Different Conceptualizations of River Basins to Inform Management of Environmental Flows

Environmental flows are a critical tool for addressing ecological degradation of river systems brought about by increasing demand for limited water resources. The importance of basin scale management of environmental flows has long been recognized as necessary if managers are to achieve social, economic, and environmental objectives. The challenges in managing environmental flows are now emerging and include the time taken for changes to become manifest, uncertainty around large-scale responses to environmental flows and that most interventions take place at smaller scales. The purpose of this paper is to describe how conceptual models can be used to inform the development, and subsequent evaluation of ecological objectives for environmental flows at the basin scale. Objective setting is the key initial step in environmental flow planning and subsequently provides a foundation for effective adaptive management. We use the implementation of the Basin Plan in Australia's Murray-Darling Basin (MDB) as an example of the role of conceptual models in the development of environmental flow objectives and subsequent development of intervention monitoring and evaluation, key steps in the adaptive management of environmental flows. The implementation of the Basin Plan was based on the best science available at the time, however, this was focused on ecosystem responses to environmental flows. The monitoring has started to reveal that limitations in our conceptualization of the basin may reduce the likelihood of achieving of basin scale objectives. One of the strengths of the Basin Plan approach was that it included multiple conceptual models informing environmental flow management. The experience in the MDB suggests that the development of multiple conceptual models at the basin scale will help increase the likelihood that basin-scale objectives will be achieved

Coding “What” and “When” in the Archer Fish Retina

Traditionally, the information content of the neural response is quantified using statistics of the responses relative to stimulus onset time with the assumption that the brain uses onset time to infer stimulus identity. However, stimulus onset time must also be estimated by the brain, making the utility of such an approach questionable. How can stimulus onset be estimated from the neural responses with sufficient accuracy to ensure reliable stimulus identification? We address this question using the framework of colour coding by the archer fish retinal ganglion cell. We found that stimulus identity, “what”, can be estimated from the responses of best single cells with an accuracy comparable to that of the animal's psychophysical estimation. However, to extract this information, an accurate estimation of stimulus onset is essential. We show that stimulus onset time, “when”, can be estimated using a linear-nonlinear readout mechanism that requires the response of a population of 100 cells. Thus, stimulus onset time can be estimated using a relatively simple readout. However, large nerve cell populations are required to achieve sufficient accuracy

Contrast Adaptation Contributes to Contrast-Invariance of Orientation Tuning of Primate V1 Cells

BACKGROUND: Studies in rodents and carnivores have shown that orientation tuning width of single neurons does not change when stimulus contrast is modified. However, in these studies, stimuli were presented for a relatively long duration (e. g., 4 seconds), making it possible that contrast adaptation contributed to contrast-invariance of orientation tuning. Our first purpose was to determine, in marmoset area V1, whether orientation tuning is still contrast-invariant with the stimulation duration is comparable to that of a visual fixation. METHODOLOGY/PRINCIPAL FINDINGS: We performed extracellular recordings and examined orientation tuning of single-units using static sine-wave gratings that were flashed for 200 msec. Sixteen orientations and three contrast levels, representing low, medium and high values in the range of effective contrasts for each neuron, were randomly intermixed. Contrast adaptation being a slow phenomenon, cells did not have enough time to adapt to each contrast individually. With this stimulation protocol, we found that the tuning width obtained at intermediate contrast was reduced to 89% (median), and that at low contrast to 76%, of that obtained at high contrast. Therefore, when probed with briefly flashed stimuli, orientation tuning is not contrast-invariant in marmoset V1. Our second purpose was to determine whether contrast adaptation contributes to contrast-invariance of orientation tuning. Stationary gratings were presented, as previously, for 200 msec with randomly varying orientations, but the contrast was kept constant within stimulation blocks lasting >20 sec, allowing for adaptation to the single contrast in use. In these conditions, tuning widths obtained at low contrast were still significantly less than at high contrast (median 85%). However, tuning widths obtained with medium and high contrast stimuli no longer differed significantly. CONCLUSIONS/SIGNIFICANCE: Orientation tuning does not appear to be contrast-invariant when briefly flashed stimuli vary in both contrast and orientation, but contrast adaptation partially restores contrast-invariance of orientation tuning

Changes in organic-matter dynamics and physicochemistry, associated with riparian vegetation loss and river regulation in floodplain wetlands of the Murray River, Australia

Extensive clearing of floodplain forests potentially reduces organic matter available to floodplain wetlands. Furthermore, on rivers regulated to provide irrigation water in summer, floodplain wetlands that were previously inundated in spring, now flood in summer/autumn. In the Murray&ndash;Darling Basin, Australia, this has changed the timing of organic matter entering the aquatic phase, since leaf fall peaks in summer. Field surveys and mesocosm experiments on floodplain wetlands on the River Murray revealed faster processing rates of leaves in summer/autumn than spring, and no difference between cleared and forested wetlands. Temperature and leaf carbon : nitrogen ratio could not explain these differences, and instead, changes to leaf chemistry associated with &lsquo;terrestrial ageing&rsquo; between peak leaf fall in summer and inundation in spring is more likely. The results indicated that the reduction of input of organic matter through riparian tree clearing and changing the timing of inundation interact to alter organic-matter standing stocks and rates of decomposition in floodplain wetlands. Restoring both natural timing of high flows and riparian vegetation might be required for recovery of these wetlands.<br /

Effects of discharge regulation on slackwater characteristics at multiple scales in a lowland river

The spatial and temporal dynamics of physical habitat in rivers is driven by the interaction between channel morphology and discharge. However, little is known about how altered discharge affects the dynamics of habitat patches such as slackwaters. This study investigated the influence of discharge on the availability, stability, quality, and diversity of slackwaters in a southeastern Australian lowland river. The area, spatial configuration, permanence, and within-patch characteristics of slackwaters of two reaches in a regulated section and two reaches in a largely unregulated section of the river were compared. There was less slackwater area and it was less permanent at higher discharges and in the two regulated reaches than at lower discharges and in the largely unregulated reaches. Individual slackwaters were more homogenous in relation to within-patch characteristics in the regulated than in the largely unregulated reaches. However, variability in the spatial configuration of slackwaters and within-patch characteristics and diversity at the reach scale were not related to discharge. We suggest that channel morphology, rather than discharge, is the main driver of these characteristics. </jats:p

Effects of discharge regulation on slackwater characteristics at multiple scales in a lowland river

The spatial and temporal dynamics of physical habitat in rivers is driven by the interaction between channel morphology and discharge. However, little is known about how altered discharge affects the dynamics of habitat patches such as slackwaters. This study investigated the influence of discharge on the availability, stability, quality, and diversity of slackwaters in a southeastern Australian lowland river. The area, spatial configuration, permanence, and within-patch characteristics of slackwaters of two reaches in a regulated section and two reaches in a largely unregulated section of the river were compared. There was less slackwater area and it was less permanent at higher discharges and in the two regulated reaches than at lower discharges and in the largely unregulated reaches. Individual slackwaters were more homogenous in relation to within-patch characteristics in the regulated than in the largely unregulated reaches. However, variability in the spatial configuration of slackwaters and within-patch characteristics and diversity at the reach scale were not related to discharge. We suggest that channel morphology, rather than discharge, is the main driver of these characteristics