Minor Contribution by Biomineralizing Phytoplankton to Surface Ocean Biomineral Pools in the Late Stratified Period

Vertical distributions of biogenic silica (bSi), particulate inorganic carbon (PIC) and key biomineral-forming phytoplankton indicate vertical zoning, or partitioning, during the late summer stratified period in the northeast Atlantic. Coccolithophores were generally more numerous in the surface mixed layer, whilst PIC concentrations were more homogenous with depth throughout the euphotic zone. Diatoms were notably more abundant and more diverse in the lower euphotic zone beneath the mixed layer in association with subsurface maxima in chlorophyll-a, bSi and oxygen concentrations. The four dominant coccolithophore species (Emiliania huxleyi, Gephyrocapsa muellerae, Syracosphera spp., and Rhabdosphaera clavigera) represented 78 ± 20% (range 31–100%) of the observed community across all sampled depths yet simultaneously contributed an average of only 13% to measured PIC pools. The diatom community, which was dominated by Pseudo-nitzschia spp. and by a species tentatively identified as Nanoneis longta, represented only ~1% of the bSi pool on average, with contributions increasing within the chlorophyll maximum. Despite a slow gradual deepening of the surface mixed layer in the period prior to observation, and adequate nutrient availability beneath the mixed layer, biomineral pools at this time consisted largely of detrital rather than cellular material.

A method for the analyses of carbon stable isotope and content of particulate inorganic carbon in inland waters

<p>Studies on particulate inorganic carbon (PIC) in inland waters are relatively scarce due to the low concentration of PIC which makes it difficult to be measured accurately. In other studies, a characteristic ratio of PIC in total suspended solids in the water column has been proposed to estimate the river PIC flux to the sea, and a titration method to measure the PIC fluxes in karst rivers has been reported. Therefore, we used the Gas Bench Ⅱ-IRMS coupled technique method to analyze the δ<sup>13</sup>C<sub>PIC</sub> and PIC concentration in inland waters. The method has the advantage of being suitable for the accurate determination of the isotopic composition of trace PIC samples.</p><p>The purging time and carbon content of samples are the important factors affecting experimental accuracy. This study proposed the optimal purge time and the lowest carbon content of the inland water sample. The samples in the experiment included laboratory calcium carbonate standard (99.95 % purity) and PIC samples from the Wujiang River. The PIC samples from Wujiang River were collected on glass fiber filters. Datasets from the experiment demonstrated that the ideal purge time is 500-700 s, and at least 25 μg C should be included in the sample. The instrument signal value is low and the isotopic value fluctuates widely when the purge time is less than 500 s. The phosphoric acid cannot be injected into the sample bottle due to the high pressure in it when the purge time is more than 700 s. Therefore, a purging time of 600 s was used for the field sample analyses. The peak area displayed by the device is correlated with the carbon content in the sample, and the datasets show a good linear relationship between the peak area and carbon content in the sample when the sample be analyzed contained more than 25 μg of inorganic carbon. The carbon content of the sample can be calculated from the peak area of the same batch of calcium carbonate standard. While the peak signal is too low to detect the sample accurately when the C content is less than 25 μg. Therefore, the sample should contain more than 25 μg for the field sample analyses. This study will help to provide a reference for the method of determining the PIC content and isotopic composition in inland water.</p>

Inorganic and black carbon hotspots constrain blue carbon mitigation services across tropical seagrass and temperate tidal marshes

Total organic carbon (TOC) sediment stocks as a CO2 mitigation service require exclusion of allochthonous black (BC) and particulate inorganic carbon corrected for water–atmospheric equilibrium (PICeq). For the first time, we address this bias for a temperate salt marsh and a coastal tropical seagrass in BC hotspots that represent two different blue carbon ecosystems of Malaysia and Australia. Seagrass TOC stocks were similar to the salt marshes with soil depths < 1 m (59.3 ± 11.3 and 74.9 ± 18.9 MgC ha− 1, CI 95% respectively). Both ecosystems showed larger BC constraints than their pristine counterparts did. However, the seagrass meadows’ mitigation services were largely constrained by both higher BC/TOC and PICeq/TOC fractions (38.0% ± 6.6% and 43.4% ± 5.9%, CI 95%) and salt marshes around a third (22% ± 10.2% and 6.0% ± 3.1% CI 95%). The results provide useful data from underrepresented regions, and, reiterates the need to consider both BC and PIC for more reliable blue carbon mitigation assessments.

Inorganic and black carbon hotspots constrain blue carbon mitigation services across tropical seagrass and temperate tidal marshes

Total organic carbon (TOC) sediment stocks as a CO2 mitigation service requires exclusion of allochthonous black (BC) and particulate inorganic carbon corrected for water– atmospheric equilibrium (PICeq). For the first time, we address this bias for a temperate salt marsh and a coastal tropical seagrass in BC hotspots. Seagrass TOC stocks were similar to the salt marshes with soil depths < 1 m (59.3 ± 11.3 and 74.9 ± 18.9 MgC ha-1, CI 95% respectively) and sequestration rates of 1.134 MgC ha-1 yr-1. Both ecosystems showed larger BC constraints than their pristine counterparts. However, the seagrass meadows’ mitigation services were largely constrained by both higher BC/TOC and PICeq/TOC fractions (38.0% ± 6.6% and 43.4% ± 5.9%, CI 95%) and salt marshes around a third (22% ± 10.2% and 6.0% ± 3.1% CI 95%). The results demonstrate a need to account for both BC and PIC within blue carbon mitigation assessments.

Cocccolithophore contribution to carbonate export to the deep sea in the Australian-New Zealand sector of the subantarctic Southern Ocean

&lt;p&gt;Coccolithophores are ubiquitous marine unicellular algae belonging to the Class Prymnesiophyceae, division Haptophyta. They are distinct from other phytoplankton groups in their capacity to produce minute calcite platelets (termed coccoliths) with which they cover their cells. During the Cretaceous and throughout the Cenozoic era, pelagic sedimentation of carbonate was largely controlled by coccolithophores as evidenced by their major contribution to deep-sea oozes and chalks. In the modern Southern Ocean, coccolithophores represent an important component of the phytoplankton communities and carbon cycle. However, their contribution to total Particulate Inorganic Carbon (PIC) for large regions of the Southern Ocean remains undocumented.&lt;/p&gt;&lt;p&gt;Here we report the Particulate Inorganic Carbon (PIC) and coccolithophore fluxes collected over a year by sediment traps placed at two sites of the subantarctic Southern Ocean. We present coccolith mass estimates of the most abundant coccolithophore species and quantitatively partition annual PIC fluxes amongst heterotrophic calcifying plankton and coccolithophores. Our results reveal that coccolithophores account for approximately half of the annual PIC export in the subantarctic Southern Ocean. Moreover, in contrast to satellite estimations, that mainly reflect coccospheres and detached coccoliths of Emiliania huxleyi, less abundant but larger species make the largest contribution to CaCO&lt;sub&gt;3&lt;/sub&gt;. Lastly, comparison of our data with previous studies suggest that projected environmental change in the Southern Ocean may result in a decline of coccolithophore PIC production and export.&lt;/p&gt;&lt;p&gt;&amp;#160;&lt;/p&gt;

Coccolithophore biodiversity controls carbonate export in the Southern Ocean

Southern Ocean waters are projected to undergo profound changes in their physical and chemical properties in the coming decades. Coccolithophore blooms in the Southern Ocean are thought to account for a major fraction of the global marine calcium carbonate (CaCO3) production and export to the deep sea. Therefore, changes in the composition and abundance of Southern Ocean coccolithophore populations are likely to alter the marine carbon cycle, with feedbacks to the rate of global climate change. However, the contribution of coccolithophores to CaCO3 export in the Southern Ocean is uncertain, particularly in the circumpolar subantarctic zone that represents about half of the areal extent of the Southern Ocean and where coccolithophores are most abundant. Here, we present measurements of annual CaCO3 flux and quantitatively partition them amongst coccolithophore species and heterotrophic calcifiers at two sites representative of a large portion of the subantarctic zone. We find that coccolithophores account for a major fraction of the annual CaCO3 export, with the highest contributions in waters with low algal biomass accumulations. Notably, our analysis reveals that although Emiliania huxleyi is an important vector for CaCO3 export to the deep sea, less abundant but larger species account for most of the annual coccolithophore CaCO3 flux. This observation contrasts with the generally accepted notion that high particulate inorganic carbon accumulations during the austral summer in the subantarctic Southern Ocean are mainly caused by E. huxleyi blooms. It appears likely that the climate-induced migration of oceanic fronts will initially result in the poleward expansion of large coccolithophore species increasing CaCO3 production. However, subantarctic coccolithophore populations will eventually diminish as acidification overwhelms those changes. Overall, our analysis emphasizes the need for species-centred studies to improve our ability to project future changes in phytoplankton communities and their influence on marine biogeochemical cycles.

Blue carbon sequestration dynamics within tropical seagrass sediments: long-term incubations for changes over climatic scales

Determination of blue carbon sequestration in seagrass sediments over climatic time scales (&gt;100 years) relies on several assumptions, including no loss of particulate organic carbon (POC) after 1–2 years, tight coupling between POC loss and CO2 emissions, no dissolution of carbonates, and removal of the recalcitrant black carbon (BC) contribution. We tested these assumptions via 500-day anoxic decomposition and mineralisation experiments to capture centennial parameter decay dynamics from two sediment horizons robustly dated as 2 and 18 years old. No loss of BC was detected, and decay of POC was best described for both horizons by near-identical reactivity continuum models. The models predicted average losses of 49 and 51% after 100 years of burial for the surface and 20–22-cm horizons respectively. However, the loss rate of POC was far greater than the release rate of CO2, even after accounting for CO2 from particulate inorganic carbon (PIC) production, possibly as siderite. The deficit could not be attributed to dissolved organic carbon or dark CO2 fixation. Instead, evidence based on δ13CO2, acidity and lack of sulfate reduction suggested methanogenesis. The results indicated the importance of centennial losses of POC and PIC precipitation and possibly methanogenesis in estimating carbon sequestration rates.