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

Abstract

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 μm 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 m the open ocean might include: (1) temperature effect on respiration; (2) seasonal differences in bacterial growth efficiency; and (3) seasonal differences m 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.