Excess fertilizer that makes it into our oceans through urban runoff can increase the growth rate of plankton. As a result, algae naturally found in the water can grow at an accelerated pace or "bloom" and accumulate into dense, visible patches near the surface of the water.
"Red Tide" is a common name for such a phenomenon where certain phytoplankton species contain reddish pigments and "bloom" such that the water appears to be red. While "Red Tides" are generally not harmful to humans, certain types can release strong neurotoxins that can be ingested by shellfish and passed on to humans who eat the infected shellfish. For more information visit Surfrider.org.