Temporal offset in oceanic production and respiration processes implied by seasonal changes in atmospheric oxygen: The role of heterotrophic microbes

Recent data on seasonal variation in the concentration of atmospheric oxygen are independent evidence for a marked annual cycle in the production/respiration (P/R) ratio of the biotic community of the ocean in both the northern and the southern hemispheres. Based on the oxygen data, the P/R ratio tends to be >1 during a 3 to 5 mo period from late winter to early spring, and <1 during the rest of the year. The amount of oxygen which accumulates in the atmosphere during spring as a result of ocean biology implies a seasonal unrespired production of, on average, 50 to 60 g C m(-2). This amount of fixed carbon is approximately one third of annual oceanic primary production, and several-fold greater than measured sinking fluxes of particulate organic matter in the open sea. Size-fractioned respiration rates in seawater imply that <5 mu m sized microbes, in particular bacteria, play a major role in the establishment of the seasonal P/R cycle. Hypotheses to explain less microbial respiration compared to primary production in spring than in summer/fall in the open ocean might include: (1) temperature effect on respiration; (2) seasonal differences in bacterial growth efficiency; and (3) seasonal differences in quality of organic substrates. These processes may result in marked seasonal variation in abundance of metabolically active bacteria. Elucidating the mechanisms that contribute to the seasonal cycle in P/R ratios in the world ocean should be a goal of future research in microbial oceanography.