Extracellular enzyme activity and uptake of carbon and nitrogen along an estuarine salinity and nutrient gradient

Amino acid oxidation (AAO) and peptide hydrolysis (PH) are processes affecting the recycling of organic material and nutrients. We compared extracellular AAO and PH rates to C and N uptake rates along estuarine gradients of salinity, nutrients and productivity in the Pocomoke River, a subestuary of the Chesapeake Bay. This estuary is seasonally depleted in inorganic N, and rich in dissolved organic material (DOM) throughout the year. AAO, PH, and N uptake rates measured in 1999 and 2000 were not limited to particular size fractions measured, or to auto- or heterotrophic groups of organisms. At a station near the turbidity maximum, where chlorophyll a biomass was highest, smaller (<1.2 mum) size-fractions contributed <20% of the AAO in May and up to 80% in August when AAO rates were similar to10 times lower. Most PH was in the larger (>1.2 mum) size-fraction, except at the least saline station in August of both years. Rates of AAO and PH were not linearly correlated with each other seasonally or spatially. Uptake of NH4+ dominated total N uptake (>50%) at all but the freshwater station, although uptake of organic compounds was measurable at all sites. Rates of dissolved free amino acid uptake, measured using dually labeled compounds, were substantial (up to 11% of the total N uptake) and contributed both C and N for growth. Dual labels unambiguously demonstrated that uptake rates of amino acid C and N were uncoupled; amino acid N was taken up preferentially to amino acid C even when rates were corrected for N uptake from AAO. Conceptual models of DOM cycling should include the realization that enzymatic processes and uptake of DOM occur in both 'microbial' and larger size fractions. Thus, competition between bacteria and phytoplankton mixotrophs may be an important factor determining the relative uptake of C and N from amino acids and other organic substrates.