Habitat heterogeneity and aquatic invertebrate distribution were investigated along the Tagliamento River in NE Italy, a 7th order braided river draining the southeastern corner of the limestone-dolomite Alps. Due to a lack of high dams, minimally altered geomorphic setting, complex and extensive flood plains, and highly dynamic flow regime, the Tagliamento River has been designated the last morphologically intact river corridor in the European Alps. The goal of this thesis was to document environmental heterogeneity and aquatic invertebrate communities along this semi-natural river corridor. Unique to this study is information on invertebrate communities and environmental heterogeneity across flood plains along the entire downstream gradient.

The downstream gradient (1100 to 5 m above sea level) was characterized by changes in geomorphology, substrate sorting processes, and increasing nutrient concentrations. The lateral dimension (main channel to floodplain edge) of six geomorphologically distinct reaches along the continuum was investigated in detail. In each reach, physical, chemical, and biological (zoobenthos and algae) samples were collected monthly or quarterly (invertebrates) from five sites across the flood plain for one year.

Habitat heterogeneity along the river continuum was a manifestation of several factors (geomorphic, mineral substrate, nutrient concentration, and thermal conditions) operating across-scales. Spatial complexity and diversity of aquatic habitats within a flood plain were related to hydrogeomorphology. Substratum diversity was a function of floodplain configuration; however, variation was scale dependent. For example, at the reach-scale (among habitats), differences between habitats were greatest in lowland flood plains, while at the habitat-scale (within habitats) variation was consistently greater in headwater reaches. Thermal variation among habitats was greatest in lowland flood plains (15 °C difference among water bodies)

Short-term habitat turnover (1 year) across a flood plain due to flooding, quantified by global positioning system surveys and geographic information system analysis, was remarkably high (nearly 62% in the braided headwater flood plain) and declined along the downstream gradient. In contrast to turnover, change in braiding, sinuosity, and habitat composition was low in all reaches. The shifting mosaic steady-state model did apply to flood plains along the Tagliamento.

Local and regional zoobenthos patterns were partitioned hierarchically using alpha (local), beta (similarity among communities), and gamma (regional) diversity and community concordance to environmental heterogeneity was assessed using a multivariate statistical approach. Many hypotheses of aquatic invertebrate diversity predict certain ‘hot-spots’ of diversity along the continuum. Along the Tagliamento, zoobenthos diversity was similar along the downstream gradient when the lateral dimension was included. Species replacement along the river continuum occurred, indicating differential species responses to environment-specific factors. The result was distinct community structure in lateral habitats along the continuum.

Evident from this study was (1) hydrology structured the environment and thereby dictated the biotic response, (2) environmental heterogeneity manifested at different scales (and through different variables) depending on hydrogeomorphology, and (3) flood plains and headwater tributaries harbored redundant and unique habitats, both of which contributed equally to high levels of aquatic invertebrate diversity. Http://webhome.crk.umn.edu/~arsco002/