Nitrogen Metabolism of the Shallow and Deep-Water Phytoplankton in a Subalpine Lake

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Abstract
Experiments utilizing 15N were performed during the 1979 and 1980 ice-free seasons in the shallow and deep-chlorophyll layers of Castle Lake (California) to determine the effects of dissolved inorganic nitrogen (DIN) concentration on the rates of DIN assimilation. The half-saturation constant (Kt) for assimilation of NO3- was about 12 .mu.g N l-1 in the epilimnion (3 m) and mid-hypolimnion (20 m), and increased to about 50 .mu.g N l-1 in the aphotic-lower hypolimnion (25 m). A similar pattern was evident for NH4+ in 1979 (Kt = 2.7, 2.6, and 9.3 .mu.g l-1 at 3, 20 and 25 m, respectively) but not 1980 (Kt = 7.0, 14.0, and 6.0 .mu.g N l-1 at 3, 20 and 25 m, respectively). The trend in Kt values paralleled the availability of DIN to the phytoplankton at the various depths. Relatively low NH4+ enrichments (.simeq. 5 .mu.g NH4+-N l-1) strongly inhibited assimilation of NO3- at 3 m. Assimilation of NO3- was less sensitive to NH4+ at 20 m (.simeq. 40 .mu.g NH4+ N l-3 was required to inhibit NO3- assimilation) and was not affected by NH4+ concentration up to about 75 .mu.g N l-1 at 25 m. This strategy appears advantageous to the phytoplankton because NH4+ is the primary source of nitrogenous nutrition at 3 m while NO3- may be a more important nitrogen source below 20 m. The seasonal persistence of the deep-water phytoplankton maximum in Castle Lake appears to be dependent to a large extent on the adaptations of these organisms to the ambient DIN supply.