Wardang Nearshore Marine Biounit
2022 Aquatic Ecosystem Condition Report
On the map, zoom in and click on the dots to view underwater video at each site
- Score has changed from Poor to Good with increases in seagrass percent cover and reef health
- Habitats in the shallow embayments of Hardwicke bay reflect systems under pressure from nutrient enrichment
About the biounit
The Wardang biounit extends from Island Point north of Port Victoria, down the western side of the Yorke Peninsula to Corny Point. The biounit faces west into Spencer Gulf and Wardang Island is offshore from Port Victoria. The biounit is exposed to moderate wave energies, particularly on the outside of Wardang Island, while Hardwicke Bay is relatively sheltered and may be affected by low flushing.
Wardang biounit has only a small number of coastal towns with the largest being Port Victoria, with 397 permanent residents in 2021. Population growth within the region has been small since the last census in 2016, with an increase of just 45 permanent residents. Sewage treatment from these small coastal towns is via septic tanks, which in sandy soils has the potential to transport nutrients into groundwater and travel towards the coast.
The region is surrounded by agricultural land with cereal crops dominating the entire peninsula. There are no recognised creeks that regularly flow to the ocean meaning limited agricultural surface runoff.
An investigation of potential threats to water quality for Wardang predicted that it was likely to be in Good condition.
A total of 7 sites were monitored in waters between 2–15 m deep during autumn 2022 to assess the condition of the Wardang biounit. Monitoring conducted in autumn 2016 assessed the biounit to be in a Poor condition, and in autumn 2022 Wardang was assessed to be in a Fair condition. One-third of the habitat monitored was seagrass, and of which, one-third was mature recruiting species Halophila spp. and Zostera spp.
The shallow and sheltered sites in proximity to Hardwicke Bay were observed to reflect a system that is under pressure from nutrient enrichment which in these types of habitat could be the natural state and cyclical. As waters are shallow, warm and have reduced flushing. Wardang Island (m0152) was the only reef habitat that was dominated by big brown canopy forming algae.
A total of 7 sites were monitored in autumn 2022 to assess the condition of the Wardang biounit – of the habitat monitored, 33% was seagrass, 44% reef and 23% bare sand.
The condition of reef systems within Wardang biounit has remained quite variable. Port Rickaby (m0136) has shown some improvement in condition with a reduction in turfing algae and an increase in larger canopy forming brown algae, although data could be subject to analyst bias and advances in camera technology allowing for better species identification. Wardang (m0152) remained intact and healthy, dominated by Cystophora spp. and other erect coarse branching brown algae species. Point Pearce (m0129), Hardwicke Bay (m0130) and Port Victoria (m0131) all reflect degraded reef systems, dominated by turfing algae and opportunistic algae with very little to no presence of erect coarse branching algae species.
Seagrass condition was quite poor throughout the biounit, and where present it was generally heavily covered with epiphyte. Point Souttar (m0128), which was once bare sand, now boasts large and mature meadows of Zostera spp. and Halophila spp.. Although the species was first recorded in the 2016 monitoring, percent cover has more than tripled. Observationally compared to 2016, the plants within the meadows are more mature, because these species are ephemeral in nature a loss at this site may be expected in future.
Port Rickaby (m0136) is a variable site containing mixed meadows of seagrass in between areas of reef. Along with improvement in reef condition at this site, seagrass condition has improved with significant increases in Amphibolis spp. and Posidonia.
With the exception of Wardang Island (m0152), epiphyte loads have increased by around 40% throughout the biounit, suggesting excess nutrients in the system. Analysis of chlorophyll pigment sizes in phytoplankton communities, indicate that the biounit is transitioning into mesotrophic conditions with the larger micro-pigments becoming more prominent.
Port Rickaby (m0136) and Point Souttar (m0128) are sites that had an increase in seagrass percent cover, epiphyte load and the largest increases in larger chlorophyll pigments. Nutrient enrichment indicators thrive in environments like Hardwicke Bay (m0130) as they are shallow, warm and have reduced flushing. With prolonged exposure to high epiphyte loads there is a risk seagrass loss may occur, although it is not known if seagrass recruitment and dieback are cyclic and driven by a natural nutrient cycle process in these habitats.
Water chemistry at the time of sampling showed turbidity, nitrogen and phosphorus were at acceptable levels throughout the biounit.
Pressures and management responses
Coastal development is likely to increase the number and density of septic tanks which contribute nutrients into shallow ground waters that flow to the sea
Development in coastal townships has increased significantly over the last few years, however most new development occurs in subdivisions where stormwater is required to be retained on site via rain water tanks and any overflow is managed through Council’s stormwater disposal system.
Coastal development will increase, however wastewater is disposed of in either Council’s Community Wastewater Management Schemes (CWMS) or disposed of in accordance with the SA Health Commission requirements where soakage trenches are to be located greater than 100 meters from the high water mark to reduce the risk from pathogens.
There has been a significant increase in the number of visitors coming to coastal towns and parks. Yorke Peninsula council have installed a number of new bush camping toilets. Toilets in high use areas are sealed system with no soakage, toilets in low use areas are all compostable