An integrated fish-plankton aquaculture system in brackish water.

Integrated Multi-Trophic Aquaculture takes advantage of the mutualism between some detritivorous fish and phytoplankton. The fish recycle nutrients by consuming live (and dead) algae and provide the inorganic carbon to fuel the growth of live algae. In the meanwhile, algae purify the water and generate the oxygen required by fishes. Such mechanism stabilizes the functioning of an artificially recycling ecosystem, as exemplified by combining the euryhaline tilapia Sarotherodon melanotheron heudelotii and the unicellular alga Chlorella sp. Feed addition in this ecosystem results in faster fish growth but also in an increase in phytoplankton biomass, which must be limited. In the prototype described here, the algal population control is exerted by herbivorous zooplankton growing in a separate pond connected in parallel to the fish-algae ecosystem. The zooplankton production is then consumed by tilapia, particularly by the fry and juveniles, when water is returned to the main circuit. Chlorella sp. and Brachionus plicatilis are two planktonic species that have spontaneously colonized the brackish water of the prototype, which was set-up in Senegal along the Atlantic Ocean shoreline. In our system, water was entirely recycled and only evaporation was compensated (1.5% volume/day). Sediment, which accumulated in the zooplankton pond, was the only trophic cul-de-sac. The system was temporarily destabilized following an accidental rotifer invasion in the main circuit. This caused Chlorella disappearance and replacement by opportunist algae, not consumed by Brachionus. Following the entire consumption of the Brachionus population by tilapias, Chlorella predominated again. Our artificial ecosystem combining S. m. heudelotii, Chlorella and B. plicatilis thus appeared to be resilient. This farming system was operated over one year with a fish productivity of 1.85 kg/m2 per year during the cold season (January to April).

A new bacterial strain mediating As oxidation in the Fe-rich biofilm naturally growing in a groundwater Fe treatment pilot unit.

A bacterial strain B2 that oxidizes arsenite into arsenate was isolated from the biofilm growing in a biological groundwater treatment process used for Fe removal. This strain is phylogenetically and morphologically different from the genus Leptothrix commonly encountered in biological iron oxidation processes. T-RFLP fingerprint of the biofilm revealed that this isolated strain B2 corresponds to the major population of the bacterial community in the biofilm. Therefore, it is probably one of the major contributors to arsenic removal in the treatment process.

Mixing and its impact on faecal coliform removal in a stabilisation pond.

Faecal coliform removal in stabilisation ponds is highly dependent on shortest water retention times. Tracer tests have been performed in a 3,300 m2 and 1.0 m deep pond, located in Southern France, to measure the retention times and bring light on the main influencing factors and mechanisms. Tracer concentrations were monitored at the outlet and 60 locations within the pond--at the surface, mid depth and the bottom of the water column. Pond water temperatures were measured at different depths and locations, together with pH, DO and redox potential. Wind velocity and rainfall were recorded. Water quality was monitored at the inlet, outlet and within the pond. Water retention times were shown to be strongly affected by weather conditions. Windy periods appeared to favour mixing regardless of the season. In sunny periods of spring and summer, a clear stratification was observed during daytime and vanished gradually during the night, suggesting alternation of mixed and stratified hydrodynamic patterns. This alternation was shown to influence microorganism contents within and at the outlet of the pond. Accurate prediction of shortest water retention times and disinfection performance requires 3D unsteady state fluid dynamic models that are able to take the influence of wind and water temperature distribution into account.