Title: Scale dependent effect of fine sediments on the structure and
function of temperate stream macroinvertebrates.
Speaker: Stefano Larsen
Date: Thursday 5 Mar 2009
Time: 10:00 am - 11:00 am
Venue: DBS Conference Room

Abstract:
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.

Background:
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.