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

20 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 25 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- 30 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 35 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. For this work, ECMWF ERA-5 wind fields were extracted every three hours with a spatial resolution of 0.5° in 260 latitude and longitude, on 29 vertical pressure levels between 50 and 1000 hPa. Back trajectories starting at different altitudes corresponding to cloud layers observed on Mira35 radar profiles have been calculated, with a temporal resolution of 15 minutes and a total duration of 120 hours. In addition to back trajectory plots, the global origin is quantified by counting the number of trajectory points in each of the following nine geographic sectors: north-northeast (NNE), east-northeast (ENE), east-southeast (ESE), south-southeast (SSE), south-southwest 265 (SSW), west-southwest (WSW), west-northwest (WNW) and north-northwest (NNW), and one nearby area. Points located over land and sea are also counted, by separating those which are close to the ground from those which are in the free troposphere, taking as a limit 2 km above the ground. We reported a high variability of the chlorophyll and phycocyanin-rich cell concentrations according to the cloud reflectivity periods. Indeed, the abundance of photosynthetic cells containing chlorophyll-a regularly decreased between CR1a and CR1c with a start at 4371 cell. L -1 to 506 cell. L -1 at the end of the event. The abundance of cells containing phycocyanin showed similar variations with higher concentrations during CR1a et b to finally reached 0 cell. L -1 at the end of the event. of microalgae cyanobacteria key an innovative method using flow cytometry to separate the concentration of chlorophyll, phycocyanin, and phycoerythrin-rich cells on fresh rain samples, we bring new information about the scavenging process of photosynthetic cells in the rainwater.