Chair: Ana Queiros

Anna McLaskey ^(1)*, Katherina L. Schoo ⁽²⁾, Julie E. Keister ⁽¹⁾, Brooke A. Love ⁽²⁾, M. Brady Olson ⁽²⁾

¹ University of Washington, School of Oceanography, Seattle, WA, 98105, USA
² Western Washington University, Shannon Point Marine Center, Anacortes, WA, 98221, USA

Background
Most studies of elevated pCO₂ have focused on single species, with few studies linking individuals across trophic levels, despite the importance of indirect effects in determining ultimate ecosystem outcomes. Copepods are an important link between primary production and higher trophic levels in marine ecosystems. Recent studies show that increased pCO₂ can alter the physiology and biochemistry of some phytoplankton species, including fatty acids, an indicator of food quality that influences copepod reproductive outcomes. Changes to phytoplankton may be an important mechanism through which ocean acidification affects copepods.

Methods
We investigated impacts of elevated pCO₂ on copepod populations mediated through changes in phytoplankton quality in the laboratory. Phytoplankton were grown under three different pCO₂ conditions (400, 800, and 1200 μatm) for approximately five generations and fed to adult female copepods held at the same pCO₂ level as their prey for 3-8 days. The biochemistry of the phytoplankton was analyzed, particularly for fatty acids. After acclimation we measured copepod grazing rates, respiration rates, egg production, hatching success, and larval development. Calanus pacificus, a large, high-lipid species, was fed the dinoflagellate Prorocentrum micans and the diatom Ditylum brightwellii, each at 12˚C. Acartia hudsonica, a smaller, low-lipid species, was fed Rhodomonas salina at 12˚C and 17˚C.

Findings
Although phytoplankton lipids increased at elevated pCO₂, C. pacificus had significantly lower clutch sizes fed P. micans at 1200 compared to 400 μatm, with no effects on hatching or larval development. There were no effects on C. pacificus fed D. brightwellii. Preliminary results from A. hudsonica experiments indicate a trend towards slower larval development with elevated pCO₂ at 12˚C and faster development with elevated pCO₂ at 17˚C.

Conclusions
Prey quality is an important mechanism through which OA can impact marine ecosystems; understanding effects on the phytoplankton-copepod link will be critical to our knowledge of OA impacts.