Beate Sommer

Macroinvertebrate cycles of decline and recovery in Swan Coastal Plain (Western Australia) wetlands affected by drought-induced acidification

Management of wetlands influenced by the Gnangara groundwater mound, Western Australia, is becoming increasingly challenging due to an ongoing decline in the regional groundwater table. A number of these groundwater-dependent wetlands have acidified (with adverse effects on the extant macroinvertebrate fauna) due to the oxidation of pyritic sediments. One management option in such cases is artificial augmentation of surface water in order to maintain or reinstate anaerobia in the sediments. This paper documents cycles of macroinvertebrate decline and recovery over 12 years of monitoring in three Gnangara mound wetlands affected by drought-induced acidification, one of which is being artificially augmented. Acidification did not result in a reduction of the total number of macroinvertebrate families present, however, there were clearly identifiable groups of acid-sensitive taxa (amphipods and isopods, ostracods, chydorid and daphnid cladocerans, mayflies, oligochaetes, clams and snails) and acid-tolerant taxa (sandfly larvae, macrothricid cladocerans and water boatmen). In the artificially augmented wetland, the effects of acidification were reversed: acid-sensitive taxa reappeared and acid-tolerant taxa decreased in numbers. Moreover, there were a number of taxa that appeared for the first time since augmentation, and summer family richness increased markedly. This study has shown that artificial augmentation of wetland water levels can be a successful recovery strategy for recently acidified systems, but this will depend on a number of factors, and the ‘recovered’ state will be at least slightly different from the original state.

Wetland invertebrate richness and endemism on the Swan Coastal Plain, Western Australia.

Regional assessments of inland aquatic biota, determined from compilations of mostly descriptive and distributional data, can reveal information to justify local, regional or even global conservation priorities. In the present study, a regional assessment of the wetlands of the Swan Coastal Plain (SCP), Western Australia, was conducted to test the hypothesis that the relatively recent (in geological terms) formation of the wetlands has yielded relatively homogeneous aquatic assemblages. A compilation of aquatic invertebrate taxa from 18 studies of 66 wetlands has shown a surprisingly rich fauna (considering the comparatively small survey area and the degree of anthropogenic alteration of the SCP). Some of this richness is associated with increased sampling effort and proximity to other wetlands, and the habitat complexity and degree of permanence of the wetlands has contributed to richness, community assemblage composition, rarity and endemism patterns. Although taxonomically rich, local and regional endemism is relatively low compared with other regional assessments. The caves and the springs of the bioregion have a distinctive faunal assemblage; however, the assemblages of surface water wetlands do appear more homogeneous. Although iconic wetlands demand priorities for conservation action, management should be directed at the entire suite of wetlands to ensure wetland ecosystem integrity.