Ocean Acidification Impact to Copepod Populations Mediated by Changes in Prey Quality

Research shows that elevated atmospheric pCO₂ and subsequent ocean acidification (OA) have physiological consequences for individual organisms. However, existing research does not adequately address how OA effects to individuals are coupled across trophic levels. Pelagic copepods form an important link between primary production and higher trophic levels, making it crucial to understand how these organisms are affected by OA directly and through changes in their phytoplankton prey. Recent studies indicate that OA affects phytoplankton by altering their physiology and biochemistry, often increasing growth rate and fatty acid content. As copepod grazing and fecundity can vary with these prey characteristics, OA mediated changes in phytoplankton quality may be an important indirect mechanism through which OA acts on copepod populations and, ultimately, marine food webs. We carried out experiments investigating the direct and indirect effects of OA on copepod (Acartia hudsonica) production at two different temperatures, 12°C and 17°C. Phytoplankton (Rhodomonas salina) were grown under three different pCO₂ conditions (400, 800 and 1200 ppm) for five generations and fed to adult female copepods maintained under the same three pCO₂ levels. Phytoplankton physiology and biochemistry were analysed, focussing on the fatty acid composition. We found significant increases in phytoplankton lipids with increasing pCO₂ levels, allowing us to test our hypothesis that OA effects on copepods may be mediated by food quality. Following an acclimation period copepod grazing, respiration, fatty acid composition, egg production, hatching rate and the ontogenetic development of nauplii were analysed. The same experiments were performed with the copepod Calanus pacificus feeding on a diatom (Dytilum brightwelii) and a dinoflagellate (Prorocentrum micans) using the same experimental design at 12°C.