Chlorophyll content of Plešné Lake phytoplankton cells studied with image analysis

Using image analysis, chlorophyll autofluorescence was measured in single cells of green alga Monoraphidium dybowskii and in filaments of cyanobacteria (Pseudanabaena sp. and Limnothrix sp.) in the vertical profile of small acidified mountain lake Plešné jezero (Plešné Lake) from May to November of 2003. Cell chlorophyll autofluorescence was converted to cell chlorophyll content using a conversion factor determined by comparing the total autofluorescence of phytoplankton in a microscope field with spectrophotometrically determined total chlorophyll concentration; the conversion factor did not differ between epilimnion (0.5 m depth) and hypolimnion (9 m depth). Vertical patterns of chlorophyll concentration and of cellular chlorophyll content depended on water column mixing: during the period of stable thermal stratification, a metalimnetic peak in total chlorophyll concentration was present and cellular chlorophyll contents in the metalimnion and hypolimnion were notably elevated compared to the surface. Monotonous vertical profiles of both total chlorophyll concentration and cell chlorophyll content were typical for the period of water column overturn. During the stratification period, hypolimnetic Monoraphidium cell chlorophyll content was on average twice as high (maximum difference 2.7-fold) compared to surface values (of 3.2–12.9 fg µm−3), while in filamentous cyanobacteria (surface cell chlorophyll content of 2.2–13.3 fg µm−3), the difference was much higher — six-fold on average, with an 11.6-fold maximum value. The values measured with image analysis in 2003 were compared to unpublished values of total phytoplankton biomass-specific chlorophyll concentrations obtained using manual phytoplankton biomass determination and spectrophotometric chlorophyll measurement in 1998 at the same locality. Good agreement was found in seasonal patterns and vertical profiles of chlorophyll between both seasons.