Interdisciplinary strategy to assess the impact of meteorological variables on the biochemical composition of the rain and the dynamics of a small eutrophic lake under rain forcing

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.
