The underwater light-field of lakes with marked physico-chemical and biotic diversity in the water column

Errors in calculation of radiant fluxes of downwelling underwater photosynthetically available radiation (PAR) from force-fitting statistical values of $K¯$ d to vertical broad-band PAR profiles are compounded by vertical heterogeneity in such factors as turbidity, gilvin (= gelbstoff) and chlorophyll. Examples are given from two Tasmanian meromictic lakes where vertical zonation of gilvin and microorganisms in the mixolimnia and across the chemoclines produces a markedly heterogeneous water column. Localised concentrations of gilvin produce kinked profiles of downwelling PAR (400–700 nm) and microbial plates in the vicinity of the chemoclines act as false, reflective bottoms, abruptly extinguishing residual PAR by absorption and scattering.