28 SEPTEMBER 2023 • WORLD AQUACULTURE • WWW.WAS.ORG primary producers (O, Fig. 6). Marked daily variations in oxygen concentration were observed: the increase in O2 concentrations during the day (due to the release of oxygen by photosynthesis of primary producers) offset and exceeded respiration by heterotrophs and the respiration of all organisms (both autotrophs and heterotrophs) during the night. The impact of reared oysters was observed through the concentrations of nutrients (DIN and PO4 3-) that increased due to releases from oyster metabolism. Phytoplankton concentrations increased following the release of nutrients by the oysters. A first peak was observed at the beginning of the experiment, after which phytoplankton biomasses decreased before again increasing. This could be linked to a shift in dominance from micro phytoplankton (>20 µm) consumed by oysters, to pico phytoplankton (<5µm), which is too small to be retained during oyster filtration. Conclusions Inside the mesocosms, the link between oysters and sediment is a determining factor for triggering anoxia and nutrient releases such as nitrogen, phosphate and sulfides. In the first experiment, oxygen levels decreased in the Control but no anoxia was observed. Respiration by microbial organisms in the water and the oxygen demand from the sediment was not enough to upset the system, as the oxygen produced by photosynthesis satisfied the demand. In the second experiment, in the presence of reared oyster without any sediment, oxygen levels also decreased at the beginning but without causing anoxia. The combined respiration of microbial organisms and oysters was not enough to trigger an anoxic event. In the absence of sediment and with no oyster mortality event, no releases of sulfide were observed in the chambers. However, when the presence of reared oysters was combined with the presence of sediment in the first experiment, the system rapidly shifted to anoxia leading to massive oyster mortality (Figure 7). The respiration of microbial organisms, oysters and the oxygen demand from the sediment was not offset by the production of oxygen through photosynthesis. In anoxic conditions, with high concentrations of nutrients and sulfides, a bloom of small green algae was favored. This treatment promoted the presence of small organisms able to use anaerobic metabolism, thus favoring a microbial loop rather than the transfer of biomass to higher trophic levels. To conclude, we were able to reproduce an anoxic crisis and analyze its consequences using mesocosms. This study confirms the important role of the benthic compartment and of benthic-pelagic coupling in the oxygen cycle, especially in shallow ecosystems. This study only considered bare sediment. It will be important to study the effect of different benthic communities on the onset of anoxia. Benthic communities could play a determining role in the onset FIGURE 4. Photos of the in-situ mesocosms of the first experiment from September 9, 2020 to September 22, 2020; on the left and the second experiment from September 17, 2021 to September 28, 2021, on the right. ©Nicolas Cimiterra FIGURE 5. Fluctuations in concentrations of dissolved oxygen (mg·L-1) in the three treatments (N: Natural environment; C= control with no oysters; and O= Oyster mesocosm) at the two different depths (B= Benthic and P= Pelagic), including the sediment, during the course of the September 2020 experiment in the Thau lagoon. From Le Ray et al.2023. FIGURE 6. Changes in mean dissolved oxygen concentrations (mg·L-1) during the three treatments (N: Natural environment; C= control with no oysters; and O= Oyster mesocosm), excluding the sediment, during the course of the September 2021 experiment in the Thau lagoon.