I like diving with lots of plankton, but:
1) The plankton has to be diverse; the greater the diversity, the greater the variety of large living things there are in the water, like zooplankton and, in turn, fish. You can see this difference in the variety of sea life between the waters of
Puerta Galera and those of the
Red Sea: in the plankton drags I've done at Puerta Galera, I've seen a
huge variety of phytoplankton (100+ species) whereas those I collected from a more recent trip to the Red Sea was, by comparison, nearly bereft of phytoplankton. This also explains why the Red Sea has such excellent water clarity- visibility often exceeding 30 meters.
2) Near shore plankton blooms of one specific phytoplankton are often a bad thing. Generally associated with excess agricultural nitrogen input, explosions of a specific phytoplankton, known as algal blooms, decrease water visibility, consume available oxygen (and kill fish), release poisons in the water and the food chain, and smother everything on the seafloor with excess organic material. Red tides, for example, do a lot of damage....do NOT snorkel during a Red Tide Alert. On the other hand, pure, fresh water run-off from undeveloped land can lead to useful inputs of much needed oxygen and dissolved organic matter to local water. Healthy algal blooms are also important, especially out in the open ocean (where no one is snorkeling!).
Hope that helps!
