Ocean acidification is one of the most pressing marine science issues of our time, with potential biological impacts spanning all marine phyla. Rising levels of atmospheric pCO2 are increasing the acidity of the world oceans. It is generally held that average surface ocean pH has already declined by ~0.1 pH units relative to the pre-industrial level (Orr et al., 2005; translating to a ~26% increase in acidity), and is projected to decrease 0.3 to 0.46 units by the end of this century, depending on CO₂ emission scenarios (Caldeira and Wickett, 2005; translating to a doubling to near-tripling of the acidity of sea water).  The effects of such a change could be profound.  The overall goal of this work was to parameterize how changes in CO₂ concentration could alter the biological and alkalinity pumps of the world ocean (how CO2 gets fixed and transported to the deep sea as sinking particulate matter as well as dissolved inorganic carbon species). Specifically, the direct and indirect effects of ocean acidification were examined within a simple, controlled predator/prey system containing a single prey phytoplankton species (coccolithophores) and a single predator (mesopelagic ocean grazers).  Experiments were designed to elucidate both direct effects (i.e. effects of ocean acidification on only the individual organisms) and interactive effects (i.e. effects on the combined predator/prey system).  Interactive experiments with phytoplankton prey and zooplankton predator were a critical starting point for predicting the overall impact of ocean acidification in marine ecosystems. 

     To meet these goals we designed and constructed a state-of-the-art facility with algal and copepod growth chambers and highly-controlled pH and alkalinity levels to ensure that conditions within the chambers were well described and tightly monitored for dissolved inorganic carbon levels. We measured growth and calcification rates in two species of coccolithophores (calcifying algae thought to be susceptible to ocean acidification)  and the developmental rates, morphological and behavioral effects on three species of copepods. 

     Our results demonstrated that counter to our expectations, for the estuarine coccolithophore, Pleurochrysis carterae, increased CO₂ concentration had no significant impact on its growth rate or photosynthetic rate.  Moreover, P. carterae calcification rates significantly increased in elevated pCO₂ concentrations of 750 ppm.  This trend of increased calcification at higher pCO₂ conditions fits into a recently developed model, which demonstrates that coccolithophores have a calcification optima.  Our evidence also indicated differing biological control on the water chemistry by two species of coccolithophores, P. carterae and Emiliania huxleyi, when at high cell densities.  That is, photosynthesis of P. carterae tends to drive the pH up, while the calcification of E. huxleyi drives the pH down at high cell densities.  Models to accurately predict future pH in a high CO₂ world need to factor in this diversity of response.

     For the 3 species of copepods tested, there was no significant effect of pCO2 on development times, lipid accumulation, feeding rate, or metabolic rate. Small but significant treatment effects were found in body length and mass (in terms of dry, carbon and nitrogen mass), notably a somewhat larger body size at the mid-pCO₂ treatment; that is, a putatively beneficial effect. Based on these results, and a review of other studies of Calanus spp, we conclude that the present parameterizations of vital rates in models of copepods population dynamics, used to generate scenarios of abundance and distribution of this species under future conditions, do not require an “ocean acidification effect” adjustment.

Research Team. The team of scientists in this work comprised expertise in coccolithophore ecology, biogeography and physiology (Balch) and copepod ecology and physiology (Fields). The project also had a post-doctoral fellow (White) and a highly capable technical staff to provide the necessary support to cultivate and monitor cultures during experiments. Eleven supervised undergraduate students had projects associated with this work. 

Broader Impact. Our finding have been disseminated through five primary mechanisms: (a) 13 peer-reviewed publications published plus one legislative document for the state of Maine on the effects of ocean acidification on Maine’s marine economy, (b) 17 presentations were made at national and international scientific conferences, (c) student projects (11 undergraduate students+ 2 MS students) (d) public outreach (Bigelow Open House, Other public venues – >1000  people were exposed to information about ocean acidification), and (e)involvement with policy stakeholders (NERACOOS, NECAN, Maine OA Commission). Data are publically available from the Biological and Chemical Oceanography Data Management Office (BCO-DMO) at http://www.bco-dmo.org/project/514415.

Last Modified: 11/17/2017
Modified by: William M Balch