Wind drives fast changes of light climate in a large, shallow re-established lake.

With ever greater frequency, wetlands and shallow lakes that had been diverted for agriculture are being re-established to reduce nutrient loss and greenhouse gas emission, as well as to increase biodiversity. Here, we investigate drivers of water column light attenuation (K) at multiple time scales and locations in Lake Fil, Denmark, during the first five years after its re-establishment in 2012. We found that K was generally high (overall mean: 3.4 m), with resuspended sediment particles and colored dissolved organic matter being the main contributors. Using daily time series of light attenuation recorded at four stations, we used a generalized additive model to analyze the influence of wind speed and direction on K. This model explained a high proportion of the variation (R = 0.62, RMSE = 0.74 m, and MAE = 0.55 m) and showed that higher wind speed increased K on the same day and, with smaller influence, on the next day. Furthermore, we found a significant influence of wind direction and an interaction between wind speed and wind direction, a combination that suggests that short-term variations in light climate depends on the interplay between wind direction and sources of particles. Wind from non-prevailing directions thus influence K more, as it can activate previously deposited particles. The maximum colonization depths of submerged vegetation occurred at ~2-6% of sub-surface light from 2014 to 2016 and peaked at 1.2 m in 2016. The fast, day-to-day variation of K in Lake Fil reveals the importance of wind on light climate and in turn biological elements such as phytoplankton and submerged macrophyte development in shallow lakes. The implications are essential for the prior planning and management of future lake re-establishment.