Ecological outcomes of managed flooding and control structures at Webster’s Lagoon

"June 2009".

Project Number: Ecological outcomes of managed flooding & control structures at Webster’s Lagoon - M/BUS/268 & M/BUS/293.

MDFRC item.

This project reports on ecological responses to the re-instatement of a wet-dry hydrological regime in Webster’s Lagoon via the construction of a regulator at the connection between Webster’s Lagoon and Toupenein Creek. Webster’s Lagoon is a large (c. 80 ha) wetland located on Lindsay Island in northwest Victoria. Construction of Lock and Weir No. 6 in 1930 and subsequent management of the river in this region changed the hydrology of Webster’s Lagoon from an ephemeral to a permanently inundated wetland with a direct connection (at its western end) to the Murray River via Toupenein Creek. Webster’s Lagoon is considered to be of high ecological value on both regional and state levels (SKM and Roberts 2003)and is a high priority site for management intervention. During July 2006, a block bank and regulator was constructed at the western end of the lagoon at the junction of Toupenein Creek and Webster’s Lagoon. Between completion of the regulator in October 2006 and June 2008, Webster’s Lagoon was disconnected from Toupenein Creek and allowed to dry completely. A managed flood was introduced in June 2008 via delivery of 252 ML of environmental water into the wetland. This initial allocation was supplemented by a surcharge filling (283 ML) in October 2008. It was anticipated that construction of a regulator (fitted with a carp screen) and the subsequent imposition of a wet-dry regime would lead to an improvement in the ecological values of the wetland and consequently the icon site. The ecological indicators reported upon here include water quality, macro- and micro- invertebrates, fish and wetland vegetation. Water quality The primary objective of this component of the project was to investigate water quality trends at Webster’s Lagoon. It was anticipated that construction of a regulator and the subsequent imposition of a wet-dry regime would lead to changes in key water quality parameters, and that these changes may have cascade effects on other components of the wetland ecosystem. Because of the low natural organic matter (NOM) loading on the base of Webster’s Lagoon at the time of this study, it is considered likely that a substantial component of the nutrient release observed originated from the wetland soil. Oxygen drawdown did not occur upon re-flooding, however a series of hypoxic periods were observed as the water temperatures decreased. Reductions in turbidity and chlorophyll a were also evident and are considered to be positive ecological outcomes. Acid sulphate soils Assessments were made on the risk of Webster’s Lagoon suffering from acidification following the reinstatement of a wet-dry regime after long-term permanent inundation. Sediments were also assessed post-watering to allow an assessment of the impact of repeated ponded floods on soil salinity. The data collected indicates that there is currently a low risk of the oxidation of any sulfidic material present leading to the liberation of sufficient acid to cause a drop in surface water pH and the subsequent acidification of the wetland. The continuation of a regular wetting/drying regime is considered likely to aid in the control of any sulfidic material present. Macro- and microinvertebrates The primary objective of this component of the project was to investigate temporal trends in the macro- and micro- invertebrate community at Webster’s Lagoon. It was anticipated that a shortterm (flood-pulse) would increase the density and diversity of macro- and micro- invertebrates A “re-setting” of the macro and micro- invertebrate community characterised by a Successional pattern similar to that expected for an ephemeral wetland was observed, providing a food source for larval and planktivorous fish, predatory macroinvertebrates and many waterbirds. Fish The aim of this component of the project was to investigate the influence of constructing a regulator(fitted with a carp screen) at the inlet to Webster’s Lagoon; imparting a drying cycle on the wetland which resulted in complete drying of the site; followed by an inundation phase whereby water was delivered initially through the screened regulator and then by pumping to surcharge the wetland above the standing water level maintained by the Lock 6 weir pool. Results have indicated a significant change in community composition within Webster’s Lagoon. The installation of a carp screen has led to a considerable reduction in the presence of carp with only 5 juveniles captured during the surveys (compared to 166 when permanently inundated). Cumbungi Pre-existing stands (established when the wetland was permanently inundated) had been reduced to desiccated stands that had collapsed upon themselves by the time the pre-flooding survey was undertaken. Consequently, it is concluded that the drying phase led to a substantial reduction in the above ground condition of the cumbungi stands at Webster’s Lagoon. However, following the managed flood, cumbungi was recorded within zones where stands had previously been recorded. Consequently, although the extent of the stands was greatly reduced, cumbungi was not eliminated from the site. It is also of note that a large proportion of the remaining dry foliage material was deliberately burnt for control purposes by Parks Victoria in the days preceding flooding. It is considered likely that the burning of the remnant stands prior to flooding may have contributed to the re-establishment of these stands. Understorey vegetation The objective of this component of the project was to investigate the potential to reinstate a flood dependent understorey vegetation community within the wetland. It was anticipated that the reinstatement of a wet- dry phase would result in an increase not only in the total number of species recorded, but also an increase in plant community diversity through reinstating Successional patterns (e.g. aquatic-dry-aquatic). Presence/absence data was compared before and after the inundation event resulting in an increase in the diversity of aquatic macrophytes, and significant changes in species composition within Webster’s Lagoon. It is of note that a number of significant (i.e. vulnerable, rare and known) plants and noxious weeds (i.e. Field dodder, Cuscuta campestris) were recorded. River red gum saplings The establishment of river red gums within the base of wetlands and creek lines has become a major management issue in recent years, and is an issue present at Webster’s Lagoon. The potential for managed floods to control river red gum saplings within the wet/dry zone via prolonged inundation was investigated. Changes in the size and visual condition of the saplings were documented before and after inundation. Results indicated that the construction of a regulator and the subsequent imposition of an extended period of inundation phase did not negatively impact the existing population of river red gum saplings within Webster’s Lagoon. Furthermore, the observation that additional germination has occurred on the wetland base following the managed flood suggests that the existing population may increase without direct management intervention.