Jane Chambers

Loss of water challenges aquatic animal survival, and although it happens annually in seasonal wetlands, its effect on faunal dynamics is poorly understood. We studied these dynamics in detail in a single wetland, South Lake. The aims of these experiments and surveys were to observe faunal occupancy over different hydroperiods and invertebrate response. Although located in suburban Perth it is relatively undisturbed, drying to a pool before completely drying out during summer. We have sampled South Lake over multiple years and hydroperiods, including the dry phase. Temperature loggers were placed in and around South Lake, logging ambient air, water and sediment temperature throughout multiple hydroperiods, showing some extreme high temperatures in summer. During spring, invertebrate diversity was very high. As the water level declined, and then dried out, diversity declined and large invertebrate predators/scavengers fed on stranded invertebrates. Before surface water completely disappeared predatory insects pupated and flew away. Other invertebrates used resting stages to survive drying, hatching once South Lake reflooded. When ‘dry’ South Lake retains deep crevices in the sediment that are connected to the groundwater, providing a refuge for taxa such as amphipods and isopods that do not have resting stages and cannot leave the wetland. Thus, the decline and reassembly of the invertebrate community during drying and reflooding depends on three processes: flying insects with resilience traits that leave and return; taxa requiring at least perennial dampness that make micro-scale movements into sediment crevices; and taxa with resting stages that remain in the dry sediment.

Ellen Brook contributes only ~7% of flow, but 39% and 28% of the total phosphorus and nitrogen load into the Swan River, Perth, Western Australia (Swan River Trust 2009). To reduce this nutrient export, riparian vegetation is promoted as a best management practice. International literature has shown significant benefits of riparian vegetation to reduce incoming nutrients and improve water quality. The riparian paradigm relies on there being sufficient slope to generate flow through the riparian vegetation to allow plant/sediment and water interaction. However, in Ellen Brook the majority of the catchment is flat and dominated by sandy soils, resulting in little subsurface flow but rather vertical rise and fall of shallow groundwater. To assess whether riparian vegetation is functioning to reduce nutrient input into streams under these conditions, three rows of nested piezometers (1.5m and 2.5m) were installed in a transect across the riparian zone and into the paddock. Nutrient concentrations were significantly (p=>0.05) higher and more variable in shallower groundwater. From the first flush of winter rains to the dry early summer period, redox potentials declined in shallow groundwater to highly reducing values (-33.71mv to -121.55mv). Nitrate decreased exponentially from the paddock (x= 733µg.N/L) to the stream (x=42 µg.N/L). Ammonium concentrations remained relatively stable over time and space. This indicates the potential for nitrogen removal in the riparian zones of Ellen Brook through denitrification. Organic carbon and soluble reactive phosphorus concentrations peaked in the middle of the riparian zone (TOC: x=113 µg.C/L, SRP: x= 691 µg.P/L) and decreased closer to the stream (TOC: x= 77 µg.C/L, SRP: x=341 µg.P/L). Understanding the mechanisms behind phosphorus distribution throughout the riparian zone is being investigated through groundwater hydrological testing, and is pivotal for assessing the effectiveness of riparian vegetation as a best management practice.

Recent literature suggests that the initial concept of two contrasting ecological regimes (macrophyte or phytoplanktondominated) in nutrient-enriched shallow lakes should be extended, based on observations that such systems may be dominated by free-floating plants, submerged charophytes, submerged angiosperms, green algae or cyanobacteria at different points along a gradient of eutrophication. The implications of this for management and restoration are that different levels or thresholds of a controlling variable (such as nutrients) may be necessary to cause a shift in dominance depending on the characteristics of the dominant taxa. Each of the plant types listed above occur simultaneously in spring in different locations in the Vasse Wonnerup Estuary. This paper explores the drivers for dominance of different plant communities in these shallow waterbodies, focussing on potential thresholds (particularly total phosphorus concentrations and N:P ratios), that might explain the efficacy of nutrient reduction measures, the likelihood of catastrophic loss of macrophytes from the wetlands or guide restoration of macrophytes in currently phytoplankton-dominated systems.