The Thau lagoon is a Mediterranean lagoon exploited by shellfish farming under oligotrophication and impacted by anoxic crisis. In situ experiments using mesocosms were carried out to investigate 1) the triggering and consequences of anoxia on nutrient, sulfides and planktonic organisms in an oyster farming site, and 2) the role of oyster ropes and sediment on anoxia and consequences. The principle of experiments was to confine the lagoon waters in mesocosms, in presence or in absence of reared oysters and bare sediment, to trigger or not anoxia by asphyxia (Figure 1).
In the controls, oxygen concentrations decreased daily but remained above hypoxic concentrations, in presence or in absence of sediment. In contrast, in presence of sediment and oyster rope, anoxia appeared after 54 hours. All the oysters had died between days 6 and 9, after the beginning of the prolonged anoxia and following the appearance of ∑H2S (up to 295 µmol·l-1). At 9 d, a peak of nutrients was observed with up to 390 µmol·l-1 for NH4+ and 17 µmol·l-1 for PO43-. These releases favoured a bloom of phytoplankton (11.8 µg·l-1, 106 cells·L-1), mainly composed of picoeucaryotes. In absence of sediment, the mesocoms did not switch to anoxia, despite the presence of oysters. Although oxygen concentrations decreased during the first days, they then increased to reach supersaturation levels (almost 150%). No H2S releases and massive oyster mortality were observed in these conditions. Increase of NH4+ and PO43- were also observed on day 1 with maximum concentrations of 15.6 μmol·l-1 and 0.4 μmol·l-1, but 25-fold lower than concentrations observed in presence of oysters and sediment. A phytoplankton bloom, dominated by picoeukaryotes was also observed on day 4 at 9.6 μg·l-1, corresponding to abundances of 1222 x 106 cells·L-1.
Both oyster ropes and sediment triggered oxygen depletion and nutrient releases in relation to respiration, excretion and mineralisation processes. Oyster ropes favoured picophytoplankton bloom by filtration and excretion in normoxia or through flesh decomposition in anoxia. Without sediment, no anoxia and H2S releases were observed, wondering which is the role of benthic habitat on trigger of massive oyster mortality. These results were completed with a series of in situ benthic mesocosms experiment, to highlight the role of several benthic habitat (bar sediment, Zostera, Halopithys) to trigger or preserve the ecosystems from anoxia and oyster massive mortality.