FEATURES OF THE UNDERWATER LIGHT CLIMATE JUST BELOW THE SURFACE IN SOME NEW-ZEALAND INLAND WATERS

1. Using sampling rates of 8-64 Hz we found clear indications of exten sive and high frequency fluctuations of underwater photosynthetically active radiation (PAR) just below the surface (0.016-1.1 m) in some Ne w Zealand water bodies. High variability and flashing occurred down to at least 3 m depth. 2. PAR variability increased under the influence of bright sunshine if wind roughening of the surface took place. Conco mitantly, the average PAR levels declined by about 10%. However, even when the surface was shaded, high variability of PAR persisted. 3. Und er a calm surface, PAR irradiance followed a log normal distribution. This occurred independently of the presence of direct sunlight. Howeve r, when the surface was roughened by wind in sunshine, PAR immediately switched to a Gumbel (extreme value type EV1) distribution. 4. Neithe r wave action nor wave focusing of incident irradiance would explain t he wide range of PAR close to the water surface, although both factors add to the PAR variability. 5. The data indicate that transmittance t hrough the surface is highly variable at the temporal and spatial scal es studied, and that the irregularity of the air-water interface is in strumental in bringing about the observed fluctuations of PAR just bel ow the surface.