Estimates Of Particulate Organic Carbon Research Articles

Determination of particulate organic carbon (POC) in seawater: The relative methodological importance of artificial gains and losses in two glass-fiber-filter-based techniques

Particulate matter in aquatic systems is an important vehicle for the transport of particulate organic carbon (POC). Its accurate measurement is of central importance for the understanding of marine carbon cycling. Previous work has shown that GF/F-filter-based bottle-sample-derived concentration estimates of POC are generally close to or higher than large-volume in-situ-pump-derived values (and in some rare cases in subzero waters are up to two orders of magnitude higher). To further investigate this phenomenon, water samples from the surface and mid-water Northeast Atlantic and the Baltic Sea were analyzed. Our data support a bias of POC concentration estimates caused by adsorption of nitrogen-rich dissolved organic material onto GF/F filters. For surface-ocean samples the mass per unit area of exposed filter and composition of adsorbed material depended on the filtered volume. Amounts of adsorbed OC were enhanced in the surface ocean (typically 0.5 μmol cm − 2 of exposed filter) as compared to the deep ocean (typically 0.2 μmol cm − 2 of exposed filter). These dependencies should be taken into account for future POC methodologies. Bottle/pump differences of samples that were not corrected for adsorption were higher in the deep ocean than in the surface ocean. This discrepancy increased in summer. It is shown that POC concentration estimates that were not corrected for adsorption depend not only on the filtered volume, true POC concentration and mass of adsorbed OC, but also on the filter area. However, in all cases we studied, correction for adsorption was important, but not sufficient, to explain bottle/pump differences. Artificial formation of filterable particles and/or processes leading to filterable material being lost from and/or missed by sample-processing procedures must be considered. It can be deduced that the maximum amounts of POC and particulate organic nitrogen (PON) that can be artificially formed per liter of filtered ocean water are ∼ 3–4 μM OC (5–10% of dissolved OC) and ∼ 0.2–0.5 μM ON (2–10% of dissolved ON), respectively. The relative sensitivities of bottle and pump procedures, and of surface- and deep-ocean material, to artificial particle formation and the missing/losing of material are evaluated. As present procedures do not exist to correct for all possible biasing effects due to artificial particle formation and/or miss/loss of filterable material, uncertainties of filtration-based estimates of POC concentrations need further testing. The challenge now is to further constrain the magnitude of the biasing effects that add to the adsorption effect to reduce the uncertainties of estimates of POC concentrations, inventories and fluxes in the ocean.

Variability of particulate organic carbon concentration in the north polar Atlantic based on ocean color observations with Sea‐viewing Wide Field‐of‐view Sensor (SeaWiFS)

We use satellite data from Sea‐viewing Wide Field‐of‐view Sensor (SeaWiFS) to investigate distributions of particulate organic carbon (POC) concentration in surface waters of the north polar Atlantic Ocean during the spring–summer season (April through August) over a 6‐year period from 1998 through 2003. By use of field data collected at sea, we developed regional relationships for the purpose of estimating POC from remote‐sensing observations of ocean color. Analysis of several approaches used in the POC algorithm development and match‐up analysis of coincident in situ–derived and satellite‐derived estimates of POC resulted in selection of an algorithm that is based on the blue‐to‐green ratio of remote‐sensing reflectance Rrs (or normalized water‐leaving radiance Lwn). The application of the selected algorithm to a 6‐year record of SeaWiFS monthly composite data of Lwn revealed patterns of seasonal and interannual variability of POC in the study region. For example, the results show a clear increase of POC throughout the season. The lowest values, generally less than 200 mg m−3, and at some locations often less than 50 mg m−3, were observed in April. In May and June, POC can exceed 300 or even 400 mg m−3 in some parts of the study region. Patterns of interannual variability are intricate, as they depend on the geographic location within the study region and particular time of year (month) considered. By comparing the results averaged over the entire study region and the entire season (April through August) for each year separately, we found that the lowest POC occurred in 2001 and the highest POC occurred in 2002 and 1999.

Lipids as biomarkers for carbon cycling on the Northwest Shelf of Australia: results from a sediment trap study

Sediment traps were deployed on the Northwest Shelf (NWS) of Australia in November 1996, to determine fluxes of organic matter and inorganic elements from the photic zone to deeper waters in a transect extending from Exmouth Shelf to Exmouth Plateau. Infiltrex II water samplers collected particulate and dissolved organics from the water column near the trap sites. Surface sediments and sediment cores were also collected over the study region. Lipid biomarkers were used to determine the sources of organic carbon and its cycling processes on the NWS. Dry weight fluxes from the traps ranged from 124 to 616 mg m −2 day −1and particulate organic carbon (POC) fluxes ranged from 22 to 42 mg m −2 day −1. The biogenic lipids consisted of biomarkers indicative of marine zooplankton, phytoplankton and bacteria, plus traces of land plant markers. A large contribution of unresolved complex material (UCM), which is indicative of petroleum, was detected at four times the biogenic hydrocarbon flux at shallow stations, and up to seven times the biogenic hydrocarbon flux at the most offshore station. There is essentially no river input, and only trace aeolian-derived material to contribute to primary production on the NWS of Australia. Most of the organic matter produced are rapidly recycled in the water column and the small fraction of lipids that settle to the sediments is already partially degraded and undergoes further rapid degradation in the surface sediments. Natural oil seeps also provide utilisable organic carbon to the system. The production and vertical flux rates of organics determined in this study are comparable to those reported in studies of shallow traps in oceanic areas from long-term studies in the Arabian Sea, and other coastal margins such as the Bay of Biscay (France) and California (USA). In offshore areas, most “living” lipid materials passed through the GFF filters thus invalidating POC estimates based on high volume sampling. To adequately assess living (particulate) carbon, gentle filtration of low volume seawater samples is more accurate, as shown by this solid phase absorption study of individual lipid biomarkers.

234Th and particle cycling in the central equatorial Pacific

US JGOFS-EgPac 234Th data sets for 1992 boreal spring (Survey I, TT007) and fall (Survey 11, TT011) cruises from 12°N to 12°S along 140°W were used to determine rates of 234Th and particle cycling using a thorium sorption model and three coupled particle-thorium models. Sampling methodology had a large impact on model results — estimates of particulate organic carbon varied by a factor of 3 between bottle and in-situ filtration techniques. Adsorption rate constants and residence times from the thorium sorption model showed strong depth, latitudinal and seasonal variability which we were able to attribute to changes in particle concentration. A reevaluation of the `particle concentration effect' on the adsorption rate constant, k′, showed that our values of k′ increased with particle concentration and were consistent with other study sites with similar particle concentrations. Recycling of particulate organic carbon in the euphotic zone of the central equatorial Pacific was 2–10 times faster than sites previously studied. Calculations of adsorption rate constants from the thorium sorption, coupled particle- 234Th and phytoplankton models were extremely dependent on the model treatment of remineralization. Results from the coupled particle- 234Th model, where particles have a constant ]ability, suggested that 234Th recycled three to four times between the dissolved and paticulate phases before being removed from the euphotic zone. Aggregation rate constants and sinking rates in the central equatorial system were compared with other sites using the size-fractionated model developed by Clegg and Whitfield (1991, Deep-Sea Research, 38, 91–120). Removal of particles by sinking from the equatorial euphotic zone depended on a mechanism of differential recycling of organic matter in the euphotic zone in which only a fraction of the particles are remineralized and the more refractory particles sink.