Perennial rivers and streams make a disproportionate contribution to global carbon (C)cycling. However, the contribution of intermittent rivers and ephemeral streams, which
1. Despite widespread recognition of the role of body size in fish trophic ecology, little attention has been focused on this issue in isotopic studies, particularly in tropical systems. 2. We used analyses of stomach contents and stable isotopes to examine size-related shifts in diet in a terapontid fish assemblage in the Australian wet-dry tropics. Stomach content analysis identified substantial ontogenetic dietary shifts in all species, corresponding to changes in body size-isotope trajectories for two species. Shifts away from relatively specialised diets of heavily 13 C-depleted insect larvae to consumption of a range of items across multiple basal carbon sources appeared to be the proximate cause of observed isotopic changes. 3. Allochthonous organic matter in the form of C3 riparian vegetation was particularly important to smaller terapontids before larger fish shifted to a broad range of dietary items and similarly broad range of basal carbon sources. 4. While there was general agreement between d 13 C and stomach content analysis, there was minimal concurrence between the latter and d 15 N isotopic derivation of estimates of trophic position. Due to factors such as omnivory, isotopically overlapping basal sources and uncertainties about rates of isotopic fractionation in both predator and prey species, stomach content analysis provides an essential complement to isotopic methodologies in tropical systems. 5. Given that basal sources supporting any individual species can change markedly with ontogeny, consideration of intraspecific, size-related variation is necessary in isotopic studies of food web structure.
The impact of large-scale mining on the landscape is a permanent legacy of industrialisation and unique to the Anthropocene. Thousands of lakes created from the flooding of abandoned open-cut mines occur across every inhabited continent and many of these lakes are toxic, posing risks to adjacent communities and ecosystems. Sustainable plans to improve water quality and biodiversity in ‘pit lakes’ do not exist due to: (1) confusion as to the ultimate use of these lakes, (2) involvement of ecologists only after the lake is filled and (3) pit lake ecology struggling to reach the primary literature. An integrated approach to pit lake management engages ecologists in pit lake design, prioritising ecological progress and passive treatment in mine closure planning, ultimately empowering communities with post-mining options
Climate change and human pressures are changing the global distribution and the extent of intermittent rivers and ephemeral streams (IRES), which comprise half of the global river network area. IRES are characterized by periods of flow cessation, during which channel substrates accumulate and undergo physico‐chemical changes (preconditioning), and periods of flow resumption, when these substrates are rewetted and release pulses of dissolved nutrients and organic matter (OM). However, there are no estimates of the amounts and quality of leached substances, nor is there information on the underlying environmental constraints operating at the global scale. We experimentally simulated, under standard laboratory conditions, rewetting of leaves, riverbed sediments, and epilithic biofilms collected during the dry phase across 205 IRES from five major climate zones. We determined the amounts and qualitative characteristics of the leached nutrients and OM, and estimated their areal fluxes from riverbeds. In addition, we evaluated the variance in leachate characteristics in relation to selected environmental variables and substrate characteristics. We found that sediments, due to their large quantities within riverbeds, contribute most to the overall flux of dissolved substances during rewetting events (56%–98%), and that flux rates distinctly differ among climate zones. Dissolved organic carbon, phenolics, and nitrate contributed most to the areal fluxes. The largest amounts of leached substances were found in the continental climate zone, coinciding with the lowest potential bioavailability of the leached OM. The opposite pattern was found in the arid zone. Environmental variables expected to be modified under climate change (i.e. potential evapotranspiration, aridity, dry period duration, land use) were correlated with the amount of leached substances, with the strongest relationship found for sediments. These results show that the role of IRES should be accounted for in global biogeochemical cycles, especially because prevalence of IRES will increase due to increasing severity of drying events.
We analyzed basal sources, trophic levels, and connectance in dry-season food webs on 4 rivers in the upper Burdekin catchment in the dry tropics of northeastern Australia. The region is characterized by episodic summer rainfall, and most of the annual river flow occurs in a short period. In the dry season, rivers typically contract into a series of water holes of varying permanence and hydrologic connectivity. We used stable-isotope and stomach-content analyses to identify trophic levels of macroinvertebrates and fish, and we used a mixing model (SIAR) to identify foodweb basal sources at each site. We found substantial variability among sites in basal-source contributions, trophic position of individual taxa, and foodweb structure, and sites from the same river often were as different as sites from different rivers. Important basal sources at different sites included allochthonous tree litter, autochthonous algae and macrophytes, and Fe-fixing bacteria. Many relationships between consumers and basal sources were not resolved in the mixing model, mainly because of extensive omnivory or isotopic overlap among sources. Nevertheless, our results show high variability of dry-tropics river communities that extends beyond previously described macroinvertebrate assemblages to the broader food web. However, the main components of the upper trophic levels were similar across sites, such that different lower trophic levels supported similar assemblages of top consumers. These tropical rivers were defined by omnivory and ecological opportunism, which may be adaptations to seasonal hydrological variability
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