title: Aquatic biodiversity and the ecological value of on-farm water storages on irrigation farms
Creators: name: Lutton, Susan J.
Publisher: name: Griffith University
Year: 2010
Subjects: subject: ecology
subject: irrigation
subject: aquatic
subject: biodiversity
subject: wetlands
subject: morphology
subject: hydrology
subject: fish

description: Globally, natural wetlands are under threat from water resource development reflecting the need to support a growing population. In the Border Rivers Catchment in Queensland, Australia, a large irrigation industry coupled with a highly variable flow regime has necessitated the building of large on-farm water storages and often associated destruction or isolation of their natural counterparts. With the decline in abundance of natural wetlands, the presence of these storages on the floodplain has raised the question of their suitability as alternative aquatic habitat. This project aimed to investigate the diversity of storages and the structure and function of the aquatic assemblages they support compared with nearby natural wetlands. These results were then used to recommend best management practice for optimising both diversity and ecosystem function in storages. Initially the physical variety of water storages in the Border Rivers Catchment was described and their morphology and hydrology compared to that of natural wetlands. Storages and natural wetlands formed two distinct groups based on morphology. Storages tended to be large, deep structures with a more regular shape, while natural wetlands were irregular and shallow with large perimeters. Although there was a degree of variability amongst storage sites, most fell into one group and were considered to be a ‘typical’ storage in this region. After classifying the storages based on hydrology and morphology, the spatial and temporal biodiversity of the different storage groups was compared with natural wetlands. Spatial patterns (between waterbody types) in fish and macroinvertebrate assemblages composition were explored between August and October 2005 in three waterbody types; natural wetlands, typical storages and a group of storages with similar morphology to typical storages but lacking tailwater (‘no tailwater’ storages). In total nine fish species were collected comprising seven natives and two exotic species. Only one species, the eel-tailed catfish, Tandanus tandanus, was specific to natural wetlands. Although fish species diversity was similar between the two storage types and natural wetlands, there was significant spatial variation in total abundances, with typical storages having ten times the average catch size of that found in natural wetlands. In both storage types catches were dominated by bony bream, Nematalosa erebi, while in natural wetlands there was a more even distribution of species. The percentage of exotic species was much lower in both storage types (&lt; 8%) compared with natural wetlands (&gt; 40%). There were also significant spatial differences in macroinvertebrate assemblages; 24 samples from natural wetlands were represented by more than 14,500 individuals across 84 different taxa. In comparison, only 34 taxa from 2,142 individuals and 34 taxa from 7,611 individuals were collected in 15 samples from typical storage sites and 12 samples from ‘no tailwater’ sites respectively. Seventeen taxa were common to all three waterbody types while 45 taxa were specific to natural wetlands. None of the measured environmental variables explained the observed variation in macroinvertebrate assemblages between waterbody types. Temporal patterns in macroinvertebrate and zooplankton assemblages were also investigated comparing natural wetlands and typical storages during the 05/06 and 06/07 cotton seasons. Specifically, macroinvertebrate and zooplankton assemblages from typical storages were less diverse than those found in natural wetlands. Temporal variation in aquatic assemblages was also apparent with macroinvertebrate and zooplankton assemblages of both waterbody types following a similar pattern through time. However, there were no strong associations between the environmental data collected and the observed assemblage composition. To further investigate the spatial variation observed in aquatic assemblages, two sources of colonisation of storages were examined, namely the aquatic assemblages found in extracted river water and those hatching from the egg bank. Eight different fish species including three exotics were collected during sampling of pumped river water. As was observed in storages, the catch was dominated by N. erebi and the contribution of exotic species to the total catch was extremely low (&lt; 1%). Fish with a standard length of over 200 mm survived the extraction process. The eight samples of pumped river water were represented by 22 macroinvertebrate taxa and 332 individuals. This was a reduced diversity than collected from typical storages (34 taxa). In comparison, zooplankton assemblages had similar diversity to those observed in the typical storages, 25 taxa compared with 22 taxa. Sediment samples collected from the floodplain had a more diverse but less abundant egg bank (46,463 individuals from 20 taxa) than those collected from dry typical storages (70,600 individuals from 16 taxa). The observed spatial differences in morphological and hydrological features and aquatic assemblages suggested that there may also have been variations in aquatic processes between waterbody types. Stable isotope analyses were used to investigate the major sources of energy fuelling the aquatic food webs of natural wetlands and typical storages. In general all components of the food web in typical storages were 13C and 15N-enriched in comparison with natural wetlands. The importance of source carbon to consumers varied greatly between species, sites, and waterbody types. Algae, in the form of biofilm and floating algae, along with zooplankton were the major sources of organic carbon to aquatic food webs, contributing 50% of organic carbon across natural wetlands and 54% across typical storages. Storages primarily function as water supplies and their associated management makes them mostly unsuitable as ‘replacement’ wetlands. However, given the large numbers of storages across the catchment, if managed effectively, they may provide an additional source of aquatic habitat and help maintain regional biodiversity. To maximise the biodiversity of storages it will be essential to reduce the morphological homogeneity of storages across the landscape and increase habitat diversity within storages. In the future, improved design of new storages and alterations to existing storages and their management could help overcome this problem of low diversity of habitat. As a group, storages in the Border Rivers Catchment are still fundamentally different to natural waterbodies, with storages being a mostly homogeneous group. If we are to sustain the aquatic biodiversity in the Border Rivers Catchment and other similar irrigation regions it will be necessary to preserve the spatial and temporal variation in habitat evident in natural wetlands.