Rapid communicationRole of aquaculture pond sediments in sequestration of annual global carbon emissions
Introduction
Efforts to quantify carbon sequestration in inland water bodies have focused on inland seas, natural lakes, and large river impoundments (Mulholland and Elwood, 1982, Dean and Gorham, 1998). A recent study in Iowa (Downing et al., 2008) suggested that small, agriculturally-eutrophic impoundments bury carbon at an average rate of 2122 g m−2 yr−1 – five times higher than in large, river impoundments, 30 times more than in small, natural lakes, and over 400 times greater than in inland seas and large natural lakes (Mulholland and Elwood, 1982, Dean and Gorham, 1998). The combined water surface area of small impoundments in farming areas was estimated at 21,000 km2 in the United States and 77,000 km2 globally (Smith et al., 2002, Downing et al., 2006), and these impoundments may bury more carbon than the world’s oceans (Downing et al., 2008).
The area of agriculturally-eutrophic impoundments used for estimating carbon sequestration (Downing et al., 2008) did not include aquaculture ponds. According to statistical data on aquaculture production maintained by the Food and Agriculture Organization (FAO) of the United Nations, there are 110,830 km2 of aquaculture ponds worldwide (Verdegem and Bosma, 2009). Aquaculture ponds also may be important in global, carbon sequestration.
Aquaculture ponds do not have large external sediment loads typical of river reservoirs or small, watershed ponds in agricultural or other rural areas (Boyd, 1995). However, earthwork of aquaculture ponds is eroded by rain, waves, and water currents generated by mechanical aerators, activities of culture species, and harvesting operations. Manure, grass, and other agricultural wastes traditionally have been applied to ponds as organic fertilizer to increase aquatic animal production, but high-quality, pelleted feeds are rapidly replacing fertilizers as a means of achieving greater production (Boyd and Tucker, 1998). Fertilizers and feeds contain inorganic nutrients that stimulate organic carbon production by phytoplankton photosynthesis in ponds (Boyd and Tucker, 1998).
Coarse, soil particles suspended by internal erosion settle near edges of ponds while smaller particles tend to settle in deeper areas (Boyd, 1995). Organic matter from dead plankton, organic fertilizers, uneaten feed, and excrement of culture species settles on pond bottoms and gradually mixes with soil particles. Aquaculture management favors microbial decomposition of organic matter. For example, organic matter inputs usually have a narrow carbon:nitrogen ratio, ponds with acidic, bottom soils are limed, and mechanical aeration avoids oxygen-depletion at the sediment–water interface (Boyd and Tucker, 1998). Much recently-settled organic detritus is discharged when ponds are drained for harvest (Ayub et al., 1993). After draining, pond bottoms usually are dried to enhance soil aeration and accelerate decomposition of labile organic matter (Boyd, 1995). Nevertheless, a layer of sediment with an organic carbon concentration higher than that of the original pond bottom soil and with a characteristic profile of well-defined strata or horizons develops (Munsiri et al., 1995).
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Materials and methods
The United States Agency for International Development funded the Pond Dynamics Aquaculture Collaborative Research Support Program (ACRSP) from 1984 to 2008. As part of this project, sediment cores were taken from 233 aquaculture ponds of known age at 25 locations in nine countries according to the methodology described by Munsiri et al. (1995). Ponds were used to culture a total of eight common species or species groups. Forty-two of the ponds were located on research stations while the rest
Results and discussion
Ponds age ranged from 1 to 52 yr (Table 1) with an average age of 14.9 ± 10.4 yr. Sediment depth varied from 4 to 36 cm (average 16.8 ± 11 cm), and the average rate of sediment accumulation was 1.44 ± 0.79 cm yr−1 (range 0.5–3.7 cm yr−1). Sediment had been removed on one or more occasions from some of the ponds in Thailand, but sediment depth was correlated with pond age (r = 0.78; P < 0.01). The annual rate of sediment deposition averaged 14,400 cm3 m−2 (144 m3 ha−1). According to a study by Steeby et al. (2004),
Acknowledgements
This research was a component of the AquaFish Collaborative Research Support Program (CRSP) and was supported by USAID Grant No. LAG-G-00-96-90015-00 and by funds from Auburn University. The Aquaculture CRSP Accession Number is 1369. The opinions expressed herein are those of the authors and do not necessarily reflect the views of the US Agency of International Development.
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