Niall M. Connolly

The effect of fine sedimentation on tropical stream macroinvertebrate assemblages: a comparison using flow-through artificial stream channels and recirculating mesocosms

In-situ artificial stream channels and ex-situ laboratory mesocosms were used to measure the responses of macroinvertebrate assemblages, from upland and lowland tropical streams, to high loads of fine clay sediment. Significant responses were observed mainly in the in-situ channels in the upland stream, where densities and the number of taxa were lower in the treatment channels than in controls. There was no evidence of any taxon being particularly sensitive to sedimentation, with a general decrease in densities across several taxa and differences only detectable for abundant taxa. Animals moved downstream in response to the treatment, but only a short distance within the channels. However, further colonization after the treatment was impeded in the treatment channels. In the mesocosm experiments, the upland macroinvertebrate assemblage demonstrated some negative effects; however, the lowland fauna was very tolerant to sedimentation, even when exposure was extended to 15 days. Together, the mesocosm and in-situ experiments indicate that there is a behavioural response to sedimentation because of a change in the habitat, and that the lowland macroinvertebrate assemblage is more tolerant of sedimentation, at least in the short term.

Ecology of streams in a biogeographic isolate—the Queensland Wet Tropics, Australia

Global studies of streams are needed to develop general ecosystem and management models. We reviewed research that tested ecological models in streams of the Queensland Wet Tropics bioregion (QWT), which makes up 0.26% of Australia but supports distinctive and high biodiversity, most of which is of Gondwanan or marine origin. QWT streams have seasonal but perennial flow, high insolation, and higher diversity of riparian vegetation, invertebrates, and fish than temperate streams. Consistent physical conditions sustain biological processes through the year, and predictable wet seasons, but unpredictable floods, have shaped a resilient and opportunistic biota. Stream food webs are dominated by predators, and prey turnover is rapid. Small streams are heterotrophic and become autotrophic as canopies open in larger streams. Predation and competition influence assemblage composition most in the dry season, when habitats contract and densities increase. Riparian clearing, weed invasion, and agricultural contamination affect table lands and floodplains. High temperatures exacerbate weed growth and eutrophication, but contaminants may be diluted by high flows from forested catchments. Climate change probably will cause warming and greater hydrological seasonality, threatening endemic species. The biophysical characteristics of QWT streams are found elsewhere in the tropics, but the species pool is not. QWT streams are important because of their insular and remnant nature. Patterns and processes can differ between QWT and comparable temperate systems because of biogeographic and biophysical characteristics and their interactions with anthropogenic effects, exacerbated by the tropical climate. Research in the QWT both affirms and contradicts theories of stream ecology, underpins conservation and management needs of tropical streams, and provides points of reference for comparative studies in stream ecology, conservation, and management.

Nutrient enrichment of a heterotrophic stream alters leaf litter nutritional quality and shredder physiological condition via the microbial pathway

Streams receiving agricultural runoff are typically enriched with nutrients, which variously impact stream communities. We examined the effects of phosphate and nitrate enrichment on leaf litter breakdown, microbial biomass and the nutrition of an invertebrate shredder to determine how nutrients are transferred through the stream detrital food web. Using artificial streams, individuals of Anisocentropus kirramus (Trichoptera: Calamoceratidae) were fed leaves of Apodytes brachystylus (Icacinaceae) under different nutrient regimes. We measured the amount of leaf material consumed or decomposed and the microbial biomass colonising the leaves. The dry mass, and protein, lipid and carbohydrate composition of A. kirramus larvae were determined after 28-day feeding on the leaves. Supplements of phosphorus, but not nitrogen, enhanced leaf breakdown, microbial growth and growth of larvae. Microbial biomass and dry mass of larvae increased with nutrient enrichment and they were significantly correlated. Thus, the phosphorus supplement was transmitted through the detrital food web via the microbial pathway, resulting in higher nutritional quality of leaves and enhanced physiological condition of the shredder. Understanding such subtle relationships is important in determining the impacts of anthropogenic contaminants on freshwater ecosystems.