Determining environmental risks to high priority wetlands in the South East

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.