LIGHT PENETRATION IN RELATION TO PHYTOPLANKTON CONTENT OF EUPHOTIC ZONE OF LOUGH-NEAGH, N IRELAND

A strong spectral variation in underwater light penetration is found in Lough Neagh, particularly in the wavelengths available for photosynthesis (400-700 nm). Using a spectroradiometer and colored filters to analyze the underwater light climate, the depth of the euphotic zone varied between 1-3 m and was largely attributable to changes in algal density between 26-92 mg chlorophyll [chl] a m-3 in the open lake and between 48-300 mg chl a m-3 in a shallow almost enclosed area, Kinnego Bay. Although self-shading of the algal cells is important at high crops the contribution from non-algal sources becomes increasingly significant at low crop concentrations. The factors controlling the algal content of the euphotic zone for the open lake (70-174 mg chl a m-2) and Kinnego Bay (114-380 mg chl a m-2) are discussed and compared with other well-mixed eutrophic lakes.