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 hypothesize stream metabolism will show low resistance to storm events because GPP and ER will be either enhanced by inputs of limited resources (small storms) or hindered 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 Oct 2018–Feb 2023) and estimated metabolic rates prior, during and after 35 of them. Individual storm events had different magnitude (discharge from 0.6 to 872.4 L s^-1), duration (from 4 to 32 days) and precipitation intensity (from 1.3 to 31.4 mm h^-1). Considering all events, 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. Changes in GPP were unrelated to any hydrological parameter, 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 = 0.49), but level-off at ca. 6 days, suggesting that the positive effect of resource inputs on stream metabolic activity limited over time. Our findings support the idea that storm events act as triggers of stream metabolism and highlight how changes in hydrological regimes could impact stream functioning and its role in global biogeochemical cycles.