The biological utilization will be measured and the origins will be identified for dissolved organic matter (DOM) fueling microbial metabolism in riverine saltmarsh estuaries. Because most heterotrophic activity in estuaries is bacterial in origin, this research will focus on sources of DOM used by bacteria in order to elucidate a major pathway of energy use in coastal ecosystems. Stable isotope signals will be utilized to identify the ultimate source of DOM used by bacteria, tracing organic matter flow from riverine, saltmarsh, and phytoplankton sources of DOM that have distinct stable isotopic compositions. To trace DOM use, a bioassay has been developed that involves growth of natural bacteria in filtered water coupled with subsequent determination of the delta 13C value of bacterial biomass, and in some cases, delta 15N. The isotopic measurements will indicate the source of DOM utilized. Bacterial growth rates and yields in the bioassay indicate the nutritional quality and quantity of the labile substrate source. Experiments will be extended throughout an annual cycle in the Parker River of northern Massachusetts to determine the isotopic composition of estuarine DOM that is ultimately derived from phytoplankton, Spartina, and upland plants, and to document the relationships between these sources of organic matter and bacterial growth via stable isotopic tracer measurements. Additional experiments will be conducted in a broad suite of other estuarine systems with substantially different presumptive sources of organic carbon in order to broaden the comparison and generalize the research results. The research group for this project is excellent. Facilities for the research are first class. Productive collaborations will be pursued. Practically, the research will provide information pertinent to the understanding of biological productivity of estuaries. Such estuaries are often characterized is the fragile nurseries for the commercial fishing industry.
