Warming influences nutrient removal in mesocosms across different substrates

Abstract

Climate change is increasing stream temperatures globally, likely impacting stream ecosystem processes. We conducted a stream warming experiment at the Notre Dame Experimental Mesocosm Facility using N = 24 recirculating mesocosms to examine the effect of warming on biofilm colonization of organic and inorganic substrata and rates of nutrient removal. We conducted an experiment with three temperature treatments (20°C, 23°C, and 26°C) in mesocosms containing either decomposing sugar maple ( Acer saccharum ) leaves or cobble substrate under shaded and unshaded conditions, respectively. We measured biofilm chlorophyll a (Chl a ) on cobble, community respiration on leaf disks, and removal of ammonium (NH 4 + ‐N) and soluble reactive phosphorus weekly for 21 d. Warming did not significantly alter Chl a or community respiration (generalized linear mixed model, p > 0.05). For nutrients, however, warming increased NH 4 + ‐N removal for both leaves and cobble. For cobble, NH 4 + ‐N removal was higher at 26°C (mean = 3.5 h −1 with a 75% credible interval of [3.1, 4.0]) compared to 20°C (2.3 h −1 [1.9, 2.7]; Bayesian regression model). For leaves, NH 4 + ‐N removal was also higher at 26°C (1.1 h −1 [1.0, 1.2]) compared to 20°C (0.5 h −1 [0.4, 0.6]). In contrast, warming only altered soluble reactive phosphorus removal for cobble; removal was higher at 26°C at 1.9 h −1 [1.7, 2.2] compared to 20°C (1.1 h −1 [0.8, 1.3]). Warming did not alter soluble reactive phosphorus removal for leaves. We show that warming‐induced changes in nutrient removal are mediated by substrate, and that NH 4 + ‐N removal may be more sensitive to elevated temperatures than soluble reactive phosphorus removal, with implications for altered nutrient cycling under a warmer future.

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