Particulate Organic Carbon In Waters Research Articles

Mapping particulate organic carbon in lakes across China using OLCI/Sentinel-3 imagery

Remote sensing monitoring of particulate organic carbon (POC) concentration is essential for understanding phytoplankton productivity, carbon storage, and water quality in global lakes. Some algorithms have been proposed, but only for regional eutrophic lakes. Based on in-situ data (N = 1269) in 49 lakes across China, we developed a blended POC algorithm by distinguishing Type-I and Type-II waters. Compared to Type-I, Type-II waters had higher reflectance peak around 560 nm (>0.0125 sr−1) and mean POC (4.65 ± 4.11 vs. 2.66 ± 3.37 mg/L). Furthermore, because POC was highly related to algal production (r = 0.85), a three-band index (R2 = 0.65) and the phytoplankton fluorescence peak height (R2 = 0.63) were adopted to estimate POC in Type-I and Type-II waters, respectively. The novel algorithm got a mean absolute percent difference (MAPD) of 35.93 % and outperformed three state-of-the-art formulas with MAPD values of 40.56–76.42 %. Then, the novel algorithm was applied to OLCI/Sentinel-3 imagery, and we first obtained a national map of POC in 450 Chinese lakes (> 20 km2), which presented an apparent spatial pattern of “low in the west and high in the east”. In brief, water classification should be considered when remotely monitoring lake POC concentration over a large area. Moreover, a process-oriented method is required when calculating water column POC storage from satellite-derived POC concentrations in type-II waters. Our results contribute substantially to advancing the dynamic observation of the lake carbon cycle using satellite data.

Top-down control over dissolved organic carbon in the bottom water of the Weddell Sea and its implication for the continental shelf pump

Dense water out of the Antarctic shelves is expected to drive the transport of carbon into the deep Southern Ocean via the formation of Antarctic Bottom Water. However, bottom water formation’s capacity to sequester carbon into the deep ocean is poorly constrained. Here, dissolved organic carbon (DOC), dissolved black carbon (DBC), and particulate organic carbon (POC) were examined to reveal the influence of the Weddell Sea Deep Water (WSDW) on DOC transport during its flowing out of the Weddell Sea. High DOC concentrations (>60 μM-C) and low DBC/DOC ratios (<1.5%) were observed in surface water near the South Orkney Islands, ascribing to sea ice melt-induced phytoplankton blooms. Seawater at the mid-deep depths exhibited a higher DOC concentration (averaging 48.1 ± 3.7 μM-C) than the incoming water source, resulting from the release of DOC from sinking particles. Bottom water had higher DOC concentration compared to the mid-deep layer water (t-test, p < 0.005), while the DBC concentrations were comparable. In addition, the excess DOC (relative to WSDW) in bottom water showed a close relation with POC in surface water. These results reveal a top-down control over the DOC concentration in bottom water through a quick sinking of diatom detritus and subsequently solubilization in bottom water and/or sediment. With an estimate, the WSDW carries 5.1 ± 2.8 Tg-C/yr of excess DOC out of the Powell Basin, representing an important carbon source to the deep Southern Ocean. This study highlights the key role of the Antarctic continental shelf pump in carbon sequestration.

Suspended particulate organic carbon and its carbon isotopic composition in the surface water around the Antarctic Peninsula during summer 2017–2018

AbstractThe concentration of suspended particulate organic carbon (POC) and its carbon isotopic composition (δ13CPOC) were analysed in this study with the aim of exploring the sources and factors influencing levels of POC in the surface water around the Antarctic Peninsula. The scanning electron microscopy results suggest that diatom particles formed the main component of suspended particulate matter, indicating that POC was mainly from in situ primary production. The high concentrations of chlorophyll a and POC in sea water mainly occurred in nearshore and sea-ice edge regions, which might be controlled by nutrient and reactive iron inputs stemming from sea-ice melting. The δ13CPOC in the study area is significantly lower than that in low-latitude waters, with a range of -31.8‰ to -22.8‰ (mean -28.9‰), which was controlled by the high CO2 concentration in the Southern Ocean and might be influenced by phytoplankton growth rates and assemblages. This study helps us to understand material cycling in the Antarctic region under the conditions of global climate change.

Multi-temporal variability forecast of particulate organic carbon in the Indonesian seas.

The current global condition characterized by high levels of CO2 is altering the carbon cycle and elemental biogeochemistry, resulting in subsequent global warming, climate change, ocean acidification, and the indirect response of deoxygenation. The features of Indonesia's coastal ecosystems and continental shelf waters also contribute to spatio-temporal ocean carbon variability. For instance, the level of particulate organic carbon (POC) will change annually, and thus, over a decadal period, ocean dynamics may affect the temporal variability of POC. Motivated by such conditions, future forecasting is needed to envision the productivity of Indonesian seas by predicting vital parameters such as POC. This research aimed to forecast the temporal variability of POC in Indonesian waters. The Seasonal Autoregressive Integrated Moving Average (SARIMA) forecasting model was used by considering the lowest value of the Akaike information criterion (AIC) and the mean absolute percentage error/MAPE (threshold < 10%). Using the highest correlation coefficient (threshold: 0.75), we obtained the best fit for forecasting POC temporal variability. Hindcast POC data (2002-2020/2021) was used to train the forecasting model. The result shows that forecasting of POC temporal variability can be conducted up to 2030. The validity of prediction is ensured for less than 5years forward after 2020 with correlation coefficients of 0.65 and 0.83 for seasonal and monthly POC, respectively. The hindcast and forecast estimates of POC in the Indonesian seas show a decreasing trend. The present study emphasizes the different forecasting results obtained using the different approaches of annual versus inter-annual variability. A sustained research effort is still required to assess POC forecasting for its potential benefits in marine system monitoring and assessment.

The influence of increasing sea surface temperature on particulate organic carbon in Indonesian waters based on MODIS Aqua satellite

Both Sea Surface Temperature (SST) and Particulate Organic Carbon (POC) have important roles in the marine ecosystem and carbon cycle. Therefore, we analyze the influence of SST on POC in Indonesian water. We used SST and POC products from the moderate-resolution imaging spectroradiometer (MODIS) Aqua satellite from 2003 to 2021 (nineteen years). Regression and a temporal anomaly correlation approach were applied to assess the effect of SST on POC in Indonesian waters. Our results show that the southeast Indonesian waters, south of Java Island and around the Riau Islands, mostly have a high SST variation. On the other hand, the high variation of POC mostly appeared in Indonesian coastal. During the last nineteen years, the increasing SST anomalies (SSTA) in Indonesian waters have been followed by decreasing POC anomalies (POCA). These results indicate that rising SST reduces the ocean’s ability to absorb carbon in Indonesian waters. A strong negative relation between SSTA on POCA was found from April to August and October to January in a seasonal timescale. In the interannual timescale, our result reveals that the strong negative relationship between SSTA and POCA is likely linked to the El Niño Southern Oscillation, the Indian Ocean Dipole, and global warming.

A novel semianalytical remote sensing retrieval strategy and algorithm for particulate organic carbon in inland waters based on biogeochemical-optical mechanisms

The estimation of particulate organic carbon (POC) concentrations from satellite images can provide crucial spatiotemporal continuous observation data for the carbon cycle and ecological environmental governance. Here, we developed a novel inversion algorithm for deriving POC in inland water based on remote sensing and geochemical isotopes, which is summarized as follows. First, we developed empirical relationships between the phytoplankton absorption coefficient and endogenous POC concentration (CendPOC) and between the nonalgal particulate absorption coefficient and terrestrial POC concentration (CterPOC). Second, based on the valid relationships, semianalytical retrieval models were established to estimate CendPOC and CterPOC. Third, the proportions of endogenous POC (RendPOC) and terrestrial POC (RterPOC) to the total POC concentration (CPOC) were derived using a three-band empirical model. Finally, CPOC was obtained by dividing CendPOC by RendPOC (RendPOC ≥ 0.5) or dividing CterPOC by RterPOC (RendPOC < 0.5). Validation with field data shows that our proposed algorithm can accurately derive CPOC (0–20 mg/L), with a root mean square deviation (RMSD), median bias (MB), median absolute percent difference (MAPD), and median ratio (MR) of 1.15 mg/L, −0.05 mg/L, 24%, and 0.98, respectively. Synchronous validation based on Sentinel-3/OLCI images confirmed the accuracy, with RMSD, MB, MAPD, and MR values of 0.41 mg/L, −0.16 mg/L, 28%, and 0.91, respectively. The algorithm was applied to ocean and land color sensor (OLCI) images to reveal the temporal and spatial variations in POC in Lake Taihu.

Spatial and temporal variations in particulate organic carbon in Indonesian waters over two decades

Particulate organic carbon (POC) is one of the essential variables in oceanic biogeochemical processes. Assessing the spatiotemporal variability of POC will help deepen our understanding of the carbon cycle. In this study we collected moderate-resolution imaging spectroradiometer (MODIS) Aqua satellite data for surface POC in Indonesian waters during the period 2002–21, together with in situ data from the south-western Sumatran waters (SSW), Sunda Strait, Sumba–Savu waters and the Halmahera Sea and surrounds. These data were analysed to gain an understanding of the spatial and temporal variations in POC. Generally, high POC concentrations in Indonesian waters were found at coastal and continental shelves connected to the estuaries of large rivers; we assume that the POC concentrations in these areas are affected by terrestrial input. Meanwhile, in Indonesian waters apart from these estuaries, marine-derived materials are the main components of POC. The seasonal pattern of POC in Indonesian waters generally shows highest levels during the south-east monsoon and lowest levels during the north-west monsoon. Trends in POC concentrations are positively correlated with chlorophyll-a and negatively correlated with sea surface temperature. This study suggests that the combination of natural physicochemical processes and anthropogenic perturbations has significant effects on spatial and temporal variations in POC in Indonesian waters.

Biomass of plankton and macrobenthos and benthic species diversity in relation to environmental gradients in a nationwide coastal survey

Using Korean coastal survey data, we examined the relationships between plankton and benthic biomass and environmental parameters. The nitrate in coastal waters was mainly derived from the influx of fresh water, whereas much of the ammonium originated from decomposing organic matter in the sediments and river discharge. Phytoplankton and zooplankton concentrations were positively related to the amount of suspended particulate matter (SPM), but decline when SPM becomes high (> 10 ppm). Particulate organic carbon (POC) comprised about 40% of the SPM, suggesting that organic matters associated with river-borne sediments and re-suspended particles are a large source of coastal water POC. Zooplankton abundance was more closely related to the distribution of phytoplankton than to that of POC. Benthic biomass showed a dome-shaped relationship with the water column POC concentration, but no clear relationship with sediment POC distribution. The biodiversity of macrobenthic animals gradually decreases when POC exceeds 1% of the sediment weight. Reduction of siltation and POC could lead to enhanced benthic–pelagic coupling via increased plankton biomass and productivity, as well as overall increased benthic diversity in coastal environments.

A two-step approach to mapping particulate organic carbon (POC) in inland water using OLCI images

Particulate organic carbon (POC) plays an important role in the carbon cycle in freshwater ecosystems, and estimating the POC concentration with remote sensing is a useful method of analyzing the spatiotemporal distribution of POC in inland waters. However, the complex origins of POC in inland waters require the application of remote sensing methods at large scales, and these methods are often challenging to implement. Therefore, a two-step method based on classification was developed to estimate the POC concentrations in inland water bodies. A total of 331 samplings were collected from 7 inland waters, Taihu Lake, Chaohu Lake, Dianchi Lake, Dongting Lake, Hongze Lake, Hengshui Lake, and the Jiajiang River, at different times. The POC concentration and other optical parameters were analyzed. A determination method using OLCI bands 6 and 11 was proposed to identify the POC originating from phytoplankton and from other origins. As a result, the water was classified into two water types. In water type I, the POC was mainly derived from phytoplankton, and in water type II, and the POC mainly originated from non-algal materials. Therefore, specific POC estimation algorithms were developed for different water types. OLCI bands 8, 12 and 16 were used for water type I, and bands 8, 11 and 12 were applied for water type II. An estimation accuracy comparison with two unclassified estimation methods, semi-analytical algorithm and empirical algorithm using an independent dataset was conducted, and the results showed that the MAPE decreased to 27.25% from 55.56% and 60.87%, and the RMSE decreased to 2.13 mg/L from 3.33 mg/L and 3.20 mg/L. The accuracy assessment demonstrated that the estimation performance of the two-step method was significantly improved. Finally, this method was validated using two OLCI images acquired on December 8th, 2016, and July 24th, 2017, for mapping the POC concentration in Hongze Lake and Taihu Lake.

Distribution of planktonic biogenic carbonate organisms in the Southern Ocean south of Australia: a baseline for ocean acidification impact assessment

Abstract. The Southern Ocean provides a vital service by absorbing about one-sixth of humankind's annual emissions of CO2. This comes with a cost – an increase in ocean acidity that is expected to have negative impacts on ocean ecosystems. The reduced ability of phytoplankton and zooplankton to precipitate carbonate shells is a clearly identified risk. The impact depends on the significance of these organisms in Southern Ocean ecosystems, but there is very little information on their abundance or distribution. To quantify their presence, we used coulometric measurement of particulate inorganic carbonate (PIC) on particles filtered from surface seawater into two size fractions: 50–1000 µm to capture foraminifera (the most important biogenic carbonate-forming zooplankton) and 1–50 µm to capture coccolithophores (the most important biogenic carbonate-forming phytoplankton). Ancillary measurements of biogenic silica (BSi) and particulate organic carbon (POC) provided context, as estimates of the biomass of diatoms (the highest biomass phytoplankton in polar waters) and total microbial biomass, respectively. Results for nine transects from Australia to Antarctica in 2008–2015 showed low levels of PIC compared to Northern Hemisphere polar waters. Coccolithophores slightly exceeded the biomass of diatoms in subantarctic waters, but their abundance decreased more than 30-fold poleward, while diatom abundances increased, so that on a molar basis PIC was only 1 % of BSi in Antarctic waters. This limited importance of coccolithophores in the Southern Ocean is further emphasized in terms of their associated POC, representing less than 1 % of total POC in Antarctic waters and less than 10 % in subantarctic waters. NASA satellite ocean-colour-based PIC estimates were in reasonable agreement with the shipboard results in subantarctic waters but greatly overestimated PIC in Antarctic waters. Contrastingly, the NASA Ocean Biogeochemical Model (NOBM) shows coccolithophores as overly restricted to subtropical and northern subantarctic waters. The cause of the strong southward decrease in PIC abundance in the Southern Ocean is not yet clear. The poleward decrease in pH is small, and while calcite saturation decreases strongly southward, it remains well above saturation ( &gt; 2). Nitrate and phosphate variations would predict a poleward increase. Temperature and competition with diatoms for limiting iron appear likely to be important. While the future trajectory of coccolithophore distributions remains uncertain, their current low abundances suggest small impacts on overall Southern Ocean pelagic ecology.

Evaluating Optical Proxies of Particulate Organic Carbon across the Surface Atlantic Ocean

Empirical relationships between particulate organic carbon (POC) and inherent optical properties (IOPs) are required for estimating POC from ocean-colour remote sensing and autonomous platforms. The main relationships studied are those between POC and particulate attenuation (cp) and backscattering (bbp) coefficients. The parameters of these relationships can however differ considerably due to differences in the methodologies applied for measuring IOPs and POC as well as variations in particle characteristics. Therefore it is important to assess existing relationships and explore new optical proxies of POC. In this study, we evaluated empirical relationships between surface POC and IOPs (cp, bbp and the particulate absorption coefficient, ap) using an extensive dataset collected during two Atlantic Meridional Transect (AMT 19 and 22) cruises spanning a wide range of oceanographic regimes. IOPs and POC were measured during the two cruises using consistent methodologies. To independently assess the accuracy of the POC-IOPs relationships, we predicted surface POC for AMT-22 using relationships developed based on independent data from AMT-19. We found typical biases in predicting POC ranging between 2-3%, 4-9%, and 6-13% for cp, bbp and ap, respectively, and typical random uncertainties of 20-30%. We conclude that 1) accurate POC-cp and POC-bbp relationships were obtained due to the consistent methodologies used to estimate POC and IOPs and 2) ap could be considered as an alternative optical proxy for POC in open-ocean waters, only if all physiological variability in the POC:chl ratio could be modeled and used to correct ap.

Regional-scale algorithm to estimate the particulate organic carbon (POC) in inland waters using Landsat-5/TM images

A regional-scale algorithm was developed in order to test if the Landsat-5/TM can be used to estimate the particulate organic carbon (POC) in oligotrophic-to-mesotrophic inland water. To develop the POC algorithm two fieldworks were conducted, the first in May and the second in September 2009. The algorithm was calibrated using the dataset from September and validated using the dataset from May. The results showed that the best calibration was obtained using a polynomial fitting function (R2 of 0.80, p < 0.0001). This model was validated with a normalized root mean square error (NRMSE) of 6.21%. The algorithm was then applied in two Landsat-5/TM images from April and July 2009. The spatial distribution of POC obtained from the satellite images reveals a strong dependence of POC concentrations with the weather conditions. These results allowed us to conclude that there is a great potential to study the temporal dynamics of POC in inland waters using Landsat-5/TM images.

Remote Sensing Observation of Particulate Organic Carbon in the Pearl River Estuary

River estuaries are connectors of terrestrial and marine ecosystems. Riverine particulate organic carbon (POC) is discharged into oceans after a series of biogeochemical reactions in estuaries. Satellite monitoring of POC will improve our understanding of the carbon dynamics of these water bodies. Based on in situ data from four seasonal survey cruises, we developed an algorithm for estimating POC concentrations in the Pearl River Estuary (PRE). Reflectance ratios, Rrs(678)/Rrs(488) and Rrs(748)/Rrs(412), were set as inputs to calculate POC concentration in the PRE. The algorithm was then applied to MODIS/AQUA data to inverse POC concentrations in the PRE from 2002 to 2014. Additionally, sources, impact factors, and seasonal distributions of POC were also investigated. Phytoplankton contributed more to POC in off-shore waters than that in in-shore waters in autumn and spring, but showed the opposite pattern in winter. Under the influence of freshwater from the Pearl River, underwater topography, tides, winds, etc., the seasonal POC concentrations along a specific section, vertical to water depth gradient, decreased in different seasons. These decreases could be described by exponential functions (y = aebx, b &lt; 0). The distribution of POC concentrations in the PRE resulted from complex physical and biogeochemical processes, which can change spatially and seasonally.

Modern carbon burial in Lake Qinghai, China

The quantification of carbon burial in lake sediments, and carbon fluxes derived from different origins are crucial to understand modern lacustrine carbon budgets, and to assess the role of lakes in the global carbon cycle. In this study, we estimated carbon burial in the sediment of Lake Qinghai, the largest inland lake in China, and the carbon fluxes derived from different origins. We find that: (1) The organic carbon burial rate in lake sediment is approximately 7.23gm−2a−1, which is comparable to rates documented in many large lakes worldwide. We determined that the flux of riverine particulate organic carbon (POC) is approximately 10 times higher than that of dissolved organic carbon (DOC). Organic matter in lake sediments is primarily derived from POC in lake water, of which approximately 80% is of terrestrial origin. (2) The inorganic carbon burial rate in lake sediment is slightly higher than that of organic carbon. The flux of riverine dissolved inorganic carbon (DIC) is approximately 20 times that of DOC, and more than 70% of the riverine DIC is drawn directly and/or indirectly from atmospheric CO2. (3) Both DIC and DOC are concentrated in lake water, suggesting that the lake serves as a sink for both organic and inorganic carbon over long term timescales. (4) Our analysis suggests that the carbon burial rates in Lake Qinghai would be much higher in warmer climatic periods than in cold ones, implying a growing role in the global carbon cycle under a continued global warming scenario.

Various links between landslide debris and sediment flux during earthquake and rainstorm events

The variation of riverine dissolved ions is an important clue to understand the connection between earthquake and rainstorm-triggered landslide to the transference of sediment flux. The 2006 ML6.2 Taitung earthquake increased the annual sediment discharge of 27Mt in 2006; more than 2-fold the annual average 12Mt/yr of the preceding decade in the Luye catchment of southeastern Taiwan. During the same period of time, the proportion of area disturbed by landslides to total catchment area increased from 1.3% before to 2.3% after the earthquake. There was huge amount of vegetable delivered with the transport of landslide debris into channel to increase the concentrations of particulate organic carbon in water. However, those sediment flux produced from landslides in wet seasons usually included more particulate silicate material to dilute the occupied ratio of particulate carbon in sediment, which was lower in wet seasons but increased 4-fold in dry seasons. Besides, even through the variations of dissolved ions were relatively stable whether in wet or dry seasons because huge amount of fine material could be provided for whole year, we also found the concentrations of Na+, K+ and Mg+ had significantly increased during the rainstorms following the 2006 Taitung earthquake. This increase should still be attributed to the delivery of landslide debris into the channel, which had a positive impact on the sediment flux after earthquake and subsequent typhoon events.

Carbon isotopes and lipid biomarker investigation of sources, transport and degradation of terrestrial organic matter in the Buor-Khaya Bay, SE Laptev Sea

Abstract. The world's largest continental shelf, the East Siberian Shelf Sea, receives substantial input of terrestrial organic carbon (terr-OC) from both large rivers and erosion of its coastline. Degradation of organic matter from thawing permafrost in the Arctic is likely to increase, potentially creating a positive feedback mechanism to climate warming. This study focuses on the Buor-Khaya Bay (SE Laptev Sea), an area with strong terr-OC input from both coastal erosion and the Lena river. To better understand the fate of this terr-OC, molecular (acyl lipid biomarkers) and isotopic tools (stable carbon and radiocarbon isotopes) have been applied to both particulate organic carbon (POC) in surface water and sedimentary organic carbon (SOC) collected from the underlying surface sediments. Clear gradients in both extent of degradation and differences in source contributions were observed both between surface water POC and surface sediment SOC as well as over the 100 s km investigation scale (about 20 stations). Depleted δ13C-OC and high HMW/LMW n-alkane ratios signaled that terr-OC was dominating over marine/planktonic sources. Despite a shallow water column (10–40 m), the isotopic shift between SOC and POC varied systematically from +2 to +5 per mil for δ13C and from +300 to +450 for Δ14C from the Lena prodelta to the Buor-Khaya Cape. At the same time, the ratio of HMW n-alkanoic acids to HMW n-alkanes as well as HMW n-alkane CPI, both indicative of degradation, were 5–6 times greater in SOC than in POC. This suggests that terr-OC was substantially older yet less degraded in the surface sediment than in the surface waters. This unusual vertical degradation trend was only recently found also for the central East Siberian Sea. Numerical modeling (Monte Carlo simulations) with δ13C and Δ14C in both POC and SOC was applied to deduce the relative contribution of – plankton OC, surface soil layer OC and yedoma/mineral soil OC. This three end-member dual-carbon-isotopic mixing model suggests quite different scenarios for the POC vs SOC. Surface soil is dominating (63 ± 10 %) the suspended organic matter in the surface water of SE Laptev Sea. In contrast, the yedoma/mineral soil OC is accounting for 60 ± 9 % of the SOC. We hypothesize that yedoma-OC, associated with mineral-rich matter from coastal erosion is ballasted and thus quickly settles to the bottom. The mineral association may also explain the greater resistance to degradation of this terr-OC component. In contrast, more amorphous humic-like and low-density terr-OC from surface soil and recent vegetation represents a younger but more bioavailable and thus degraded terr-OC component held buoyant in surface water. Hence, these two terr-OC components may represent different propensities to contribute to a positive feedback to climate warming by converting OC from coastal and inland permafrost into CO2.

Model-based remote sensing algorithms for particulate organic carbon (POC) in the Northeastern Gulf of Mexico

Hydrographic data, including particulate organic carbon (POC) from the Northeastern Gulf of Mexico (NEGOM) study, were combined with remotely-sensed SeaWiFS data to estimate POC concentration using principal component analysis (PCA). The spectral radiance was extracted at each NEGOM station, digitized, and averaged. The mean value and spurious trends were removed from each spectrum. De-trended data included six wavelengths at 58 stations. The correlation between the weighting factors of the first six eigenvectors and POC concentration were applied using multiple linear regression. PCA algorithms based on the first three, four, and five modes accounted for 90, 95, and 98% of total variance and yielded significant correlations with POC with R 2 = 0.89, 0.92, and 0.93. These full waveband approaches provided robust estimates of POC in various water types.Three different analyses (root mean square error, mean ratio and standard deviation) showed similar error estimates, and suggest that spectral variations in the modes defined by just the first four characteristic vectors are closely correlated with POC concentration, resulting in only negligible loss of spectral information from additional modes. The use of POC algorithms greatly increases the spatial and temporal resolution for interpreting POC cycling and can be extrapolated throughout and perhaps beyond the area of shipboard sampling.

Seasonal and interannual changes in particulate organic carbon export and deposition in the Chukchi Sea

Particulate organic carbon (POC) export fluxes were estimated in the shelf‐slope region of the Chukchi Sea using measurements of 234Th−238U disequilibria and the POC/234Th ratio in large (&gt;53‐μm) particles. These export fluxes were used in conjunction with rates of primary productivity and benthic carbon respiration to construct a POC budget for this shelf‐slope region. Samples were collected along a series of shelf‐basin transects in the spring (May–June) and summer (July–August) of 2004. These stations were previously occupied during the ice covered (spring) and open water (summer) seasons of 2002, allowing for an interannual comparison of export flux. In contrast to 2002, when open water POC fluxes were significantly higher than in the ice‐covered period, POC export fluxes in 2004 were similar during the spring (average = 19.7 ± 24.8 mmol C m−2 d−1) and summer (average = 20.0 ± 14.5 mmol C m−2 d−1). The high POC fluxes measured during the spring are attributed to a plankton bloom, as evidenced by exceptionally high rates of primary productivity (average = 124.4 ± 88.1 mmol C m−2 d−1). The shelf‐slope budget of particulate organic carbon indicates that 10–20% of primary productivity was exported below 50 m but was not consumed during benthic carbon respiration or burial and oxidation in underlying sediments. Furthermore, a water column−sediment budget of 234Th indicates that particulate material is retained in shelf sediments on a seasonal basis.

Inorganic and organic sinking particulate phosphorus fluxes across the oxic/anoxic water column of Cariaco Basin, Venezuela

Phosphorus (P) is a vital nutrient that is essential for all organisms and may limit growth on both modern and geologic timescales. The major removal mechanism of P from marine systems is via the transformation of dissolved P into sinking particulate P pools. Although most particulate P is remineralized before it reaches the seafloor, little is known about the processes that control its breakdown into dissolved phases. In this study, the P composition of sinking particles captured by five sediment traps distributed through the oxic and anoxic water column of the Cariaco Basin, Venezuela is examined. Samples were collected from January 1996 to December 2004. Total particulate P (TPP), particulate inorganic P (PIP), and particulate organic P (POP) fluxes varied considerably over the course of the nine year study, yet there were no significant seasonal differences in the overall flux of POP. In contrast, PIP, which comprises a major portion of TPP (averaging 52 ± 19% across all depths) had fluxes that were 30% higher during non-upwelling periods. Poor relationships between PIP and biologically derived constituents, e.g. particulate organic carbon (POC), suggest that most of this material was derived from non-biological sources, namely terrestrial runoff from rivers. Rapid remineralization of POP occurred relative to POC and PIP in oxic surface waters, whereas PIP was quickly remineralized relative to POC and POP in subsurface anoxic waters. This suggests a significant and alternating source of particulate P to the dissolved P pool that depends on oxygen availability. Thus, particulate P release to the dissolved phase and the upwelling of dissolved P into the euphotic zone is a potentially important positive feedback mechanism for enhanced primary production and carbon sequestration in continental margin sediments regardless of oxic versus anoxic conditions.

Satellite estimation of marine particulate organic carbon in waters dominated by different phytoplankton taxa

The relationship between in situ remote sensing reflectance (Rrs) and particulate organic carbon (POC) was assessed for the continental shelf waters of the southwestern Ross Sea, Antarctica. This sector of the Southern Ocean supports two distinct phytoplankton communities, one dominated by diatoms and the other dominated by the haptophyte Phaeocystis antarctica. In both communities, Rrs exhibited a statistically significant (p &lt; 0.05) positive linear correlation with POC concentration, although the slope of the regression was over twofold higher in waters dominated by P. antarctica. These distinct relationships between POC and Rrs are likely due to differences in the cell size and composition, as well as detrital degradation products, of the dominant taxa within each of these phytoplankton communities. Taxon‐specific POC‐Rrs relationships were translated into a bio‐optical algorithm for estimation of surface POC concentration from remotely sensed estimates of Rrs. Applying this algorithm to satellite‐derived Rrs from the Ross Sea, where distributions of P. antarctica and diatoms are reasonably well known, yields distributions of POC and POC:chlorophyll ratios that are in good agreement with field measurements. Performance of the algorithm can be improved by in situ characterization of the detrital contribution to total particle backscatter.