Investigating the drivers and control of filamentous algae and restoration of aquatic plants in the Coorong

Ruppia tuberosa (Ruppia) is a keystone species for the southern Coorong – contributing to maintaining water quality and trapping sediment while providing habitat for invertebrates and fish, and food for waterbirds. Beginning in late spring and continuing over the summer, excessive filamentous algal growth in the Coorong negatively impacts on Ruppia growth and seed production throughout the system, particularly in the southern Coorong, with flow-on effects for the Coorong ecosystem (Collier et al. 2017). These impacts are exacerbating the long-term recovery of the Coorong ecosystem following the large-scale losses of the Ruppia community in the Coorong throughout the Millennium Drought despite successful translocation efforts (Collier et al. 2017). The current state of the previously robust Ruppia community, has been described as being in a vulnerable state requiring significant effort to restore resilience and multiple ecological functions (Brookes et al. 2018). Field observations over the last 4-5 years suggest that the southern Coorong has remained in the Hypersalinity–High Nutrient state (Figure 1 – upper right panel).

Figure 1. Conceptual diagram summarising alternative states for the southern Coorong based on observed ecological conditions (from Collier et al. 2017).

The dominance of filamentous algae impacts ecological functions of the southern Coorong through:

• filamentous algal mat forming species gaining an advantage over Ruppia through the combined, complex interactions of nutrient availability, salinity, water temperature and water level (Collier et al. 2017)
• increased prevalence of filamentous algae, compared to Ruppia, altering nutrient cycling away from relatively stable fixed carbon (i.e. Ruppia) persisting throughout the season to more rapid dynamic trophic cycling typical of eutrophic systems (filamentous algae and micro-biota)
• algae accumulating on shorelines, forming mats that prevent shorebirds from accessing food resources from within mudflats
• filamentous green algae forming mats at the water surface shading plants and entangling Ruppia flowers resulting in them breaking away from stems, leading to Ruppia failure to set seed
• failed reproduction of Ruppia contributing to a depleted seedbank and lowered resilience of the population, which affects the invertebrates, fish and waterbird for which it provides food and/or habitat
• the breakdown of algae that accumulates on shorelines reducing oxygen levels within the sediment, affecting the invertebrate communities, and thus food for key fish and waterbird populations (Dittmann et al. 2017, Paton et al. 2017, Collier et al. 2017)
• filamentous green algae creating a barrier at the water’s surface impeding foraging by birds (Paton et al. 2018).