Carbon and nitrogen budgets in shrimp ponds of extensive mixed shrimp-mangrove forestry farms in the Mekong delta, Vietnam

Abstract

Mass balance estimates of carbon and nitrogen flux through two extensive shrimp ponds in the Mekong delta, Vietnam, were constructed to identify major sources and sinks of organic matter potentially available for shrimp production. Nutrient transformations in the sediments were measured to further assess rates of decomposition and burial and quality of organic matter. Tidal exchange was the major pathway for inputs and outputs of carbon and nitrogen in both ponds, with net primary production, nitrogen fixation and precipitation being minor inputs. No fertilizers or artificial feeds were added to either pond. The nutrient budgets identified burial and respiration as the next most important outputs after tidal exchange losses of particulate and dissolved carbon and nitrogen. There was no measurable denitrification in either pond, and volatilization was negligible. Mineralization efficiency of carbon in the water column was high (> 100%) in pond 23 reflecting rapid respiration rates; efficiency was lower (36%) in pond 12 waters. Mineralization efficiency of sediment nutrients averaged 34% for C and 41% for N in the pond with a higher annual shrimp yield (pond 12); lower mineralization efficiencies (11% for C, 10% for N) were calculated for the lower yield pond (pond 23). High burial efficiencies for both C (66–89%) and N (59–90%) in the sediments of both ponds suggest that little organic matter was shunted into biological production. Conversion efficiency for shrimp averaged 16% for C and 24% for N from pond 12, and 6% for C and 18% for N from pond 23. The high quantity but low quality of organic matter entering the ponds coupled with other factors, such as poor water quality, limits shrimp productivity. On average, nutrient outputs were greater than inputs in both ponds. This imbalance partly explains why shrimp yields are declining in these ponds.

Carbon dynamics in the Mekong Delta

“…The phytoplankton primary production was probably sustained by high inorganic nutrients inputs from shrimp farming pods typically observed in adjacent channels (for example Cardozo and Odebrecht, 2014) or within the ponds themselves (Alongi et al, 1999a). Yet, the 10 more negative δ 13 C-DIC values in the side channels indicate sustained CO 2 production from organic matter degradation related to the shrimp ponds (Alongi et al, 2000), consistent with higher pCO 2 values and lower %O 2 in the side channels compared to the adjacent estuarine channels. The distinctly higher CH 4 values would indicate that part of the organic matter degradation in the side channels occurs in 15 sediments.…”

Shrimp‐based livelihoods in mangrove silvo‐aquaculture farming systems

“…Regular (Tran ) but not too frequent water exchange can improve water quality and make the pond's primary production more autotrophic (Alongi et al . ). Likewise, the use of a settling pond can further improve water quality, thus increasing the pond's primary production (Alongi et al .…”

“…Likewise, the use of a settling pond can further improve water quality, thus increasing the pond's primary production (Alongi et al . ). Planting mangrove on platforms which are not under tidal influence limits mangrove's contribution to its biodiversity function, as does the restricted water exchange.…”

Carbon dynamics and CO<sub>2</sub> and CH<sub>4</sub> outgassing in the Mekong delta

“…The phytoplankton primary production was probably sustained by high inorganic nutrient inputs from shrimp farming ponds typically observed in adja- cent channels (for example Cardozo and Odebrecht, 2014) or within the ponds themselves (Alongi et al, 1999a). However, the more negative δ 13 C-DIC values in the side channels indicate sustained CO 2 production from organic matter degradation related to the shrimp ponds (Alongi et al, 2000) (MW test p = 0.0253 in December 2003 and p = 0.0040 in April 2004). This is consistent with generally higher pCO 2 values and lower %O 2 in the side channels compared to the adjacent estuarine channels.…”