Importance of heterotrophic bacterial assimilation of ammonium and nitrate in the Barents Sea during summer

In a transect across the Barents Sea into the marginal ice zone (MIZ), five 24-h experimental stations were visited, and uptake rates of NH4+ and NO3- by bacteria were measured along with their contribution to total dissolved inorganic nitrogen (DIN) assimilation. The percent bacterial DIN uptake of total DIN uptake increased substantially from 10% in open Atlantic waters to 40% in the MIZ. The percentage of DIN that accounted for total bacterial nitrogen production also increased from south to north across the transect. On average, at each of the five 24-h stations, bacteria accounted for 16-40% of the total NO3- uptake and 12-40% of the total NH4+ uptake. As a function of depth, bacteria accounted for 17%, 23%, and 26% of the total NH4+ assimilation and 17%, 37%, and 36% of the total NO3- assimilation at 5, 30, and 80 m, respectively. Bacteria accounted for a higher percentage of total NO3- uptake compared to total NH4+ uptake in 12 out of 15 samples. Bacterial productivity explains a substantial amount of the variability associated with bacterial DIN uptake, but the relationship between bacterial production and bacterial DIN uptake is best explained when the data from the open Atlantic water stations are grouped separately from the MIZ stations. The percentage of DIN that accounts for bacterial N production is approximately four-fold higher in 24 h MIZ stations compared with open Atlantic stations. This suggests that bacteria play a larger role in NO3- utilization, particularly in the MIZ, than previously hypothesized and that bacterial uptake of NO3- should not be ignored in estimates of new production. Understanding processes that affect autotrophic based new production, such as heterotrophic bacterial utilization of NO3-, in polar oceans is of particular significance because of the role these regions may play in sequestering CO2. (C) 2002 Elsevier Science B.V. All rights reserved.