Red tides, also known as harmful algal blooms, or HAB's, are the result of a massive multiplication, or "bloom", of microscopic, single-celled algae named Karenia brevis, (aka K. brevis), or single-celled organisms called dinoflagellates.
Most oceanographers and scientists prefer the term harmful algal blooms over the term red tide because the water is not always discolored when the blooms cause damage and because it is often harmless even when the water is discolored.
There are still many differing theories as to the cause of these phenomena. .
One such theory put forward by Raphael Kudela, assistant professor of ocean studies at the University of California, Santa Cruz, is that the increased urea content in the sewage run-off in urban, agricultural, and aquicultural areas may play a more crucial role in triggering and/or sustaining harmful algal blooms than was previously considered.
Although marine biologists routinely monitor the sea's ecosystems for above normal concentrations of common inorganic nutrients, urea has not been on the list of substances to be monitored.
Laboratory studies have shown that dinoflagellates use organic urea as a nutrient. It has also been determined that dinoflagellates prefer urea to other inorganic forms of nitrogen. .
High levels of nutrients combined with abundant sunlight can make these single-celled plants grow incredibly fast. They have even been known to double in size within a twenty-four hour period.
Although some species of this phytoplankton produce strong neurotoxins when they bloom, most of these "blooms" are not harmful.
As phytoplankton is the ocean's primary producer, these toxins are transferred up the food chain and can even effect humans. (Kudela, UCSC, 2000).
What is so baffling about red tides is that humans cannot control them. Some believe that the blooms are triggered by just the right temperature, salinity, and nutrient levels in the water.