Seasonal Variability in Phytoplankton Responses to Water Accommodated Petroleum Hydrocarbons in the Northern Gulf of Mexico

Seasonality in the relative abundance of phytoplankton groups is primarily due to environmental variables such as light availability, nutrient availability and flux as well as shifts in higher trophic level abundance. This variation is amplified in shallow coastal waters, where the biology and physical characteristics are influenced by dynamic temporal variation in freshwater plume influx. The flooding of the Gulf of Mexico with organic matter from the Deepwater Horizon Oil spill in April 2010 highlighted the lack of regional baseline knowledge regarding phytoplankton succession and the effect of petrocarbon loading on different phytoplankton groups. To understand the effect of perturbations on seasonal succession, we are conducting monthly multi-day grow out experiments using water from the Alabama Coast, in the northern Gulf of Mexico. These assessments are conducted using treatments of water accommodated fraction of crude oil (WAF, MC252 Surrogate Crude Oil) and chemically enhanced WAF (CEWAF, Crude + Dasic International Slickgone NS®) relative to controls. We are examining changes in photophysiology, productivity, biomass accumulation and community composition. Preliminary results show significant photochemical quenching of chlorophyll fluorescence after one day of exposure to 10% WAF. In addition, significant divergences in chlorophyll concentration were observed among oil exposure treatments. These results imply strong sublethal effects for the aggregate phytoplankton community. Ongoing time-series results will be presented to examine whether similar sublethal effects are observed throughout the year and whether the magnitude of this variation is affected by the encountered community composition and hydrographic conditions. Given that phytoplankton mediate the transfer of carbon to higher trophic levels, understanding the effect of perturbations on seasonal relative phytoplankton abundance has implications for ecosystem resilience as a whole.