Nene Nearshore Marine Biounit
2015 Aquatic Ecosystem Condition Report
- On the map, zoom in and click on the dots to view underwater video at each site
- The biounit is subject to high wave energy
- No seagrass habitat was found
- Reefs were in good condition with extensive and diverse algal communities
About the biounit Nene
The Nene biounit is in the Otway bioregion and extends eastwards from Cape Banks near Carpenter’s Rocks to Cape Northumberland. The biounit is dominated by limestone reefs close to shore that attenuate wave energy from the Southern Ocean prior to reaching the rocky shores and sandy bays.
The entire southeast region of South Australia is a highly modified landscape. The catchments throughout Nene have been largely cleared for agriculture and forestry with very little native vegetation remaining, resulting in increased surface water runoff often laden with nutrients and sediments. Agricultural productivity from land adjacent to Nene is significant for the Southeast, which contributes approximately $5 billion to the South Australian economy.
There are small townships on the coast of Nene including Carpenter’s Rocks and Blackfellows Caves, which have small populations likely to swell during holiday periods. The townships treat sewage using onsite disposal, which in high densities can result in nutrients entering the groundwater and subsequently flows to the coast.
The Finger Point wastewater treatment plant (WWTP) collects wastewater from South Australia’s largest regional centre, Mount Gambier, and discharges it into coastal waters east of Cape Douglas, across a large intertidal reef into the high-energy waters.
The Nene biounit is sparsely populated with very few pollutant inputs and the biounit was predicted to be in Excellent condition, based on an assessment of threats to the nearshore habitats.
Three sites were monitored between 2 – 15 m deep during autumn and spring in 2015 to assess the condition of the Nene biounit. There are large areas within the biounit that are deeper than 15 m, which are not included as a part of this evaluation.
The sites monitored were dominated by diverse reef habitats with generally high cover of canopy algae.
The findings suggest that the nearshore marine habitats are generally in Very good condition: the habitat structure is considered natural, but some detectable changes compared to excellent condition. Habitat changes are unlikely to be altering ecosystem function. Any detrimental effects are limited to small localised areas and are likely to be reversible. The high wave energy, lack of substantial areas of restricted flow and low population appear to be having minimal impact on the condition of habitats in Nene.
Three sites were monitored in Nene, with 90% of the area classified as reef habitat, sand accounted for 10% and there was no seagrass observed. The reefs monitored were found to display extensive and diverse algal communities. However, the rocky reefs in the Southeast have had relatively little scientific monitoring compared to locations closer to Adelaide, which has resulted in sparse information about their response to disturbance. This limits our ability to interpret some of these findings as degradation or whether this is the natural state. Further work is needed to develop more detailed conceptual models for this area.
Douglas Point (m0546) and Finger Point (m0549) were reefs that were not dominated by the typical large brown canopy forming species that are known to be indicative of a reef in good condition (eg Ecklonia radiata, Cystophora spp, Sargassum spp. etc). However, they were diverse and multi-layered communities, rather than dominated by turfing species as seen in disturbed reefs. At this point in time, there is insufficient evidence to suggest that these reefs are impacted, and likely to represent a natural state that is different to reefs closer to Adelaide.
Soluble nitrogen measured in the water column was largely below the limit of reporting which is typical of very nutrient poor waters. Similarly, organic nitrogen and phytoplankton very also very low consistent with very little land based discharges into these coastal waters and the high energy and flow through the region. Towards the south of Nene sites had elevated turbidity in autumn with three sites (m0546, m0547, m0549) all having measurements of 1.2 NTU compared to values less than 0.44 NTU in spring. The results in autumn may reflect the high wave energy of the biounit.
Pressures and management responses
|The Finger Point wastewater treatment plant collects, treats and then discharges nutrient rich wastewater to the nearshore marine environment and may be impacting on the ecosystem||
SA Water monitors its discharges from the Finger Point WWTP to ensure the plant is performing and to enable an assessment of any changes in the discharge which might increase the pressure on the receiving environment. These reports are submitted annually to the EPA for review.