Rochelle Petrie

Microfaunal communities in three lowland rivers under differing flow regimes.

Microfaunal samples were collected from within the channels of three rivers in north eastern Victoria, Australia (the Murray, Ovens and Broken Rivers) as a component of a study examining the effects of flow on the biota of lowland rivers in Australia. Samples were collected from the water column of the river channel and slackwaters and from the layer of water immediately above the bottom sediment of the slackwaters. There was no connectivity between the river channel and the floodplain wetlands for all three rivers during the sampling period. Substantial numbers of microfauna were resident in the slackwaters of all three rivers, with the greatest densities occurring close to the bottom sediment, with densities often exceeding 1000 animals l−1 whereas in the plankton samples densities were usually less than 500 animals l−1. The presence of large and diverse microfaunal communities and the lack of connectivity between the river channel and associated floodplain wetland indicate that these communities are capable of persisting and recruiting within riverine channel slackwaters.

From fresh to saline: a comparison of zooplankton and plant communities developing under a gradient of salinity with communities developing under constant salinity levels

In Australia, many freshwater wetlands are becoming saline. Knowing which elements of a biotic community will persist as wetlands turn saline is relevant to their future management. We simulated gradual and sudden increases in salinity in outdoor mesocosms to test the hypotheses that: (1) aquatic plant and zooplankton communities exposed to a gradient of increasing salinity over time would initially resemble freshwater communities, but as the salinity increased they would resemble communities found in more saline systems; and (2) that a gradual change in salinity over 6 months influences zooplankton and plant communities in the same way as a sudden salinity change. Below 1000 mg L–1, as salinity increased gradually, communities rich in species and numbers of individuals resembled freshwater communities. However, as the salinity exceeded 1000 mg L–1, taxa were progressively lost and communities became less diverse. When salinities exceeded 3000 mg L–1 the diversity decreased rapidly and few taxa remained at 5000 mg L–1. Both sudden and gradual increases in salinity induced similar decreases in diversity. We predict that as natural wetlands become more saline, few freshwater biota will survive once the salinity exceeds 5000 mg L–1. In the long term, such salinised wetlands would need to be recolonised by salt-tolerant taxa for a functional wetland to persist.

River metabolism and carbon dynamics in response to flooding in a lowland river

Lowland riverine–floodplain systems often have significant but irregular inputs of allochthonous carbon. However, the importance of this carbon to riverine systems remains poorly understood. We assessed open water dissolved organic carbon (DOC) concentrations, metabolism and biofilm stable isotope (δ13C) signatures, upstream and downstream of an extensive floodplain forest on the Murray River, Australia, before and after a flood event. Prior to flooding, all sites had similar concentrations of DOC, rates of metabolism and biofilm δ13C signatures. During the flood DOC concentration increased up to three-fold downstream of the forest, gross primary production (GPP) increased at all sites, but community respiration (CR) increased only at the downstream sites, resulting in decreased in NPP downstream and a slight increase upstream. Biofilm δ13C signatures became depleted by between 4 and 7‰ downstream of the forest during the flood, reflecting a rapid incorporation of allochthonous carbon into the biofilm. These results indicate that flooding led to a substantial increase to the energy budget of the Murray River through the provisioning of large quantities of allochthonous carbon and that terrestrial carbon was processed within the river biofilms. Allochthonous carbon assimilation within biofilms during flooding provides a potential pathway for allochthonous carbon to be incorporated into the metazoan foodweb.

Carbon and nutrient subsidies to a lowland river following floodplain inundation

Despite the perceived importance of floodplain inundation to the functioning of lowland rivers, there is limited understanding of the contribution that floodplains make to the main river channel during floods. In 2010, substantial flooding occurred throughout south-eastern Australia, which provided an opportunity to quantify the export of biological material and nutrients from a floodplain back in to the main river channel. We quantified the amounts of zooplankton, phytoplankton, dissolved organic carbon and nutrients within the main river channel of the River Murray immediately upstream of the Barmah–Millewa Forest, and at two sites immediately downstream of the forest during two flood events in July and October of 2010. Results demonstrated that although a smaller flood event in July did not contribute substantially to an increase in the measured parameters, a much larger flood in October contributed 0.4 tonnes (t) of phytoplankton; 7t of zooplankton and 300t of dissolved organic carbon. This suggests that small floods will provide minimal resource subsidies back into the main channel after the cessation of flooding. In comparison, larger floods that result in large volumes of floodplain water returning to the river will provide substantial subsidies of terrestrially derived resources.

Seed bank dynamics in wetland complexes associated with a lowland river

The existence of a dormant “bank” of plant seeds plays an important role in maintaining and preserving species and genetic diversity. However, information on the spatial heterogeneity of the pool of dormant seeds among wetland complexes along riverine systems is limited. In this study we collected sediment from 18 wetlands within six wetland complexes along the Murray River, Australia. The germinable, residual and viable seed banks in each wetland complex were assessed by undertaking a germination trial, counting seeds (morphotypes) and viability testing. A diverse and viable seed bank exists within all the complexes however seed bank communities differed among wetland complexes. There was no difference in the viability of seeds within the sediment profile, however more seeds occurred in the surface layers of sediment and communities differed with sediment profile depth. In general, the number of species germinating was fewer than the number of seed morphotypes counted. Management actions need to be targeted not only at preserving the extant plant communities but also to ensure the seed bank is replenished. This may also involve allowing wetlands to dry sufficiently to enable cracking of wetland sediment and seeds to become buried to maximise the longevity of the seed store.