Red tide is the term given to the waters in which there has been a prolific multiplication of microalgae so that the sea takes on a red, brown or even green colour from the pigment secreted by the these organisms. 1 At present, there are some 2,000 species of the marine form of these microorganisms, known as dinoflagellates, inhabiting both boreal and tropical seas (Taylor 1987:1). They are simple, single-celled organisms capable of propulsion by means of a unique flagella arrangement that permits them to move both horizontally and vertically at a maximum rate of about one metre per hour or in excess of 10 metres per day (Anderson 1994:56). Normally, these dinoflagellates live in small, scattered communities in the sea, with many species able to remain encysted or dormant in bottom sediment for years when water conditions are not supportive. But they are also able to reproduce rapidly by asexual fission (simple division) in more favourable conditions created by intense sunlight, changing salinity levels and opportune weather conditions such as heavy rains or winds that increase nutrient availability (Spector 1984:217-218; Caburian 1988). 2 Many species, in fact, are capable of doubling their numbers in a day (Pearce 1989:155). In dense concentrations (a millimetre of seawater can contain tens of thousands of cells), these microorganisms form massive, mat-like colonies of mono-specific planktonic blooms that carpet the water’s surface. They proliferate especially in discrete water masses such as estuaries, coastal areas or former mangrove swamps where the integrity of the water composition can be maintained for longer (Daiwey 1992; Spector 1984:220-222). Most of these blooms are harmless and a few have even given their name to some of the planet’s principal bodies of water, most notably in the case of the Red Sea. 3 Others, however, are far less benign, the discolouration of the water acting more as a warning of toxicity. 4 Over the last 30 years, red tides have come to represent a natural hazard of increasing magnitude and gravity to both coastal and marginal urban populations in the Philippines.
Currents and tides act as the ‘dynamic transport mechanism’ for these blooms’ dispersal but the actual distribution of dinoflagellates is largely determined by hydrological processes such as the frontal systems associated with current boundaries and upwelling (Spector 1984:216, 222). More