Impacts of Coastal Acidification on Louisiana Plankton

Anthropogenic input of carbon dioxide (CO2) has an increasingly acidifying effect on the world’s oceans with the potential to impact biogeochemical cycles and food web dynamics. Coastal Louisiana is an area highly vulnerable to changing climate, but the unique local water chemistry created by input from the Mississippi and Atchafalaya Rivers makes the manifestation of elevated CO2 and its biological implications difficult to predict. Louisiana estuaries are highly productive and support a large commercial market for the eastern oyster, Crassostrea virginica. This study explores the use of experimental microcosms to expose natural phytoplankton assemblages and larval oysters to elevated pCO2. Spring and fall phytoplankton were collected from two biogeochemically distinct Louisiana estuaries and cultured in lab for 16 weeks while bubbling with CO2 enriched air corresponding to current (400 ppm) and future (1000 ppm) pCO2 levels. Spring phytoplankton assemblages increased in diatoms over the first 8 weeks, but after 14 weeks of incubation transitioned to cyanobacterial dominance regardless of pCO2 level, likely due to a nutrient imbalance. Fall phytoplankton assemblages also increased in diatoms over the first 8 weeks, but after 14 weeks returned to their original community structure, showing evidence of adaptation to elevated pCO2 exposure. Over the course of a 6-day pilot study, resilience was also observed during early larval oyster development, as D-stage C. virginica survived and grew at the same rate in control and elevated pCO2 cultures. In Louisiana, spring and fall phytoplankton blooms in conjunction with water temperature are critical in dictating the timing of oyster spawning. When low pH is compounded with seasonal salinity, temperature, and nutrient variations, it has the potential to influence the phytoplankton community during a critical oyster spawning time, creating a mismatch. Coastal acidification could affect the success of larval oysters by changing the availability and quality of its food source, phytoplankton