton community was dominated by copepods (Calanus finmarchicus, Temora longicornis, Pseudocalanus elongatus, Oithona similis), followed by meroplanktonic larvae. Over the course of the experiment, the total zooplankton abundance in the water column decreased in all nine mesocosms, mainly due to decreasing copepod abundances. Only Pseudocalanus elongatus, appendicularians and medusea increased in abundance. Species composition did not change with CO2 concentration; however, some taxa (e.g. bivalves and gastropods) were less abundant in mesocosms with high CO2 levels.
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