Program

The effects of elevated CO₂ concentration and temperature were studied in nutrient-induced phytoplankton blooms in indoor mesocosms with surface seawater collected from Stony Brook Harbor, NY. At various times during the blooms, aggregation experiments in roller tanks were conducted using seawater from the differently-treated mesocosms. We measured phytoplankton biomass, particulate organic carbon and nitrogen production, and gel particle concentrations in the mesocosms. We measured sinking velocity, size, excess density and gel particle (TEP and CSP) content of aggregates formed in the roller tanks. No significant differences between treatments were observed in overall nutrient uptake, Chl-a concentration, gel particle concentration or aggregate formation, abundance or composition. However, the roller tank experiments showed that aggregates formed during the bloom peak at higher CO₂ and the present temperature were smaller and had lower sinking velocity than aggregates formed under present CO₂ conditions. We suggest that the effect of CO₂ on aggregate properties may vary at different stages of the phytoplankton bloom and that these variations should be considered in making conclusions about effects of the future ocean on the biological pump. The absence of a clear CO₂ effect during these mesocosm experiments may be more representative of coastal environments, which naturally have a wide range of temperature and CO₂ conditions and are dominated by diatoms that have low sensitivity to high CO₂ and temperature, and might not be directly applicable to open ocean environments.