The largest visual impact will be on birds and coastal shorelines, I've heard the spill was as little as 1000 or as much as 5000 gallons. Impact will be greatly determined by duration of the cleanup. Things like plankton won't be a major concern due to their irregular distribution. Reefs and fish that remain stationary in the area can be affected heavily, of course clams, etc.. that filter water will feel the greatest impact.