Photosynthetic Behaviour in Stratified and Unstratified Lake Populations of a Planktonic Diatom

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Abstract
Photosynthetic behavior is described for four populations of Asterionella formosa, developed as spring maxima during two successive years in two English lakes. Rates of gross phytosynthesis were determined from exposures in lake and laboratory. Their variation is described in relation to environmental factors and population density, with special attention to effects from vertical stratification. Maximum photosynthetic rates, and the light intensities required for saturation, did not differ greatly between the lakes or from corresponding features measured earlier with cultured material. The photosynthetic capacity was often lower, but still considerable, during long periods when further cell multiplication was blocked by Si depletion. The influence of temperature (5-17[degree] C) on photosynthetic rate was negligible at low intensities, but at saturating intensities an average Q10 of 2.1 was found. Vertical differentiation of population density and several physiological characteristics developed rapidly after thermal stratification in the lakes. The physiological differentiation was chiefly controlled by light, and is interpreted in terms of "sun" and "shade" behavior plus effects of illumination on cells in a silica-deficient medium. Deeper cells showed an increased content of chlorophyll a, a decreased carotenoid/chlorophyll a ratio, and contracted chromatophores; their photosynthesis usually reached light-saturation at slightly lower intensities and was more susceptible to depression at high intensities. Dark pretreatments of surface samples could often reduce their carotenoid content and sometimes increase their photosynthetic capacity. Reductions in capacity at light saturation, and in population density, often contributed to the decline of photosynthetic capacity or pigments was not certainly found. The depth-distribution of photosynthetic activity in stratified populations could be assessed from the distribution of photosynthetic capacity per unit water volume, which was strongly correlated with population density, and depth-profiles of relative rates of photosynthesis obtained from homogeneous material. Other effects from physiological differentiation with depth were of secondary importance for photosynthetic productivity.