Spatial Variability in Light Yields Colimitation of Primary Production by Both Light and Nutrients in a Forested Stream Ecosystem
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Colimitation of primary production is increasingly recognized as a dominant process across aquatic and terrestrial ecosystems. In streams, both nutrient availability and light availability have been shown to independently limit primary production, but colimitation by both light and nutrients is rarely considered. We used a series of nutrient-diffusing substrates (NDS) bioassays deployed across a range of light availability conditions in a single-study stream over two summers to determine the light level at which the limiting factor for benthic periphyton accrual transitioned from light to nutrients. Stream periphyton accrual was nutrient-limited in high-light patches, and light-limited in low-light patches, with the transition from being predominantly light-limited to being predominantly nutrient-limited occurring when daily light fluxes exceeded 3.5 mol m−2 day−1. We quantified light at each NDS bioassay location and at 5 m intervals throughout our two adjacent 160 m study reaches—one in structurally complex old-growth riparian forest and one bordered by more uniform second-growth forest. Although both reaches were colimited overall, the resource (light or nutrients) dominating limitation differed between the two riparian forest age/structure conditions. In the old-growth section, about three quarters of the reach was predominantly nutrient-limited, whereas in the second-growth reach only about a quarter of the streambed was nutrient-limited. In this stream, colimitation of benthic periphyton accrual by light and nutrients at the reach scale was an emergent property of the ecosystem that manifested as a result of high heterogeneity in riparian forest structure.
Keywordsnutrient-diffusing substrate nitrogen limitation light limitation colimitation benthic primary production HJ Andrews riparian forest stream light habitat heterogeneity
We thank Brian VerWey, Emily Heaston, Katherine Pospisil, and Chris Kopet for help in the field and in the lab. We thank Brian VerWey and Lindsey Thurman for feedback on early drafts of the manuscript. Summer 2015 fieldwork was supported by NSF DEB 1547628 awarded to DRW. SMC was supported by an NSF Postdoctoral Research Fellowship in Biology (DBI 1401954). Funding for author EMP was provided through the HJ Andrews Research Experience for Undergraduates (REU) program, and facilities were provided by the HJ Andrews Experimental Forest research program—both funded by the National Science Foundation’s Long-Term Ecological Research Program (DEB 1440409), US Forest Service Pacific Northwest Research Station, and Oregon State University.
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