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. The marine planktonic copepod, Calanus finmarchicus, is a predominant species in zooplankton assemblages of the western North Atlantic and a key link in the transfer of energy from phytoplankton to fish. Here I investigate effects of lower ocean pH and higher surface temperatures expected over the coming decade on C. finmarchicus egg hatching success. This is particularly relevant in the Gulf of Maine, where C. finmarchicus resides at the southern edge of its biogeographic range. 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. C. finmarchicus eggs were exposed to acidified seawater of varied pH (ranging from pH 6.54- 8.07) at 6°C in order 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. Eggs were obtained directly from females immediately after capture from the Gulf of Maine or by sorting females captured from live tows into acrylic tubes with 500 (im mesh (egg separation containers) and providing them with a superabundant mixture of phytoplankton to promote egg laying. Hatching success was measured by counting how many of the eggs put into petri dishes hatched into nauplii. 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