Incorporation of brown tide into an estuarine food web

Abstract

In Laguna Madre, Texas, USA, a monaspecific brown tide bloom began in January 1990 and was still persisting at the time of this writing. Immediately following the start of the bloom, abundance, biomass and diversity of benthos declined, and have remained low for 6 yr. One explanation for the decline is that the brown tide organism is a poor food source. To determine whether the brown tide was incorporated into the estuarine food web, benthic invertebrates and fish were studied 14 mo after the bloom onset using stable carbon isotope ratios. Fish and benthos were collected from 2 areas, a seagrass habitat in Laguna Madre, and a muddy bottom habitat in the adjacent Alazan Bay. The muddy bottom fauna had a strong brown tide signature, indicating the incorporation of brown tide or brown tide detritus into the food web. The higher-biomass seagrass-fauna had heavier isotope values, reflecting incorporation of seagrass carbon in addition to brown tide. The top predators, Sciaenops ocellatus and Pogonias cromis, have different niches, but were able to switch food sources and thrive during the brown tide bloom. Brown tide appears to be able to support an estuarine food web, but at the expense of benthic diversity. The loss of benthic diversity could be due to allelopathy or the inability of some species to assimilate brown tide. Habitats with extensive seagrass beds maintain higher productivity and diversity than muddy habitats. However, seagrass habitats are endangered because brown tide reduces light levels, inhibits seagrass growth, and is causing seagrass decline.