Direct observations of diel biological CO<sub>2</sub> fixation on the Scotian Shelf, northwestern Atlantic Ocean

Abstract

Abstract. Much of the variability in the surface ocean's carbon cycle can be attributed to the availability of sunlight, triggering surface heat flux and photosynthesis, which in turn regulate the biogeochemical cycling of carbon over a wide range of time scales. The critical processes of this carbon cycle regulation, occurring at time scales of a day or less, however, have undergone few investigations, most of which have been limited to time spans of several days to months. Optical methods have helped to infer short-term biological variability, but complementing investigations of the oceanic CO 2 system are lacking. We employ high-frequency CO 2 and optical observations covering the full seasonal cycle on the Scotian Shelf, northwestern Atlantic Ocean, in order to unravel diel periodicity of the surface ocean carbon cycle and its effects on annual budgets. Significant diel periodicity in the surface CO 2 system occurs only if the water column is sufficiently stable as observed during seasonal warming. During that time biological CO 2 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 grazing. 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 CO 2 periodicity in the surface waters.

A high-resolution time-depth view of dimethylsulphide cycling in the surface sea

“…Coupling between autotrophs and heterotrophs is possibly driven by phytoplankton activity cycles56 and involves heterotrophic bacteria that use phytoplankton-released metabolites57, but also protist grazers5859 and even viruses58. These coupling/decoupling cycles are thought to exert diel rhythmic forcing on biogeochemical fluxes60; however, observations like ours, showing diel rhythms in open-ocean biogeochemical processes, are very scarce335761.…”

Sediment-water column fluxes of carbon, oxygen and nutrients in Bedford Basin, Nova Scotia, inferred from <sup>224</sup>Ra measurements

“…Assuming a DIC : O 2 ratio of 1 : 1, this term added 0.3 mmol DIC m −3 day −1 to each layer, or removed O 2 at a rate of −0.3 mmol O 2 m −3 day −1 . This chosen respiration rate is small compared to prior estimates on the nearby Scotian Shelf (Thomas et al, 2012), and lies in the low end of the range (0.02-75 mmol O 2 m −3 day −1 ) listed in a global compilation study of water-column respiration rates (Robinson and Williams, 2005) Fig. 6).…”

The effect of seasonality in phytoplankton community composition on CO 2 uptake on the Scotian Shelf

“…7(c) along with their respective error estimates. NCP values from Thomas et al (2012) are also shown for comparison. Error bars for this study represent the combination of uncertainties calculated using standard propagation of error techniques.…”

“…(b) Seasonal pattern of the rate of change of phytoplankton carbon, ΔC p . (e) ΔC p and net community production (NCP) with error estimates shown for this study, andThomas et al 2012.…”