Relations of phytoplankton in situ primary production, chlorophyll concentration and underwater irradiance in turbid lakes

In order to study the relationships among primary productivity, chlorophyll content and light conditions (underwater quantum irradiance, diffuse attenuation coefficient) in turbid lakes, we performed a series of measurements in three Estonian lakes (Peipsi, Vortsjarv and Harku) during 2003-2005. We focused on (1) comparison of in situ vertical profiles of planar and scalar quantum irradiances in the water, (2) extremely high values of chlorophyll content and primary production in some turbid lakes and (3) analysis of the formation of the vertical profiles of primary production (shape and the depth of its maximum value) as a function of incident irradiance. The following parameters were measured: (a) primary production at different depths, (b) incident planar quantum irradiance in the region of 400-700 nm, (c) underwater downwelling planar and scalar quantum irradiances at different depths in the same waveband, (d) relative transparency of water (Secchi depth), (e) concentrations of chlorophyll a and (f) spectra of beam attenuation coefficient for unfiltered and filtered water in the wavelength range of 350-700 nm (from water samples). During 36 measurement days (14 in Peipsi, 14 in Vortsjarv and 8 in Harku), and we collected reliable dataset for 53 measurement series. Our data showed the relative difference between underwater planar and scalar quantum irradiance in a range of about 25-65% implying big differences in primary production models if instead of the preferable scalar quantum irradiance planar irradiance data are used. The results showed a large variation of vertical profiles of primary production in relation to illumination conditions. The depth of maximum primary production depended on water turbidity and incident irradiance, and was located at the water surface in case of low illumination. The relationship between vertically integrated primary production and chlorophyll concentration could be described by a power function with an exponent of 0.64 (R-2 = 0.81).