Abstract
Estuarine tidal creeks are an important conduit for freshwater run-off into the coastal ocean. In Long Bay, South Carolina, tidal creeks terminate in swashes—broad sandy fields constantly reworked by discharged creek water. We examined the role of a highly permeable sandy column in altering the nutrient loading of the passing water at Singleton Swash, Myrtle Beach, South Carolina. Seasonal transects along the swash’s primary channel documented gradients in physical and biogeochemical parameters. The nutrient and chlorophyll a concentrations were higher in the sediment than in the overlying water, consistent with coastal sediments as a major site of organic matter degradation, nutrient regeneration, and benthic primary productivity. Oxygen, nutrient, and chlorophyll concentrations exhibited a strong seasonal component, explained by a photosynthesis–respiration balance shift between summer and winter. The conservative mixing model approach to elucidate the sink–source patterns was moderately informative due to the lack of a gradual salinity gradient from land to ocean, due to substantial tidal flushing and observable nutrient-rich surface freshwater discharges along the channel that fueled substantial submerged aquatic macroalgal growth. Future studies should focus on the role of benthic photosynthesizers, both microbial and macroalgal, in retaining land-derived nutrients in irrigation freshwater inputs prior to them reaching the coastal ocean.
Highlights
Populations in coastal regions of the U.S are continuously expanding at a rate of almost 10% every ten years [1]
∆C = CMeas − CModel where C can be replaced by the formula or notation of the compound under investigation, CMeas is the measured concentration at non-end-member stations, and CModel is the theoretical concentration predicted by the linear conservative mixing model, as follows: CModel = mModel × SalMeas + bModel where SalMeas is the measured salinity at a non-end-member station, m is the slope of the linear model and b is the intercept of the linear model
The sedimentary and physical properties across the study site are summarized in Table 1 and Overall, the sediment on the shore and along the primary channel is classified as moderately sorted medium sand with symmetrical skewness and a high permeability (>10−12 m2 )
Summary
Populations in coastal regions of the U.S are continuously expanding at a rate of almost 10% every ten years [1]. A key driver of sandy column function is its high permeability, which results in rapid filtration of organic matter, nutrient regeneration, and primary productivity as the regenerated nutrients are transported rapidly to the sediment surface and overlying water column [9,10,11,12]. The implications of this rapid cycling exchange on biogeochemistry and ecology have been studied extensively over a variety of spatial and temporal scales [13,14,15]. Our study fundamentally explores how a living, highly-permeable medium, such as a sandy beach, may interact with and modify the biogeochemical composition of surface runoff entering the coastal ocean
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