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%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.

The exopolymer secretions (EPS) layer surrounding Aureoumbra lagunensis cells affects growth, grazing, and behavior of protozoa

“…The bloom was formed by Aureoumbra lagunensis (Stockwell, DeYoe, Hargraves, and Johnson), a small pelagophyte that shares some similarities to the species that forms brown tides along the coast of Long Island (DeYoe et al 1997). It has been suggested that hypersaline conditions in the Laguna Madre may favor brown tide blooms (Buskey et al 1998). Several lines of evidence support this hypothesis.…”

“…These findings lead us to hypothesize that the increased EPS mucus layer surrounding A. lagunensis cells may prevent effective grazing by microzooplankton. Previous studies (Buskey and Hyatt 1995) found lower growth rates of protozoa fed A. lagunensis compared to other similarly sized algal species, but the mechanism causing the reduced growth was unknown.…”

“…Several lines of evidence support this hypothesis. First, A. lagunensis can grow at its maximum rate at salinities as high as 70 PSU (Buskey et al 1998). Second, fewer phytoplankton species were found under hypersaline conditions, hence there were few potential competitors.…”

“…Second, fewer phytoplankton species were found under hypersaline conditions, hence there were few potential competitors. Third, the high salinity caused a decline of microzooplankton and benthic filter-feeders; therefore, grazing pressure was reduced in hypersaline waters (Buskey et al 1997). A. lagunensis possesses a mucus layer of exopolymeric secretions (EPS) outside its cell wall (DeYoe et al 1997).…”

“…All three species are common components of the microzooplankton community in the brown tide impacted waters of the Laguna Madre. The heterotrophic dinoflagellate Oxyrrhis marina was isolated from the Aransas Ship Channel near the University of Texas Marine Science Institute in Port Aransas as described in Buskey et al (1998). The two species of ciliates, Euplotes sp.…”