Nitrogen and Warming Effects on Nitrous Oxide Production Associated with Coastal Macroinvertibrates

Bivalve shellfish potentially reduce excess nitrogen in the water column, however they can also be involved in the emission of nitrous oxide (N2O), a potent greenhouse gas. Environmental controls on N2O production from bivalves have not been well quantified. We tested responses of N2O production by three bivalves (Mytilus edulis, Mercenaria mercenaria and Crassostrea virginica) to nitrogen (N) loading and/or warming after immediate (1 day) and short-term (14-28 days) exposure. This two-factor laboratory study had four treatments: (1) ammonium nitrate (N) addition (targeting 100μM-N), (2) warming (22oC), (3) N addition + warming and (4) no N addition or warming (control, 19oC). Potential N2O production rates were higher in response to N additions for all bivalves, particularly with short-term exposures. Warming had a small but significant impact on N2O production from M. mercenaria, confounded by a significant interaction of exposure X warming and exposure X nitrogen X warming. Similarly, C. virginica also showed a significant interaction of exposure X warming, indicating that longer exposure to warming may influence N2O production from this species. M. edulis showed the highest N2O production rates, reaching 252 nmol N2O ind-1 hr-1, more than an order of magnitude higher than the previously largest reported rates. However, mass-specific rates (7.5 nmol N2O g-1 hr-1) were the same order of magnitude as previous studies. Notably, N2O production associated with M. edulis were obtained while the organisms had poor health, which likely induced high respiration rates and was probably caused by hypoxic water conditions. We also examined the influence of macro-epifauna on the N2O production associated with M. edulis via removal of macro-epifauna. There was no significant difference in N2O produced by M. edulis with and without epifauna, which suggests that N2O production may be largely due to gut microbial activity and microbial biofilms on the shells from M. edulis. In summary, our study indicates a strong influence of N on the potential N2O emissions rates of prominent bivalves, which should be considered when incorporating them into coastal N mitigation strategies