ABSTRACT: 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 under control. Laboratory studies of microzooplankton grazers feeding on brown tide indicate that this alga is nutritionally inadequate to support the growth of several species including the ciliate Strombidinopsis sp., the heterotrophic dinoflagellate Noctilucascintillans or the rotifer Brachionusplicatilus. The presence of brown tide also inhibits the growth of some species (Noctilucascintillans, Brachionusplicatilus) even when other nutritionally adequate food species are present. Some species that grow on brown tide grow best at low cell concentrations, including the ciliates Fabreasalina and Euplotes sp.; as algal densities increase, growth rates decrease. Laboratory studies of egg release of adult female Acartiatonsa 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 similarly sized phytoplankton. The brown tide is toxic to early naupliar stages of Acartiatonsa but not to adults. The brown tide alga appears to be toxic to some species of planktonic grazers and a poor food for others; the reduced grazing by the planktonic community may be a contributing factor to the persistence of this bloom.

KEY WORDS: Harmful algal blooms . Zooplankton . Brown tides

Published in MEPS Vol. 126 (1995) on October 5
ISSN: 0171-8630. Copyright © Inter-Research, Oldendorf/Luhe, 1995