Quantifying nutrient and suspended solids fluxes in a constructed tidal marsh following rainfall: The value of capturing the rapid changes in flow and concentrations

Etheridge, J. R., et al., 2015. Ecological Engineering

Original research (primary data)
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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.

Case studies

Basic information

  • Case ID: INT-154-1
  • Intervention type: Restoration
  • Intervention description:

    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.

  • Landscape/sea scape ecosystem management: Yes
  • Climate change impacts Effect of Nbs on CCI Effect measures
    Reduced water quality  Positive % nitrate retained by marsh over a period of 10 days.
  • Approach implemented in the field: Yes
  • Specific location:

    The study site was a constructed marsh in Carteret County, North Carolina (34.82◦ N 76.61◦ W).

  • Country: United States of America
  • Habitat/Biome type: Saltmarsh |
  • Issue specific term: Not applicable


  • Notes on intervention effectivness: Effectiveness determined based on assumption that without the restored marsh, no retained nutrients would occur therefore anything measured in the study would be additional to what would have been without it. (but note this was not explicitly written by the authors, we are inferring this was their intent)
  • Is the assessment original?: Yes
  • Broadtype of intervention considered: Not applicable
  • Compare effectivness?: No
  • Compared to the non-NBS approach: Not applicable
  • Report greenhouse gas mitigation?: No
  • Impacts on GHG: Not applicable
  • Assess outcomes of the intervention on natural ecosystems: No
  • Impacts for the ecosystem: Not reported
  • Ecosystem measures:
  • Assess outcomes of the intervention on people: No
  • Impacts for people: Not reported
  • People measures:
  • Considers economic costs: No
  • Economic appraisal conducted: No
  • Economic appraisal described:
  • Economic costs of alternative considered: No
  • Compared to an alternative: Not reported

Evaluation methodology

  • Type of data: Quantitative
  • Is it experimental: No
  • Experimental evalution done: Not applicable
  • Non-experimental evalution done: Empirical case study
  • Study is systematic: