Methane distribution, sources, and sinks in an aquaculture bay (Sanggou Bay, China)

Abstract

From 2012 to 2015, we investigated methane (CH4) distribution, air-sea fluxes, and sediment-water fluxes in an aquaculture bay (Sanggou Bay, China), and estimated the input of CH4 from potential land sources including rivers and groundwater. Surface water CH4 in the bay ranged from 3.0 to 302 nM, while bottom CH4 was usually higher due to sediment release. Water column CH4 in summer and autumn was 3 to 10 times that in spring and winter due to seasonal variation in water temperature and land source inputs. Surface CH4 was higher in kelp and scallop polyculture zones than in other culture zones and outside the bay, suggesting the influence of aquaculture activities. CH4 concentrations were 123 to 2190 nM in rivers around the bay, and 1.6 to 405 nM in groundwater along the shoreline; both showed great spatial and temporal variations. Sediment-water CH4 fluxes ranged from 0.73 to 8.26 µmol m-2 d-1, with those in bivalve culture zones higher than in polyculture zones. Sea-air CH4 fluxes ranged from 2.1 to 123.2 µmol m-2 d-1 (mean 48.2 µmol m-2 d-1) and showed seasonal variations. CH4 budget in Sanggou Bay showed that groundwater input (4.2 × 105 mol yr-1) was the largest source of CH4, followed by sediment release (2.6 × 105 mol yr-1) and riverine input (1.4 × 105 mol yr-1), while sea-to-air release (2.5 × 106 mol yr-1) and export from the bay to the Yellow Sea (8.8 × 105 mol yr-1) were the dominant CH4 sinks. Net water column production-oxidation was estimated preliminarily to produce 1.7 × 105 mol CH4 yr-1. However, there was a great imbalance of sources and sinks, with an apparent missing source of 2.4 × 106 mol yr-1 that was mostly due to an underestimate of in situ water column production and CH4 release from the sediments.