Stream ecosystems as monitors of tropical forest catchments

[Extract] This project was initiated to provide empirical data on water quality in streams of the Wet Tropics Biogeographical Region, on the benthic invertebrate communities of the streams, and the relationship, between the two. The proposal aimed to provide a classification of Wet Tropics streams on the basis of their invertebrate communities, and to explain community composition in terms of biogeographic, physical, chemical and biological properties. The main outcome was planned to be a manual for monitoring water quality using invertebrates. The approach was partly based on the classification of streams in Britain (e.g. Wright 1995). Subsequent to the commencement of this study, an Australia wide scheme, following similar principles, was established. This scheme, the Monitoring River Health Initiative (MRIIl) was established within the Land and Water Resources Research and Development Corporation (L WRRDC). One of the main premises of the scheme is that stream invertebrate species respond differentially to different conditions of habitat, water quality and disturbance and so can give an integrated indication of the recent history of a particular site. There are many case studies that support this premise, including studies in the Wet Tropics (Pearson and Penridge 1987). However, there is little experimental evidence at the community level to back up monitoring schemes, by calibrating responses of invertebrate populations and communities to different levels of contamination or disturbance. Consequently, LWRRDC commissioned research to investigate and quantify experimentally the effects of selected impacts on stream communities, including a project based on tropical streams, JCUll, which ran in parallel with JCU8. Certain studies were shared between the two projects, and will therefore be referred to here. The focus of JCU8 on streams of the Wet Tropics arose from the following: 1. on the basis of detailed studies of two streams, it was known that Wet Tropics streams have very high biodiversity on national and world scales (Pearson et al. 1986, Lake et al. 1994); 2. the Wet Tropics World Heritage Area (WTWHA) was inscribed on the basis of several attributes, including biodiversity, ecological processes and evolutionary processes; 3. informed management of the WTWHA and contiguous catchments in the Wet Tropics bioregion was critical to sustaining these special values; 4. catchment use is a major threat to these values: in particular, agriculture, infrastructure development and tourism and recreation impact on catchments and directly on streams; 5. stream water quality reflects catchment integrity, and water quality may be best monitored using invertebrate communities, which integrate various and variable contaminants over time; 6. management agencies required better understanding of the biota of Wet Tropics streams, and of their utility in monitoring; there was thus a need to develop tools for assessment and monitoring. This report outlines the approach of this project, its results, and their application. The project could not be completed within the original time frame for several logistical reasons, but it is continuing so that the original goals are met. In particular, a monitoring manual is only partly finished, and scientific publications are only just beginning to appear. Nevertheless, the aims are being addressed in continuing work, with the manual expected to be completed early in 1999

Stream macroinvertebrate assemblage uniformity and drivers in a tropical bioregion

1. Understanding spatial and environmental drivers of undisturbed stream assemblages is important for separating natural and human-induced changes, but has rarely been attempted for an entire tropical bioregion. 2. We sampled riffle macroinvertebrate assemblages and measured associated biophysical variables in post-wet and dry seasons from 68 streams of orders 1-5 across the Australian Wet Tropics, a small bioregion (18 497 km 2) defined by its warm moist climate and closed-forest vegetation. 3. As climate and landscape were relatively uniform across the bioregion, we predicted that assemblages would be similar, with turnover (beta diversity) unrelated to distance, and with composition determined mainly by habitat. 4. We identified 93 higher taxa. Density and richness of macroinvertebrates were greatest in the dry season because of habitat contraction and minimal flow disturbance. Richness was greatest in higher order streams. 5. Relative abundance and richness of functional groups showed minor effects of catchment, lithology and stream order, and a positive relationship with altitude for richness of collectors, predators and shredders, and for abundance of shredders. 6. Distance-based linear modelling showed that among-assemblage differences were explained by landscape-scale variables (9.5% of the variation), stream size (13.0%), riparian characteristics (9.6%), water quality (4.6%), substratum (21.1%) and organic resources (22.4%); for models of individual taxa and functional groups, habitat and organic-resource variables also predominated. 7. Similarities among site assemblages differed little among catchments and there was no relationship between pairwise similarities of catchments and their geographical distances. Nestedness analysis confirmed that samples and catchment groupings were nested subsets of the total set of samples. 8. Across the Australian Wet Tropics, uniformity of assemblages (identified to family or above) conformed to the bioregional classification, probably as a result of the great age of the region. The habitat variables that most influenced macroinvertebrate assemblages support the idea that a suite of biophysical influences is common to stream macroinvertebrate assemblages globally

Invertebrate responses to land use in tropical streams: discrimination of impacts enhanced by analysis of discrete areas

We identified influences of land-use disturbances on invertebrate assemblages in streams draining eight areas of the Great Barrier Reef catchment in tropical Australia (~15.7–22°S), a region of high biodiversity. We used distance- based linear modelling (DistLM) to analyse assemblage data (103 taxa), richness and the SIGNAL2 taxon sensitivity index. DistLM of assemblages explained ~40% of variation across all samples and 7–54% of variation in individual areas. DistLM of richness and SIGNAL2 explained respectively 19–81 and 26–95% of variation. Explanatory variables were land use (especially cropping and grazing v. forest), riparian width, instream habitat, climate (drier south) and water quality (conductivity greater in south). Local impacts of activities such as mining were evident in models of individual areas. A detailed comparison of streams with contrasting riparian management demonstrated a 25% loss of richness, but no change in SIGNAL2 score. Accounting for local environmental gradients and using measures appropriate to the type of disturbance improved identification of impacts and could form a framework for future regional monitoring of stream ecological condition. The impacts identified may be mitigated by remediation such as riparian rehabilitation, although management at catchment scales is required to be effective

Crayfish process leaf litter in tropical streams even when shredding insects are common\ud

Comparisons of leaf-litter processing in streams suggest that tropical streams have fewer leaf shredders than temperate streams and that insect shredders might be replaced by other taxa such as Crustacea in tropical systems. Australian wet-tropical streams have abundant insect shredders, and also abundant crayfish, which may contribute to litter processing. We monitored litter input and retention in a Queensland rainforest stream to determine availability of litter in different seasons, and we conducted experiments to test the hypothesis that crayfish were important contributors to litter processing. Litter fall peaked in the late dry season and litter accumulated steadily in pools, whereas in riffles, the standing crop was maintained at a threshold level. All accumulated litter was washed from the stream during a flood. The crayfish, Cherax cairnsensis, readily fed on leaves. Its processing rate was related negatively to leaf toughness and positively to leaf nitrogen content. The crayfish assimilated up to 28.5% of the material processed at 24°C, and none at 11°C. These results confirm that there are alternatives to insect shredders in tropical streams, even when insect shredders are abundant

Crayfish process leaf litter in tropical streams even when shredding insects are common\ud

Comparisons of leaf-litter processing in streams suggest that tropical streams have fewer leaf shredders than temperate streams and that insect shredders might be replaced by other taxa such as Crustacea in tropical systems. Australian wet-tropical streams have abundant insect shredders, and also abundant crayfish, which may contribute to litter processing. We monitored litter input and retention in a Queensland rainforest stream to determine availability of litter in different seasons, and we conducted experiments to test the hypothesis that crayfish were important contributors to litter processing. Litter fall peaked in the late dry season and litter accumulated steadily in pools, whereas in riffles, the standing crop was maintained at a threshold level. All accumulated litter was washed from the stream during a flood. The crayfish, Cherax cairnsensis, readily fed on leaves. Its processing rate was related negatively to leaf toughness and positively to leaf nitrogen content. The crayfish assimilated up to 28.5% of the material processed at 24°C, and none at 11°C. These results confirm that there are alternatives to insect shredders in tropical streams, even when insect shredders are abundant