A scientific evaluation to inform nutrient removal options for the Coorong

The Coorong is culturally, environmentally and economically important at local, national and international scales but has experienced a long-term decline in its ecological condition due to reductions in inflows and increased rates of sedimentation. Whilst there has been recovery of some elements of the Coorong ecosystem associated with increased inflows since the end of the Millennium Drought in 2010, the South
Lagoon has not recovered to the levels expected. There has been a shift of the ecosystem state from being dominated by aquatic plants (e.g. Ruppia) to extensive phytoplankton and filamentous algae associated with eutrophication (Mosley et al. 2020). Eutrophication can be defined as an increase in the supply of organic matter to an ecosystem (Nixon 2009, Le Moal et al. 2019), and is a major and ongoing concern in many aquatic systems worldwide (Cloern 2001, McDowell et al. 2020). The accumulation of organic matter can cause many deleterious impacts including high availability of nutrients that promote algae growth, depletion of dissolved oxygen, toxicity (e.g. from high sulfide and/or ammonia concentration, and harmful algal species) and loss of both submerged aquatic vegetation (such as seagrasses) and benthic invertebrate communities (low biodiversity) that contribute to food webs and broader ecosystem health (McGlathery et al. 2007, Nixon 2009). Internationally, efforts have been undertaken to revert eutrophic conditions in coastal ecosystems, with re-oligotrophication (lowering of nutrient levels) often taking several decades to achieve ecological regime shifts (e.g. in Mediterranean, Le Fur et al. 2019, Derolez et al. 2020)