NEW PRODUCTION IN THE SUB-ARCTIC PACIFIC-OCEAN - NET CHANGES IN NITRATE CONCENTRATIONS, RATES OF NITRATE ASSIMILATION AND ACCUMULATION OF PARTICULATE NITROGEN

Changes in water column nitrate and particulate nitrogen (PN) concentr ations and rates of nitrate assimilation at 50-degrees-N 145-degrees-W were measured over a four-month interval for 1984,1987 and 1988. Rate s of nitrate depletion in the upper 80m of the water column averaged 1 2mg N m-2d-1, but most of the net depletion occurred during May when r ates were high (approximately 75 mg N m-2d-1) compared to later in the year. Particulate nitrogen (collected on GF/F filters) increased 2- t o 3-fold during the month of May and accounted for 30-60% of the net n itrate depletion for May. Mean rates of PN accumulation for the 4-mont h intervals were 2.4mg N m-2d-1 and accounted for about 20% of the net nitrate depletion. Rates of nitrate assimilation (measured in incubat ion bottles with N-15) averaged 45.0+/-4.5mg N m-2d-1 (mean+/-SD), and appeared to decrease between May and September. A good correspondence between in situ and incubation estimates of nitrate assimilation was found for the 4-month comparison, but not for the month of May when ne t changes in nitrate concentrations were greatest. Vertical and horizo ntal inputs of nitrate are about the same order of magnitude as biolog ical removal, thus the high input of nitrate into the euphotic zone co ntributed to the continuously high nitrate concentrations in this regi on. Seasonal changes in nitrate and PN were significant and need to be considered in comparisons of new and export production.