Oil Spills at the Water Surface
The paper states that because most kinds of oil are less dense than water, most spilled oil floats on the water surface. It spreads out and is pushed across the water by wind and currents. How spilled oil affects near-surface creatures depends on just when and where the oil spills those creatures might or might not even be in the area at the location and time of a spill. In short it describes how oil affects near-surface creatures depends on how vulnerable those creatures are to the effects of oil.