Abstract
Periphyton communities of a boreal stream were exposed to different light and nutrient levels to estimate energy transfer efficiency from primary to secondary producers using labeling with inorganic 13C. In a one-day field experiment, periphyton grown in fast-flow conditions and dominated by opportunistic green algae were exposed to light levels corresponding to sub-saturating (forest shade) and saturating (open stream section) irradiances, and to N and P nutrient additions. In a two-week laboratory experiment, periphyton grown in low-flow conditions and dominated by slowly growing diatoms were incubated under two sub-saturating light and nutrient enrichment levels as well as grazed and non-grazed conditions. Light had significant positive effect on 13C uptake by periphyton. In the field experiment, P addition had a positive effect on 13C uptake but only at sub-saturating light levels, whereas in the laboratory experiment nutrient additions had no effect on the periphyton biomass, 13C uptake, biovolume and community composition. In the laboratory experiment, the grazer (caddisfly) effect on periphyton biomass specific 13C uptake and nutrient content was much stronger than the effects of light and nutrients. In particular, grazers significantly reduced periphyton biomass and increased biomass specific 13C uptake and C:nutrient ratios. The energy transfer efficiency, estimated as a ratio between 13C uptake by caddisfly and periphyton, was positively affected by light conditions, whereas the nutrient effect was not significant. We suggest that the observed effects on energy transfer were related to the increased diet contribution of highly palatable green algae, stimulated by higher light levels. Also, high heterotrophic microbial activity under low light levels would facilitate energy loss through respiration and decrease overall trophic transfer efficiency. These findings suggest that even a small increase in light intensity could result in community-wide effects on periphyton in boreal streams, with a subsequent increase in energy transfer and system productivity.
Highlights
According to stream ecosystem theory, riparian vegetation has the potential to have strong system-level effects on stream biota, through its organic inputs and physical influences [1]
Depth and velocity of the stream varied substantially due to precipitation: the field experiment was conducted under medium flow conditions (25–26 May 2010), whereas material for the laboratory experiment was collected at low flow (9 June 2010)
We found no support for the hypothesized positive effect of nutrient addition on carbon uptake below saturated light levels in the laboratory experiment
Summary
According to stream ecosystem theory, riparian vegetation has the potential to have strong system-level effects on stream biota, through its organic inputs and physical influences [1]. Anthropogenic processes have a high potential to alter the light and nutrient regimes in boreal streams. Deforestation of catchments leads to increases in terrestrial inputs of dissolved and particulate organic matter in streams and reduces watershed retention of deposited nitrogen [9,10]. The increased nutrient inputs to the streams could stimulate algal growth; suspended particulate matter and water coloration due to dissolved organic carbon may limit light for photosynthesis [11]. Development of forest management tools, such as leaving riparian buffers [17,18], requires knowledge of limiting factors for primary productivity and sensitivity of local periphytic communities to light and nutrient alterations in the streams, with effects of the rest of the stream food chain
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