EFFECTS OF THE TEXAS (USA) BROWN-TIDE ALGA ON PLANKTONIC GRAZERS

The Laguna Madre of south Texas, USA, has experienced a dense, nearly monospecific phytoplankton bloom since January 1990 referred to as the 'brown tide'. Zooplankton populations declined following the outbreak of the bloom and planktonic grazers have failed to bring the bloom un der control. Laboratory studies of microzooplankton grazers feeding on brown tide indicate that this alga is nutritionally inadequate to sup port the growth of several species including the ciliate Strombidinops is sp., the heterotrophic dinoflagellate Noctiluca scintillans or the rotifer Brachionus plicatilus. The presence of brown tide also inhibit s the growth of some species (Noctiluca scintillans, Brachionus plicat ilus) even when other nutritionally adequate food species are present. Some species that grow on brown tide grow best at low cell concentrat ions, including the ciliates Fabrea salina and Euplotes sp.; as algal densities increase, growth rates decrease. Laboratory studies of egg r elease of adult female Acartia tonsa indicate that the brown tide is a poor food for these copepods; egg release rates are similar to those of starved copepods and less than those of copepods fed other similarl y sized phytoplankton. The brown tide is toxic to early naupliar stage s of Acartia tonsa but not to adults. The brown tide alga appears to b e toxic to some species of planktonic grazers and a poor food for othe rs; the reduced grazing by the planktonic community may be a contribut ing factor to the persistence of this bloom.