Assessment and valuation of nitrogen mitigation ecosystem services in natural and restored wetlands of the Texas coastal bend

Eutrophication leads to poor water quality, hypoxia, and biodiversity loss in aquatic ecosystems, which is a major issue in the Gulf of Mexico and its surrounding estuaries. It is largely caused by excess nutrients in aquatic environments. With population rising along the Texas coast, there is an increase in nitrogen-based nutrients output, through wastewater and agricultural runoff. It is critical to reduce nutrients input to nitrogen-limited waters to improve water quality and eliminate eutrophication. Wetlands are important ecosystems that offer many ecosystem services including nitrogen mitigation. Denitrification is the main pathway for removing excessive nitrogen-based nutrients in wetland sediments. In this study, nitrogen mitigation was quantified through potential denitrification measured in five wetland sites, two restored and three natural ones. Our results showed that the age of wetlands is a major factor regulating denitrification rates, with lowest average annual rates found in two restored sites, Egery Flats and the Nueces Bay restored marsh (11.46 and 10.85 kg N∙ha-2∙yr-1, respectively). Significantly higher rates were found in natural wetland sites, with mean annual rates of 22.5, 29.39, and 39.27 kg N∙ha-2∙yr-1 measured for the Aransas River Estuary, Oso Bay marsh, and the Naval Airbase Bridge, respectively. Temperature was another influencing factor for denitrification rates in three of the five sites. The seasonal denitrification rates measured in this study were used to quantify the economic value of nitrogen mitigation ecosystem services in the two restored wetland sites. The replacement cost was $36,565∙yr-1, and $8,125∙yr-1, for Egery Flats and Nueces Bay restored marsh, respectively, which is an equivalent value of $13.55∙kg N-1 removed.