Direct observations of diel biological CO2 fixation on the Scotian Shelf, northwestern Atlantic Ocean

[摘要] Much of the variability in the surface ocean's carbon cycle can beattributed to the availability of sunlight, triggering surface heat flux andphotosynthesis, which in turn regulate the biogeochemical cycling of carbonover a wide range of time scales. The critical processes of this carboncycle regulation, occurring at time scales of a day or less, however, haveundergone few investigations, most of which have been limited to time spansof several days to months. Optical methods have helped to infer short-termbiological variability, but complementing investigations of the oceanicCO₂ system are lacking. We employ high-frequencyCO₂ and optical observations covering the full seasonal cycleon the Scotian Shelf, northwestern Atlantic Ocean, in order to unravel dielperiodicity of the surface ocean carbon cycle and its effects on annualbudgets. Significant diel periodicity in the surface CO₂system occurs only if the water column is sufficiently stable as observedduring seasonal warming. During that time biological CO₂drawdown, or net community production (NCP), is delayed for several hoursrelative to the onset of photosynthetically available radiation (PAR), dueto diel cycles in chlorophyll a concentration andto grazing. In summer, NCP decreases by more than 90%, coinciding withthe seasonal minimum of the mixed layer depth and resulting in thedisappearance of the diel CO₂ periodicity in the surfacewaters.

[学科分类] 地球化学与岩石