In Situ Measurements of Nitrogen Cycling Across an Aquaculture Chronosequence

Aquaculture is increasing globally, yet its long-term environmental impact is not yet known. Using a novel, in situ approach we measured rates of N cycling across a chronosequence (or space for time substitution) in an oyster aquaculture farm in a temperate, coastal lagoon (Ninigret Pond, RI, USA). We hypothesized that rates of denitrification would increase as a function of age until a certain age when the sediments would no longer support this important ecosystem service.

Water samples were collected and analyzed for fluxes of N₂ and N₂O using membrane inlet mass spectrometry and gas chromatography methods, respectively. There were significantly higher fluxes of N₂ at sites below oyster culture (475.4-736.9 μmol N₂-N m^-2 hr^-1) compared to the control site (75.7 μmol N₂-N m^-2 hr^-1). Contrary to our hypothesis, there did not appear to be any pattern between age of culture and rate of denitrification. Alongside denitrification, we observed N₂O uptake for all ages of culture, with the greatest magnitude at the middle-aged site (3-4 years; -828.5 μmol N₂O m^-2 hr^-1), suggesting oyster aquaculture may stimulate sediment to become a sink for N₂O. We will discuss our results in terms of site environmental characteristics as well as the potential for oyster aquaculture to remove reactive nitrogen in coastal lagoons.