The exchange of organic material between the Murray River Channel and Barmah Forest during the 2005 floodplain watering - M/BUS/102 AA185
MDFRC Consultancy report
The release of the 2005/2006 Barmah-Millewa Forest Environmental Water Allocation (513GL) supplemented by natural flows from the Kiewa and Ovens Rivers between October 2005 and March 2006, resulted in peak flows of up to 28 000 ML/day in the Murray at Tocumwal between October 2005 and December 2005. Significant areas of floodplain were inundated. This study quantified the exchange of material between the Barmah-Millewa Forest and the Murray River channel by measuring concentrations of materials in the river sites upstream and downstream of the forest, and their impact on primary and secondary production. It also provided data to refine the Blackwater Model developed for the Barmah-Millewa Forest. Water quality In response to the changes in flow and inundation of the floodplain there were marked changes in water quality parameters at both of the sites downstream of the forests. Total nitrogen, total phosphorus, dissolved organic carbon and electrical conductivity increased in the Murray and Edward Rivers while total suspended solids, particulate organic carbon, dissolved oxygen, pH and chlorophyll a decreased at these sites over the same period. Blackwater modelling The increase in dissolved organic carbon and the decrease in dissolved oxygen were consistent with a blackwater event. The Blackwater Model predicted significant changes in dissolved oxygen and dissolved organic matter associated with the flood. The predicted dissolved oxygen concentrations are higher than those we would expect on the floodplain, based on the concentrations we measured in the rivers at the time of the flood. Both the starting dissolved oxygen and the minimum dissolved oxygen were higher than that measured, reflecting the fact that some reduction in water quality had occurred before the water entered the floodplain, and this is not reflected in the model. The current model uses historical data to predict temperatures and forecast blackwater conditions, but it may be modified so that actual temperatures may be directly entered, potentially reducing some of the variability in the predicted dissolved oxygen concentrations and, linking this model with an appropriate hydrological model would improve its ability predict the impact of blackwater on the rivers. Transfer of materials between the river and the floodplain There was evidence of lateral transfer of materials between the main channel of the river and the floodplain. The heavier suspended particulates remained trapped on the floodplain as the water is dispersed, while soluble nutrients and carbon were rapidly released into the water on the floodplain and flowed back to the river. The abundance of planktonic microinvertebrates declined during the flood but increased rapidly during the falling limb of the hydrograph potentially due to bacterial productivity associated with the dissolved carbon and nutrient pulse. There was no effect on the taxonomic richness of the microinvertebrate communities. All sites were Barmah-Millewa Floodplain Watering 2005/2006 dominated by small taxa (rotifers) characterised by rapid growth and population renewal rates and less likely to be preyed upon by planktivorous fish due to their size compared with the larger microcrustaceans. Flooding and productivity The 2005/2006 allocation of water to the Barmah-Millewa Forest led to a significant exchange of material between the floodplain and main channel that appeared to stimulate production. This highlights the need for consideration of main channel impacts in the allocation of environmental flows to icon sites Two issues will need to be resolved by future monitoring. First, the extent to which the exchange of organic matter, nutrients and organisms between the floodplain varies in response to variations in the flooding regime. Second, the downstream or long term ecological significance of the material returned to the main channel, as it is possible that this material may have an impact on processes such as algal production or initiate a cascade up the food-web.