Revealing the strong effect of seasonality on wind-induced nearshore current dynamics in large Lake Geneva captured by multi-annual observations

Multi-annual studies focusing on seasonal nearshore current dynamics and its effect on nearshore-offshore exchange in large lakes are scarce, even though such information is essential for determining the long-term evolution of a lake's ecosystem. To help address this lack, 8-year-long, near-bottom temperature and current data at 26 m depth, combined with meteorological data, were collected in the northern nearshore zone of Lake Geneva. An analysis revealed that persistent, well developed alongshore currents in the nearshore zone are structured as events driven by two strong, dominant winds coming from the northeast and from the southwest. Current events typically last from several hours to ∼3 days. Alongshore near-bottom currents can exceed 20 cm s −1 causing bursts of sediment resuspension and increased horizontal mixing. Mean alongshore-current direction changed consistently on a seasonal scale from predominantly westward during summer to eastward during winter. Correlated with westward alongshore transport, Ekman transport leads to coastal downwelling mainly in summer. In winter, downwelling at the downwind shore dominated and was correlated with eastward alongshore transport. Thus, during both seasons, offshore transport prevailed near the bed. Offshore currents reached up to 20 cm s −1 . Waters of different origin passed through the nearshore zone as reflected by near-bottom summer temperatures continuously changing, within hours, from near-surface values to those recorded at greater depth. Such three-dimensional exchange and mixing between the nearshore zone and the l ake interior can significantly affect a lake's bio-geo-chemical equilibrium. The findings of this study reveal the importance of seasonal variability on long-term nearshore zone dynamics.