Abstract
Depth profiles of horizontal light-beam attenuation and in situ fluorescence recorded at 1 to 2 cm depth intervals in Lake Vechten contained pronounced maxima related to phytoplankton and bacterioplankton layers. This was confirmed by microscopy and HPLC analysis of pigments in discrete samples. Algae were sparse throughout the oxic water column compared with green and purple sulphur bacteria in the anoxic zone of the lake, but plankton layers comprising diatoms, green algae, euglenophytes and cryptomonads were observed. Maximum concentrations of phototrophic bacteria, with dominant representatives Synechococcus, Thiopedia, Chloronema and Chlorobium, occurred very close to one another and within the anoxic zone at depths of pigment maxima and major fluorescence peaks. Plankton layers affected downwelling irradiance in two ways: first, by markedly increasing overall light extinction in the vicinity of the layers, e.g. between 4 m and 8 m the minimum wavelength-specific extinction coefficient, which was at 580 nm (k580) increased from 0.5 m−1 to 1.5 m−1; secondly, changes in the shape of extinction (k) spectra corresponded to in vivo absorbance characteristics of photosynthetic pigments of plankton in the layers. It is inferred that field microbial ecological studies may greatly benefit from simultaneous use of in situ optical techniques.
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