Light-dependency of nitrate uptake by phytoplankton over the spring bloom in Auke Bay,Alaska |
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Authors: | J. Kanda D. A. Ziemann L. D. Conquest P. K. Bienfang |
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Affiliation: | (1) The Oceanic Institute, Makapuu Point, P.O. Box 25280, 96825 Honolulu, Hawaii, USA;(2) Present address: Ocean Research Institute, University of Tokyo, 1-15-1 Minamidai, Nakano-ku, 164 Tokyo, Japan |
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Abstract: | The effect of light intensity on nitrate uptake by natural populations of phytoplankton was examined by 15N traceruptake experiments during the spring (March–May 1987) in Auke Bay, Alaska. The data were fit to a rectangular hyperbolic model which included a term for dark uptake. Three types of curves described nitrate uptake as a function of light intensity. The first (Type I) had a low half-saturation light intensity (KI), low chlorophyll-specific uptakes rates, no dark uptake and occasional photoinhibition. These were observed during a period of biomass decrease, accompanied by low daily light and strong wind, prior to the major bloom. The second type (Type II) had relatively high KI, high chlorophyll-specific uptake rates, and no dark uptake. Type II curves were observed during most of the period prior to nitrate depletion in the surface waters. Types I and II both appeared prior to nitrate depletion in the water and reflected variations in the light history of the phytoplankton population. The third type (Type III) occurred in nitrate-deplete conditions, when nitrate uptake was less dependent on light intensity (i.e., high rates of dark uptake and lower KI). Decreased light-dependency during this period was coupled with physiological nitrogen deficiency in the population. Comparing these parameters to those of photosynthetic carbon fixation, KIvalues of nitrate uptake were generally higher than those of photosynthesis prior to nitrate depletion, and lower during nutrient-deplete conditions. |
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