Geographe Bay and Seagrass Beds
Biological and physical values
Geographe Bay is highly valued and used extensively for recreation by the local community and visitors to the area. The sheltered waters provide a range of opportunities such as safe boating, fishing and swimming; whale watching; and many other water-based pursuits.
Geographe Bay is predominantly north facing and provides sheltered conditions from the prevailing south-westerly swells for much of the year. The embayment is formed by a broad, shallow intercontinental shelf and has a mainly sandy base overlying limestone, of which much has been colonised by seagrass meadows. The influence of the Leeuwin Current enables a combination of tropical and temperate fauna species to occur. The combination of these features has created a marine embayment with an unusually diverse combination of marine flora and fauna that is still being explored.
Geographe Bay receives ephemeral surface flow from 16 waterways that dissect the Geographe Bay catchment. Of these, only the Lower Vasse, Lower Sabina, Abba and Ludlow rivers drain into the Vasse Wonnerup Wetlands before discharging through the Wonnerup Inlet into Geographe Bay. A network of seasonal streams flow into the Toby Inlet before draining into the bay. All other waterways flow directly into Geographe Bay either through their natural outlets or artificially constructed drains with the exception of Gynudup Brook and Tren Creek, which flow first to the Capel River.
Geographe Bay also receives flow from groundwater sources.
Geographe Bay supports the most extensive seagrass meadows in temperate Western Australia (DEC 2006). Larger meadows occur in Shark Bay in the state’s north, but these contain a range of tropical as well as temperate species. The seagrass meadows of Geographe Bay are not only large but also highly diverse, with 10 species from the five genera Amphibolis, Posidonia, Halophila, Heterozostera and Thalassodendron being identified (Elscott & Bancroft 1998; Walker et al. 1995). Most of the meadows in Geographe Bay are comprised of Posidonia sinuosa but mixed meadows of P. angustifolia, Amphibolis graffiti and A. antarctica also occur. A further feature of note is that some seagrass species are found at unusually great depths in Geographe Bay. Thalassodendron pachyrhizum has been found growing at 45 m while Posidonia and Amphibolis species have both been recorded at 27 m (Elscott & Bancroft 1998).
Seagrass meadows provide many important ecological functions that make them vital components of the ecosystems of which they form a part. The leaves of seagrass provide a refuge for fish and invertebrates, as well as a substrate for the growth of algae – which in turn provides an important food source for many marine animals (Orth & Van Montfrans 1983). Following annual senescence (shedding) of leaves, the detritus formed by the leaves also provides a food source, primarily for aquatic invertebrates. Some fish and invertebrate species also consume seagrass leaves directly. Seagrass plants provide an important stabilising role for sediment in their environment: Posidonia species are particularly good at this since their large underground biomass is very resistant to wave action (Edgar 2001). Seagrass meadows also directly help nutrient cycling by taking up nutrients through their leaves and rhizomes, and indirectly when the algae that colonise their leaves absorb nutrients (Short 1986).
Brochure: Seagrass communities of Geographe Bay (UWA, 2009). Click to download (3.7 MB)
Coral and reef habitats
Well-developed coral communities occur between Dunsborough and Cape Naturaliste among low-relief rocky substrate. This area supports 14 species of seven genera – of which two species are endemic to Western Australia (Elscott & Bancroft 1998). Ten of those species are tropical and of these, five have their southern limit at Cape Naturaliste. In addition to these natural occurrences, the Busselton Jetty has created conditions for the colonisation of various soft coral species that would normally only occur under rock ledges where they are protected from light.
As well as the coral communities, numerous patches of low-relief limestone reef occur throughout Geographe Bay interspersed among the seagrass meadow (DEC 2006).These corals and patch reefs provide important and well-used fish and invertebrate habitat in the bay. Both the coral and reef communities have the potential to be adversely affected by elevated nutrients should algal assemblages cause the smothering of corals or substrate.
Geographe Bay supports an extensive array of marine fauna – ranging from the large and charismatic humpback whales to highly diverse and unusual species of sponges. A recent survey of fish in Geographe Bay using Baited Remote Underwater Video recorded 76 species of fish from 54 genera (Westera et al. 2007). The same research project recorded seven sea-star species and 12 ascidian species. In addition, an incredibly diverse array of sponges was collected, with an expectation that 40 to 60species will be identified. Marine mammals known to use the bay include the New Zealand fur seal, which has a colony at Cape Naturaliste, and large populations of resident bottlenose dolphins. A variety of whale species including the humpback and southern right whale shelter their young and feed in the bay’s protected waters during their annual southern migration in spring.The rare and endangered blue whale, the largest living mammal on earth, also feeds in the bay during November. Many other marine fauna species also occur in the bay including shark, octopi, squid and crabs. Elevated nutrients have the potential to impact on a wide range of marine fauna through disruptions to food-web linkages. Such disruptions can result from disturbance of important habitats such as seagrass meadows and coral communities.
Long-term monitoring of the marine life of Geographe Bay and the surrounding areas that make up the Ngari Capes Marine Park was undertaken to (1) broaden our understanding of the region’s marine environment by developing a long term, quantitative database for fish and algae community changes over time and (b) create a benchmark against which changes to marine biodiversity following the establishment of marine parks can be determined (Westera et al, 2008).
From this study, two interactive excel spreadsheets relevant to Geographe Bay were developed to enable users to query the survey findings and look at trends in data from over four years of work by the Centre for Marine Futures at UWA.
Patterns and Indicators for Fish Assemblages in the Proposed Ngari Marine Park
Get to know the marine fish life of the bay including facts on fish species present such as where it sits in the food chain and how vulnerable it is to fishing.
Click on image to open excel spreadsheet (2 MB).
Benthic communities of the Southwest (Capes) Region
Find out about the diversity of benthic communities at six locations from Geographe Bay to Flinders Bay. Users can choose from assemblage indicators (species richness and total biomass) or consider the most abundant and most widespread species.
Click on image to open excel spreadsheet (82 MB).
The many values of the bay has been recognised through the inclusion of the southern portion of Geographe Bay (from Capel River and extending south west) in the Ngari Capes Marine Park. The marine park extends from the eastern boundary of the City of Busselton and west to Cape Naturaliste and south to include the Cape to Cape coastline and the Hardy Inlet. Learn more about the Ngari Capes Marine Park here.
With high levels of regional growth forecast to continue, combined with significant increases in tourists and locals recreating by the coast, the threats to marine and coastal communities, including Geographe Bay, are likely to intensify accordingly. Contaminants and the introduction of exotic species in the marine and coastal environs are also likely to increase if effective management strategies are not put into place to deal with increasing demands. The threat to natural resources is also likely to increase in the near future with greater numbers of fishers and improvements in technology that increase the capacity to locate and capture fish (Limbourne & Westera 2006).
Assessing the current impact of large nutrient loads on the Geographe Bay ecosystem is a complex task. Seagrass was certainly lost from Geographe Bay between 1958 and 1976, though the cause is unclear (Searle & Logan 1978; Conacher 1993). This was a period that coincided with intensive agricultural development in the catchment, but it is also possible that erosion processes within the bay were responsible for the change. Losses of seagrass cover that ranged from 17 to 45 per cent (depending on the location) were followed by a gradual recovery in most offshore areas between 1978 and 1993 (Conacher 1993).
Inshore, scientists are not yet able to confirm the status of the seagrass. At sites close to nutrient-rich-water input such as Wonnerup, Quindalup and near Dunsborough, a reduction in seagrass cover may still be occurring but this seems to be balanced by gains in other areas (DAL Science and Engineering 2004). It is possible that these nearshore losses of seagrass cover are related to shoreline changes rather than elevated nutrients or are simply a reflection of natural variability, though these issues require further research. The techniques used to monitor changes in seagrass cover to date are not well suited to measuring small changes, since there are large errors associated with the mapping processes. Researchers from the University of Western Australia are currently establishing ecological benchmarks so that changes in the seagrass ecosystem can be more closely monitored and assessed over time (Westera et al. 2007).
Geographe Bay and Climate Change
Climate change is also expected to heavily impact the SW in the near future with changes to rainfall patterns and small changes in sea-level likely to have profound impacts across the SW region.
SWCC Strategic Priority
Geographe Bay, including its seagrass beds, are identified as a priority asset under the Coasts and the Marine Environment theme.
GeoCatch has been working on the ‘Bay OK’ project for a number of years now. The ‘Bay OK’ mission is to encourage the community to work together to improve water quality of the Geographe Catchment’s waterways and wetlands in order to protect the Geographe Bay for the future. GeoCatch has a range of ideas on how you can by being Bay Ok at home, school, on the farm, in business and in the garden. Find out more here.
- Department of Water (2010). Vasse Wonnerup Wetlands and Geographe Bay. Water Quality Improvement Plan.
- Limbourne A.J. and Westera M.B. (2006). A review, gap analysis and assessment of current information relating to marine and coastal environments in the SW Region. Part A of Project C1-G1: A coastal and marine management planning framework for the South West Catchments COuncil. School of Plant Biology, UWA.