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 without interruption in the Laguna Madre of Texas from January 1990 through October 1997. This is the longest continual phytoplankton bloom of which we are aware. Although the factors leading to the initiation of this bloom have been well documented, the factors contributing to its persistence are still being investigated. Two physical characteristics of the Laguna Madre may play an important role: the long turnover time for waters in this coastal lagoon (similar to 1 year) and the hypersaline conditions that usually exist (40-60 PSU) due to evaporation exceeding 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 of its protozoan grazers. Historical data from before the onset of the brown tide provide evidence for the suppression of microzooplankton populations and mesozooplankton growth caused by hypersalinity. The brown tide alga will grow in a remarkably wide range of salinities ranging from 10 to 90 PSU. Maximum growth rates are achieved at salinities ranging 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 conditions of the Laguna Madre may, therefore, favor the brown tide alga over other phytoplankton species that do not grow well under hypersaline conditions, and also suppress the growth and feeding rates of potential grazers.