Effects of pH and temperature on egg hatching success of the marine planktonic copepod, Calanus finmarchicus

Calanus finmarchicus is a predominant planktonic copepod in the northern North Atlantic Ocean, where it is a fundamental link in the transfer of energy from phytoplankton to fish. Here, we investigate whether ocean acidification at present-day and future levels will cause a significant decrease in the egg hatching success (HS) of C. finmarchicus in the Gulf of Maine. Eggs spawned by female C. finmarchicus collected from the coastal Gulf of Maine were incubated in seawater acidified by addition of CO2 to selected pH levels at 3.5 °C (in a single experiment), 6 °C and 14–15 °C (in multiple experiments). HS was unaffected by pH between 6.58 and 8.0 at 3.5 and 6 °C, and between 7.1 and 8.0 when incubated at 15 °C. A significant interactive effect between temperature and pH on HS was found using a two-way ANOVA of the data from experiments at 6 °C and 14–15 °C, temperatures that are experienced in summer in the Gulf of Maine. HS of eggs spawned from C. finmarchicus females immediately after capture from a coastal station was significantly reduced at pH ≤ 7.0 when incubated at 14–15 °C, although HS of eggs collected from well-fed females in the laboratory in water from the Damariscotta Estuary was not significantly reduced at pH levels as low as 6.6 at 15 °C. This finding is consistent with the hypothesis that parental history and possibly maternal provisioning can influence capability of eggs to adjust to lower pH environments. While an interaction between pH and temperatures experienced by C. finmarchicus at the southern edge of its biogeographic range was observed, the pH at which this interaction occurred is substantially lower than pH levels predicted for the surface ocean over the surface ocean.