Riparian forest controls on light and the longitudinal transitions from light to nutrient limitation of stream periphyton

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Limitation of primary production by two or more factors together (i.e., co-limitation) is increasingly recognized as the predominant condition at the ecosystem scale.  In stream ecosystems, numerous studies have documented co-limitation by multiple nutrients, and beyond nutrient-limitation alone, studies have also noted the potential for light and nutrients to co-limit primary production in streams. In this study, we used sets of 6 to 12 nutrient diffusing substrate arrays paired with stream light measurements to determine light versus nutrient limitation of stream periphyton at multiple streams in the HJ Andrews LTER in the Cascade Mountains in western Oregon.  The streams encompassed a range of background nutrient levels with mean mid-summer nitrate concentrations as low as 3 ug/L and as high as 65 ug/L.  We were able to identify both light limited and nutrient limited areas within the same stream reach, even when background nutrients were low.  Patchiness in stream light led to variability in the limiting factor for periphyton growth along each reach. Based on responses across the set of nutrient diffusing substrate arrays in each stream, we identified a potential light availability threshold range in each system where periphyton shifted between being predominantly light limited versus predominantly nutrient limited.  Given the variability in light availability along these streams (based on the extensive set of light measurements from a subset of the streams), we concluded that these ecosystems are collectively co-limited by light and nutrients but that co-limitation at the reach scale may be a product of spatial variability in single-factor limitation.