Biomass and nitrogen uptake by heterotrophic bacteria during the spring phytoplankton bloom in the North Atlantic Ocean

Abstract

N biomass and N uptake by heterotrophic bacteria and phytoplankton were examined during the North Atlantic spring bloom (May 1989). Phytoplankton N had reached its zenith by the time the authors arrived at the station and increased only slightly during the first week of the study, while particulate nitrogen (PN) nearly doubled (increase of 450 mg N m −2), apparently due to the growth of heterotrophic organisms other than bacteria. During the second week, bacterial N doubled, increasing by 280 mg N m −2. These increases in biomass N and total PN were large compared to NO 3 − uptake and to N export from the upper 100 m. To examine the role of bacteria in greater detail, we estimated total N uptake and the uptake of free amino acids (DFAA), NH 4 +, and NO 3 −. The ratio of total N uptake by bacteria to total N uptake by phytoplankton varied greatly and was often high (0.2–0.9). When uptake was corrected for phytoplankton activity, heterotrophic bacteria accounted for 22–39% and 4–14% of NH 4 + and NO 3 − uptake. DFAA, NH 4 + and NO 3 − supplied 12–34%, 19–29% and 2–8% respectively, of the nitrogen needed for bacterial production. Preference indices further indicated that NO 3 − was the N source least preferred by heterotrophic bacteria. Nitrogen uptake fuelled a buildup of bacterial biomass and other suspended material that needs to be considered in reconciling new production and N export from the upper layers of the ocean during the spring bloom.