Effects of landuse intensification on stream basal resources and invertebrate communities

Human population growth and development have degraded freshwater ecosystems through direct negative effects on basal resources and higher trophic levels. Knowledge of how basal resources are influenced by anthropogenic activities is needed to examine foodweb responses and to evaluate negative effects of human disturbance. We combined quantitative sampling of all putative resources and consumers and naturally occurring stable isotopes of resource C and N to investigate how land use influences characteristics of basal resources (leaf litter, fine particulate organic matter [FPOM] suspended in water, FPOM deposited in pools, and epilithon) and the invertebrate communities in 9 streams of British Columbia (Canada) on a seasonal basis. Study streams were grouped as forest, cropland, and urban, based on percentages of land use in their catchments. We tested for seasonal and spatial differences in biomass, isotopic variability, and overlap of different basal resources and their relationships with invertebrate communities. Our results show that landuse intensification promotes wholesale degradation, which includes deterioration of water quality, changes to the quantity and quality of basal resources, and shifts in community structure of benthic invertebrates. Stream basal resources showed large variations in isotopic signatures caused by landuse changes, with the largest isotopic variability observed in cropland streams and the smallest in urban streams. Invertebrate communities had fewer taxa and reduced functional diversity in impaired streams (i.e., cropland and urban) via simplification of feeding styles of consumers, probably because of bottom-up controls. Our study highlights the importance of quantifying the isotopic variability of basal resources to estimate effects on food webs over time and along landuse intensification gradients.