Effects of irradiance on nitrogen uptake by phytoplankton: comparison of frontal and stratified communities

Rates of NOT and urea uptake by phytoplankton in the shallow and deep chlorophyll layers of the Strait of Georgia, British Columbia (Canada) were measured, over a gradient of photosynthetic photon flux densities (PPFD), using the 15N tracer technique.The results of these experiments could be fitted with a hyperbolic function similar to the Michaelis-Menten equation and included a term for dark uptake.Half-saturation constants (KLT) for light-dependent uptake of urea and NOT ranged from 0 to 14 % of the surface PPFD, and dark uptake of both urea and NOT ranged from 0 to 58 % of the uptake at saturating PPFD.Although the importance of dark uptake increased with increased N limitation, the dramatic difference in phytoplankton community composition between the N-replete frontal waters and the N-depleted stratified waters precludes a simple explanation of PPFD effect@) on N uptake based solely on phytoplankton N status.The results of this study are compared to those reported for other aquatic systems.