Effects of natural surfactants on the spatial variability of surface water temperature under intermittent light winds on Lake Geneva

Abstract

The spatial variability of lake surface water temperature (LSWT) between smooth and rough surface areas and its potential association with the natural surfactant distribution in the surface microlayer were investigated for the first time in a lake. In spring 2019, two different field campaigns were carried out in Lake Geneva to measure: i) the enrichment factor of fluorescent dissolved organic matter (FDOM) as a proxy for biogenic surfactants, and ii) LSWT and near-surface water temperature profiles while simultaneously monitoring water surface roughness in both cases. Results indicate that, under intense incoming short-wave radiation and intermittent light wind conditions, the atmospheric boundary layer (ABL) was stable and the accumulation of heat due to short-wave radiation in near-surface waters was greater than heat losses by surface cooling, thus creating a diurnal warm layer with strong thermal stratification in the water near-surface layer. A threshold wind speed of 1.5 m s-1 was determined as a transition between different dynamic regimes. For winds just above 1.5 m s-1, the lake surface became patchy, and smooth surface areas (slicks) were more enriched with FDOM than rough areas (non-slick) covered with gravity-capillary waves (GCW). Sharp thermal boundaries appeared between smooth and rough areas. LSWT in smooth slicks was found to be more than 1.5°C warmer than in rough non-slick areas, which differs from previous observations in oceans that reported a slight temperature reduction inside slicks. Upon the formation of GCW in non-slick areas, the near-surface stratification was destroyed and the surface temperature was reduced. Furthermore, winds above 1.5 m s-1 continuously fragmented slicks causing a rapid spatial redistribution of LSWT patterns mainly aligned with the wind. For wind speeds below 1.5 m s‑1 the surface was smooth, no well-developed GCW were observed, LSWT differences were small, and strong near-surface stratification was established. These results contribute to the understanding and the quantification of air-water exchange processes, which are presently lacking for stable Atmospheric Boundary Layer conditions in lakes.

A Persistent Submesoscale Frontal Slick: A Novel Marker of the Mesoscale Flow Field in a Large Lake (Lake Geneva)

“…In addition, TLB long-term records indicate that the location and the orientation of submesoscale slicks in the widest part of the lake change; this reflects the dynamics of the underlying gyre pattern (Cimatoribus et al, 2019). A frontal slick such as the one observed here can be classified as part of a broader category of submesoscale currents commonly observed in oceanic surface waters and recently in Lake Geneva (Hamze-Ziabari, Foroughan, et al, 2022;Hamze-Ziabari, Razmi, et al, 2022).…”

“…The surface micro‐layer water samples showed that the smooth band on the warm side of the LSWT front had higher concentrations of surfactants (Figure S2 in Supporting Information ), emphasizing the role of natural surfactants in manifesting frontal dynamics by inhibiting GCW development, as confirmed in the RGB images taken by the ZiviCat when crossing the frontal slick (Movie ). A previous study in Lake Geneva showed that smooth slicks are associated with biogenic surfactants (Foroughan et al., 2022) and that they suppress GCW development.…”

Monitoring Mesoscale to Submesoscale Processes in Large Lakes with Sentinel-1 SAR Imagery: The Case of Lake Geneva

“…An increase in the viscous properties of the surface layer can have an The effect of SST on the NRCS is relatively small compared to that due to surfactants [23,30], which can be up to 5-10 dB [40]. Surfactants originate from biological activities and terrestrial sources introduced by catchments around the lake [52]. Due to the divergence of the flow field at the center of a cyclonic gyre/eddy, an accumulation of terrestrial source material therein seems unlikely.…”

“…Due to the divergence of the flow field at the center of a cyclonic gyre/eddy, an accumulation of terrestrial source material therein seems unlikely. This suggests that the source of surfactants at the center of the lake is probably autochthonous [52]. A field study in Lake Tanganyika revealed that nutrient concentrations and phytoplankton chlorophyll levels increased in the pelagic upwelling zone [75].…”

“…Large lakes subject to the Coriolis force, such as Lake Geneva, are characterized by complex large-scale rotational current patterns [51], and thus 2D high-resolution SAR imagery can provide new insights into these mesoscale/submesoscale processes. Natural slicks, which contain a higher concentration of biogenic surfactants than rough surface areas, are ubiquitous in Lake Geneva [52], and can be observed under light to moderate wind speeds ranging from 1.5 to 5 m s −1 . Detecting and monitoring large-scale currents in Lake Geneva using SAR imagery thus appears feasible.…”