Zooplankton in the Murray estuary and Coorong during flow and no-flow periods

From 2001 to 2010, River Murray flows were extremely low because of drought and over allocation. Barrage releases to the Murray Mouth were small and intermittent, with an extended period of closure and no flow from December 2001 to September 2003 and from February 2006 to November 2010. We studied the zooplankton communities in the Murray estuary and Coorong North Lagoon from 2003 to 2008 to identify the distribution and abundance of zooplankters in flow and no-flow periods, to compare zooplankton communities to those in other Australian estuaries and to consider the role of zooplankton in trophodynamics. Most of our samples were taken with a 59 mu m net. In September/October, 2003 a barrage release brought freshwater zooplankters into the Murray estuary. By November and December, salinities had risen and only estuarine and hypermarine species were collected. In the spring/summer of 2004/05, salinities ranged from 16 to 88 parts per thousand (ppt) and a limited estuarine/hypermarine community was present. By 2007/08, salinities had risen to be 35-130 ppt and the distribution of the estuarine/hypermarine community was restricted and the Australian brine shrimp Parartemia zeitziana was collected. Overall, the zooplankton community was of low diversity compared to other estuaries in south-east Australia and lacked several of the common copepods. Rotifers, especially Synchaeta spp., were well represented especially in the hypermarine regions. The diversity and abundance of zooplankters was the highest during the period of River Murray outflow. The low abundance of adult calanoid copepods during the no-flow period might suggest that large bodied individuals were removed by high fish predation. The abundance of copepodites and nauplii shows that small mesh nets are required to see the complete zooplankton community and also that copepodites and nauplii can be surprisingly abundant when adults are rare.