Primary production during nutrient-induced blooms at elevated CO 2 concentrations

Abstract

A CO2 enrichment experiment (PeECE III) was carried out in 9 mesocosms in which the seawater carbon- ate system was manipulated to achieve three different levels of pCO2. At the onset of the experimental period, nutrients were added to all mesocosms in order to initiate phytoplank- ton blooms. Primary production rates were measured by in-vitro incubations based on 14C-incorporation and oxygen production/consumption. Size fractionated particulate pri- mary production was also determined by 14C incubation and is discussed in relation to phytoplankton composition. Pri- mary production rates increased in response to nutrient addi- tion and a net autotrophic phase with 14C-fixation rates up to 4 times higher than initial was observed midway through the 24 days experiment before net community production (NCP) returned to near-zero and 14C-fixation rates dropped below initial values. No clear heterotrophic phase was observed during the experiment. Based on the 14C-measurements we found higher cumulative primary production at higher pCO2 towards the end of the experiment. CO2 related differences were also found in size fractionated primary production. The most noticeable responses to CO2 treatments with respect to primary production rates occurred in the second half of the experiment when phytoplankton growth had become nutrient limited, and the phytoplankton community changed from di- atom to flagellate dominance. This opens for two alternative hypotheses that the effects are either associated with mineral nutrient limited growth, and/or with a change in phytoplank- ton species composition. The lack of a clear net heterotrophic phase in the last part of the experiment supports the idea that a substantial part of production in the upper layerwas not de- graded locally, but either accumulated or exported vertically.