A dynamic nutrient budget of subsystem interactions in a Salt Marsh Estuary

In tidal salt marsh estuaries, the different habitats of the ecosystem interact primarily through the tidal creek water column. These interactions include nutrient and materials exchanges with the salt marsh, oyster reefs, creek bottoms, and adjacent uplands. Nutrient budgets are often used to synthesize these kinds of subsystem exchange data, and are usually based on annual totals without accounting for nutrient variability at finer temporal resolutions. In this paper, we present a dynamic budget of carbon (C), nitrogen (N), and phosphorus (P) for the North Inlet estuary, South Carolina that synthesizes subsystem flux data in a new way. We have developed a dynamic budget that uses a tidal hydrology model to generate daily areas of inundated intertidal habitat (i.e. vegetated marsh and oyster reef) from tidal heights calculated hourly and combines them with flux data to determine a net daily input to, or removal from, the water column. Daily surpluses or deficits of each nutrient were compared with daily rates of change in observed tidally-averaged nutrient concentrations. Particular emphasis was placed on evaluating budget output from the intertidal subsystems. We compared our total annual budgets to values from syntheses of two North Inlet flux studies. Although areas of marsh inundated were 150-200 times greater than areas of oyster reef inundated, interactions per unit volume of estuarine water column were comparable in magnitude for soluble reactive P (SRP), particulate organic C (POC), and dissolved organic C (DOC). The marsh dominated the ammonium (NH⁺₄) and nitrate + nitrite (NN) exchanges in the summer but the NH⁺₄ and POC output were particularly sensitive to changes in oyster reef area. Winter and spring DOC release by the marsh coincided closely (in timing and magnitude) with the peak in DOC concentrations observed in the North Inlet estuary, suggesting that forest stream inputs of DOC are not nearly as important as has been hypothesized. Comparison of our budget predictions to a previous synthesis of the same subsytem flux data confirmed the power of using tidal hydrology to estimate subsystem interactions between sampling times. These comparisons also emphasized the importance of (1) water column processes to NH⁺₄ dynamics (2) subtidal benthic fluxes to DOC dynamics, and (3) external inputs to NN dynamics. By incorporating our best current knowledge of estuary-wide subsystem areas, the dynamic budget also allowed us to link subsystem flux data to the results of a study quantifying exchanges between the estuary and the coastal ocean. That comparison indicated the shortcomings of any site-specific extrapolation to whole-system conclusions where a homogeneous ecosystem must be assumed. We used the differences between our total annual C, N, and P budgets and reported exports of those constituents from the system to generate hypotheses and suggest future research efforts both at North Inlet and southeastern salt marsh estuaries in general.