| Summary: | 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, ...
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