Ecosystem measures of river health and their response to riparian and catchment degradation

1. Measurements of ecological patterns are often used as primary biological indicators of river health. However, these patterns provide little informa tion about important stream ecosystem processes (e.g. the sources and fate of energy and nutrients). The direct measurement of these processes is cons idered fundamental to the determination of the health of stream and river e cosystems. 2. In this paper we used two basic approaches to assess stream ecosystem re sponse to catchment disturbance and, particularly, to the loss of riparian vegetation in different forested biomes across Australia. Benthic gross pri mary production (GPP) and respiration (R-24) provided measures of the amoun ts of organic carbon produced and consumed within the system, respectively. Stable isotope analysis was used to trace the fate of terrestrial and inst ream sources of organic matter in the aquatic food web. In a focal catchmen t in SE Queensland, additional measurements were taken of riparian attribut es, catchment features and water quality. 3. Baseline measurements of GPP and R-24 from undisturbed forest streams pr ovided reference values for healthy streams for comparison with sites where the catchment or riparian vegetation had been disturbed. These values of m etabolism were low by world standards in all biomes examined. Preliminary d ata from the Mary River catchment in SE Queensland indicated that these par ameters were sensitive to variations in riparian canopy cover and, to a les ser extent, catchment clearing, and predictive models were developed. The r atio P : R (GPP : R-24) was used to determine whether sites were net consum ers (P < R) or producers (P > R) of carbon but this was not considered a re liable 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 R-24 especially when instream primary producers shi fted from palatable unicellular algae to prolific filamentous green algae a nd 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 qual ity in some of the streams studied.