Seasonal Dynamics of Organic Carbon in the Deep Water Column of the Eastern North Pacific

Organic carbon, a product of primary production, is a vital factor for the sequestration of atmospheric carbon dioxide. Here, we consider the variability of organic carbon (OC) on various spatial and temporal scales in the northeastern Pacific. Typically, total (TOC) and dissolved organic carbon (DOC) samples are collected by our laboratory during basin-crossing U.S. GO-SHIP cruises that commonly occur in summer once a decade, thus limiting our understanding of temporal variability in organic carbon concentrations, especially in productive waters. To fill this deficit in knowledge, samples for TOC and DOC were collected on transects near Ocean Station Papa (50°N, 145°W), including the Canadian DFO Line P and NASA’s EXPORTS programs. Heterogeneity vertically, horizontally, and seasonally was observed in OC distributions in the meso- and bathypelagic water column along Line P. Relatively high concentrations of organic carbon were measured at depths down to 3500 meters, particularly in spring-time, the input of which supports the deep microbial community and aids in carbon sequestration. Sporadic deep "hot spots" of carbon were also observed, suggesting high carbon production and export at times. Smaller scale variability in the upper 500 meters, on the order of days/weeks and ~15 kilometers, was observed during the EXPORTS cruise, suggesting that changes in OC can occur rapidly across small areas. These observations shed new light on the degree of variability in organic carbon in waters underlying productive systems; we see short-lived additions of OC to the deep water column with sinking particles, as well as differences in distributions within short distances.