Using the raz‐rru method to examine linkages between substrate, biofilm colonisation and stream metabolism in open‐canopy streams

Abstract

Abstract Understanding the mechanisms that control gross primary production (GPP) and ecosystem respiration (ER) is important in open‐canopy streams, particularly as their prevalence increases across the landscape with the expansion of human land use. We measured resazurin (raz) transformation to resorufin (rru) as an indicator of ER in two contrasting experimental stream systems to examine the linkages between underlying substrate, biofilm colonisation and stream metabolism in open‐canopy systems. We implemented a gradient of biofilm coverage (0%–100%) in 20 recirculating streams and found that raz transformation (kτ) increased proportionally with algal biomass and gross primary production (GPP). We then conducted multiple short‐term additions of raz over a 5‐month biofilm colonisation sequence in four groundwater‐fed, experimental streams (Q = 1.5 L/s, 50 m long) at the Notre Dame Linked Experimental Ecosystem Facility (ND‐LEEF). Streams at ND‐LEEF were lined with two different substrate sizes (pea gravel vs. cobble) and two levels of heterogeneity (alternating sections vs. well‐mixed). We found that longitudinal patterns of kτ varied with substrate type and heterogeneity on the majority of sampling days. Temporal patterns of kτ over the trajectory of biofilm growth were similar among streams, with “peak” kτ occurring after three months of biofilm growth; kτ decreased with the onset of algal senescence and decreasing temperatures. Raz transformation was also positively correlated with algal biomass and GPP in the streams at ND‐LEEF, which varied with substrate composition. Overall, the raz‐rru method identified spatial and temporal variability in the linkages between substrate, algal biofilms, and metabolism in open‐canopy streams.