Title: Scale dependent effect of fine sediments on the structure and function of temperate stream macroinvertebrates
Speaker: Stefano Larsen
Date: 5 Mar 2009, Thursday
Time: 10.00 am – 11.00 am
Venue: DBS Conference Room
Catchment agriculture, urbanisation, forestry and miming can increase sediment delivery to streams and rivers and alter sediment quality with profound consequences for aquatic organisms. Sediment effects are manifold and organisms at all trophic levels are affected, for example through a reduction of available light for primary producers and visual predators, alteration of substratum structure and habitat quality for benthic organisms, decline in feeding efficiency of filter feeders and grazers and reduced oxygen supply to salmonids eggs via interstitial occlusion. In reality, quantitative understanding of the mechanisms involved and the sensitivity of organisms is poor, for many reasons. First, anthropogenic alteration of sediment regime range from whole catchment modification (agriculture) to local habitat alteration (livestock
trampling), so that exact sediment sources are seldom identified readily. Second, over large spatial extent, ecological effects can be masked by natural variability.
In this study we employed two consecutive surveys at different spatial resolutions to understand the apparent causes and extent of sediment deposition in the Usk cathment, Wales (UK), and to identify the scale at which ecological effects on stream macroinvertebrates are evident.
At both scales, in-stream fine sediments were directly related to the extent of bank erosion 500 m upstream; in turn, sedimentation and bank erosion were negatively correlated to woodland cover. At the larger spatial extent the influence of land-use and altitude were most evident, with richness declining as rough grazing or woodland was replaced by grassland. Also, modified reaches gained resilient taxa typified by small body size, polivoltinism and ovoviviparity. No apparent sediment effect was observed at this scale.
Conversely, at the local patch-scale sediment deposition clearly affected community composition and richness of sensitive taxa. Moreover, fine sediments were accompanied by changes in the representation of behavioural traits (e.g. feeding modes, locomotion) and an overall reduction in functional diversity. We conclude that in this upland locations fine sediments are associated with local bank erosion and that detection of ecological effects is scale dependent. This implies that sediment effects may be influenced by larger catchment controls and thus the assessment of diffuse anthropogenic sediment effects can benefit from a scale specific approach.
I am mostly interested in applied ecology and conservation, particularly in freshwater habitats. Both my Masters project and my early working experiences aimed at identifying anthropogenic influences, such as land-use, on freshwater and riparian organisms, namely macroinvertebrates and riparian birds in central Italy. I am currently undertaking a Ph.D. at Cardiff University, Wales (UK) studying the ecological effects of fine sediment entrainment and deposition on stream invertebrates and fish in upland Welsh streams. The research involved both extensive field surveys and manipulative experiments to assess the main causes of altered sediment regimes and the effect on the structure and function of invertebrate communities. Assessing how the representation of species life-history and behavioural traits is changed by habitat modification allows a deeper understanding of the processes behind observed patterns enhancing the predictability and transportability of conservation strategies.