Changes in Microbial and Phytoplankton Communities in Response to Oil and Nutrients in the Northern Gulf of Mexico: Correlating Experiments With Field Observations.

Natural hydrocarbon seeps account for around 47% of the oil released into the environment, with seeps in the Gulf of Mexico releasing up to 1.1×10⁸ L oil year^-1. Our previous work in the Gulf of Mexico has linked natural hydrocarbon seepage occurring at depths exceeding 1000 meters to elevations in surface and sub-surface chlorophyll concentrations in the upper water column via plume-driven upwelling of nutrient rich waters from depth. Subsequent experiments further revealed synergistic impacts of oil, nutrients, and microbial predation on surface microbial growth rates and metabolism - suggesting an additional top-down control on surface bacteria and microbes. While the magnitude of each contribution remains unclear, both, bottom-up and top-down mechanisms associated with natural oil seepage are evident. We build on our previous findings with results from microcosm experiments aimed at dissecting out the influence of nitrate, phosphate, silica, and crude oil on microbes in surface waters. Changes in phytoplankton populations, bacterial growth rates, and the subsequent drawdown of nutrients in these microcosms are presented here in context of field measurements made at natural seeps.