Wastewater influences nitrogen dynamics in a coastal catchment during a prolonged drought

Abstract

Ecosystem function measurements can enhance our understanding of nitrogen (N) delivery in coastal catchments across river and estuary ecosystems. Here, we contrast patterns of N cycling and export in two rivers, one heavily influenced by wastewater treatment plants (WWTP), in a coastal catchment of south Texas. We measured N export from both rivers to the estuary over 2 yr that encompass a severe drought, along with detailed mechanisms of N cycling in river, tidal river, and two estuary sites during prolonged drought. WWTP nutrient inputs stimulated uptake of N, but denitrification resulting in permanent N removal accounted for only a small proportion of total uptake. During drought periods, WWTP N was the primary source of exported N to the estuary, minimizing the influence of episodic storm-derived nutrients from the WWTP-influenced river to the estuary. In the site without WWTP influence, the river exported very little N during drought, so storm-derived nutrient pulses were important for delivering N loads to the estuary. Overall, N is processed from river to estuary, but sustained WWTP-N loads and periodic floods alter the timing of N delivery and N processing. Research that incorporates empirical measurements of N fluxes from river to estuary can inform management needs in the face of multiple anthropogenic stressors such as demand for freshwater and eutrophication.

Figure 1

Map of the Mission and Aransas river watersheds flowing into Copano Bay, Texas. Squares denote upper river samplings sites, also the location of the USGS gauging station for each river. Circles denote tidal river sampling sites and diamonds denote estuary sites (lower inset shows tidal river sites and estuary sites in more detail). Open triangles show all waste‐water treatment plant discharge points in each catchment permitted to release > 0.5 million gallons d⁻¹.

Figure 2

Discharge (A), ${\text{NH}}_4^{+}$ export (B), and ${\text{NO}}_3^{-}$ export (C) based on routine sampling and LOADEST modeling for the period from 01 August 2010 to 01 August 2012. Discharge and nutrient concentration samples are collected from river sites.

Figure 3

Characterization of N export for the Mission (a) and Aransas (b) Rivers during periods of flood and baseflow from 01 August 2010 to 01 August 2012 based on sampling at the Mission and Aransas River sites. River sampling locations drain 67% and 30% of the Mission and Aransas River watersheds, respectively. During the drought, tributaries below the sampling points were dry, and for the Aransas River, WWTP inputs above the sampling site dominate flow.

Figure 4

Molar dissolved inorganic nitrogen (DIN) to soluble reactive phosphorus (SRP) based on routine sampling in the Aransas and Mission Rivers from 01 August 2010 to 01 August 2012.

Figure 5

Salinity (A), nitrate ( ${\text{NO}}_3^{-}$) concentrations (B), ammonium ( ${\text{NH}}_4^{+}$) concentrations (C), dissolved organic nitrogen (DON) concentrations (D), dissolved organic carbon (DOC) concentrations (E) and soluble reactive phosphorus (SRP) concentrations (F) at all river, tidal river, and estuary sites from August 2010 to August 2012. Samples collected monthly except for more frequent intervals following storm events.

Figure 6

Fates of N in the Mission river system (A), Aransas river system (B), and central estuary site (C). Positive fluxes of N are out of the sediment, negative fluxes are into the sediment. River sites were measured on 10 July 2011 and 18 July 2011, tidal river sites on 12 July 2011, and estuary sites on 29 June 2011.

Figure 7

Nitrate ( ${\text{NO}}_3^{-}$) concentrations (A), potential denitrification rates (B), sediment oxygen demand (C), and denitrification to ${\text{NO}}_3^{-}$ ratio (D) across the river‐estuary sites. River sites were measured on 10 July 2011 and 18 July 2011, tidal river sites on 12 July 2011, and estuary sites on 29 June 2011. ${\text{NO}}_3^{-}$ was below detection in the western Copano Bay site, therefore the denitrification: ${\text{NO}}_3^{-}$ could not be calculated for this site. Dashed vertical lines denote the Mission and Aransas river systems.