Air temperature more than drought duration affects litter decomposition under flow intermittency

Stream intermittency - periodic sequences of water flow cessation and resumption - occurs throughout the year, across seasons. Even though temperature is a known regulator of litter decomposition in both terrestrial and aquatic environments, comparative experiments on drought durations at distinct temperatures on microbial-mediated decomposition in streams experiencing intermittency are still lacking. Here, three drought temperatures (5, 15 and 25 °C) and two durations (short: 2.5 weeks; long: 5 weeks) were applied in a microcosm study to oak leaf discs colonized in a reference stream; mass loss and associated microbial parameters (fungal biomass, microbial activity, and sporulation rates) were evaluated following re-submersion for 2 weeks. Higher mass loss was found at 15 °C than 25 °C. A prolongation of the drought exposure period had no effect on mass loss, suggesting an early (≤ 2.5 weeks) inhibitor effect of drought on microbial-mediated leaf degradation. Fungal biomass was highest at 25 °C following a short drought, and decreased with a longer drought period at both 15 °C and 25 °C. Microbial activity was not affected by either drought duration or temperature. Sporulation rates and fungal diversity were significantly reduced by the longer drought period; in the short treatment, maximum values were found at 15 °C. In contrast to longer droughts, aquatic fungal communities during short dry periods seem to invest in energetically-expensive physiological responses to desiccation (e.g., ergosterol production) promoting biomass accrual at the expense of mass loss and reproductive output. Under more severe desiccation (higher duration and temperature), the lower diversity of fungal communities seem to result in negative legacy effects for fungal growth and reproductive capacity after flow resumption. These results suggest that native riparian vegetation, through its ability to regulate temperature in streams, may be critical in protecting freshwaters from intensified severity of drought periods in streams experiencing intermittency.