Effects of removal of riparian vegetation on algae and heterotrophs in a Mediterranean stream

Abstract

The effect of removal of a riparian strip on aquatic autotrophic (algae) and heterotrophic (bacteria, macroinvertebrates) organisms was monitored in a Mediterranean stream during the canopy growing period. Community composition, biomass and metabolic activities were compared with those recorded during a pre-riparian removal period and in a forested stretch downstream. Higher irradiance was associated with Cladophora increase in the logged section. Algal biomass increased up to ten times, and productivity was up to four times higher than in the pre-removal period and the forested section. Bacterial communities showed higher ectoenzymatic activities (β-glucosidase, β-xylosidase) in the logged section than in forested conditions. Moreover the coincidence between the maxima of β-glucosidase and chlorophyll-α suggests that bacterial activity was enhanced by the higher availability of high-quality algal material. The macroinvertebrate community had higher density and biomass in the logged section than in the forested section and in the pre-removal period. Scrapers and filterers become dominant after riparian removal, while shredders, predators and collectors did not show significant changes either between sites or periods. Responses of environmental variables and biotic communities indicate that the changes occurring in the stream due to riparian removal could be considered bottom-up controlled, as increased illumination was the main mechanism responsible.