The influence of organic carbon on oxygen dynamics and bacterial sulfate reduction in inland shrimp ponds

Abstract

Experiments conducted in saline aquaculture ponds demonstrated that organic matter was the primary factor influencing sediment sulfate reduction rates. Changes in sediment oxygen demand (SOD), sulfate reduction rates, and sulfate reducing bacteria (SRB) abundances were monitored in nine 0.04-0.08 ha earthen shrimp ponds during a 17-week growing season. Organic matter (feed) was added at low, medium, and high rates in a block design. The aquaculture site was located in an and inland region near Imperial, Texas, and utilized saline groundwaters of lower salinity and proportionally higher sulfate concentrations than undiluted seawater. SOD was measured in situ with an enclosed chamber; sulfate reduction rates were measured in cores using injected []SO4-2, and SRB counts were estimated using most probable number analyses. Shrimp growth rates averaged 1. 2 g shrimp-1 week-1, with a mean harvest weight of 20.8 g shrimp-1 (C.V.= 4.5%). Shrimp survival in the ponds was low, however, ranging from 10.2-16.5% (mean = 12.4%). Shrimp yield averaged 473 kg shrimp ha-1 (C.V.= 34%), and was much lower than that for coastal shrimp ponds. No significant treatment difference ([] = 0.05) in shrimp growth or survival could be found. Furthermore, no significant correlation could be demonstrated between shrimp survival and rates of sulfate reduction. Over the growing season SOD increased four-fold, SRB populations increased two orders of magnitude, and sulfate reduction rates increased over two orders of magnitude, with highest values occurring at the end of the season. Sulfide flux measurements demonstrated that sulfate reduction represented a greater proportion of SOD at the end of the season (mean: 84%) than in the beginning (< 2%). Sediment porewater sulfate concentrations were high at all depths (24.7-3 1.0 MM) and did not appear to limit sulfate reduction. Sediment depth profiles showed close parallels between organic matter content and sulfate reduction rates (r.2 = 0.66). Furthermore, significant differences (p < 0.05) in sulfate reduction were found between ponds that had been used for aquaculture in the previous season versus newly constructed ponds. Differences were consistent with the hypothesis that sulfate reduction was regulated by organic matter quantity and reactivity.