The influence of sediment biogeochemistry on water quality and shrimp health in integrated rice-shrimp ponds.

Abstract

Integrated rice–shrimp ponds (IRSPs) are common in areas of southeast Asia where saltwater intrudes into rice fields in the dry season, leading to rice planting in the wet season, and shrimp farming in the dry season or throughout the year. The rotation of rice with shrimp in this system is thought to increase environmental sustainability and contribute to food security. Nevertheless, the productivity of IRSPs is generally low and unreliable, and technical solutions do not address the impacts of climate change. Consequently, farmers experience reduced livelihood and lifestyle opportunities. The understanding of the biogeochemistry in shrimp ponds has focused on water column processes. However, shrimp live at the sediment–water interface where chemical and biological processes in and on the sediment, and the effects on water quality, can impact shrimp health, and overall pond productivity. This PhD research examines the sediment biogeochemical processes particularly relevant to shrimp health in IRSPs, i.e. nutrients and oxygen. Options for modifying conditions to improve IRSPs productivity are also discussed. The study examined ISRPs in two districts of Ca Mau Province, Vietnam. In the nutrient budget study, 12 farms in Cai Nuoc District were examined. It became clear that this district was marginal for sustainable rice production; hence in order to examine biogeochemical processes in more detail, two ISRPs in Thoi Binh District, where rice production was more sustainable, were investigated. A combination of measurements of parameters in the water and sediment were combined with nutrient and oxygen flux measurements in situ (pond side) to understand the dynamic nature of these systems. The first step in understanding biogeochemical processes was to estimate nutrient budgets in IRSPs. The study showed that the main nutrient input (92% of the N input, 57% P and 95% C) came from intake water, while water discharge accounted for the highest output (75% of the N output, 41% P and 57% C). Hence, most of the nutrients were not assimilated in the ponds. The main reason for this poor assimilation was low shrimp densities and survival (6.3 ± 2.2%); thus the nutrients did not convert to shrimp biomass. This, combined with the low rice harvest (mostly no rice crop due to high salinity affecting rice production), supports the conclusion that IRSPs seemed to be inefficient systems. Furthermore, fertilizer addition only accounted for 8% N, 43% P and 5% C of the total input, and thus fertilizer addition was likely to be an unnecessary expense. The study also found that IRSPs had periods of low dissolved oxygen (DO) concentration which may have a critical negative effect on shrimp survival. To understand the causes of low DO, oxygen fluxes were examined at two IRSPs during a two-year period. The key finding was that a high percentage of oxygen demand at a whole pond scale was from the sediment; hence sediment oxygen demand (SOD) drove low DO concentrations in the water column. Moreover, oxygen demand was considerably higher than oxygen production within the IRSPs, indicating high bacterial activity relative to algal production. SOD rates were significantly positively correlated (p < 0.05) with chlorophyll a concentrations in the water column. These findings suggest that algal production in the water column, rather than benthic algal production, or other organic loadings, provided an organic carbon source driving SOD. Sediment nutrient pathways were also examined at the same two IRSPs to understand drivers of poor water quality. The study showed that the IRSPs had low denitrification efficiency. Denitrification rates were significantly positively correlated with chlorophyll a concentrations in the sediment, suggesting carbon availability was a key driver. Nitrate, ammonium and phosphate concentrations in the water column were high despite low sediment nutrient fluxes. Given the low sediment nutrient fluxes, and low N removal by denitrification, high nutrient loads in the ponds were likely derived from incoming water. Despite the low mean values for denitrification, the fact that values at some sites within the ponds were high suggests that there was scope to enhance denitrification, via C addition, and hence to improve water quality. In summary, this study examines key nutrient biogeochemical fluxes in IRSPs and shows, for the first time, that sediment was a main driver of low oxygen conditions in these ponds. High nutrient loads, which also ultimately helped drive low oxygen conditions, were the result of poor quality incoming water and inefficient N removal via denitrification. Nutrients were mostly in-fluxing into the sediment; hence sediment nutrient fluxes were relatively low and were minor contributors to water column nutrients. These key findings indicate that rather than contributing to eutrophication in adjacent waterways, IRSPs were net nutrient removal mechanisms.