Nene Nearshore Marine Biounit
2021 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 condition hasn’t changed and were in good condition with extensive and diverse algal communities
About the biounit
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, contributing a gross value in agricultural production of approximately $3.5 billion in 2018-19.
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, 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.
Using reduced site data, in 2015 the Nene biounit was in Good condition, based on monitoring conducted over autumn.
Three sites were monitored in waters between 2 – 15 m deep during autumn 2021 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 communities with generally high cover of canopy forming algae. There was no seagrass meadows observed in this biounit which is consistent with State and National data.
The findings suggest that the condition of the Nene biounit has not changed since last monitoring period and is still scored as Good. Indicators of nutrient enrichment have reduced since 2015. 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 the Nene biounit, with 83% of the habitat classified as reef, sand accounted for 17%, no seagrass was recorded. 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.
Similarly to 2015 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. Cape Douglas (m0547) was dominated by the larger brown canopy forming algae’s, suggesting that location may play a role in reef community structure.
At the time of sampling, phytoplankton community structures (using Fp ratios and size classes) show that the amount of micro pigments present have reduced since 2015 and smaller pigments such as nano and pico are more prevalent. This is an indication that the waters of the Nene biounit are becoming less nutrient enriched. Organic and soluble nitrogen were slightly elevated from 2015, although higher it is not a sign of nutrient enrichment as the biounit is subject to high wave energy with lots of mixing and small resident times.
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.
SA Water plans to upgrade infrastructure at the Finger Point including improvements in sludge management processes which will assist in maintaining the quality of treated water being discharged as well as an upgrade to the nearshore outfall to aid dispersion and mixing