Impact of thermal fluctuations on phytoplankton: an experimental multi-trait analysis across species.

Thermal fluctuations affect the physiology and ecology of organisms. To date, most studies quantifying the effects of temperature on phytoplankton have used mean constant conditions, thus overlooking the role of short-term thermal fluctuations. Here, we use a multi-trait analysis to quantify how thermal regime (constant vs. fluctuation by ±3°C around mean temperature) alters the effect of temperature (18 vs. 22°C) on the growth, elemental composition, photosynthetic performance and metabolism of five phytoplankton species (, , , sp. and ) during exponential growth and stationary phases. Our results showed a high degree of inter-trait and inter-specific variability in the response to the temperature treatments. The carbon-based growth rates tended to be reduced by thermal fluctuations (by 20-29%), particularly under warming conditions. By contrast, thermal fluctuations increased the photosynthesis rates up to 25%, regardless of the growth phase. The carbon-to-nitrogen and carbon-to-chlorophyll ratios, maximum photochemical yield of photosystem II and relative maximum electron transport rates did not show a clear response to interactions between thermal fluctuations and temperature. These results suggest that, when assessing phytoplankton responses to temperature, it is essential to consider both thermal fluctuations and multi-trait analysis.