Relationships between the surface concentration of particulate organic carbon and optical properties in the eastern South Pacific and eastern Atlantic Oceans

International audienceWe have examined several approaches for estimating the surface concentration of particulate organic carbon, POC, from optical measurements of remote-sensing reflectance, <i>R_rs(?)</i>, using field data collected in tropical and subtropical waters of the eastern South Pacific and eastern Atlantic Oceans. These approaches include a direct empirical relationship between POC and the blue-to-green band ratio of reflectance, <i>R_rs(?_B)/R_rs</i>(555), and two-step algorithms that consist of relationships linking reflectance to an inherent optical property IOP (beam attenuation or backscattering coefficient) and POC to the IOP. We considered two-step empirical algorithms that exclusively include pairs of empirical relationships and two-step hybrid algorithms that consist of semianalytical models and empirical relationships. The surface POC in our data set ranges from about 10 mg m^-3 within the South Pacific Subtropical Gyre to 270 mg m^-3 in the Chilean upwelling area, and data on phytoplankton pigments, suspended particulate matter, and the backscattering ratio suggest a considerable variation in the composition of particulate assemblages in the investigated waters. The POC algorithm based on the direct relationship between POC and <i>R_rs(?_B)/R_rs</i>(555) promises reasonably good performance in the vast areas of the open ocean covering different provinces from hyperoligotrophic and oligotrophic waters within subtropical gyres to eutrophic coastal upwelling regimes characteristic of eastern ocean boundaries. The best error statistics were found for power function fits to the data of POC vs. <i>R_rs</i>(443)<i>/R_rs</i>(555) and POC vs. <i>R_rs</i>(490)<i>/R_rs</i>(555). For our data set that includes over 50 data pairs, these relationships are characterized by the mean normalized bias of about 2% and the normalized root mean square error of about 20%. We recommend that these algorithms be implemented for routine processing of ocean color satellite data to produce maps of surface POC with the status of an evaluation data product for continued work on algorithm development and refinements. The two-step algorithms also deserve further attention because they can utilize various models for estimating IOPs from reflectance, offer advantages for developing an understanding of bio-optical variability underlying the algorithms, and provide flexibility for regional or seasonal parameterizations of the algorithms

Analysis of apparent optical properties and ocean color models using measurements of seawater constituents in New England continental shelf surface waters

Author Posting. © American Geophysical Union, 2004. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Journal of Geophysical Research 109 (2004): C03026, doi:10.1029/2003JC001977.We used budgets of absorption (a), scattering (b), and backscattering (bb) for particles and chromophoric dissolved organic matter (CDOM) to investigate sources of seasonal variations in apparent optical properties (AOPs) of New England continental shelf surface waters. Spectral a, b, and bb budgets for particles were estimated from flow cytometric measurements of eukaryotic pico/nanophytoplankton, Synechococcus, heterotrophic prokaryotes, detritus, and minerals; AOPs were modeled with Hydrolight radiative transfer software. For late summer and spring, our modeled values of the diffuse attenuation coefficient (Kd) and remote sensing reflectance (Rrs) were on average within 15% and 9%, respectively, of independent measurements. This close agreement allowed us to examine how different seawater constituents contributed to AOP variability. Higher values of Kd in the spring, compared to summer, were due to higher absorption by eukaryotic phytoplankton (aeuk) and CDOM (aCDOM), which coincided with higher nutrient levels and less stratified conditions than in the summer. Differences in the spectral shape of Rrs between the seasons were caused by a combination of differences in aeuk, aCDOM, and bb from non-phytoplankton particles (minerals and detritus combined). For non-phytoplankton bb the major seasonal difference was a higher inverse wavelength dependence in the summer due to the effects of small organic detritus. We applied two semianalytical ocean color models to our data, in order to evaluate whether the assumptions and parameterizations inherent in these models are applicable for New England shelf waters. We show how differences between observed and modeled chlorophyll a specific phytoplankton absorption, aCDOM, and non-phytoplankton bb cause errors in chlorophyll a concentration and IOPs retrieved from reflectance inversion models.Financial support was provided by ONR grants N00014-95-1-0333 and N00014-96-1-0965 (H. Sosik and R. Olson), NASA grants NAGW- 517, NAG5-7538, and NAG5-8868, and a NASA Earth System Science Fellowship (R. Green)

Sources of increase in lowermost stratospheric sulphurous and carbonaceous aerosol background concentrations during 1999–2008 derived from CARIBIC flights

This study focuses on sulphurous and carbonaceous aerosol, the major constituents of particulate matter in the lowermost stratosphere (LMS), based on in situ measurements from 1999 to 2008. Aerosol particles in the size range of 0.08–2 µm were collected monthly during intercontinental flights with the CARIBIC passenger aircraft, presenting the first long-term study on carbonaceous aerosol in the LMS. Elemental concentrations were derived via subsequent laboratory-based ion beam analysis. The stoichiometry indicates that the sulphurous fraction is sulphate, while an O/C ratio of 0.2 indicates that the carbonaceous aerosol is organic. The concentration of the carbonaceous component corresponded on average to approximately 25% of that of the sulphurous, and could not be explained by forest fires or biomass burning, since the average mass ratio of Fe to K was 16 times higher than typical ratios in effluents from biomass burning. The data reveal increasing concentrations of particulate sulphur and carbon with a doubling of particulate sulphur from 1999 to 2008 in the northern hemisphere LMS. Periods of elevated concentrations of particulate sulphur in the LMS are linked to downward transport of aerosol from higher altitudes, using ozone as a tracer for stratospheric air. Tropical volcanic eruptions penetrating the tropical tropopause are identified as the likely cause of the particulate sulphur and carbon increase in the LMS, where entrainment of lower tropospheric air into volcanic jets and plumes could be the cause of the carbon increase

Phytoplankton spring bloom initiation: The impact of atmospheric forcing and light in the temperate North Atlantic Ocean

The spring bloom dominates the annual cycle of phytoplankton abundance in large regions of the world oceans. The mechanisms that trigger blooms have been studied for decades, but are still keenly debated, due in part to a lack of data on phytoplankton stocks in winter and early spring. Now however autonomous underwater gliders can provide high-resolution sampling of the upper ocean over inter-seasonal timescales and advance our understanding of spring blooms. In this study, we analyze bio-optical and physical observations collected by gliders at the Porcupine Abyssal Plain observatory site to investigate the impact of atmospheric forcing and light conditions on phytoplankton blooms in the temperate North Atlantic. We contrast three hypotheses for the mechanism of bloom initiation: the critical depth, critical turbulence, and dilution-recoupling hypotheses. Bloom initiation at our study site corresponded to an improvement in growth conditions for phytoplankton (increasing light, decreasing mixing layer depth) and was most consistent with the critical depth hypothesis, with the proviso that mixing depth (rather than mixed layer depth) was considered. After initiation, the observed bloom developed slowly: over several months both depth-integrated inventories and surface concentrations of chlorophyll a increased only by a factor of ~2 and ~3 respectively. We find that periods of convective mixing and high winds in winter and spring can substantially decrease (up to an order of magnitude) light-dependent mean specific growth rate for phytoplankton and prevent the development of rapid, high-magnitude blooms

Satellite observations of seasonal and regional variability of particulate organic carbon concentration in the Barents Sea

SummaryThe Nordic and Barents Seas are of special interest for research on climate change, since they are located on the main pathway of the heat transported from low to high latitudes. The Barents Sea is characterized by supreme phytoplankton blooms and large amount of carbon is sequestered here due to biological processes. It is important to monitor the biological variability in this region in order to derive in depth understanding whether the size of carbon reservoirs and fluxes may vary as a result of climate change. In this paper we analyze the 17 years (1998–2014) of particulate organic carbon (POC) concentration derived from remotely sensed ocean color. POC concentrations in the Barents Sea are among the highest observed in the global ocean with monthly mean concentrations in May exceeding 300mgm−3. The seasonal amplitude of POC concentration in this region is larger when compared to other regions in the global ocean. Our results indicate that the seasonal increase in POC concentration is observed earlier in the year and higher concentrations are reached in the southeastern part of the Barents Sea in comparison to the southwestern part. Satellite data indicate that POC concentrations in the southern part of the Barents Sea tend to decrease in recent years, but longer time series of data are needed to confirm this observation

Bio‐optical variability associated with phytoplankton dynamics in the North Atlantic Ocean during spring and summer of 1991

Bio‐optical data recorded from April 30 to July 19, 1991, using a mooring located in the open ocean (59°35.6°N, 20°57.9°W) are described and interpreted. Five multi‐variable moored systems (MVMS) were deployed in the upper 90 m to obtain concurrent, co‐located measurements of horizontal currents, water temperature, photosynthetically available radiation (PAR), transmission of light at 660 nm (c660), and stimulated chlorophyll fluorescence. In addition, meteorological and subsurface temperature data (12 depths from 80 to 310 m) were collected. When the mooring was deployed, surface waters were weakly stratified and there was little evidence of a phytoplankton bloom. Soon after the deployment, a marked increase in phytoplankton concentration occurred simultaneously with an increase of near‐surface water temperature. The most striking observation was a period (year days 128–140) of strong mixed layer depth variability (daily amplitude of about 40 m) during which phytoplankton standing stock reached its maximum. During this period, phytoplankton biomass was mixed down to deeper waters at nighttime. As a result, the variability of the bio‐optical parameters was extremely high, and deepwater phytoplankton concentration was much greater than would have been expected from the productivity estimates. Later, phytoplankton concentrations declined sharply in response to extremely stormy weather around year day 140. Once the storm passed (after day 143), surface waters stratified and the phytoplankton stock increased again, but the depth integrated biomass concentration did not reach as high values as before the storm. During this strong thermal stability period, fluorescence and c660 signals in near‐surface waters were much higher than at depth, and displayed a diel cycle which was well correlated with PAR