Calcareous nannoplankton and benthic and planktonic or pelagic diatoms are major photosynthetic, skeletal-secreting prosists at the base of the food chain. Major impacts on their populations by spill oil and dispersants will be felt throughout the trophic system up to the vertebrate level. We propose to extend baseline studies of these and related groups previously carried out in the NE Gulf of Mexico off Florida, from inland bays and estuaries to the shelf and slope. Our sampling regime will, to the extent possible, be carried out in collaboration with investigators from other institutions, such as those from the University of South Florida on their 8-day R/V Weatherbird II cruise from Pensacola to Tampa Bay. We will compare the diatom, calcareous nannoplankton, and related water-column protist populations (exclusive of foraminifers) as they existed before and after the Deepwater Horizon spill. Of particular interest will be the much studied Perdido Bay where spill oil has already strongly impacted the outer beaches by the time of this writing. Other such areas of intense previous study along the Florida Panhandle by members of our team extend east to Apalachee Bay, where our sampling from small boats will be aided by generous offers of assistance we are receiving from local volunteers and environmental organizations. We will also culture some of our more abundant diatom and nannoplankton taxa and expose them to various concentrations of dispersed oil and the dispersants from the Deepwater Horizon spill site after which they will be examined via scanning electron microscopy to detect possible skeletal abnormalities resulting from stress and physiological damage to their cells.