Algal bloom off Yucatan Peninsula, Mexico

From a post on Earth Snapshot, hosted by Chelys:

Clearly visible from their bright green color, phytoplankton flourish in the Gulf of Mexico off the coasts of the states of Campeche (top) and Yucatan (bottom), in Mexico.

The bloom extends particularly far out into the gulf between the city of Campeche (the light brown spot along the shoreline, near the center) and Ciudad del Carmen near the Términos Lagoon (lower left).

Upon closer observation of the lagoon, a more intense, circular green algal bloom becomes evident on the northern end, as well as some brown sediments flowing out into the gulf on the southwestern end.

Algal blooms are not healthy for aquatic systems, as explained in a Wikipedia entry:

Algal blooms are the result of an excess of nutrients, particularly phosphorus. Excess carbon and nitrogen have also been suspected as causes, but research has shown that this is not the case. When phosphates are introduced into water systems, higher concentrations cause increased growth of algae and plants. Algae tend to out-compete plants under these conditions, and many plant species may begin to die. This dead organic matter becomes food for bacteria that decomposes it. With more food available, the bacteria increase in number and use up the dissolved oxygen in the water. When the dissolved oxygen content decreases, many fish and aquatic insects cannot survive. This results in a dead area.

Going to the original post and clicking to enlarge the photo makes the post's comments clearer.