THE EFFECTS OF OCEAN ACIDIFICATION AND CLIMATE CHANGE ON THE REPRODUCTIVE PROCESSES OF THE MARINE COPEPOD Calanus finmarchicus

            Ocean acidification and climate change can affect a variety of marine species by lowering ocean pH and increasing ocean temperatures. The copepod Calanus finmarchicus is very abundant throughout the western North Atlantic and an essential link between primary producers (phytoplankton) and larger consumers (fish) in the marine food web. I tested the hypothesis that the combination of lower water column pH and high surface layer temperatures in which eggs may be spawned, especially in fall, reduces egg hatching success and thereby the capacity for C. finmarchicus to sustain high abundance in the Gulf of Maine. This is particularly relevant in the Gulf of Maine, where C. finmarchicus resides at the southern edge of its range.

            In this study, C. finmarchicus eggs (collected from females collected in the Gulf of Maine) were exposed to acidified seawater of varied pH (ranging from pH 6.54- 8.07) at 6˚C in order to simulate scenarios of ocean pH in the future. Different pH levels were obtained by the bubbling in gas mixtures of O₂, N₂, and CO₂ at predetermined rates. Temperature trials using cold rooms and a water bath were conducted to investigate the effect of temperature alone on hatching success. Temperature treatments ranged between 6- 26˚C. The acidified trials were then rerun at 15˚C to test for a synergetic effect on hatching success from lowered pH and elevated temperature. All experimental work took place at the Darling Marine Center in Walpole, Maine between May 2011 and August 2012.

            The temperature trials showed that hatching success is not significantly reduced by temperatures up to 19˚C. Average hatching success dropped below 15% at 21˚C and declined to 0% at 24˚C or higher. The pH trials at 6˚C showed no significant effect of pH on the hatching success of C. finmarchicus eggs down to an electrode-measured pH of about 6.73. The pH trials at 15˚C show a significant effect of pH on the hatching success of C. finmarchicus eggs at an electrode-measured pH of about 7.05 and below. These results are consistent with the null hypothesis that the anticipated pH decrease over the next century will not affect C. finmarchicus hatching success.