Determining environmental risks to high priority wetlands in the South East

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Project Partners:

The University of Adelaide, Flinders University, and SARDI

Research Theme:



Project Overview

Ewens Ponds, located in the South East of South Australia, are exceptionally clear-water wetlands dominated by macrophytes, which provide critical habitat for protected aquatic species including the critically endangered Glenelg Spiny Crayfish (Euastacus bispinosus).

Regional changes in land use from native vegetation to pasture, and alteration of the hydrology due to increased water extraction, decreased the quantity and quality of groundwater flowing into Ewens Ponds. Episodic occurrence of cyanobacterial blooms and epiphytic algal growth are initial warning signals of deteriorating water quality. Similar freshwater ecosystems have responded in a drastic way to increasing nutrients shifting from a clear-water macrophyte-dominated state to a phytoplankton-dominated state, with concomitant reduction in ecosystem health.

There is increasing concern that pelagic and epiphytic phytoplankton might outcompete macrophytes in Ewens Ponds causing habitat degradation and loss of endangered species. The uniqueness of the ponds and their regional and global importance are motivators for their protection and the maintenance of suitable water quality and flow.

The project aims to identify the most significant threats to high value wetlands in the South East.

  1. Determine the causes of elevated alkalinity and high pH in the South East wetlands. Possible contributory factors to be considered are evapoconcentration of inorganic carbon and depletion of CO2 by algal photosynthesis.
  2. Identify if spikes in the pH of water in these wetlands are a cause for concern, and whether they can be controlled through management of agricultural practices or the operations of the drainage network.
  3. Determine the nutrient budget and sources of nutrients to the clear water ponds of the South East (Ewens Ponds) and model the impact of these nutrients on water quality in the ponds to determine the likely consequences for water clarity and macrophyte growth.
  4. Recommend management guidelines and actions that will reduce the risks to high value wetlands in the South East. 

Progress Update and Key Findings

The key findings related to the risk of regime shift in Ewens Ponds were:

  • Residence time is one of the most important factors controlling future environmental risk. A low residence time of about 9.5 h (flow rate ~1.2 m3 s-1) flushes out of the system pelagic phytoplankton cells that might develop at the surface. Additionally, it avoids stratification, even during the summer, maintaining high oxygenation in the water column. In this highly oxygenated environment there is almost no internal release of phosphorus from the sediment.
  • Nutrient monitoring and bioassay experiments showed that the system is phosphorus limited (e.g. TN:TP ratio of about 420) and nitrogen is extremely high 5.8 ± 0.5 mg L-1. However, sediment flux experiments revealed that there is sufficient total phosphorus available in the sediment to represent a risk. If the flow rate decrease and anoxic conditions develop in the bottom layers, the TP released in 20 days would be able to support pelagic algal growth and reduce light availability for macrophytes of about 50% at their maximum depth of colonization.
  • The combined effect of TP and flow on pelagic phytoplankton development and consequently on light availability for macrophyte growth, were evaluated by modelling. This allowed estimating TP and flushing rate thresholds to maintain clear water and preserve the macrophyte community.
  • The risk in Ewens Ponds is also associated with a possible increase of TP groundwater input in the future. It has been estimated that a spike in nutrients entering the Ponds might be observed between 2026 and 2037. This was obtained relating the calculated water age of the Ponds with the Australian trends in fertilizer use in the last decades. This risk might still be mitigated by flow rates, because, even with increasing TP, pelagic phytoplankton growth will be unlikely to occur if the present flushing rate is maintained.
  • An additional risk for the clear water system conservation is represented by epiphytic algal growth. The development of epiphytic algae might occur at higher flow rate than pelagic algae and it might cover the macrophyte shading the light necessary for their development and compromise ecosystem function. With no limitation of phosphorus the epiphytic algal growth rate could drastically increase up to 5 times the current rates.
  • Many wetlands in the South East are undergoing pressures similar to Ewens Ponds, such as increasing nutrients and flow reduction. For example, several highly valued wetlands, which are groundwater dependent, were identified as being located within groundwater development risk zones in the South East. Information available on total nitrogen (TN) and total phosphorus (TP) observed in South East wetlands were collated and analysed.
  • Conservation planning for Ewens Ponds should focus on maintaining high flow and limiting phosphorus inputs. 

The key findings related to the alkalinity were:

  • Sulfate concentrations in the wetlands studied were low or negligible and would be unlikely to generate large increases in pH. The addition of organic matter caused the available sulfate to be reduced but caused an acidification rather than an alkalinisation of both soil and sediment. Thus, there was no evidence that sulfate reduction was a significant factor in the high pH.
  • Mesocosms experiment results suggested that the high pH is in part due to high carbon dioxide demand from the growing plants coupled with the to slow diffusion of carbon dioxide from the atmosphere which is insufficient to replenish the carbon dioxide sequestered by the plants.
  • The geochemical modelling based on water analyses established that evapoconcentration could cause moderate increases in pH, and in the case of Little Reedy could explain pH up to 9.5. Higher pH in lakes elsewhere has been shown to be associated with low levels of calcium and magnesium.
  • Environmental sampling, mesocosm studies and geochemical modelling indicated that the high pH in the wetlands studied is a natural phenomenon driven by productive plant growth. Despite the high pH, these wetlands support a diversity of plant and animal life. 

Project Impacts

This project provided scientific knowledge to anticipate drastic change in ecosystem dynamic avoiding consequent loss of habitat function for threatened species.

Project outcomes allowed identifying the stressors that interact controlling water clarity and alkalinity in South East wetlands. This enabled support of water quality and conservation management decisions. Additionally, approach and tools adopted can easily be picked up by government and managers to reduce and manage risks. 

Research Outputs

Phytoplankton Growth Dilution Model

Causes of high alkalinity in South East wetlands

Determining environmental risks to Ewens Ponds in the South East

Project News