Disruption of grazer populations as a contributing factor to the initiation of the Texas brown tide algal bloom

Abstract

Disruption of grazer populations, or mismatches between phytoplankton blooms and population dynamics of their planktonic and benthic grazers, may play an important role in the initiation of some harmful algal blooms. The Laguna Madre of Texas has experienced a persistent, nearly monospecific phytoplankton bloom since January 1990 commonly referred to as the “brown tide.” Whole‐water samples collected in the area where the bloom began reveal the pattern of bloom initiation and dispersion during the first few months of the bloom. Enumeration of microzooplankton from these same samples also reveals that protozoan grazer populations were depressed and nearly eliminated before the bloom began. Benthic biomass and species diversity also declined before the onset of the bloom. The disruption of planktonic and benthic grazers may have been due to extreme hypersaline conditions caused by an extended period of drought (salinities > 60‰). A massive fish kill caused by an abnormal period of below freezing temperatures released a pulse of nutrients into an already disturbed environment allowing this nuisance bloom to become established. Planktonic and benthic grazers have been unable to contribute to the demise of this bloom, which has persisted for >7 yr without interruption.