ALBERT

Notes: 1. Measurements of ecological patterns are often used as primary biological indicators of river health. However, these patterns provide little information about important stream ecosystem processes (e.g. the sources and fate of energy and nutrients). The direct measurement of these processes is considered fundamental to the determination of the health of stream and river ecosystems.2. In this paper we used two basic approaches to assess stream ecosystem response to catchment disturbance and, particularly, to the loss of riparian vegetation in different forested biomes across Australia. Benthic gross primary production (GPP) and respiration (R24) provided measures of the amounts of organic carbon produced and consumed within the system, respectively. Stable isotope analysis was used to trace the fate of terrestrial and instream sources of organic matter in the aquatic food web. In a focal catchment in SE Queensland, additional measurements were taken of riparian attributes, catchment features and water quality.3. Baseline measurements of GPP and R24 from undisturbed forest streams provided reference values for healthy streams for comparison with sites where the catchment or riparian vegetation had been disturbed. These values of metabolism were low by world standards in all biomes examined. Preliminary data from the Mary River catchment in SE Queensland indicated that these parameters were sensitive to variations in riparian canopy cover and, to a lesser extent, catchment clearing, and predictive models were developed. The ratio P : R (GPP : R24) was used to determine whether sites were net consumers (P 〈 R) or producers (P 〉 R) of carbon but this was not considered a reliable indicator of stream health on its own.4. Although forest streams were typically net consumers of carbon (P 〈 〈 R), stable isotope analysis of metazoan food webs indicated a high dependence on inconspicuous epilithic algae in some biomes.5. A dramatic decline in the health of forest streams was observed when GPP substantially exceeded R24, especially when instream primary producers shifted from palatable unicellular algae to prolific filamentous green algae and macrophytes. These sources of instream production do not appear to enter aquatic food webs, either directly through grazing or indirectly through a detrital loop. Accumulation of these plants has led to changes in channel morphology, loss of aquatic habitat and often a major decline in water quality in some of the streams studied.

Notes: 1. The catchments of many tropical lowland streams in far north Queensland have been extensively cleared for the cultivation of sugar cane to the extent where very little of the native riparian vegetation remains. Stream channels are often choked by a matrix of introduced pasture grass (Brachiaria mutica, or para grass) and accumulated sediment from cropland erosion.2. Detailed transects across Bamboo Creek, a fourth order cane-land stream, revealed an estimated sediment load of 20 000 t km–1. This has resulted in an estimated 85% reduction in the predicted bankful discharge of the original stream channel. Channel capacity has been reduced from 2.3 times to 0.3 times the predicted Q50 flood discharge of 140 m3 s–1.3. Shade cloth treatments of 50% and 90% across the stream were used to mimic the effect of shading by riparian vegetation. Three months of shading resulted in a substantial reduction in the height and standing biomass of para grass in both shade treatments, compared to open plots (0% shade). The most dramatic effect was in the 90% treatment, where a mean reduction of 63% in height and 52% in total biomass was recorded. This was despite high net primary production of para grass in the open plots of 2.8 g dry wt m–2 day–1, which resulted in a overall increase of 11% and 28% in plant height and total biomass, respectively.4. These data suggest that restoration of native riparian vegetation will be an effective long-term means of controlling invasive macrophytes in disturbed cane-land stream channels. Reduction of excessive macrophyte growth and the mobilisation of accumulated sediment are essential to the restoration of natural hydrological and ecological processes.

Notes: Abstract We used multiple stable-isotope analysis to investigate the importance of seagrasses, mangroves and other primary sources (macroalgae, seston) to the food webs supporting penaeid prawns in the Embley River estuary and adjacent off-shore waters in the north-eastern Gulf of Carpentaria, Australia. Mangroves, seagrass and macroalgae/seston were well separated on the basis of their δ13C values in both the dry and the wet seasons. In contrast, only macroalgae and seston (phytoplankton and zooplankton), which had similar δ13C values, were separated by their δ15N values. The primary source of carbon supporting food webs of several species of juvenile penaeid prawns clearly depended on the location within the estuary. The δ13C values of juvenile prawns (Penaeus esculentus, P. semisulcatus and Metapenaeus spp.) in seagrass beds were close to those of seagrass and seagrass epiphytes, particularly in the dry season. This was despite the proximity of the seagrass to mangroves and the presence of mangrove detritus in the seagrass beds. Juvenile prawns (P. merguiensis, P. semisulcatus, and Metapenaeus spp.) in an upstream mangrove creek had δ13C values that were midway between those of mangroves and seagrass, and close to those of macroalgae and seston during the dry season. Mangroves could have made a significant contribution to the carbon assimilated by juvenile prawns at this site, but only if it is assumed that the remainder of the carbon is ultimately derived from a seagrass source. The δ13C values of adults of three species of prawns in offshore waters were very similar and were much higher than those of mangroves. The considerable amount of mangrove/terrestrial carbon exported from tropical Australian estuaries during the wet season is therefore unlikely to contribute to offshore food webs supporting adult prawns. Furthermore, the contribution of mangrove/terrestrial sources to the food webs of juvenile prawns appears to be limited to a very small spatial scale – within the mangrove fringe of small creeks and mainly during the wet season.