Department Surface Waters - Research and Management

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.

We investigate the impact on remote sensing reflectance by the vertical non-uniformities of water constituents. Reflectance simulated for 210 pairs of in situ measured chlorophyll-a and turbidity profiles (z = 0–20 m) from Lake Geneva are compared to simulations for uniform constituent gradients and non-uniform profiles approximated by Gaussian curves, orthogonal layers and steady gradients. Relevant concentration ranges are between 0 and 17 mg m⁻³ for chlorophyll-a and 0 and 4.6 g m⁻³ for total suspended matter within the photic layer. Our results show that mesotrophic lakes are specifically sensitive to non-uniformities with 20% of the 210 samples used in this study showing deviations of the spectral angle > 5° between a uniform assumption and observations which mostly occur for deeper-laying water constituents. By stressing the different use of blue and red parts of the spectrum, we argue further that algorithms are affected by variable vertical structures of algal and inorganic particles. Finally, we demonstrate that approximation models of the vertical structure of water constituents are a good solution to better account for non-uniformities in the development of invertible bio-optical models.

Directives and legislations worldwide aim at representatively and continuously monitoring the ecological status of surface waters. In many countries, chlorophyll-α concentrations (CHL) are used as an indicator of phytoplankton abundance and the trophic level of lakes or reservoirs. In-situ measurements of water quality parameters, however, are time-consuming, costly and of unknown but naturally limited spatial representativeness. In addition, the variety of the involved lab and field measurement methods and instruments complicates comparability and reproducibility.
Taking Lake Geneva as an example, 1234 satellite images from the MERIS sensor on the Envisat satellite from 2002 to 2012 are used to quantify the spatial and temporal variations of CHL concentrations. Based on histograms of spring, summer and autumn CHL estimates, the spatial representativeness of two existing in-situ measurement locations is analysed. Appropriate sampling frequencies to capture CHL peaks are examined by means of statistical resampling. The approaches proposed allow determining optimal in-situ sampling locations and frequencies. Their generic nature allows for adaptation to other lakes, especially to establish new survey programmes where no previous records are available.