Impact of mussel farming on CO2, CH4 and N2O emissions in a coastal area.

Coastal areas including fjords, emit substantial amounts of methane (CH4) and nitrous oxide (N2O), that may partially offset their carbon dioxide (CO2) sink potential. Expanding coastal marine aquaculture may impact greenhouse gas dynamics. The role of mussel farming as a CO2 sink or source and its potential for nutrient removal is well investigated, but its effects on sea-air greenhouse gas emissions remain unclear. Here we resolve greenhouse gas emissions from a Swedish fjord with mussel farms, covering temporal (pre- and postharvest) and spatial (mussel farm versus control area) variability. The sediment-water fluxes of dissolved inorganic carbon, CH4 and N2O were on average 2.2 times (without the outlier in winter), 2.7 times and 6.0 times higher in the farm compared to the corresponding control site, leading to enhanced bottom water concentrations. The sea-air fluxes ranged from -8.3 to 12.8 mmol CO2 m-2 d-1, from 2.9 to 39.2 μmol CH4 m-2 d-1, and from -1.8 to 2.1 μmol N2O m-2 d-1. CO2 emissions were 15 % lower and 11 % greater in farms (small farm and large farm respectively) than control sites, CH4 emissions were comparable between farms and controls and mussel farms lowered N2O emissions by 46 % compared to controls. We estimated low aquatic emission intensities with -2.98 g CO2 equivalents per kg edible meat for the smaller farm and 1.74 g CO2 eq. for the larger farm. Overall, our findings highlight a large temporal and spatial variability of greenhouse gas emissions and reveal the relatively small impacts of mussel farms on total emissions.