Physiological response of the copepod Pseudocalanus sp. in the Baltic Sea at different thermal scenarios

We studied the physiological response of Pseudocalanus sp. under four different temperature elevation regimes: +0, +2, +4 and +6 °C above the decadal average temperature in the Western Baltic Sea. We measured fecal pellet (FP) production rates, which was taken as a proxy of ingestion, egg production (EPR) and respiration rates. Experiments lasted from mid‐February to end April, corresponding most of the observations to the postspring bloom phase. We combined small scale incubations with the use of big (ca. 1400 L) mesocosms, which have previously been shown to be appropriate when studying phyto‐ and zooplankton succession, and the water used for the incubations was taken from the mesocosm tanks. Given that the phytoplankton succession varied between the four thermal scenarios, we evaluated (excepting in the case of the respiration rates, where incubations were carried out using 0.2 μm filtered water) both the temperature and the associated food concentration effects. Respiration and ingestion rates were found to increase with temperature. As for EPR, they also increased with temperature during the bloom, but remained at low and constant values during the postbloom in all the four treatments due to the food limitation. Linked to the temperature rise, we also detected an increase in instantaneous mortality rates and a reduction in the net growth efficiency. Finally, we discuss the potential implications of our findings for the spring phyto‐ and zooplankton succession under the forecasted climate warming, as well as for the fisheries in the Baltic Sea, where Pseudocalanus sp. is a key species.