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

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%0). 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. Harmful algal blooms in estuarine and neritic environments have been increasing in frequency in recent years (Anderson 1989; Smayda 1989; Hallegraeff 1993); this increase has been related to the increasing anthropogenic influences on coastal waters, especially alterations in nutrient supplies and nutrient ratios in coastal waters. The role of phytoplankton grazers in the population dynamics of these blooms is not well understood, but it seems likely that declines in grazer populations may contribute to the initiation of some harmful algal blooms and that grazers and pathogens may play a role in terminating blooms. Zooplankton grazing has often been suggested to play an important role in phytoplankton bloom dynamics. Early studies focused on copepods and diatom blooms. Fleming (1939) suggested that diatom populations could be controlled by grazers, and Riley (1946; 1947) produced a mathematical model that demonstrated the potential of grazer populations to maintain low populations of phytoplankton. Martin (1965) suggested that the spring bloom of diatoms in Narragansett Bay was able to become established in part due to low abundances of grazers and that the winter-spring bloom ended when rising spring temperatures become favorable to zooplankton reproduction. Benthic grazers may also help control phytoplankton biomass, especially in shallow environments (Cloern 1982; Officer et al. 1982). However, remarkably little is known about the role of grazers in the bloom dynamics of harmful algal bloom species. A dense bloom of a small unidentified alga has persisted