Why is there a red tide?
Unfortunately, although scientists understand what red tides are, often they don’t know why they form at particular times. Red tides are natural occurrences; the plankton community becomes dominated by one or a few species at extraordinarily high concentrations. The net growth rate of the cells (including accumulation by swimming) is higher than their net loss rate (due to grazing, etc.). The growth can be fueled by nutrients brought to the surface by upwelling. There is some evidence that development and growth of red tides can be facilitated by discharges of treated sewage from municipal sewage treatment plants (containing nitrates, urea and other nutrients) and also from “urban runoff” that contains fertilizers washed off of landscaped areas. Why are red tides so dense? The organisms that make red tides (at least in southern California) tend to be able to swim. They might be swimming upward to photosynthesize, and downward to take up nutrients. If they are all doing the same thing,
