Spatial Variability in Light Yields Colimitation of Primary Production by Both Light and Nutrients in a Forested Stream Ecosystem

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 lightlimited 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.

Key words: nutrient-diffusing substrate; nitrogen limitation; light limitation; colimitation; benthic primary production; HJ Andrews; riparian forest;
stream light; habitat heterogeneity.