Diagenetic Processes in Aquaculture Ponds Showing Metal Accumulation on Shrimp Gills

Abstract

The gill is the organ by which many toxic metals are taken up by crustaceans. Iron is known to precipitate at its surface, a phenomenon recently observed in some tropical aquaculture ponds. The present study uses a field approach to understand better the environmental conditions and ecological processes involved in this deposit. Because shrimp are exposed to reduced products originating from organic waste accumulated in the sediment, spatial variation in pH, redox potential and concentrations of dissolved metals in pore water were investigated in these ponds. Total organic carbon, acid volatile sulfide and pyrite were also analyzed in the solid phase. Fe2+ in pore waters showed high spatial variability between ponds and within the same pond with concentrations up to 1,193 μmol l–1. Behaviors of Fe2+, Mn2+ and Co2+ in pore water were similar. Four geochemical environments were identified, based on their physico-chemical characteristics. Highest concentrations for Fe2+, Mn2+ and Co2+ in sediment pore water occurred in slightly acidic and suboxic conditions. When the sediment became anoxic, the H2S produced reacted with Fe2+ and/or Co2+ to form acid volatile sulfide and pyrite. When pH increased, the concentration of free H2S rose up to 736 μmol l–1. With neutral and suboxic conditions, dissolved metal concentrations could be controlled by their precipitation as oxides and hydroxides. The production of pyrite suggested the existence of a possible process of sediment acidification between two crop periods through the production of sulfuric acid. This acidification could increase with pond age and be the cause of the accumulation of reduced metal after 30 years of aquaculture activity.

Iron deposits turn blue shrimp gills to orange

“…Fe concentrations for shrimp without OG were in the range (80 to 852 μg.g − 1 d.w) of the values reported in literature. As regards shrimp with OG, the value was similar to that reported for one of the most polluted harbors in the world (Lewtas et al, 2014).…”

“…No data from the literature were found on Co concentration in shrimp gills. By way of comparison with our study, Cr concentrations was reported to be highest for the greentail prawn, Metapenaeus bennettae, harvested in Sydney harbor (Lewtas et al, 2014). They were highest in the gill tissue, followed by hepatopancreas, exoskeleton and tail muscle.…”

“…The emergence of OG after 11 to 13 weeks occurs following the autotrophy to heterotrophy switch of sediment biogeochemistry (Hochard et al, 2019), when food inputs are at their maximum. Acidification of the pond environment could occur during rearing linked to this change in environmental conditions, inducing the solubilization and accumulation of Fe, Co and Mn, as recently shown at the water-sediment interface in ponds with OG (Lemonnier et al, 2021). Partial harvest after 17 weeks induces a decrease of food inputs into the system and thus a reduction of its eutrophication level.…”

“…Under acidic conditions, Al exposure induces structural abnormalities in the gill for penaeid shrimp juveniles (Russell et al, 2019). Taking into account the above considerations, we hypothesize that acidic conditions in ponds (Lemonnier et al, 2021) could lead to the release of metal ions, particularly Fe and Al and their deposits on the surface of the gills. Because there is a need for contact between shrimp and sediment to develop OG, as shown by the transfer experiment conducted in the present study, acidification of this compartment during rearing could be at the root of this process.…”

“…Zbinden et al, 2004). Lemonnier et al (2021) recently described pond environmental conditions and ecological processes involved in the metal contamination in shrimp pond in New Caledonia. Results showed high concentrations of reduced iron in pore water at the interface water-sediment (up to 1000 μmol.l − 1 ) during OG periods, close to the values reported for hydrothermal environments (Zbinden et al, 2004).…”