Breathing storms: Enhanced ecosystem respiration during storms in a heterotrophic headwater stream

Abstract. Hydrological disturbances following storm events influence the structure and functioning of headwater streams. However, understanding how these disturbances impact critical processes such as stream metabolism is challenging. We assessed the effect of storm events on the resistance and resilience of gross primary production (GPP) and ecosystem respiration (ER) in a heterotrophic headwater stream. We hypothesized that stream metabolism will show low resistance to storm events because GPP and ER will be either stimulated by inputs of limited resources (small storms) or suppressed by biofilm damage (large storms). We also expected resilience to decrease with the size of the storm event. To test these hypotheses, we hydrologically characterized 53 individual storm events during 4.5 years (period October 2018–February 2023) and estimated metabolic rates prior, during, and after each event. Individual storm events had different duration (from 4 to 32 d), and exhibited contrasting changes in discharge (discharge from 0.6 to 872.4 L s−1). Due to data and model constraints, we were able to estimate metabolic rates for 35 of these events, for which GPP and ER averaged 1.7 ± 1.8 and −13.4 ± 7 g O2 m−2 d−1, respectively. The two processes showed low resistance to storm events, with magnitudes increasing in 69 % and 86 % of the cases for GPP and ER, respectively. The relationship between hydrological parameters and changes in GPP was not statistically significant, while a positive relation with the magnitude of the storm event was found for ER (R2 > 0.37). Similarly, recovery times were positively related to the size of the event only for ER (R2 > 0.46). Yet recovery times were always lower than 6 d, suggesting that the positive effect of resource inputs on stream metabolic activity was limited over time. Our findings support the idea that storm events stimulate metabolic activity in headwater streams, especially ER, and highlight how changes in hydrological regimes could impact stream functioning and its role in global biogeochemical cycles.