Differences in seawater particulate organic carbon concentration in samples collected using small- and large-volume methods: the importance of DOC adsorption to the filter blank

Particulate organic carbon (POC) data collected by small-volume ( 1–2 l) bottle filtration and large-volume ( 100–600 l) methods are compared for samples from the central Arctic, Equatorial Pacific, Equatorial and South Atlantic, Gulf of Maine, and Narragansett Bay. Small-volume samples were collected using Niskin® and Go-Flo® bottles and large-volume samples were collected using in situ pumps and large-volume bottle filtration. Results indicate that small-volume bottle POC data are often greater than large-volume results, by as much as 2–4 times, in regions with low POC concentration (< 5 μM). The implication is that POC concentrations determined by small-volume bottle filtration in regions characterized by low POC concentrations, i.e., in the majority of surface open ocean and deep waters, may be erroneously high. We suggest the most likely explanation is adsorption of DOC to the filter, which is rarely quantified yet can significantly increase the filter blank. The magnitude of DOC adsorption was assessed using coastal seawater by determining the y-intercept of a plot of the organic carbon retained by a glass-fiber filter against the volume filtered. The intercept was approximately two-fold greater than the precombusted filter blank, which we attribute to DOC adsorbed to the filter. Thus, when seawater POC concentrations are similar to, or less than, the precombusted filter blank, not correcting for the true in situ blank can result in erroneously high POC concentrations. To avoid this artifact, we recommend using large-volume sampling methods, which result in a greater quantity of POC per unit area of the filter relative to the filter blank. When large-volume filtration is not possible, we recommend a simple method to evaluate the true in situ filter blank.