Management pathways for the floodplain wetlands of the southern Murray–Darling Basin: Lessons from history

The condition of floodplain wetlands of the Murray–Darling Basin (MDB) reflects the combined effects of climate variability, river regulation, vegetation clearance, and the impacts of human settlement and industry. Today, these systems are degraded, in large part due to changes in the hydroecology of waterways arising from water diversion and abstraction to sustain irrigated agriculture. The MDB Plan directs substantial investment towards the restoration of ecosystems largely via the buy‐back of water allocations, under a cap‐and‐trade system, for use as environmental flows. This region is projected to receive less winter rainfall and run‐off, which could exacerbate the impact of water diversions. Long‐term climate records suggest a higher level of resilience to drying than may be inferred from modern studies. Further, palaeoecological records of change reveal that many wetlands that are perennial today were once naturally seasonal or intermittent, and that much wetland degradation predates regulation and can be attributed to declines in water quality, rather than quantity. A mix of approaches to rehabilitate this long‐degraded system, planned and implemented over an extended period, may meet the demands of the Water Act of 2007, but also support the regional economy. An adaptive management approach offers a framework within which to map system vulnerabilities, characterize climate pressures, identify adaptation options, and monitor outcomes along a pathway to a sustainable future. Early lessons show the extent to which such a deliberative framework can assist water reform under changing socio‐economic priorities and external hydroclimatic pressures.     

Use of sediment elemental and isotopic compositions to record the eutrophication of a polymictic reservoir in central Texas, USA

Paleolimnological studies are rarely performed on reservoirs because of concern that sediments might not accurately chronicle reservoir history. Eutrophication indicators might behave differently in polymictic reservoirs and stratified natural lakes because of system and/or mixing regime differences. Particulate organic carbon (POC), particulate organic nitrogen (PON), and total phosphorus (TP) concentrations, carbon:nitrogen (C:N) and nitrogen:phosphorus (N:P) ratios, and carbon (δ¹³C) and nitrogen (δ¹⁵N) stable isotopes from a sediment core were measured to demonstrate that sufficient information can be derived from sediments to permit a historical reconstruction. The scattered POC data were likely biased by seasonal/annual variability in allochthonous organic matter (OM) loading. The upwardly increasing PON in the sediment core supported historic primary productivity (PP) data, suggesting PON could be a better PP indicator than POC. The upwardly increasing TP documented historic P enrichment. The upwardly decreasing C:N ratio identified an OM source shift from allochthonous to increasingly autochthonous sources with reservoir age. The upwardly increasing N:P ratio implied that N‐fixation rates have increased with reservoir age, to compensate for increasing N limitation as the P loading increased. The δ¹³C decreased as the PP increased with reservoir age producing an atypical relationship compared to stratified natural lakes. The OM source shifts likely biased the δ¹³C–PP relationship, and might weaken δ¹³C‐inferred PP reconstructions in similar reservoirs. The δ¹⁵N increased with reservoir age, likely resulting from dissolved inorganic N (DIN) source changes, rather than nitrate utilization. Watershed urban growth and dairy operation intensification potentially contributed greater loads of isotopically heavy DIN. This study demonstrated that paleolimnology has great potential to assist eutrophication assessment and management efforts in reservoirs.     

Fate of organic matter during natural and anthropogenic lake acidification

In Groβer Arbersee, a cirque lake in the Bavarian Forest (Germany), the acidification chronology since the late-glacial period has been studied paleolimnologically, applying subfossil diatom assemblages. Distinct phases of acidification could be detected, both natural ones by post-glacial development of soils and vegetation in the catchment and anthropogenic ones by mineral acid depositions. Whereas the first phases were accompanied (and most probably caused as well) by increases in organic carbon contents, the recent phases are characterized by losses of organic carbon contents in the lake.

In various sensitive lakes in Central Europe the fate of organic carbon (measured as loss on ignition) is documented for the anthropogenic acidification period. In lowland as well as in high alpine lakes this process is accompanied by loss of organic carbon. These losses are by no means uniform. On the contrary, they range between 10 and 90% per drop of one pH-unit, in each lake calculated from estimated preacidification pH-conditions.