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Recent observations in polar marine waters have shown that a large fraction of primary production may be lost to respiration by planktonic bacteria due to very low bacterial growth efficiencies in cold waters. Here we report that sea temperature may be a key factor (but not the only one) influencing the interaction between bacteria and primary production in North Atlantic and Arctic waters, suggesting that low primary production rates could not sustain bacterial carbon demand in the coldest Arctic waters. The use of freshly produced phytoplankton exudate by bacteria in early- and mid-summer was assessed, together with the bacterial uptake of dissolved inorganic nitrogen (DIN = nitrate and ammonium), in surface waters along a latitudinal gradient from the North Sea to the Arctic sea ice. Bacterial production was independent of the low primary production measured in the coldest waters. Under these conditions, heterotrophic bacteria can consume a large fraction of DIN and N-rich organic matter, making them strong contributors to N fluxes in these waters.

During the ANTARES 3 cruise in the Indian sector of the Southern Ocean in October-November 1995, the surface waters of Kerguelen Islands plume, and the surface and deeper waters (30-60 m) along a transect on 62 degrees E from 48 degrees 36'8 to the ice edge (58 degrees 50'S), were sampled. The phytoplankton community was size-fractionated (2 mu m) and cell numbers, chlorophyll biomass and carbon assimilation, through Rubisco and beta-carboxylase activities, were characterized. The highest contribution of 2 mu m. The mixotrophic capacity of these small cells is proposed. From sampling stations of the Kerguelen plume, a relationship was observed between the Rubisco activity per picophytoplankton cell and apparent cell size, which varied with the sampled water masses. Moreover, a depth-dependent photoperiodicity of Rubisco activity per cell for