1.) Algae is growing in some parts, not in others.
Algae needs water (easy in the ocean) sunlight, and dirt. In the deep parts, algae in the surface don't get much minerals because the bottom is so far away. You get more algae where tides or rivers wash minerals into the water, or where it is shallow.
2.) A muddy bottom, river, or tides can dissolve dirt in the water.
Because the BP oil spill has not yet reached the clear spots.
That indicates the convective nature of the ocean.