Figure 1

Amplitude and temporal frequency of the light flicker of downward irradiance are wavelength dependent. (A,B) Examples of light flicker time series of downward irradiance at 1 m depth and light wavelengths of 400 and 600 nm. Each time series constitutes of 3,000 measurements acquired over 173 s. The amplitude of the light flicker at 600 nm is larger than at 400 nm. (C) The amplitude (estimated as the coefficient of variation) of light flicker of downward irradiance decreased with growing water depth (across the 1 to 10 m depth range), and increased monotonically toward longer light wavelengths. The ratio between the amplitude at the longest and shortest wavelengths was calculated for each depth. This ratio did not vary considerably across depths, and ranged between 2.5 and 3.0 (presented next to each spectrum). (D) The frequency distribution of the flicker at a depth of 1 m differed across the light spectrum. For clear graphical presentation, the power spectrum of light flicker, normalized to the dominant frequency (1.54 Hz), is presented for different wavelengths at 50 nm intervals. See Additional file 1A-D for the frequency distribution of flicker at 2, 4, 6, and 10 m depth. (E) The frequency distribution of light flicker at 500 nm differed across water depths, with the dominant frequency (1 m, 1.54 Hz; 2 m, 1.54 Hz; 4 m, 0.83 Hz; 6 m, 0.80 Hz; 10 m, 0.67 Hz) and the relative power at high frequencies decreasing with growing depth. (F) The wavelength dependence of light flicker became weaker with growing depth. Wavelength dependence was assessed as the reciprocal of the root mean square error (RMSE) and the normalized RMSE (NRMSE) between the power distribution at 500 and 550 nm.