Frequency and amplitude of temperature oscillations can profoundly affect structure and function of ecosystems. Unless the rate of a biological process changes linearly within the range of these fluctuations, the cumulative effect of temperature differs from the effect measured at the average temperature (Jensen's inequality). Here, we measured numbers and types of spores released by aquatic hyphomycetes from oak and alder leaves that had been exposed in a Portuguese stream for between 7 and 94 days. Recovered leaves were incubated at four temperatures between 5 and 20 °C. Over this range, the sporulation response to temperature was decelerating, with an estimated optimum around 12.5 °C. Assuming a linear response, therefore, overestimates spore release from decaying leaves. The calculated discrepancy was more pronounced with recalcitrant oak leaves (greater toughness, phenolics concentration, lower N and P concentration than alder), and reached 26.6 % when temperature was assumed to oscillate between 1 and 9 °C, rather than remaining constant at 5 °C. The maximum fluctuation of water temperature over 48 h during the field experiment was approximately 3 °C, which would result in a discrepancy of up to 6 %. The composition of the fungal community (assessed by species identification of released spores) was significantly influenced by the state of decomposition, but not by leaf species or temperature. When quantifying the potential impact of global change on aquatic fungal communities, the average increase as well as fluctuations of the temperature have to be considered.