AMMONIUM, NITRATE, PHOSPHATE, AND INORGANIC CARBON UPTAKE IN AN OLIGOTROPHIC LAKE: SEASONAL VARIATIONS AMONG LIGHT RESPONSE VARIABLES1

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  • 1

    Received 20 January 1989. Accepted 12 July 1989.

  • We would like to thank D. Gudder for technical assistance and D. R. S. Lean and two anonymous reviewers for their comments on the manuscript. R. Hauer and R. Steinkraus at the Flathead Lake Biological Station provided light data. This research was supported by the Soap and Detergent Association.

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ABSTRACT

The dependence of substrate saturated uptake of 15NH4+, 15NO3, 32PO43−, and 14CO2 on photosynthetic photon flux density (PPFD or photsynthetically active radiation, 400–700 nm) was characterized seasonally in oligotrophic Flathead Lake, Montana. PO43− uptake was not dependent upon PPFD at any time of the year, whereas NH4+, NO3, and CO2 uptake were consistently dependent on PPFD over all seasons. Maximal rates of NH4+, NO3 and CO2 uptake usually occurred near 40% of surface PPFD, which corresponded to about 5 m in the lake; inhibition was evident at PPFD levels greater than 40%. NH4+, NO3 and PO43− were incorporated in the dark at measurable rates most of the year, whereas dark CO2 uptake was always near 0 relative to light uptake. CO2 and NO3 uptake were more strongly influenced by PPFD than was NH43− uptake. The PPFD dependence of PO43−, NH4+, NO3 and CO2 uptake may affect algal growth and nutrient status by influencing the balance in diel and seasonal C:N:P uptake ratios.

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