CHARACTERIZATION OF THE LIGHT-FIELD IN LABORATORY SCALE ENCLOSURES OF EUTROPHIC LAKE WATER (LAKE LOOSDRECHT, THE NETHERLANDS)

Light conditions in laboratory scale enclosures (LSE) of shallow, eutrophic Lake Loosdrecht (The Netherlands), including a method for simulating a 'natural' incident fight course, are described. Total PAR (400-700 nm) and spectral irradiance distribution were measured at sestonic chlorophyll a and dry weight concentrations greater-than-or-equal-to 100 mg m-3 and 16 g m-3, respectively. Phytoplankton was dominated by Oscillatoria spp. The euphotic depth (Z(eu)) was 0.7-1.0 m. Shortly after filling the LSE with lake water, diffuse attenuation coefficients ranged from 14 m-1 for blue to 5 m-1 for red light. Around Z(eu), attenuation in the blue region was markedly lower and irradiance reflectance (R) continued to increase; these anomalies were caused by lateral incident light from the LSE's waterbath. Spectrophotometry indicated that absorption was mainly by particles, but dissolved humic substances (gilvin) were also important. The particles were likely to be dominated by detritus absorbing more blue relative to red light. Subsurface R in lake water in the LSE had a maximum around 705 nm and low values in the blue band, but was lower than that previously reported for measurements in situ. Wash-out of detritus, presumably both dissolved and particulate fractions, by flow-through with synthetic medium, greatly affected the spectral reflectance measured outside the LSE. The maximum value of R decreased from 0.022 to 0.009, and the peak shifted to 550 nm.