Combining Techniques to Conceptualise Denitrification Hot Spots and Hot Moments in Estuaries

Degradation of aquatic ecosystems from nutrient pollution is a global issue, and quantifying nutrient removal in coastal ecosystems is a topic of interest for coastal managers worldwide. Analysing relationships between natural nitrogen removal processes, such as denitrification, and environmental variables from an ecological (rather than biogeochemical) perspective may help to identify and predict biogeochemically important habitat patches (hot spots). However, in situ measurements of denitrification that are coupled with ecosystem variables are rare. In this study, we analysed a dataset encompassing 18 estuaries, broad environmental gradients, and two methods of measuring denitrification (denitrification enzyme activity (DEA) and in situ N 2 flux quantification) to better understand natural estuarine nitrogen removal processes and to rationalise methods. Generally poor relationships between denitrification measures and environmental variables suggest strong context dependency, with different activation or limiting reactants affecting denitrification rates differentially in space and time. This research illustrates how biogeochemically important habitat patches may develop and demonstrates that single-method studies have the potential to miss hot spots or hot moments of nitrogen removal. A two-method approach that integrates both long-term (DEA) and short-term (in situ N 2 flux) conditions is more likely to lead to the identification of biogeochemically important habitat patches. A better understanding of natural nitrogen removal processes in estuaries will clarify assimilative capacity questions and feed into eutrophication mitigation management efforts in these highly valued freshwater–coastal interface areas.