Photon dependence of inorganic nitrogen transport by phytoplankton in perennially ice-covered Antarctic lakes

15N was used to examine the influence of Photosynthetic Photon Flux Density (PPFD) on NO3 − and NH4 + transport by phytoplankton in the shallow and deep-chlorophyll layers of Lakes Fryxell and Vanda, Antarctica. The response observed in Lake Vanda could be modelled with a four parameter equation previously used to model photosynthesis. Only the 9 m NH4 + transport experiments in Lake Fryxell could be modelled with this equation. Other experiments in Lake Fryxell showed either no response to PPFD or a linear increase with no saturation at the PPFD levels used. Distinct trends were observed in transport parameters both between depths and between nitrogen species. Overall, the parameters indicate that phytoplankton in these lakes possess nitrogen transport affinities similar to those reported for other aquatic systems.