Nutrient Limitation of Benthic Biofilms Along the Estuarine Salinity Gradient

Project Goals Two of the most pressing concerns for Georgia estuaries are eutrophication, caused by nutrient enrichment, and saltwater intrusion, resulting from reduced freshwater flows and accelerated sea-level rise (Alber and Flory 2002, Weston et al. 2006, Neubauer and Craft 2009, Weston et al. 2010, Alber and Sheldon 2011). Salinity and increased nutrient loads are predicted to act synergistically to alter ecosystem metabolism, but the overall consequences of this interaction are not well known (Tamminen and Andersen 2007). Classically, freshwater reaches are P limited due to iron complexation with orthophosphate, while saline reaches become progressively more N limited as the sulfate in seawater complexes iron, releasing bound P. Recent bioassays in rivers (Mallin 2000), coastal wetlands (Sundareshwar et al. 2003) and continental freshwaters (Johnson et al. 2009) show differential nutrient limitation (DNL) of autotrophs (N limited) and heterotrophs (P limited). Thus, understanding DNL of benthic and pelagic autotrophs and heterotrophs may be key to understanding eutrophication in the Altamaha River. Previous research has examined nutrient limitation of coastal ecosystems in a variety of methods including large-scale and long-term nutrient additions to tidal marshes, short-term nutrient additions in laboratory bottle assays, and speculation derived from nutrient concentrations and ecological stoichiometry. In continental freshwater habitats (streams, lakes), nutrient limitation of benthic biofilms is commonly assessed using an in situ bioassay (nutrient diffusing substrata, NDS). This technique can bridge the gap between long-term and short-term nutrient limitation experiments by examining nutrient limitation over a month in situ (mid-term nutrient limitation). To examine benthic biofilm nutrient limitation, we will deploy NDS’s at multiple sites along a salinity gradient at 5 sites corresponding to tidal fresh forests, tidal fresh marsh (GCE 7), brackish marsh (GCE 8), river influenced salt marsh (GCE 9), and marine dominated salt marsh (GCE 6). NDS’s will consist of a 2% agar solution amended with either no nutrient (+0), nitrate (+N), phosphate (+P), glucose (+C), or a combination of these nutrients in a fully factorial design (n=4 per treatment). NDS’s will be topped with an inorganic fritted glass disc and deployed for ~1 month. At each site, one set of NDS’s will be placed upright for autotrophic and heterotrophic biofilm development and one set will be place upside-down under a shade cloth to select for heterotrophic biofilm development (total n=64 per site). After retrieval, each substrate will be analyzed for gross primary production (GPP) and community respiration (CR) using a light/dark bottle oxygen assay, chlorophyll a using hot ethanol extraction, and ash-free dry mass via loss on ignition.