Use of stable isotope analyses to describe trophic dynamics of aquatic ecosystems in Galveston Bay, Texas

Food webs in estuarine ecosystems are characterized by omnivory and an abundance of detritivores in addition to seasonal changes in species and physiochemical conditions. Therefore, it is difficult to identify dominant linkages of energy flow in this very complex and dynamic environment. Attempts to construct food webs using stomach content analyses are unsatisfactory because they cannot identify assimilated dietary components. The use of stable isotope analysis, when combined with dietary data, offers a more powerful method for evaluating the trophic classification of an organism. The primary source of productivity is determined through 13C content while the trophic level is determined through 15N. This study used dual stable isotope analyses to identify the primary sources and pathways of nutrition and the trophic level for the main species in the Galveston Bay Estuary Ecosystem (GBEE) from five different sub-bays; Christmas, East, Galveston, Trinity, and West Bays. Additionally, an analysis on the effects that thawing and utilizing different storage techniques has on stable isotope signatures was conducted. The 13C analysis showed that for the eastern section of the GBEE, the food web supporting the majority of the species was based on a mixture of phytoplankton and epiphytic algae and/or detritus. For the western section of the GBEE, epiphytic algae and/or detritus are very important. Few of the species examined assimilated one basal carbon source exclusively; instead a mixture of sources at each sub-bay appeared to be used. The 15N analysis showed that nutrient cycling in the upper portion of the GBEE is heavily influenced by anthropogenic sources from the Trinity and San Jacinto Rivers whereas other secondary bay communities including Christmas and West Bay are primarily driven by in-situ production from marshes and seagrass beds. When testing the effects that different storage methods and thawing had on the isotopic signatures, it was found that there was no significant difference in storing the samples on ice or flash-freezing them and no significant difference between the control and samples left out to thaw for 1, 3, 5, and 10 days. However, it appeared that the δ15N values were becoming more enriched the longer the samples were left out to thaw, although this was not statistically significant.