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Abstract. We present an interdisciplinary investigation of the links between the macro and microphysical properties of rain, the biochemical composition of rain, meteorological parameters, and their impacts on the phytoplankton dynamics of a mountain lake. In order to document this interdisciplinary scientific question, Lake Aydat in the French Massif Central mountains was fitted with a set of high-resolution atmospheric radars, a disdrometer, and a precipitation collector in 2020. In parallel, the lake was monitored via sensors and water sampling. To illustrate the potential of this novel experimental setup, we present a case study of a rain event that occurred in September 2020 and during which, three contrasting sub-periods were identified based on the type of rain (convective or stratiform). Using our high-temporal resolution monitoring, we show that the origin of the air mass mainly influences the chemical composition of the rain, which depends on the rain type. In contrast, the photosynthetic cell concentration in the rain is mainly influenced by meteorological variables, predominantly below-cloud scavenging. The very low concentrations of photosynthetic cells found in rain events cannot directly impact the lake's phytoplankton abundance. In contrast, the rain rate directly impacted the lake's thermal stratification during the convective event. The response of the phytoplankton depends on the genus and, interestingly, three cyanobacteria, Microcystis, Coelomoron, and Merismopedia, showed a systematic response to rain events with a sudden decrease in abundance at the lake surface immediately after rain events. In contrast, the abundance of green algae (Elakatothrix), picocyanobacteria (Synechocystis and Synechococcus), and diatoms (Asterionella and Melosira) gradually increased following the rain events, but with a lower intensity compared to the cyanobacteria species. These different phytoplankton responses to the same rainfall event could play a key role in phytoplankton dynamics in the temperate zone. Our results highlight the importance of high-frequency and time resolution monitoring of both atmosphere and lake to better understand the adaptive strategies of cyanobacteria following rain events.

Abstract. We present an interdisciplinary investigation of the links between rain macro and microphysical properties, meteorological parameters, and a mountain lake to assess the impact of precipitation events on phytoplankton dynamics and, in particular, on cyanobacteria. In order to document this interdisciplinary scientific question, the Aydat lake in the French massif central mountains has been instrumented in 2020 with a set of high-resolution atmospheric radars, disdrometer, and precipitation collector. In parallel, the lake was monitored with a suite of sensors and water sampling. To illustrate the potential of this original experimental setup, we present a rain case study that occurred in September 2020 and during which three contrasted sub-periods were identified. Using our high temporal-resolution monitoring, we show that the air mass origin mainly influences the rain nutrient composition, which depends on the type of rain, convective or stratiform. Our results also highlighted a non-negligible presence of photosynthetic cells in all rains, but their very low abundance can probably not impact the phytoplankton dynamics. Nevertheless, rain events indirectly impacted phytoplankton assemblages. Indeed, among all phytoplankton genera, three cyanobacteria, Microcystis, Coelomoron, and Merismopedia, showed a similar pattern with a systematic punctual decrease of their abundance at the lake surface immediately after rain events, suggesting a different impact of rain events according to the cyanobacterial genus considered. These various phytoplankton responses subjected to the same rainfall event could play a key role in phytoplankton dynamics in the temperate zone. Our results highlight the interest in high-frequency and time resolution monitoring of both atmosphere and lake to better understand the cyanobacteria adaptive strategies following rain events.

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