Gunbower Wetland Productivity Study
Client: North Central Catchment Management Authority
The following is a summary based on the Final Report available from the Murray-Darling Basin Authority website here
In 2019-2020 three wetlands in the Gunbower Forest were provided with water for the environment to sustain over-wintering habitat for populations of small-bodied fish and waterbirds that were established the previous year. To evaluate the success of this strategy, surveys of the productivity and biological diversity of all three wetlands were completed following watering events that filled wetlands in winter-2019 and allowed them to draw-down to residual pools in early autumn-2020.
- Water-dependent understory vegetation abundance and diversity
- aquatic macro-invertebrates and micro-invertebrates abundance and diversity
- fish diversity and abundance
- turtle abundance
- waterbird and bush bird diversity and abundance
Multiple sites were surveyed in winter, spring, summer and autumn at each wetland; Reedy Lagoon, Little Gunbower wetland complex and Little Reedy wetland complex in between Koondrook and Cohuna in north-central Victoria.
Water for the environment is a precious commodity and all stakeholders need to know where it can be most effectively used to achieve ecological objectives. To learn this knowledge, we must measure the ecological-effects of each watering against the objectives. A range of management questions were identified to clarify if there are differences in the responses of flora and fauna across these three wetlands that suggest a limited water resource is best focussed on any particular wetland or types of wetland.
We surveyed 6219 occurrences of 113 plant taxa in and around the wetlands. Reedy Lagoon grew the highest volume and biomass of vegetation, had the highest level of vegetation cover and the most evidence of grazing. However, vegetation production in biomass for three Gunbower Forest wetlands was low compared to the few published values for comparable wetlands.
Macroinvertebrate diversity in 2019-2020 was similar to a previous study in Gunbower Forest comparing semi-permanent wetlands with permanent lagoons, and abundance was relatively high (McInerney et al. 2017). We suggest that antecedent watering in Reedy Lagoon, Little Reedy wetland, and Little Gunbower wetland during 2018 may have shaped the assemblage structure in 2019 to have some of the characteristics typical of more permanent wetlands.
Densities of Chironomid ("mosquito") larvae were low compared to levels suggested as required for a breeding response from Grey teal. The high abundance of colonising macro-invertebrates suggests that most, if not all, of the productivity benefit to macroinvertebrates that is derived from mobilizing terrestrial energy sources can be derived from a short draw-down period as occurred between the watering years 2018-2019 and 2019-2020.
Micro-invertebrate abundance was variable and similar to that reported in studies of other relevant wetlands, although at most sites’ densities were low compared to density-thresholds suggested as required for strong native fish recruitment. Little Gunbower wetland contained the site with the highest micro-invertebrate densities, had the most fish species and also was the only site where Golden perch were sampled. Density of micro-invertebrates may be a key indicator for evaluating watering to achieve small-fish management objectives.
Little Gunbower wetland complex had the highest turtle abundance. Very few juvenile Eastern Long-necked turtles were sampled although adult biomass was significant; possibly exceeding that of fish in the three Gunbower Forest wetlands.
We sampled 56,834 fish, from eleven species (7 native and 4 non-native), 398 yabbies (1 species) and 100 turtles from two species. The fish assemblage showed seasonal succession. Little Gunbower wetland had the richest species assemblage with almost all (98%) of the expected species present.
Common carp, an invasive species, represented only 2% of the fish sample by numbers although their larger average size meant that this was 52% of fish sampled by weight. At two of the three wetlands, Common carp were obvious casualties of stranding during draw-down however, our final autumn surveys observed a small-bodied native fish assemblage had outlasted the Common carp and remained as survivors in the residual pools in all three wetlands studied.
Waterbird response was minor and similar across all wetlands with surveys counting 368 waterbirds from twenty-five species. Waterbird foraging habitat was created by wetland watering and a local waterbird assemblage was maintained from winter–summer although waterbird abundance declined as wetland-area diminished. Bush birds adjacent to the wetlands were also recorded with 261 birds observed or heard, from 52 species, over four seasonal surveys.
The patterns of productivity and diversity measured across these three wetlands in the Gunbower Forest are, perhaps unsurprisingly, complex and variable. Productivity was not consistently highest or lowest at any single wetland. We developed a multi-criteria analysis framework to summarise and rank wetlands according to the range of ecological measures used where a difference among wetlands was detected. Using this multi-criteria analysis Little Gunbower wetland ranks highest as the analysis is influenced by Little Gunbower ranking well with several fish-related measures. We also developed a putative weighting system, weighting each measure depending on whether it relates to primary-producer, secondary-producer, or higher trophic-level. Weighting primary production measures as 3x more valuable as measures of tertiary production (or higher), changes the rank order of wetlands and Reedy Lagoon then scores as the most productive. Little Gunbower wetland was also the most unique of the three wetlands being dissimilar to other wetlands for eight out of twelve multivariate measures of diversity and abundance.
The proposed multi-criteria analysis framework is an innovative way to summarise and rank wetlands following a management intervention. It uses a broad range of ecological measures where a difference among wetlands is detected. The method considers desirable vs. undesirable measures and would allow measures to be weighted according to ecological, or social importance a priori. While ultimately the ranking of wetlands is sensitive to the weightings applied, once weightings are agreed the framework is a useful model to objectively assess watering actions against a complex matrix of measurements, simultaneously evaluating multiple management objectives.
Seven recommendations (see report) are provided regarding management and monitoring the productivity of wetlands.