Spatial and Temporal Patterns in Seagrass Abundance and Condition: Insights for Dredging Management

High levels of infrastructure development for mineral exports occur in NW Western Australia, resulting in many dredging activities with large volumes of material being dredged. Dredging impacts primary producers, including... [ view full abstract ]

High levels of infrastructure development for mineral exports occur in NW Western Australia, resulting in many dredging activities with large volumes of material being dredged. Dredging impacts primary producers, including seagrasses, through reduced light availability and burial under deposited sediments. In the NW of WA, seagrass meadows are important habitats that provide nurseries and feeding grounds for prawns and juvenile fish, many of which are commercially fished and also sustain larger marine animals such as the dugong and green turtle. Despite the value of seagrasses, little is known on the distribution, abundance, phenology and condition of seagrasses in this region. This information is important to be able to effectively predict the impacts of dredging and manage these large-scale projects. The aim of this collaborative study, was to assess the spatial and temporal patterns in seagrass composition, abundance and condition at locations across a water quality gradient in NW WA. Three locations were monitored, ranging from very turbid in-shore waters in the Exmouth Gulf to clear offshore waters in the Muiron Islands. At each location, there were two replicate sites which were monitored 7 times, over three monthly intervals, Aug 2013 – Feb 2015. Seagrass cover, species composition, abundance, flowering and nutrient content were assessed. The composition of species varied with location. Halophila ovalis was found at all sites, Thalassia hemprichii only at the offshore island location and location and Cymodocea, Syringodium and Halophila spinulosa at the turbid location. There was significant spatial variability in abundance of seagrass. Cover was low to moderate, less than 5% at the offshore island site and up to 50% in the turbid location. There were no consistent temporal patterns in seagrass abundance across locations, though two locations peaked across the summer months (Feb). Flowering was observed only from November to February. This information is providing insights into the dynamic nature of these NW WA seagrass meadows. To minimise impacts to seagrass, we would recommend minimizing stress during flowering and leading up to the peak growth period.