Quantifying the water-atmosphere flux of ammonia for the estuaries of the Texas coastal bend
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In the United States urbanization and agricultural activities within coastal watersheds have greatly contributed to excessive nutrient loading in downstream waters. As a result, a gross majority of U.S. estuaries are now considered to be ecologically impaired. Nitrogen (N) is often a limiting nutrient to primary production in estuarine waters and as such, excessive contributions have been linked to eutrophication, hypoxic events, and the emergence of harmful algal blooms (HABs). Such indicators of nutrient pollution have occurred in the surface waters of the Texas Coastal Bend, a coastal region of southeastern Texas, USA that borders the northwest Gulf of Mexico. Within that region, hypoxic episodes in areas of Corpus Christi Bay and persistent HABs in Baffin Bay have both been observed. Ammonium (NH4+) is an inorganic N species that in great enough concentrations, can directly influence such conditions as it is immediately bioavailable to primary producers. Total ammonia (NHx) refers to the combined concentration of both NH4+ and its complementary gaseous compound, ammonia (NH3). In water, NHx is partitioned between NH4+ and NH3 by the chemical and physical conditions which are present there. Further, when such aqueous concentrations of NH3 are great enough and favorable water quality and meteorological conditions exist, NH3 may be emitted from surface waters into the lower atmosphere. This water-atmosphere exchange process is bidirectional, allowing for both NH3 emission to the atmosphere, and atmospheric NH3 invasion into surface waters. Due to the two-way nature of this process, the determination of net NH3 deposition in coastal regions must factor local surface water NH3 emissions as well as ambient air NH3 concentrations to produce accurate estimates. Quantifying water-atmosphere NH3 flux was the primary objective of this study, where ten sites throughout the Coastal Bend were observed regularly during regional and local campaigns of eight and twelve months, respectively. Surface water NH4+ concentrations, atmospheric NH3 concentrations and a collection of supporting surface water and meteorological parameters were obtained to determine resulting rates of water-atmosphere NH3 flux. Across the entire Coastal Bend, a NH3 flux of 2.52 ± 3.57 ng m-2 s-1 was calculated, denoting net NH3 emission during the period of September 2018 - April 2019. Specific to the Corpus Christi area, a similarly upward water-atmosphere NH3 flux of 2.54 ± 1.23 ng m-2 s-1 was determined for the period of May 2018 - April 2019. Seasonal trends in water-atmosphere NH3 flux were evident as generally the late summer and fall months featured NH3 emission events from surface waters while winter and early spring months saw the deposition of atmospheric NH3. Individual locations displayed characteristic water-atmosphere NH3 flux signatures, including a site at San Antonio Bay where it is believed that a host of conditions unique to that estuary resulted in NH3 emission in two months during which all nearby bays displayed deposition. Within the Corpus Christi area, the NH3 fluxes of Corpus Christi Bay, the Upper Laguna Madre and the nearshore Gulf of Mexico appear to have been influenced by a collection of factors including the wet deposition of NH4+, surface water inflow and transport, and the transfer of NH4+ enriched sediment pore water into the overlying water column. Additionally, water-atmosphere NH3 emission events from the Gulf of Mexico periodically coincided with deposition at the Upper Laguna Madre, indicating a potentially important transport pathway for NH3 between coastal marine waters and a neighboring coastal lagoon. Bulk water-atmosphere NH3 estimates derived from the Corpus Christi area fluxes revealed an annual magnitude of NH3 emission that amounted to more than 30% of an earlier quantification of total N deposition to area surface waters. As a potentially substantial contributor to local ambient air NH3 concentrations, the water-atmosphere flux of NH3 requires comprehensive quantification across varying estuary systems to help guide mitigation efforts if NH3 emissions in the U.S. are ever subject to regulations similar to those set forth in European countries.
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