Chronic enrichment affects nitrogen removal in tidal freshwater river and estuarine creek sediments

Abstract

Population growth in coastal areas increases nitrogen inputs to receiving waterways and degrades water quality. Wetland habitats, including floodplain forests and marshes, can be effective nitrogen sinks; however, little is known about the effects of chronic point source nutrient enrichment on sediment nitrogen removal in tidally influenced coastal systems. This study characterizes enrichment patterns in two tidal systems affected by wastewater treatment facility (WWTF) effluent and assesses the impact on habitat nitrogen removal via denitrification. We collected intact sediment cores from prevalent habitats in a tidal freshwater river (TFZ; swamp forest) and a tidal estuarine creek system (EST; salt marsh) upstream and downstream of a WWTF outfall, and quantified dissolved gas fluxes across the sediment-water interface during wet conditions in early summer and dry conditions in late summer. Data collected during two synoptic water quality monitoring campaigns complimented laboratory experiments to provide environmental context for biogeochemical processing. The two systems exhibited different enrichment patterns such that the river-dominated TFZ system was characterized by consistently elevated nitrate + nitrite concentrations downstream of the WWTF, whereas precipitation and tidal influence affected nutrient distributions in the EST creek. Downstream sediments in TFZ exhibit an apparent saturation response, while upstream rates may be limited by other factors, such as labile organic matter availability. In contrast, downstream sediments in EST denitrify at higher rates than upstream during wet conditions that may enhance transport of effluent. This work provides information on ecosystem functioning in human-influenced environments and can be of use in developing nature-based solutions, such as water treatment wetlands, for nitrogen removal.

FIGURE 1

Map of coastal wastewater treatment facilities in eastern North Carolina, land cover within tidal freshwater river (TFZ) and EST watersheds, and prevalent wetland habitat at sediment core sampling sites. Land cover data were obtained from the National Land Cover Dataset 2019.

FIGURE 2

Ambient nitrate + nitrite (NO_x) concentrations upstream and downstream relative to wastewater treatment facility (WWTF) outfall in both tidal freshwater river (TFZ) and EST systems (top), differences between upstream and downstream concentrations (middle), and mean concentration differences grouped by conditions.

FIGURE 3

Nitrate + nitrite (NO_x) concentration gradient in estuarine creek system (EST) during wet (June 6, 2021), dry (August 17, 2021), and dry‐HW (April 26, 2021) conditions. Map credits: Esri. NASA, NGA, USGS, FEMA, State of North Carolina DOT, HERE, Garmin, SafeGraph, GeoTechnologies, Inc., METI/NASA, EPA, NPS, US Census Bureau, USDA.

FIGURE 4

System‐wide N₂‐N flux measurements (µmol m⁻² s⁻¹; upstream and downstream measurements combined) on wet and dry sampling dates in tidal freshwater river (TFZ) and estuarine creek systems (EST).

FIGURE 5

Upstream and downstream N2‐N flux measurements (µmol m⁻² s⁻¹), as well as nitrate + nitrite (NO_x) concentrations (µM), chlorophyll‐a (chl‐a) levels (mg L⁻¹), and salinity (ppt) in the tidal freshwater river (TFZ) and estuarine creek system (EST) on wet and dry sampling dates during water quality monitoring campaigns. Solid points correspond with data from core collection.

FIGURE 6

Linear regressions comparing relationships between sediment oxygen demand and denitrification in tidal freshwater river (TFZ). Panel (A, left) includes data from the wet and dry core collection dates. Panel (B, right) includes data from previous studies in the TFZ, including Von Korff et al. (; upstream sites) and Ensign et al. (; downstream sites). Solid line indicates statistically significant linear relationship (p < 0.05).