Are surface temperature and chlorophyll in a large deep lake related? An analysis based on satellite observations in synergy with hydrodynamic modelling and in-situ data

Phytoplankton growth depends on various factors, and primarily on nutrient availability, light and water temperature, whose distributions are largely controlled by hydrodynamics. Our main objective is to analyse the link between spatial and temporal variability of surface water temperature and algal concentration in a large lake by means of remote sensing and hydrodynamic modelling. We compare ten years of satellite images showing chlorophyll concentrations and surface water temperature of Lake Geneva. Our observations suggest different correlations depending on the season. Elevated chlorophyll concentrations in spring are correlated with warmer zones. But, in summer, higher chlorophyll concentrations are observed in colder zones. We show with a three-dimensional hydrodynamic model that the spatial variability of the surface water temperature reflects the upwelling and downwelling zones resulting from wind forcing. In springtime, nearshore downwellings induce locally increased surface temperature and stratification, which are associated with high chlorophyll concentration. In summertime, colder surface temperature area, often interpreted as transient upwellings, represents the thermal surface signature of wind-induced basin-scale internal waves, bringing either nutrients or phytoplankton from deeper layers to the surface. Our study suggests the latter to be the dominant process, with the basin-scale internal wave activity and associated transient summertime upwellings and downwellings having little net effects on the algal concentration. This study finally demonstrates the necessity to connect remote sensing retrievals and three-dimensional hydrodynamic modelling to properly understand the dynamic of the lake ecosystems.