Coastal tidal wetlands are perceived to provide nutrient dissipation services and serve as the final buffer between excess nutrient loads coming from nearby upland watersheds and sensitive estuarine waters. The construction and restoration of tidal marshes has the potential to benefit coastal waters. However, the water quality services of tidal wetlands have yet to be established with any certainty. This is in part due to the difficulty of monitoring these systems where flow and concentrations vary widely with tidal ebb and flood along with rainfall events mobilizing nutrients in pulses from upstream watersheds. In this article, we show over a period of 10 days following a rainfall event, the value of high temporal resolution data to characterize the complex nutrient and flow dynamics and to reliably calculate material balances in a created coastal marsh in North Carolina. Ultraviolet-visible spectrometers were used to obtain 15-min concentration data for nitrate, total Kjeldahl nitrogen, dissolved organic carbon, total suspended solids, phosphate, and total phosphorus. Our results show that a pulse of nitrate moved through the marsh from upstream agricultural production following the rainfall event and 25% (13 kg of 53 kg) of the nitrate was retained in the marsh over a period of 10 days. No other material showed a clear pulse from the upstream agricultural production. The marsh acted as a sink for total suspended solids (40 kg) and had near neutral mass balances for dissolved organic carbon, total Kjeldahl nitrogen, total phosphorus, and phosphate. Subsequent simulations indicated that different and erroneous results would have been obtained from 2, 6- or 12-h sampling intervals. These results demonstrate, even on a short term basis, why high-frequency data acquisition is necessary in these tidal marsh systems to truly quantify their impact on water quality ecosystem services.
The area was a natural wetland until it was drained and converted to agricultural production in the late 1970’s as North River Farms. During the fall of 2005 and the spring of 2006, 6.9 ha of brackish marsh and 1000 m of tidal stream were constructed on a portion of the land (Fig. 1). The goal of the marsh construction was to create habitat equivalent to that available in natural marshes, while improving the quality of water reaching the North River estuary. The stream and marsh were designed using reference-based design principles. To achieve target elevations, significant grading was required. Topsoil was stockpiled during excavation and replaced during final grading to provide suitable conditions for plant establishment. The constructed marsh was planted with Spartina alterniflora, Spartina patens, and Juncus roe- merianus at appropriate elevation ranges based on observations from local reference marshes.
|Climate change impacts
|Effect of Nbs on CCI
|Reduced water quality
|% nitrate retained by marsh over a period of 10 days.
The study site was a constructed marsh in Carteret County, North Carolina (34.82◦ N 76.61◦ W).