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

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 Acknowledgments This work was supported by the Texas Higher Education Coordinating Board (grants 3658-264, TATP-426, and 003658-019) and by the National Science Foundation (OCE 95-29750). Some field samples were collected by Dean Stockwell and Rick Kalke. Technical assistance was provided by Cammie Hyatt, Jay Peterson, Chris Collumb, and Mary Conley. This is University of Texas Marine Science Institute Contribution 975. uninterrupted in Laguna Madre, Texas, since January 1990. The organism responsible for this lengthy bloom is a small phytoplankton species (4-5-pm diam) which is presently being formally described (DeYoe et al. in prep.). It is similar in morphology and pigments to the northeast “brown tide” organism Aureococcus anopha&erens (Stockwell et al. 1993) which has been responsible for recurrent blooms in Narragansett Bay and Long Island Sound (Cosper et al. 1987). Molecular data from 18s rRNA sequences indicate that both species belong to the newly recognized class Pelagophyceae (Anderson et al. 1993), but the two species are different enough to be placed in separate genera (DeYoe et al. 1995). This alga is usually present at densities ranging from 0.5 to 5X106 cells ml-’ (Buskey et al. 1996). Laguna Madre is a large (2.15 X 10” ha), shallow (avg depth, 1.2 m), often hypersaline coastal lagoon (Armstrong 1987) that contains extensive seagrass habitat (Quammen and Onuf 1993). The brown tide bloom has severely discolored the previously clear waters of Laguna Madre and reduced the distribution of seagrasses in deeper waters due to attenuation of light (Dunton 1994; Onuf 1996). Unlike blooms of A. anophagefferens, which have appeared sporadically in several locations for periods of weeks to months, the Texas brown tide has persisted without interruption for >7 yr. Most field studies of toxic or nuisance phytoplankton blooms begin well after the initiation of the bloom, making it difficult to assess the environmental and biological factors that may have contributed to the initiation of the bloom. A previously published study (Buskey and Stockwell 1993) examined the effects of the brown tide bloom on zooplankton populations in lower Baffin Bay and upper Laguna Madre after the bloom had spread to these regions in June 1990. Analysis of previously unexamined water samples from upper Baffin Bay has allowed us to identify the area where the bloom began and the time of the bloom’s initiation to within a month. By examining the changes in physical and biological parameters leading up to the initiation of the bloom, an understanding of factors contributing to its initiation can be