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Morehead City, NC, United States

Von Korff B.H.,Water Resources University | Piehler M.F.,University of North Carolina at Chapel Hill | Ensign S.H.,Aquatic Analysis and Consulting LLC
Wetlands | Year: 2014

Denitrification in tidal freshwater river channels and their adjoining freshwater wetlands greatly affects nitrogen export from river networks, yet the relative importance of these two habitats to nitrogen export has not been examined. Knowledge of how these habitats contribute to denitrification of the river nitrogen load is critical for improving models of nitrogen transport. Denitrification rates were measured in sediments from the channel, bank, and floodplain at upstream and downstream sites of two forested tidal freshwater zones (TFZs) in North Carolina, the New River and Newport River, using membrane inlet mass spectrometry to measure N2production. Denitrification rates did not usually differ statistically between the channel, bank, and floodplain, although denitrification was highest on the floodplain at the upstream site in the Newport River. When these rates were extrapolated across the entire area of the TFZ, the channel contributed more to the N2flux than the riparian zone. These results indicate that denitrification rates are comparable between the channel and riparian zone in forested TFZs, and that the importance of the channel versus the riparian zone depends on channel and floodplain morphology. © 2014, Society of Wetland Scientists.

Ensign S.H.,U.S. Geological Survey | Ensign S.H.,Aquatic Analysis and Consulting LLC | Noe G.B.,U.S. Geological Survey | Hupp C.R.,U.S. Geological Survey
Journal of Geophysical Research: Earth Surface | Year: 2014

The hydrologic processes by which tide affects river channel and riparian morphology within the tidal freshwater zone are poorly understood yet are fundamental to predicting the fate of coastal rivers and wetlands as sea level rises. We investigated patterns of sediment accretion in riparian wetlands along the nontidal through oligohaline portion of two coastal plain rivers in Maryland, U.S., and how flow velocity, water level, and suspended sediment concentration (SSC) in the channel may have contributed to those patterns. Sediment accretion was measured over a 1 year period using artificial marker horizons, channel hydrology was measured over a 1 month period using acoustic Doppler current profilers, and SSC was predicted from acoustic backscatter. Riparian sediment accretion was lowest at the nontidal sites (mean and standard deviation = 8 ± 8 mm yr-1), highest at the upstream tidal freshwater forested wetlands (TFFW) (33 ± 28 mm yr-1), low at the midstream TFFW (12 ± 9 mm yr-1), and high at the oligohaline (fresh-to-brackish) marshes (19 ± 8 mm yr-1). Channel maximum flood and ebb velocity was twofold faster at the oligohaline than tidal freshwater zone on both tidal rivers, corresponding with the differences in in-channel SSC: The oligohaline zone's SSC was more than double the tidal freshwater zone's and was greater than historical SSC at the nontidal gages. The tidal wave characteristics differed between rivers, leading to significantly greater in-channel SSC during floodplain inundation in the weakly convergent than the strongly convergent tidal river. High sediment accretion at the upstream TFFW was likely due to high river discharge following a hurricane. Key Points Sediment accretion was highest near the head of tide and oligohaline estuary Velocity and suspended sediment were lower in the tidal fresh than oligohaline Tidal stage-velocity phase lag affected suspended sediment concentration ©2013. American Geophysical Union. All Rights Reserved.

Leech D.M.,Longwood University | Ensign S.H.,Aquatic Analysis and Consulting LLC | Piehler M.F.,University of North Carolina at Chapel Hill
Aquatic Sciences | Year: 2016

We examined seasonal and spatial patterns in dissolved organic carbon (DOC) and chromophoric dissolved organic matter (CDOM) in the Chowan River watershed, North Carolina, a blackwater river which discharges into the second largest estuary in the United States, the Albemarle–Pamlico Estuarine System. From April 2008 to May 2010, DOC concentration did not significantly vary across seasons (range 7.69–30.39 mg L−1); however, CDOM molecular size and aromaticity increased throughout the spring, decreased during the summer and fall, and remained relatively low in the winter. Spectral slope ratios suggested microbial processing of CDOM in the spring and photodegradation of CDOM in the summer and fall. Spatially, DOC and CDOM concentrations were similar in the mainstem and at the mouths of two tributaries, Bennetts Creek and Wiccacon River, but were significantly higher upstream on the tributaries. DOC concentration was positively correlated with CDOM absorbance coefficients at 254 and 350 nm; however, these optical proxies explained only ~60 % of the variance. DOC and CDOM absorption loads to the Albemarle Sound ranged from 2.63 × 1010 g year−1 and 9.84 × 1010 m2 year−1, respectively, in a dry year and 7.9 × 1010 g year−1 and 2.2 × 1011 m2 year−1, respectively, in a wet year, which are comparable to non-blackwater rivers with larger watersheds. Blackwater rivers may therefore represent “hotspots” in coastal carbon chemistry, with seasonal variations in the quality and quantity of DOC and CDOM influencing estuarine food web dynamics and net ecosystem metabolism. © 2016 Springer International Publishing

Ensign S.H.,University of North Carolina at Chapel Hill | Leech D.M.,University of North Carolina at Chapel Hill | Leech D.M.,Aquatic Analysis and Consulting LLC | Piehler M.F.,University of North Carolina at Chapel Hill | Piehler M.F.,Longwood University
Ecosphere | Year: 2014

Phytoplankton biomass is commonly used as a water quality metric in the management of anthropogenic nitrogen and phosphorus loading, yet interpretation of the long-term response of phytoplankton biomass to nutrient regulation may be confounded by changes in zooplankton grazing pressure. Zooplankton community structure may be affected by planktivorous fish whose populations are subject to losses through fishing and gains through restoration efforts.We investigated temporal changes in phytoplankton biomass, nitrogen and phosphorus concentrations, zooplankton, and planktivorous fish in the tidal fresh and oligohaline Chowan River, NC, over a 30-year period in order to compare long-term trends in these parameters and infer their relative influence on phytoplankton biomass. Data were compiled from the records of two state agencies and several academic studies and supplemented by our own two year monitoring of water quality and zooplankton in the Chowan River mainstem and two tributaries. Seasonal trend decomposition using locally weighted regression was applied to chlorophyll a (a proxy for phytoplankton biomass), nitrogen, and phosphorus, and showed that chlorophyll a declined more quickly than did nutrient concentrations over the 30-year period. Despite the long-term decline in nitrogen and to a lesser extent phosphorus, the long-term trend in phytoplankton growth rate (predicted using an empirical model involving irradiance and nutrients) remained relatively constant. Zooplankton abundance increased from the period 1981-1982 to 2008-2010, as did the predicted zooplankton community water clearance rate (a proxy for zooplankton grazing). River herring, historically the dominant planktivore, declined dramatically over the 30-year period. The data indicate that the most parsimonious explanation for the long term decrease in chlorophyll a is an increase in zooplankton and not a decrease in nutrients. This inference leads us to hypothesize that the historically large river herring population exerted a positive, indirect influence on phytoplankton biomass in the Chowan River system, and that the decline in river herring has lead to higher zooplankton abundance and subsequently lower phytoplankton biomass.[[ampi]]copy; Ensign et al.

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