Ecological responses to wetland flooding, Deniliquin NSW

"June 2003".

Project Number: Research on the private property wetlands around Deniliquin - M/BUS/69.

MDFRC item.

A post-wetting ecological survey, partially funded by the NSW Murray Wetlands Working Group Inc., was conducted at three Deniliquin wetlands (Forest Creek, Greenacres and Lynnbrae) in NSW between October and December 2002. Sampling of three wetlands commenced on the 27th of October 2002 and was conducted over 5 weeks after flooding. Water was released into the wetlands from three separate channels fed from Lake Mulwala through the main Mulwala channel. The water formed part of an environmental water allocation managed by the NSW MWWG. The aim of the study was to determine post flood responses of water quality, nutrients, phytoplankton and microcrustaceans. Throughout the study period changes in the following water quality parameters were observed. Water depth in all three wetlands ranged from 5-28 cm and were influenced by additional inflows. Temperatures ranged from 25-30°C. The Forest Creek wetland was acidic throughout most of the sampling period (pH range 3.5 -7.0), while the Greenacres and Lynnbrae wetlands were slightly basic (pH ~ 7.5). High initial post flood turbidities were recorded in the wetlands, however these declined rapidly 4 days after the inundation then continued to decrease gradually for the remainder of the sampling period. Electrical conductivity remained low, peaking at ~135 µS/cm on day 15 in the Forest Creek and Lynnbrae wetlands and 220 µS/cm on day 27 in the Greenacres wetland. Inflow salinities followed similar patterns to wetlands, however remained 5-50 µS/cm below wetland salinity. Dissolved oxygen (DO) concentrations were generally below 6 mg/L in the three wetlands. The lowest DO was recorded in the Lynnbrae wetland on day 15 (1 mg/L). Initial peaks in all nutrients examined -total nitrogen (TN), Oxides of Nitrogen (NOx), total phosphorus (TP) and filterable reactive phosphorus (FRP) -were recorded within a few hours after inundation in all three wetlands and had decreased rapidly by 5-12 hours after inundation. Secondary peaks in TN, TP and FRP were also seen on day 15 in the Forest Creek and Lynnbrae wetlands. Maximum TP concentrations of 250, 505 and 835 µgP/L and TN concentrations of 2410, 4185 and 2405 µgN/L were recorded in the Forest Creek, Greenacres and Lynnbrae wetlands, respectively. Chlorophyll trends during the sampling period were similar in the Forest Creek and Lynnbrae wetlands with 2 peaks occuring on day 15 (38 and 14 µg/L, respectively) and on the last day of the study (19 and 15 µg/L, respectively). The Greenacres wetland recorded an initial peak in chlorophyll concentrations on day 7 (16 µg/L) and a second peak on day 35 (39 µg/L). Chlorophyll peaks were mainly due to increases in blue-green algae (Anabaena sp.). Microcrustacean abundances remained below 10 individuals/L until day 27 in the Forest Creek wetland and channel sites, after which they increased to 42 individuals/L by the end of the study period. This final peak was dominated by Daphniidae and Cyclopidae taxa. Very similar trends were reported between the Greenacres and Lynnbrae wetlands in relation to timing of peaks and dominant species found at the peaks. Two peaks were recorded on days 5 and 27 in both wetlands with the initial smaller peak being dominated by Cyclopidae, Centropagidae and Bosminidae and the larger second peak consisting mainly of Cyclopidae, Centropagidae, Moinidae and Daphniidae. Temporal responses of nutrients, chlorophyll and microcrustacea in the Forest Creek and Lynnbrae wetlands followed responses reported for other wetlands (Greiger et al. 1985, Culver and Geddes 1993, Ingram et al. 1997), with an initial peak in nutrients occurring within the first day, followed by a peak in phytoplankton at 2 weeks and finally a peak in microcrustacea at 5 weeks after inundation. The Greenacres wetland demonstrated the same short response time of a few hours for nutrients, however microcrustacea and phytoplankton responses differed. Following the initial nutrient peak, a peak in microcrustacea was seen on day 27, one week prior to the phytoplankton peak. Future suggestions. Due to the very high nutrient concentrations found within all the wetlands there is an urgent need to keep out grazing animals not just from wetlands but also from adjoining channels and/or decrease possible fertilizer run-off into the channels and wetlands. Future monitoring should be extended past 35 days to more clearly identify macroinvertebrate responses, which are likely to be slower than the other parameters examined. Further filling of the wetlands performed on a natural time scale should be monitored for the above examined parameters to determine the possible change in response times and effects in concurrent wetting cycles. Monitoring future flood events occurring at different times of the year within the studied wetlands will help identify possible post flood responses to seasonal variations.