Interdisciplinary strategy to survey phytoplankton dynamics of a eutrophic lake under rain forcing: description of the instrumental set-up and first results.

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 nonnegligible 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. [ABSTRACT FROM AUTHOR]