Trophic structure and the effects of agriculture in a headwater stream in southern Ontario using stable isotopes and secondary production

Abstract

In this thesis, the effect of agricultural activity on lotic macroinvertebrate communities was evaluated at the field scale. Abundance, biomass, secondary production, species richness and diet were compared between habitats upstream and downstream of an agricultural site. Estimates of diet, obtained using stable isotopes of carbon and nitrogen, and secondary production were combined to determine the dependence of individual taxa and each habitat on allochthonous and autochthonous inputs.

Distinctive shifts in the composition and production of benthic invertebrate communities within the 400 metre study reach were consistent with the expected effects of agricultural land use. Larger, more sensitive forms, especially mayflies, caddisflies and stoneflies were replaced by smaller, more tolerant chironomids, resulting in a 30% reduction in secondary production at the downstream site. A finer and more homogenous stream bed downstream, likely due to increased sedimentation, eliminated habitat and led to poor recruitment of the dominant hydropsychid caddisfly Hydropsyche slossonae. Lower density, biomass and production by the shredders was likely a result of a reduced ability of the site to retain coarse particulate matter and lower inputs of allochthonous matter from the cleared farmland. Greater retention of fine detrital sediments also supported increased abundance, biomass and secondary production of Chironomidae.

Periphyton was grown on glass plates suspended in the water column under two conditions of light and water velocity, over two seasons, to test the hypothesis that boundary layer thickness is an important variable affecting stable isotope fractionation in benthic algae. Isotopic signatures for both carbon and nitrogen in samples of periphyton varied with light intensity and season, but not current velocity. These results suggested that isotopic fractionation in periphyton was more strongly influenced by the intensity of metabolic activity than by variations in the thickness of the benthic boundary layer.

Diatom and chlorophyte carbon signatures followed a seasonal pattern in which reduced metabolic fractionation under high growth conditions in summer led to enriched signatures. Carbon signatures of the macroinvertebrates reflected the seasonal variation in algae.

The relative dependence of different functional feeding groups on allochthonous resources followed the expected trend (i.e. shredders were most, and scrapers least, dependent on terrestrial inputs). However, many taxa exploited a wider food base than their feeding group classification implied (e.g. shredders and filterers). The dietary analysis indicated that D. nivoriunda, B. brunneicolor, Parakiefferiella sp. and Pycnopsyche sp., should be reassigned different trophic guilds.

Invertebrate diets were similar at sites 1 and 5, suggesting that inputs of autochthonous and allochthonous organic material did not limit secondary production at the downstream site. The analyses support the conclusion that altered benthic community composition and lower secondary production downstream were primarily due to a shift in stream bed morphology, a likely result of agricultural activity on the adjacent fields. Estimates of autochthonous primary production and allochthonous inputs necessary to support the macroinvertebrate community were within expected ranges for a moderately enriched headwater stream with a partial canopy.