WWW.WAS.ORG • WORLD AQUACULTURE • SEPTEMBER 2023 27 (CONTINUED ON PAGE 28) ecosystems. Study Objectives and Methods Our objectives were to investigate the triggering and consequences of anoxia, focusing on oxygen, nutrients, sulfides and microbial components, and to understand the specific roles of the oyster ropes and the sediment in oxygen dynamics. To this end, we used in situ mesocosms (2 m long, with a 40 cm diameter) to confine the water. Theoretically, confining the water would asphyxiate the system as the result of respiration by organisms such as oysters, leading to anoxia (van der Loeff et al. 1984). September 2020 experiment. The first experiment lasted 13 days and included bare sediment coupling with the water column in the mesocosms (Fig. 4). Three treatments were tested: N = Natural lagoon waters; C = Control, mesocosm with no oysters; O = Oyster, mesocosm with 30 oysters. Water samples were taken on days 0, 1, 2, 6, 9 and 13, at two different depths to detect differences caused by the sediment. September 2021 experiment. The second experiment lasted 14 days and excluded the sediment from the mesocosms. The same treatments were tested (Figure 4). This experiment was conducted to dissociate the effect of the sediment from that of water containing the oysters (Figure 1B). Water samples were taken on days 0, 2, 4, 5, 6, 7, 11 and 14. Both experiments were conducted at the same site, with similar sized oysters (2020: 95.9 ± 9.9 mm; 2021: 91.7 ± 9.1 mm) used in the treatments incorporating oysters. Oxygen probes recorded O2 concentrations (mg·L-1) at 30-minute intervals. Water samples were taken to measure DIN (Dissolved Inorganic Nitrogen), PO4 3- (phosphates), Si(OH)4 (silicates), Chl a (chlorophyll a) <5 µm, Chl a 5-20µm, Chl a >20 µm, Chl b (accessory pigment present in green algae) and Chl c (accessory pigment present in brown algae). Chl a was analyzed in different size classes to detect possible shifts of the biomass of the phytoplankton community. Changes in the System with Bentho-Pelagic Coupling The results of the first experiment were published in Le Ray et al. 2023. In the presence of sediment, containment of the water body affected oxygen concentrations with or without oysters. In the Control mesocosm with no oysters, oxygen concentrations decreased but the system did not become anoxic. The decrease in O2 was higher near the sediment, with very high daily variability. Nutrients were released due to mineralization of organic matter in the sediment. This increase in nutrient availability resulted in a 4.5 increase in phytoplankton biomass by day 6. The phytoplankton was associated with micro phytoplankton development (>20 µm) and Chl c. In the presence of oysters, oxygen concentrations decreased rapidly (Figure 5). This consumption was related to respiration by organisms and to oxygen demand from the sediment. At both depths, the system rapidly shifted to anoxia in 54 hrs. After a few days of anoxic conditions, all the oysters had died and decomposed inside the mesocosm. Moreover, sulfides were detected, the highest concentration (295 µmol.L-1) being observed when the oyster flesh decomposed. Concerning nutrients, DIN and PO4 3- dynamics were similar. In anoxic conditions, concentrations increased significantly related to releases at the sediment-water interface and through the decomposition of the dead oysters. The peak observed on day 9 coincided with our observation of decomposed oyster flesh. These nutrients impacted the phytoplankton biomass. Under anoxia and in the presence of high concentrations of sulfide, a phytoplankton bloom dominated by small (3 to 4 µm) green algae (associated with the Chl b accessory pigment) was observed. The peak abundance was associated with the peak in nutrients. Changes in the System Without Bentho-Pelagic Coupling In the second experiment, the impact of oysters in the water column was studied without considering the sediment inside the mesocosms. In the Control chamber containing no oysters, oxygen concentrations remained stable throughout the experiment, indicating low respiration and low photosynthetic levels. With oysters present, mean daily oxygen concentrations started decreasing at the beginning of the experiment, but a larger increase was subsequently observed due to the photosynthetic activity of FIGURE 3. Experimental design of the in-situ mesocosms for: A) the first experiment from September 9, 2020 to September 22, 2020; B) the second experiment from September 17, 2021 to September 28, 2021. Modified from Le Ray et al. 2023.