Habitat-specific distinctions in estuarine denitrification affect both ecosystem function and services

Abstract

Abstract. Resource limitation controls the base of food webs in many aquatic ecosystems. In coastal ecosystems, nitrogen (N) has been found to be the predominant limiting factor for primary producers. Due to the important role nitrogen plays in determining ecosystem function, understanding the processes that modulate its availability is critical. Shallow-water estuarine systems are highly heterogeneous. In temperate estuaries, multiple habitat types can exist in close proximity to one another, their distribution controlled primarily by physical energy, tidal elevation and geomorphology. Distinctions between these habitats such as rates of primary productivity and sediment characteristics likely affect material processing. We used membrane inlet mass spectrometry to measure changes in N 2 flux (referred to here as denitrification) in multiple shallow-water estuarine habitats through an annual cycle. We found significantly higher rates of denitrification (DNF) in structured habitats such as submerged aquatic vegetation, salt marshes and oyster reefs than in intertidal and subtidal flats. Seasonal patterns were also observed, with higher DNF rates occurring in the warmer seasons. Additionally, there was an interaction between habitat type and season that we attributed to the seasonal patterns of enhanced productivity in individual habitat types. There was a strong correlation between denitrification and sediment oxygen demand (SOD) in all habitats and all seasons, suggesting the potential to utilize SOD to predict DNF. Denitrification efficiency was also higher in the structured habitats than in the flats. Nitrogen removal by these habitats was found to be an important contributor to estuarine ecosystem function. The ecosystem service of DNF in each habitat was evaluated in US dollars using rates from a regional nutrient-offset market to determine the cost to replace N through management efforts. Habitat-specific values of N removal ranged from approximately three thousand U.S. dollars per acre per year in the submerged aquatic vegetation to approximately four hundred U.S. dollars per acre per year in the subtidal flat. Because of the link between habitat type and processes such as DNF, changes in habitat area and distribution will have consequences for both ecosystem function and the delivery of ecosystem services.

Changes in Sediment Characteristics upon Oyster Reef Restoration, NE Florida, USA

“…Benthic microalgae may also stimulate denitrification by contributing labile organic carbon and increasing oxygenation of surface sediments [33]. Sediments from restored oyster reefs have been shown to be active sites of denitrification [18][19][20], with the highest sedimentary denitrification rate ever reported [17]. However, OM enrichment may also cause persistent anoxia within sediments, which may cause reduced N2 production if the supply of NO3 becomes limiting [33].…”

“…However, mesocosm experiments show that physical factors such as bottom shear can influence sediment resuspension and benthic micro-algal biomass, making the system more complex and the likelihood of OM burial versus remineralization more difficult to predict [7]. Several recent studies suggest that sediments associated with natural and restored oyster reefs have high rates of denitrification and may thus represent important sites for long term nitrogen removal [17][18][19][20]. Whole-creek studies and some mesocosm studies do not parse the contributions of the oysters themselves versus the associated sediments but rather consider the reef-sediment system as a whole [3,5,17].…”

Carbon and Nitrogen Stocks and Below-Ground Allometry in Temperate Mangroves

“…Based on the conservative NZU pricing we estimate the C-value of the loss of temperate A. marina forests at US $490 ha −1 . In addition, the removal of 1 ton of N in estuarine systems has been valued at approximately US $15000 (Newell et al, 2002;Piehler and Smyth, 2011;Beseres Pollack et al, 2013). This is equivalent to US $124000 per ha −1 of A. marina forest, assuming a similar calculation applies.…”

“…Various studies have now provided a monetary value for C and N (Newell et al, 2002;Piehler and Smyth, 2011;Beseres Pollack et al, 2013;Moore and Diaz, 2015) making the value of C and N stocks in estuarine ecosystems easier to assess. The value of 1 ton of C ranges widely, from US $6.20 (New Zealand Unit, NZU, spot price, February 2016) to $220 (Moore and Diaz, 2015).…”

“…The value of 1 ton of C ranges widely, from US $6.20 (New Zealand Unit, NZU, spot price, February 2016) to $220 (Moore and Diaz, 2015). In comparison, the removal of 1 ton of N in estuarine systems has been valued at approximately US $15,000 (Newell et al, 2002;Piehler and Smyth, 2011;Beseres Pollack et al, 2013). Regardless of the pricing model used, these valuations require accurate estimates of C and N stocks within the system.…”