Variations in stable carbon isotope ratio of the copepod Acartia tonsa during the onset of the Texas brown tide

The Laguna Madre of South Texas is a shallow coastal lagoon whose dominant primary producers shifted from seagrasses to phytoplankton with the onset o f the Texas brown tide, which persisted from 1990 through 1997. Acartia ton sa is the dominant component of the mesozooplankton and forms an important link in both the phytoplankton and detritus-based pelagic food webs. Stable carbon isotope ratios of A. tonsa, as well as the two major primary produc ers: phytoplankton (as particulate organic carbon) and seagrasses, were mea sured from March 1989 to October 1991. Zoo-plankton samples were collected at four locations in the Laguna Madre: two in shallow water (c. 1 m) over s eagrass beds and two in slightly deeper water (c. 2-3 m) over a muddy botto m in a secondary bay without seagrasses. We found seasonal trends in the is otopic composition of A. tonsa collected within both habitats as well as di stinct differences between the average delta(13)C values of individuals col lected in the two regions. Isotopic ratios of animals collected during the summer months were generally 4-8 parts per thousand enriched in C-13 compar ed with those collected in the winter, at all stations. A. tonsa collected over seagrass beds were 2-5 parts per thousand more enriched in C-13 than t hose collected over muddy bottoms. These observations suggest carbon derive d from seagrasses can be an important source of nutrition for these copepod s in summer, especially for copepods living over seagrass beds. The effects of the persistent brown tide decreased the contribution of seagrasses as a carbon source for A. tonsa during the summer of 1991. The pathway by which seagrass carbon enters the diet of A. tonsa is unclear, but the two pathwa ys considered most likely are through copepods feeding on microzooplankton that have fed on bacteria nourished on seagrass carbon, or by copepods feed ing directly on particles of seagrass detritus.