Interactive Effects of Drought and Shading on Boreal Stream Metabolism and Algal Biomass in Experimental Mesocosms

• Boreal catchments are experiencing intensified land-use and climate change simultaneously, yet these stressors are seldom addressed in conjunction. It is expected that exposing multiple stressors on an ecosystem will have interactive effects, and here we aimed at testing the effects of drought and loss of shading on stream ecosystem functioning. We exposed natural substrates in experimental, outdoor flumes under a gradient of shading (from 0% to 74%) to a 2-week drought and assessed the effects on benthic algal biomass (chl-α) and whole channel metabolic rates (Gross Primary Production, GPP and Ecosystem Respiration, ER) for 5 weeks pre and 2 weeks post-drought. Drought and shading had interactive effects on all response variables, but the magnitude varied over the course of the experiment. Chl-α was initially strongly negatively correlated with shading level, but the drought and onset of autumn conditions post-drought together diminished the shading effects. The drought effect on chl-α was only significant for 1 week post-drought, after which algae in some channels had recovered. However, for metabolic rates, the effect of drought was sustained during the whole 2-week period post-drought, and we did not see recovery. We show that streams in this boreal setting, with a lack of sufficient shading (< 43%) grow an algal community dominated by fast growing, filamentous algae which promote high respiration rates and that fail to recover post-drought. Algae in the most heavily shaded streams (74%) were not filamentous, and they recovered in biomass (chl-α) post-drought, but they had low metabolic rates both pre- and post-drought. This interaction of shading and drought has implications for how we manage our forested streams and highlights the susceptibility of streams with limited shade to drought.

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NATURVETENSKAP  -- Biologi -- Ekologi (hsv//swe)

NATURAL SCIENCES  -- Biological Sciences -- Ecology (hsv//eng)

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