Biologic and Hydrographic Aspects of Red Tides

Red tides are not always red and not necessarily associated with tides. Their definition differs among locations and encompasses a variety of effects, yet all are due to monospecific blooms of microor nannoplankton. Red tides can be discolored patches or streaks of seawater accompanied by death of fishes, invertebrates, or seabirds; or they may show no signs of discoloration, nor fish and invertebrate kills. Some of the later types of blooms can still cause shellfish (bivalve mollusks) to become toxic to humans because low concentrations of toxic microorganisms can be accumulated and biomagnified in filter feeders. Seawater can vary in discoloration, depending on species composition, pigments of the causative organisms, size and concentration of organisms, time of day and sun angle, and other factors. Two types of red tides occur-toxic and nontoxic. Nontoxic red tides are associated with discolored seawater caused by high concentrations, or blooms, of phytoplankton, bacteria, ciliates, or small zooplankters. These nearmonospecific blooms can also kill marine animals by depleting dissolved oxygen in the water or by physical asphyxiation. Blooms represent population densities above background levels of 10'-103 cells liter-', and in many instances, more than 5 x 104 organisms liter-' are present. The terms red tide, toxic algal bloom, red water, and discolored water need to be clarified and specific terms adopted fo specific conditions. Toxic red tides are caused by flagell t s, mostly dinoflagellates, which produce low molecular weight, nonproteinaceous endotoxins; these red tides are not always associated with obvious seawater discoloration. Less than 20 of more than 1200 described extant dino-