Summer methane emissions from sewage water-fed tropical shallow aquaculture ponds characterized by different water depths.

Aquaculture practices are steadily increasing to meet the fish demand, especially in tropical countries like India. However, efforts to characterize the contribution of these aquaculture ponds towards greenhouse gas emission like CH are still very few. CH concentration in water [pCH(water)] and air-water CH fluxes were estimated (during the summer months) in two sewage-fed ponds having different depths situated in the East Kolkata Wetlands, India (a Ramsar site). pCH(water) in both of these ponds showed significant positive correlation with water temperature (R = 0.68 and 0.71, p < 0.05). Daily mean chlorophyll-a, turbidity, biochemical oxygen demand (BOD) and gross primary productivity (GPP) also showed positive correlation with pCH(water). This indicated that higher primary production and presence of turbid materials acted as substrates for methanogenesis, which favoured air-water CH effluxes towards atmosphere. Mean air-water CH fluxes in the ponds having depth of 1.1 m and 0.6 m were observed to be 24.79 ± 12.02 mg m h and 6.05 ± 3.14 mg m h respectively. Higher depth facilitated net heterotrophic conditions, which led to lower dissolved oxygen levels, which, in turn, led to lower rate of CH oxidation. Moreover, under reduced photosynthetically active radiation (in the pond having greater depth), the pH values were comparatively lower (~7.7), which further facilitated a favourable condition for the methanogens to grow. On the whole, it was inferred that apart from pre-established physicochemical factors, depth was also found to play a decisive role in regulating the air-water CH fluxes from these aquaculture ponds. In future, continuous sampling should be carried out (by chamber method) to take into account the ebullition CH fluxes, and more number of ponds should be sampled throughout a complete annual cycle to have a more holistic understanding about this cluster of sewage-fed aquaculture ponds.