Macroalgae culture-induced carbon sink in a large cultivation area of China.

Macroalgae culture-induced carbon sink in sediments has been little investigated. Here, total organic carbon (TOC), total nitrogen (TN), and δ13C were examined in sediments in a cultivation field of macroalgae (kelp and Gracilariopsis lemaneiformis) in Sansha Bay, Southeast China. Both proxies of C/N (TOC to TN ratio) and δ13C indicated a multisource of TOC. Based on a three-endmember model, macroalgae-derived TOC (TOCma) accounted for < 35% of the total TOC, averaging 16 ± 9% (mean ± SD). On average, terrestrial and phytoplankton-derived TOC showed much higher percentages of 24 ± 17% and 60 ± 20%, respectively (t-test, p < 0.02). A preliminary estimate suggested that TOCma represents a carbon sink of 8.2 × 103 tons per year, corresponding to about 22% of the sink associated with phytoplankton and macroalgae and 8 ± 6% of the macroalgae carbon production in Sansha Bay. Considering its production magnitude, the macroalgae-induced carbon sink seems to be insignificant, on a national or global scale, to phytoplankton, though it should be taken into account given the small cultivation area.

Dynamics of particulate black carbon in the South China Sea: Magnitude, resident timescale, sinking speed, and flux.

The dynamics of particulate black carbon (PBC) in marine environments are poorly understood. Here, radioactive 234Th was used to constrain the resident timescale, settling speed, and sinking flux of PBC (soot) in the coastal Northeastern South China Sea (NSCS). The PBC concentration varied from 0.013 µg-C L-1 to 4.340 µg-C L-1. Spatially, PBC showed an exponential decrease offshore, with a coefficient of 0.030 ± 0.004. Compiling available data, an empirical formula of PBC = a e-0.032x (x is the distance offshore) was proposed for predicting the descent of PBC offshore in coastal seas. Residence times of 0.8-13 d indicate that PBC is retained for days, implying its limited dispersal to the open sea. For the first time, the settling speed of PBC was evaluated in seawater, which averaged 8.8 ± 7.1 m d-1. These results highlight that bottle-sampled PBC falls mainly into the slow-sinking particle continuum in marine environments, due to its fine size. The sinking flux of PBC averaged 4.57 mg-C m-2 d-1 in the coastal NSCS. Using the sinking speed, the preliminarily estimated sinking rate of PBC was 23.8-1800 Tg-C yr-1 on global shelves. The crucial dynamic parameters of PBC provide insights into its internal cycling in coastal seas and can be used as model parameters for assessing global PBC.

Changes in the carbon source and storage in a cultivation area of macro-algae in Southeast China.

Macro-algae culture has recently attracted attention in China because of its capability to sequester carbon. Here, radionuclides, total organic carbon (TOC), and nitrogen (TN) were examined in a cultivation area of macro-algae in Southeast China. At the reference site, the ratio of TOC to TN (C/N, 8.1 ± 0.2, mean ± SD) did not exhibit discernible variation over the past 70 years. In contrast, in the cultivation area, C/N descended from 9.0 ± 0.2 around 1960 to 8.3 ± 0.2 between 1960 and 1990 and 7.6 ± 0.2 after 1990, coincident with the recorded kelp production in this area, indicating an influence of macro-algae culture-associated activities on carbon origin. Using a model, algal culture-associated activities contributed 23 ± 7 % between 1963 and 1990 and 53 ± 8 % between 1990 and 2022 to TOC. The burial of culture-associated TOC varied from 0.15 to 1.23 mg-C cm-2 yr-1, implying the unneglectable influence on carbon storage.

Carbon and Iron Uptake by Phytoplankton in the Amundsen Sea, Antarctica.

Freshwater components in the Southern Ocean, whether sea ice meltwater or meteoric water, influence the growth of phytoplankton by affecting water stability and supplying dissolved iron (DFe). In addition, melting sea ice stimulates phytoplankton blooms by providing ice algae. In this study, sea ice meltwater and meteoric water in the Amundsen Sea (AS) were differentiated by their stable oxygen isotopic compositions (δ18O), while the phytoplankton carbon fixation rate (CFR) and iron uptake rate (FeUR) values were determined using the 14C and 55Fe tracer assays, respectively. Our results showed that FeUR exhibits a significant positive response only to sea ice meltwater, suggesting that DFe and algae provided by sea ice melting may be the main cause. In addition, the CFR had a slightly positive response to the freshwater input and a stronger correlation with the phytoplankton biomass, suggesting that the freshwater input may have enhanced the CFR through the algae released from sea ice melting. The FeUR normalized to the phytoplankton biomass was significantly positively correlated with the mixed layer depth, suggesting that water stability regulates the phytoplankton growth and the resulting Fe demand. A higher Fe demand per unit of carbon fixation during sea ice formation leads to a higher Fe/C ratio in phytoplankton. Although no significant correlations were observed between the FeUR, CFR, and meteoric water, meteoric water may have an effect on larger phytoplankton sensitive to Fe deficiencies. The results of culture experiments with DFe addition showed that the added Fe significantly enhanced the Fe uptake, carbon fixation, and Fe/C ratio of the cells, especially for micro-phytoplankton. The more pronounced response of micro-phytoplankton means that the meteoric water input may affect the efficiency of carbon export. Our study provides the first measurements of phytoplankton Fe quotas in the AS in austral late summer and early autumn, providing insights into how meteoric water and sea ice meltwater affect seasonal changes in Antarctic ecosystems.

The biological uptake of dissolved iron in the changing Daya Bay, South China Sea: Effect of pH and DO.

The oceanic acidification and coastal hypoxia have potential to enhance biological uptake of dissolved iron (Fe) by phytoplankton. In this study, the Fe uptake rate (FeUR) in Daya Bay was significantly negatively correlated with pH and dissolved oxygen (DO) (r = -0.81 and -0.73, respectively, p < 0.001). In addition, binary regression (FeUR = -1.45 × pH - 0.10 × DO + 13.64) also indicated that both pH and DO played key roles in FeUR variations. As pH and DO decreased, Fe uptake by phytoplankton was promoted, and the contribution of nano-phytoplankton to Fe uptake increased significantly, while that of pico-FeUR decreased. These will result in the phytoplankton community to be miniaturized and Fe requirement of phytoplankton goes higher, thereby leading changes of phytoplankton composition and coastal ecosystem. This study helps to understand how Fe could affect the coastal ecosystem under the increasing anthropogenic influences.

Semi-enclosed bays serve as hotspots for black carbon burial: A case study in Jiaozhou Bay, western Yellow Sea.

The provenance of black carbon (BC) and its role in affecting contaminant cycling in both the atmosphere and aquatic environments have been extensively studied. However, the fate and cycling dynamics of BC, particularly in marine environments, are poorly understood. Herein, soot BC was determined in the semi-enclosed Jiaozhou Bay to examine the seasonal variability, residence timescale in seawater, and settling flux to sediments, together with particle-reactive 234Th. Soot BC ranged from 0.39 to 4.26 µmol-C L-1. On average, spring produced the highest value of 1.88 ± 0.31 µmol-C L-1, followed by winter (1.59 ± 0.18 µmol-C L-1), summer (0.94 ± 0.09 µmol-C L-1), and autumn (0.90 ± 0.09 µmol-C L-1). The seasonality of soot BC was similar to the activity concentration of particulate 234Th (i.e., 234ThP). The close relationships between soot BC and 234ThP (p < 0.01) provide the basis for the application of 234Th to trace the fate of soot BC. Based on the 234Th deficit with respect to 238U, the residence times of soot BC were estimated to be 41 ± 6 d and 36 ± 5 d for May-August and August-November, respectively. The shorter residence times of soot BC than that of seawater indicated that soot BC delivered to Jiaozhou Bay settled in the local sediments. Furthermore, soot BC concentrations were higher in the inflow seawater from the Yellow Sea than the outflow water from Jiaozhou Bay, implying a net input of soot BC from the Yellow Sea to Jiaozhou Bay. The soot BC fluxes were 0.266 ± 0.035 mmol-C m-2 d-1 and 0.0472 ± 0.0065 mmol-C m-2 d-1 for May-August and August-November, respectively. From the bay-scale perspective, Jiaozhou Bay had buried 0.101 ± 0.010 Gg of soot BC each year. These results indicate that the semi-enclosed Jiaozhou Bay acts as an effective trap for soot BC and particle-reactive contaminants.

A persistent increase in primary productivity east off Hainan Island (northwestern South China Sea) over the last decades as inferred from sediment records.

Sediment cores were analyzed from the continental shelf of the northwestern South China Sea aiming to understand the change history of primary productivity and provide insights into key changes of environmental conditions in this region over the past ~100 years. Multiple proxies including stable carbon isotopic composition (δ13C) of sedimentary organic matter, diatom abundance and biogenic silica burial flux were applied along with 210Pb chronology. Notably, these independent evidences consistently point to a steady increase of primary production in this region only after ~1960s. We propose that increasing atmospheric deposition due to dramatically enhanced human activities especially from China supplies essential nitrogen nutrients to the N-poor region and probably acts a major reason for the observed enhancement of marine primary production. Our study provides insights into better understanding how human perturbation may have profoundly impacted biogeochemical cycling in marginal seas in the last decades.

Responses of Two Coastal Algae (Skeletonema costatum and Chlorella vulgaris) to Changes in Light and Iron Levels1.

Iron (Fe) is essential for phytoplankton growth and photosynthesis, and is proposed to be an important factor regulating algal blooms under replete major nutrients in coastal environments. Here, Skeletonema costatum, a typical red-tide diatom species, and Chlorella vulgaris, a widely distributed Chlorella, were chosen to examine carbon fixation and Fe uptake by coastal algae under dark and light conditions with different Fe levels. The cellular carbon fixation and intracellular Fe uptake were measured via 14 C and 55 Fe tracer assay, respectively. Cell growth, cell size, and chlorophyll-α concentration were measured to investigate the algal physiological variation in different treatments. Our results showed that cellular Fe uptake proceeds under dark and the uptake rates were comparable to or even higher than those in the light for both algal species. Fe requirements per unit carbon fixation were also higher in the dark resulting in higher Fe: C ratios. During the experimental period, high Fe addition significantly enhanced cellular carbon fixation and Fe uptake. Compared to C. vulgaris, S. costatum was the common dominant bloom species because of its lower Fe demand but higher Fe uptake rate. This study provides some of the first measurements of Fe quotas in coastal phytoplankton cells, and implies that light and Fe concentrations may influence the phytoplankton community succession when blooms occur in coastal ecosystems.

Role of organic components in regulating denitrification in the coastal water of Daya Bay, southern China.

Both dissolved and particulate organic materials have been proposed to be important factors in regulating heterotrophic denitrification in various aquatic environments. However, the specific pathways and mechanisms remain elusive. In this study, water column samples were collected from Daya Bay, southern China, to examine the relationships between potential denitrification and different organic components in the water column. Bulk dissolved organic carbon (DOC) was categorized into three major components including terrigenous fluorescent (tFDOC), autochthonous fluorescent (bFDOC) and non-fluorescent (nFDOC) fractions, while the bulk particulate organic carbon (POC) was divided into terrigenous (tPOC) and autochthonous (bPOC) fractions based on an isotope mixing model. Potential denitrification derived from in situ incubation experiments under anoxic conditions was evident (ranging from 6 to 107 nmol N2 per L per h) and varied markedly among stations. When normalized to nitrate concentration, the denitrification rate (NDR) followed a positive trend with either the concentration or proportion of tFDOC, and a negative trend with the proportion of nFDOC, suggesting tFDOC was potentially favorable while nFDOC was unfavorable for denitrifying degradation. In comparison, the NDR showed a significant positive correlation with the proportion of bPOC in the bulk POC (p = 0.01), with a predictive power of >70%, indicating that the composition of POC has a substantial impact on potential denitrification. Furthermore, if both bPOC and suspended particulate matter (SPM) were considered as variables concurrently, the variability of NDR can be better predicted with a predictive power as high as 80%. Therefore, denitrifiers may preferentially utilize fresher and labile autochthonous POC instead of DOC especially in coastal waters where particles/colloids are abundant. Our results thus provide new insights for a better understanding of denitrification mechanisms in water columns and the importance of both suspended particles and POC components in regulating denitrification, especially in turbid and productive coastal environments.

N2 fixation impacted by carbon fixation via dissolved organic carbon in the changing Daya Bay, South China Sea.

We present the first concurrent measurements of N2 fixation rates (15N2 uptake), primary production (14C uptake), dissolved organic carbon (DOC) concentrations, and diazotrophic community composition derived from nitrogenase (nifH) abundance in the subtropical Daya Bay (DB) of the coastal northern South China Sea (NSCS) from 2015 to 2017. N2 fixation rates ranged from n.d. - 4.51 nmol N L-1 h-1. Such values were generally higher than those reported in the neighbouring NSCS open waters and several well-studied oligotrophic waters, thereby suggesting that N-replete conditions do not prevent N2 fixation in coastal waters. N2 fixation rates were positively and significantly correlated with the primary production and the concentration of DOC in DB in the spring and summer. Combined with other lines of evidence, we suggest that N2 fixation may be facilitated by non-diazotrophic phytoplankton via a probable regulation of the quantity and quality (bioavailability) of DOC in DB. Since DB represents a suitable site that has experienced dramatic human-induced changes in environmental conditions, our results likely provide insights in understanding how N2 fixation and relevant biogeochemical processes may respond to intensified global anthropogenic forcing in similar coastal settings.

Effects of particles on potential denitrification in the coastal waters of the Beibu Gulf in China.

Although the influence of suspended particulate matter (SPM) on denitrifying activity has been identified in river waters recently through metabolic incubations and community gene analysis, the regulations of SPM to denitrification in marine systems are still poorly understood. In the present study, the effects of suspended particle properties (including concentration, composition and size) on potential denitrification were explored in the coastal Beibu Gulf water columns based on a series of 15N-labeled incubations under artificial anaerobic condition. A gradient of oxygen (O2) concentrations was also manipulated in the incubated seawaters to test the sensitivity of denitrification to O2 exposure. According to our experiments, potential denitrification was the dominant pathway for N2 production with major contribution (>60%) recovered from the particle-associated (PA) fraction. The Highest rate occurred in the benthic nepheloid layer, where high particle content induced by sediment resuspension were observed, suggesting that resuspended particles may act as a hot spot for marine nitrogen (N) loss. Both content and lability of particulate organic carbon (POC) were tightly related to the denitrification rates, with denitrification enhanced by autochthonous POC fractions more significantly. The PA denitrification was higher on small particles (1.2-10µm) compared to the large ones (>10µm), probably due to larger specific surface area and higher specific POC content in small particles. O2 suppressed denitrifying activity for both bulk water samples and PA fractions. Although in situ denitrifying activity should be minor or neglected because of high-O2 inhibition, the novel findings of particle effects on anaerobic denitrification can still be applied to hypoxic marine environments. Our research also implies that resuspended particles from sediment may act as a hot spot for N loss, and therefore to be a first step toward future studies in high particle loaded marine regimes.

Identification of the earlier human-induced sedimentation change in Daya Bay, northern South China Sea using 210Pb and 137Cs.

Over the past 30years, the rapid development of the Chinese economy resulted in environmental problems, especially in coastal areas. To discern the effects of anthropogenic activities, 210Pb and 137Cs were examined in the sediment from Daya Bay, northern South China Sea. The specific activity of 137Cs showed a clear maximum, corresponding to 1963. 210Pb specific activity varied from 25.1 to 78.5Bq kg-1. 210Pb distribution showed a hiatus at 18-19cm with 5-6cm of older sediment (>150years), indicating direct land-originating material over a short timescale rather than natural processes. This event was attributed to the human-induced redistribution of sediment during reclamation. Based on the 137Cs-labeled 1963 and 210Pb-chronologies, this event was confined to late 1977, earlier than the generally recognized significant anthropogenic activities. Thus, information archived in the sediment or in records prior to the 1970s would be better environmental background in Daya Bay.

A potential nitrogen sink discovered in the oxygenated Chukchi Shelf waters of the Arctic.

The western Arctic Shelf has long been considered as an important sink of nitrogen because high primary productivity of the shelf water fuels active denitrification within the sediments, which has been recognized to account for all the nitrogen (N) removal of the Pacific water inflow. However, potentially high denitrifying activity was discovered within the oxygenated Chukchi Shelf water during our summer expedition. Based on 15N-isotope pairing incubations, we estimated denitrification rates ranging from 1.8 ± 0.4 to 75.9 ± 8.7 nmol N2 L-1 h-1. We find that the spatial pattern of denitrifying activity follows well with primary productivity, which supplies plentiful fresh organic matter, and there was a strong correlation between integrated denitrification and integrated primary productivity. Considering the active hydrodynamics over the Chukchi Shelf during summer, resuspension of benthic sediment coupled with particle-associated bacteria induces an active denitrification process in the oxic water column. We further extrapolate to the whole Chukchi Shelf and estimate an N removal flux from this cold Arctic shelf water to be 12.2 Tg-N year-1, which compensates for the difference between sediment cores incubation (~ 3 Tg-N year-1) and geochemical estimation based on N deficit relative to phosphorous (~ 16 Tg-N year-1). We infer that dynamic sediment resuspension combined with high biological productivity stimulates intensive denitrification in the water column, potentially creating a nitrogen sink over the shallow Arctic shelves that have previously been unrecognized.

Abundance and sinking of particulate black carbon in the western Arctic and Subarctic Oceans.

The abundance and sinking of particulate black carbon (PBC) were examined for the first time in the western Arctic and Subarctic Oceans. In the central Arctic Ocean, high PBC concentrations with a mean of 0.021 ± 0.016 µmol L(-1) were observed in the marginal ice zone (MIZ). A number of parameters, including temperature, salinity and (234)Th/(238)U ratios, indicated that both the rapid release of atmospherically deposited PBC on sea ice and a slow sinking rate were responsible for the comparable PBC concentrations between the MIZ and mid-latitudinal Pacific Ocean (ML). On the Chukchi and Bering Shelves (CBS), PBC concentrations were also comparable to those obtained in the ML. Further, significant deficits of (234)Th revealed the rapid sinking of PBC on the CBS. These results implied additional source terms for PBC in addition to atmospheric deposition and fluvial discharge on the western Arctic shelves. Based on (234)Th/(238)U disequilibria, the net sinking rate of PBC out of the surface water was -0.8 ± 2.5 µmol m(-3) d(-1) (mean ± s.d.) in the MIZ. In contrast, on the shelves, the average sinking rate of PBC was 6.1 ± 4.6 µmol m(-3) d(-1). Thus, the western Arctic Shelf was probably an effective location for burying PBC.

Influence of a Decaying Cyclonic Eddy on Biogenic Silica and Particulate Organic Carbon in the Tropical South China Sea Based on 234Th-238U Disequilibrium.

Eddies play a critical role in regulating the biological pump by pumping new nutrients to the euphotic zone. However, the effects of cyclonic eddies on particle export are not well understood. Here, biogenic silica (BSi) and particulate organic carbon (POC) exports were examined inside and outside a decaying cyclonic eddy using 234Th-238U disequilibria in the tropical South China Sea. For the eddy and outside stations, the average concentrations of BSi in the euphotic zone were 0.17±0.09 µmol L-1 (mean±sd, n = 20) and 0.21±0.06 µmol L-1 (n = 34). The POC concentrations were 1.42±0.56 µmol L-1 (n = 34) and 1.30±0.46 µmol L-1 (n = 51). Both BSi and POC abundances did not show change at the 95% confidence level. Based on the 234Th-238U model, BSi export fluxes in the eddy averaged 0.18±0.15 mmol Si m-2 d-1, which was comparable with the 0.40±0.20 mmol Si m-2 d-1 outside the eddy. Similarly, the average POC export fluxes were 1.5±1.4 mmol C m-2 d-1 and 1.9±1.3 mmol C m-2 d-1 for the eddy and outside stations. From these results we concluded that cyclonic eddies in their decaying phase have little effect on the abundance and export of biogenic particles.