Direct observations of diel biological CO 2 fixation in the oceans.

摘要: Much of the variability in the surface ocean’s carbon cycle can be attributed to the availability of sunlight, through processes such as heat fluxes and photosynthesis, which regulate over a wide range of time scales. The critical processes occurring on timescales of a day or less, however, have undergone few investigations, and most 5 of these have been limited to a time span of several days to months, or exceptionally, for longer periods. Optical methods have helped to infer short-term biological variability, however corresponding investigations of the oceanic CO2 system are lacking. We employ high-frequency CO2 and optical observations covering the full seasonal cycle on the Scotian Shelf, Northwestern Atlantic Ocean, in order to unravel diel periodic-10 ity of the surface ocean carbon cycle and its effects on annual budgets. Significant diel periodicity occurs only if the water column is sufficiently stable as observed during seasonal warming. During that time biological CO2 drawdown, or net community production (NCP), is delayed for several hours relative to the onset of photosynthetically available radiation (PAR), due to diel cycles in chlorophyll-a concentration and to 15 grazing, both of which, we suggest, inhibit NCP in the early morning hours. In summer, NCP decreases by more than 90%, coinciding with the seasonal minimum of the mixed layer depth and resulting in the disappearance of the diel CO2 periodicity in the surface waters.