THE ROLE OF HYPERSALINITY IN THE PERSISTENCE OF THE TEXAS BROWN TIDE IN THE LAGUNA MADRE

A brown tide bloom of the alga Aureoumbra lagunensis was present witho ut interruption in the Laguna Madre of Texas from January 1990 through October 1997. This is the longest continual phytoplankton bloom of wh ich we are aware. Although the factors leading to the initiation of th is bloom have been well documented, the factors contributing to its pe rsistence are still being investigated. Two physical characteristics o f the Laguna Madre may play an important role: the long turnover time for waters in this coastal lagoon (similar to 1 year) and the hypersal ine conditions that usually exist (40-60 PSU) due to evaporation excee ding precipitation. In this study, we examined the effects of salinity on the growth rates of the brown tide alga and on the growth of one o f its protozoan grazers. Historical data from before the onset of the brown tide provide evidence for the suppression of microzooplankton po pulations and mesozooplankton growth caused by hypersalinity. The brow n tide alga will grow in a remarkably wide range of salinities ranging from 10 to 90 PSU. Maximum growth rates are achieved at salinities ra nging from 20 to 60 PSU. One common grazer on the brown tide alga, the heterotrophic dinoflagellate Oxyrrhis marina, was found to grow more slowly under hypersaline conditions. The normally hypersaline conditio ns of the Laguna Madre may, therefore, favor the brown tide alga over other phytoplankton species that do not grow well under hypersaline co nditions, and also suppress the growth and feeding rates of potential grazers.