PRESERVATION OF THE LIGHT-FIELD IN TURBID LAKE AND RIVER WATER IN LABORATORY-SCALE ENCLOSURE

The laboratory-scale enclosure (LSE), as a physical model of the water column, has been introduced for studies of seston dynamics in shallow , mixed lakes. Preserving the 'natural' underwater light climate was a mong the design criteria. However, lateral exchange of light between t he LSE cylinder and its water bath caused deviations from conditions i n the water column in situ. Two modifications of the original design a re introduced by providing the LSE cylinder with (i) a masking plate a nd (ii) a reflective coating. The light fields in the original and mod ified designs are compared for Oscillatoria limnetica culture and West ern Scheldt Estuary water. These test suspensions had dry weight conce ntrations of approximately 50 mg l-1 and differed widely in scattering to absorption ratios, i.e. 4 and 12 for the culture and natural water , respectively. Using the reflective coating, the LSE provided an unde rwater light field conforming to general theory for diffuse attenuatio n and reflectance. Using the masking plate may suffice for cultures of cyanobacteria, but the reflective coating is needed for studies of ph ytoplankton growth in relation to the optical conditions in turbid lak e and river water.