From the headwaters to the sea: The role of riparian, alluvial, and tidal wetlands to filter nutrients and ameliorate eutrophication

Abstract Wetlands are known for their ability to trap sediment and eliminate pollutants from the surrounding catchment. However, less is known regarding the differential role of headwater, mid‐catchment, and coastal wetlands in filtering these materials. Soil accretion, organic carbon (C), total nitrogen (N), and total phosphorus (P) were measured in wetlands from the headwaters to the mouth of the Altamaha River, Georgia, USA to assess how sediment deposition, C sequestration, and N and P burial vary along the waterway. Soil cores (n = 2 per site) were collected from riparian, upper and lower alluvial, tidal freshwater forest and marsh, brackish marsh, and salt marsh. Two‐centimeter depth increments were analyzed for 137Cs, to determine soil accretion, bulk density, and C, N, and P concentration. Accretion exhibited a bimodel distribution with the highest rates in riparian wetlands of the headwaters (3.9 mm/year) and in tidal fresh and brackish marshes (4.7–5.4 mm/year) of the estuary. Accretion rates were considerably lower in alluvial and tidal fresh forests and salt marshes (0.9–2.5 mm/year). Carbon sequestration and N burial followed a similar trend with the greatest accumulation in soils of tidal fresh and brackish marshes (102–150 g C/m2/year, 7.1–9.5 g N/m2/year) that had not only high accretion but also high organic matter content (11%–12% C). Riparian soils with their low C content, high bulk density, and high P content had much greater sediment deposition (3310 g/m2/year) and P burial (2.75 g P/m2/year) than other wetlands along the waterway (180–1730 g sediment/m2/year, 0.23–0.90 g P/m2/year). Results suggest that, in the Altamaha River, sediment deposition and P removal are maximized in the headwaters thereby protecting downstream freshwaters from the effects of P eutrophication. Tidal fresh and brackish marshes with their high rates of N burial can aid in protecting estuaries from N enrichment, many of which suffer from the effects of N eutrophication. Results from this study are scalable to other rivers of the southeastern U.S. piedmont and coastal plain and similar rivers of this size, topography, and geology.