JOURNAL OF PLANKTON RESEARCH | VOLUME 6 | NUMBER 5 | PAGES 733-749 | 1984
© Oxford University Press
research-article |
A comparison of nitrogen and carbon metabolism in the shallow and deep-water phytoplankton populations of a subalpine lake: response to photosynthetic photon flux density
Department of Biology, Montana State University Bozeman, MT 59717, USA
Received on May 1, 1983; accepted on June 1, 1984
Rates of 
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and CO2 assimilation by the organisms in the shallow and deepchlorophyll layers of Castle Lake were measured over a gradient of photosynthetic photon flux densities (PPFD) during the 1979–1980 ice-free seasons. The results of these experiments could be fitted with a hyperbolic function in the manner of the Michaelis-Menten equation (excluding rates of dark assimilation) up to –40% of the surface PPFD after which photon inhibition occurred. The half saturation constants relative to incidence PPFD (KLT) for assimilation (
= 1.1 E m–2d–2) were about twice those for ( = 0.5 E m–2d–1). All of the KLT values correspond to depths in thelakeranging from 17–29 m(–1% of surface PPFD). Dark assimilation of both and was –50% of the assimilation at saturating PPFD implying that part of the immediate energy required for inorganic nitrogen assimilation may come from intermediary metabolism. This contention was supported for assimilation by the results of experiments performed with specific inhibitors of non-cyclic photophosphorylation and oxidative phosphorylation. The KLT values for the assimilation of CO2 were from 2–10 times higher than those for inorganic nitrogen. These values for CO2 assimilation were not significantly altered by the addition of either or during 12 h incubations.