Distribution of nitrogen fixation in the coastal Alaskan Arctic

Our understanding of high latitude nitrogen fixation lags that of temperate and tropical systems because temperature, light, and nutrient concentrations in polar regions were thought to be unfavorable for marine diazotrophs. Relatively recent studies have found, however, that the once broadly accepted “rules” for where and when nitrogen fixation occurs may not apply for all marine diazotrophs. Despite generally lower nitrogen fixation rates in cold-water than in tropical waters, active nitrogen fixation by unicellular cyanobacteria (UCYN-A) in Arctic waters has been confirmed. We investigated the extent of nitrogen fixation in late summer along the northern coast of Alaska in two consecutive summers. Size fractionated (> 3µm and < 3µm) nitrogen fixation rates were determined at several depths throughout the water column from surface to deep water. Data were also collected on ambient physical and biogeochemical parameters to determine what conditions promoted or suppressed nitrogen fixation. We found that while still generally low, nitrogen fixation rates were highest in the northern Bering Sea and southern Chukchi Sea and were mediated by the presence of light. Identifying where, when, and under what conditions nitrogen fixation occurs in cold ocean systems is critical to redefining the parameters under which marine nitrogen fixation may occur and if it is increasing in the region.