Tidal nitrogen exchanges across a freshwater wetland succession gradient in the upper Cooper River,South Carolina |
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| Authors: | H N McKellar D L Tufford M C Alford P Saroprayogi B J Kelley J T Morris |
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| Institution: | 1.South Carolina Department of Natural Resources,Freshwater Fisheries Research Laboratory,Eastover;2.Department of Environmental Health Sciences,University of South Carolina,Columbia;3.Department of Biological Science,University of South Carolina,Columbia;4.Biology Department,The Citadel,Charleston |
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| Abstract: | Tidal freshwater sections of the Cooper River Estuary (South Carolina) include extensive wetlands, which were formerly impounded
for rice culture during the 1,700s and 1,800s. Most of these former rice fields are now open to tidal exchange and have developed
into productive wetlands that vary in bottom topography, tidal hydrography and vegetation dominants. The purpose of this project
was to quantify nitrogen (N) transport via tidal exchange between the main estuarine channel and representative wetland types
and to relate exchange patterns to the succession of vegetation dominants. We examined N concentration and mass exchange at
the main tidal inlets for the three representative wetland types (submerged aquatic vegetation SAV], floating leaf vegetation,
and intertidal emergent marsh) over 18-21 tidal cycles (July 1998–August 2000). Nitrate + nitrite concentrations were significantly
lower during ebb flow at all study sites, suggesting potential patterns of uptake by all wetland types. The magnitude of nitrate
decline during ebb flow was negatively correlated with oxygen concentration, reflecting the potential importance of denitrification
and nitrate reduction within hypoxic wetland waters and sediments. The net tidal exchange of nitrate + nitrite was particularly
consistent for the intertidal emergent marsh, where flow-weighted ebb concentrations were usually 18–40% lower than during
flood tides. Seasonal patterns for the emergent marsh indicated higher rates of nitrate + nitrite uptake during the spring
and summer (> 400 μmol N m-2 tide-1) with an annual mean uptake of 248 ± 162 μmol m–2 tide–1. The emergent marsh also removed ammonium through most of the year (207 ± 109 μmol m–2 tide–1), and exported dissolved organic nitrogen (DON) in the fall (1,690 ± 793 μmol m–2 tide–1), suggesting an approximate annual balance between the dissolved inorganic N uptake and DON export. The other wetland types
(SAV and floating leaf vegetation) were less consistent in magnitude and direction of N exchange. Since the emergent marsh
site had the highest bottom elevation and the highest relative cover of intertidal habitat, these results suggest that the
nature of N exchange between the estuarine waters and bordering wetlands is affected by wetland morphometry, tidal hydrography,
and corresponding vegetation dominants. With the recent diversion of river discharge, water levels in the upper Cooper estuary
have dropped more than 10 cm, leading to a succession of wetland communities from subtidal habitats toward more intertidal
habitats. Results of this study suggest that current trends of wetland succession in the upper Cooper River may result in
higher rates of system-wide inorganic N removal and DON inputs by the growing distributions of intertidal emergent marshes. |
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