Saltwater intrusion has become a severe issue for the Mekong Delta in Vietnam, especially near the coastline. This issue has led to farmers diversifying from exclusively growing rice to adopting a mixed rice-shrimp system with rice only cultivated in the wet season. However, the nutrient (nitrogen, phosphorus and carbon) cycling and nutrient use efficiency of this system remain poorly understood. To address this knowledge gap, we examined nutrient budgets across 12 farms using integrated rice-shrimp ponds, and in some cases semi-intensive or intensive shrimp grow-out ponds (Penaeus monodon or Penaeus vannamei), over a two-year period (2014-2015). In terms of nutrient budgets, the main nutrient input (92% of the N input, 57% P and 95% C) in the integrated rice-shrimp ponds (IRSPs) came from intake water (excluding C from primary production), while water discharge accounted for the highest output (75% of N output, 41% P, 57% C, excluding C from respiration). The study showed that IRSPs had low dissolved oxygen and high nutrient concentrations which may affect shrimp production. Conversely, salinity levels in the wet season were too high for rice plants thereby affecting rice production. Shrimp survival in the IRSPs was low over the two years (6.3 ± 2.2%), which resulted in the low proportion of nutrients exported from the ponds as harvested shrimp (6% N, 5% P and 10% C). In contrast, the shrimp grow-out ponds (SGOPs), had much higher survival (77.1% for P. vannamei and 59.2% for P. monodon) in three of the six farms where the
Integrated rice-shrimp ponds (IRSPs) are common in areas of Southeast Asia where saltwater intrudes into rice fields in the dry season, enabling rice production in the wet season, and shrimp farming in the dry season or throughout the year. Previous research has highlighted that IRSPs have periods of low dissolved oxygen concentrations which may have a critical effect on shrimp survival. To understand the causes of low dissolved oxygen, this study examined oxygen fluxes at two IRSPs in Ca Mau Province, Vietnam during a two-year period (two wet seasons and two dry seasons). Sediment oxygen demand (SOD) incubations and whole-system oxygen flux measurements were conducted and compared with a range of water and sediment parameters to explain drivers for low oxygen concentrations. A high percentage of oxygen demand at a whole pond scale was from the sediment; hence SOD drove low oxygen concentrations in the water column. SOD rates were significantly positively correlated with chlorophyll a concentrations in the water column. These findings suggested that algal production in the water column, rather than benthic algal production, or other organic loading, provided an organic carbon source driving SOD. Oxygen demand was much higher than oxygen production within the IRSPs, indicating high bacterial activity and low algal production. This study has shed new light
Integrated rice-shrimp ponds (IRSPs) have periods of low oxygen and high nutrient loads which may have a critical effect on both shrimp survival and water quality in adjacent waterways. To understand drivers of poor water quality, this study examined sediment nutrient pathways at two IRSPs. The study showed that the IRSPs had low denitrification efficiency. However, denitrification rates were significantly positively correlated with chlorophyll a concentrations, suggesting carbon availability was a key driver of denitrification. Dissolved nutrient concentrations in the water column were relatively high despite low sediment nutrient effluxes. Given the low sediment nutrient effluxes, and low nitrogen (N) removal by denitrification, the high nutrient loads were likely derived from incoming water. Therefore, the IRSPs were net nutrient removal mechanisms, rather than contributing to eutrophication in adjacent waterways. There also appears to be scope to enhance denitrification for increased N removal, via carbon addition, and hence improve water quality.
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