Determining true particulate organic carbon: bottles, pumps and methodologies

Abstract

The primary means of determining particulate organic carbon (POC) concentrations in aquatic environments is by filtering from water bottles or by in situ filtration with pumps and analyzing the filters. The concentrations measured by these two methods, however, can differ by a factor of 1.2–5 in temperate waters, and by factors as large as 200 in cold, high-latitude waters. Here we report that the ratio of bottle POC to pump POC ranged between 20 and 200 in the Ross Sea during early spring and between 5 and 50 during summer. In the Antarctic Polar Front the ratio ranged between 2 and 25 in spring. A new approach to constraining POC concentrations is to use high-temperature combustion (HTC) of water samples (rather than filters), POC being the difference between measurements of total and dissolved organic carbon. POC concentrations determined by bottle filtration are in reasonable agreement with the POC concentrations obtained by HTC, independent biomass measurements, and beam-attenuation/bottle POC ratios that are similar to previous studies, thus lending credibility to the bottle POC values. Data from several studies suggest that the most likely reasons for differences between bottle and pump POC are the use of slightly larger pore-size filters with in situ pumps and higher pressure differentials across the filter during in situ pump filtration, resulting in particulate carbon being pulled through the filters. The one-to-two orders of magnitude differences in high-latitude, cold-water environments needs further investigation. If accurate measurements of POC can be obtained for calibration, beam attenuation profiles using transmissometers offer a way to quantify rapidly the distribution of POC. The JGOFS protocols for POC filtration must be modified to include a blank that accounts for DOC adsorption onto filters. The largest impact of DOC adsorption will be when POC concentrations are below B2mmol/l, which includes most sub-euphotic zone waters. These findings have important ramifications for any programs involving the particulate portion of the carbon cycle.