World Aquaculture - September 2023

26 SEPTEMBER 2023 • WORLD AQUACULTURE • WWW.WAS.ORG organisms. In this shallow farmed lagoon, oxygen demand from the sediment and the cultured oysters could both have important roles in the triggering and consequences of anoxia. This ecological crisis had an associated economic impact; for example, 4000 tons of shellfish decomposing and dying in the lagoon in 2018. The ecological consequences of anoxic events are often apparent as high concentrations of nutrients, including ammonium, phosphates, silicates, and hydrogen sulfide released from the sediment (Balzer et al. 1983) as well as the mineralization of dead organisms, including cultured shellfish. These high nutrient concentrations result in diatom blooms (Nitzschia, Cylindrotheca spp.). Diatom blooms are also a response to the release of silicates at the water-sediment interface as observed during anoxia (Balzer et al. 1983). Both the triggering and the consequences of anoxia are strongly linked to the sediment. Lagoon ecosystems are shallow, and their functioning is closely linked to benthic-pelagic coupling. The exchange of fluxes between the sediment and the water column plays a crucial role in the dynamics of dissolved oxygen (Griffiths et al. 2017). The sediment requires oxygen for the mineralization of organic matter and is a source of nutrients and a support for the attachment of macroalgae and seagrasses (Boynton et al. 2018). Distinguishing the effects of the sediment from those of the water column and the reared shellfish on oxygen levels, nutrients and phytoplankton concentrations is complicated. Nonetheless, this information is essential to better understand both the drivers and consequences of anoxia for lagoon Coastal ecosystems, such as lagoons, are highly productive and therefore ideal for aquaculture. Lagoons are partially confined and strongly influenced by their watershed, making them particularly sensitive to climate and anthropic pressures, such as rising temperatures and pollution. Under these climatic pressures, anoxia is having a major impact on coastal waters worldwide, notably provoking massive mortalities (Breitburg et al. 2018). The present study was conducted in the Thau lagoon in the South of France, which is connected to the Mediterranean Sea. Thau lagoon is of significant economic interest in the region as it is used for shellfish farming, and its oyster output (Crassostrea gigas) accounts for 10% of total French production. Oysters are reared on ropes attached to shellfish growing tables (Figures 1 and 2). Thau lagoon has already suffered many major anoxic crises. One in 1975 affected the whole lagoon and another in 2006 impacted the whole shellfish farming area. Triggering of prolonged anoxia events has been associated with eutrophication, rising air temperatures, low winds and high rainfall rates during winter (Derolez et al. 2020). Until 1992, most of these anoxic events were combined with high rates of eutrophication. Improvements made to wastewater treatment plants were undertaken between 1993 and 2002 that led to a shift from eutrophic to oligotrophic lagoon waters between 2003 and 2006. The oligotrophication of the lagoon significantly reduced the frequency of summer anoxic events; however, a climate-related anoxia was observed in 2018 following high nutrient inputs due to winter precipitation and summer heat waves with continuous low winds. This anoxia event was linked to the depletion of available oxygen caused by respiration of lagoon Role of Oysters and Sediment in Anoxia Processes in a Shellfish Farmed Lagoon Julie Le Ray, Béatrice Bec, Annie Fiandrino, Marion Richard FIGURE 1. Photo of oysters (Crassostrea gigas) drying in the open air after being glued to the ropes with cement before being attached to the shellfish growing tables. ©Julie Le Ray FIGURE 2. Photo of a shellfish growing table in the Thau lagoon, France. ©Julie Le Ray