Why is seagrass important?
Seagrasses are marine flowering plants that form extensive and ecologically significant meadows in the more sheltered waters surrounding Kangaroo Island. Healthy seagrass meadows are known to support diverse and productive marine ecosystems, including species of commercial and recreational importance such as King George whiting (Sillagonodes punctata), Southern calamari (Sepioteuthis australis) and Southern sea garfish (Hyporhamphus melanochir). Intact seagrass meadows reduce wave action during winter storms and along with seagrass wrack (dead seagrass) deposited on beaches, seagrasses protect shorelines from coastal erosion and link important nutrient pathways in coastal systems. Seagrass meadows can store more than twice the amount of carbon than the equivalent area of forest, which means healthy seagrass meadows are vitally important to help reduce the impacts of climate change. Degradation or loss of seagrass meadows can therefore have major flow-on effects for marine and terrestrial environments.
For most, the benefits of protecting healthy seagrass meadows is obvious. However the condition of seagrass communities continues to decline worldwide and it is thought that around 20 per cent of the world’s seagrass meadows have now been lost. The greatest threat to seagrass meadows on Kangaroo Island comes from land runoff entering the marine environment. Runoff high in sediment loads increases the turbidity (clouding) of the water and corresponding elevated nutrients stimulates smothering algal growth (epiphytes) on seagrass blades and competition from opportunistic macroalgae. Both of these effects reduce the amount of light available for seagrasses to photosynthesise and grow. In Nepean Bay alone, approximately 3,500 hectares of seagrass has either been lost or significantly degraded since the 1960s. Most of the seagrass lost was smooth tapeweed (Posidonia sinuosa), which is recognised as Vulnerable under the International Union for Conservation of Nature (IUCN) Red List of threatened species.
How do we monitor seagrass health?
Repeated monitoring of seagrass meadows provides a measure of condition and an ability to detect further loss or recovery over time. To achieve this, NRKI’s Coast and Marine team have been undertaking annual monitoring since 2009 using standardised techniques. Using a vessel to tow an underwater video camera, a series of transects are recorded and later analysed to assess the cover and condition of seagrass across each site. Results so far have detected no further loss of seagrass extent or condition within Nepean Bay.
What can environmental monitoring tell us?
Information collected on seagrass is used to evaluate the effectiveness of NRM funded on-ground works being implemented by land managers throughout the catchments that flow into Nepean Bay. These on-ground works, including fencing remnant vegetation, revegetating creek lines, installing stock crossings and re-locating stock watering points out of streams, are designed to help improve water quality, and help prevent nutrients and sediments from being washed into rivers.
Corresponding improvements in water quality and fish populations are being monitored, along with the health of seagrass meadows to determine how a catchment to coast approach can benefit coastal environments and communities in the future. Most of the seagrass meadows in Nepean Bay are long-lasting and even after improvements are made they can take decades to recover. Ultimately, better water quality means healthier seagrass meadows in the future.