| Estuaries are very dynamic ecosystems with regard to the transport and transformation of organic matter. Detrital organic matter is abundant in most estuaries, however, the dynamics of detritus utilization is not well understood. Two questions that remain unanswered are the sources of the detrital material and how the detritus is utilized by the different consumer groups in both the water column and surface sediments. A combination of phospholipid fatty acid analysis (PLFA) and compound specific isotope analysis (CSIA) was used to provide information on the possible sources, distribution, and transformation of sedimentary organic matter in the Altamaha and Savannah River estuaries in GA. Total PLEA concentration, a measurement of microbial biomass and community structure, was similar in both rivers. The highest microbial biomass was observed in the mesohaline zone apparently in response to an input of labile organic matter at the mixing zone. Microbial community structure differed significantly both as a function of time and type of river. A lower concentration of Gram positive indicators in the Savannah River relative to the Altamaha River, appeared to be a result of the effect of land use changes in the Savannah River basin. Selective utilization of C sources by specific microbial groups was demonstrated with CSIA. This analysis indicated that Gram positive bacteria degraded 13C-enriched material (marine C), whereas Gram negative bacteria assimilated higher concentrations of C3 material. In general, the results indicated that estuarine systems under the influence of large riverine inputs are subjected to seasonally differing regimes of microbial C cycling in response to changes in rain episodes and river discharge. However, the seasonality can be readily altered by anthropogenic alterations to river flow and other land use changes that affect the natural C cycling at the sediment-water interface of the estuarine continuum. |
