Effects of abalone (Haliotis discus hannai Ino) and kelp (Saccharina japonica) mariculture on sources, distribution, and preservation of sedimentary organic carbon in Ailian Bay, China: Identified by coupling stable isotopes (δ13C and δ15N) with C/N ratio analyses

Seasonal effects of fish, seaweed and abalone cultures on dissolved organic matter and carbon sequestration potential in Sansha Bay, China

Coastal bays serve as undeniable dissolved organic matter (DOM) reactors and the role of prevalent mariculture in DOM cycling deserves investigation. This study, based on four seasonal field samplings and a laboratory incubation experiment, examined the source and seasonal dynamics of DOM and fluorescent dissolved organic matter (FDOM) in the seawater of fish (Larimichthys crocea, LC), seaweed (Gracilaria lemaneiformis, GL) and abalone (Haliotis sp., HA) culturing zones in Sansha Bay, China. Using three-dimensional fluorescence spectroscopy coupled with parallel factor analysis (EEMs-PARAFAC), three fluorescent components were identified, i.e. protein-like C1, protein-like C2, and humic-like C3. Our results showed that mariculture activities dominated the DOM pool by seasonal generating abundant DOM with lower aromaticity and humification degrees. Accounting for 40-95 % of total fluorescent components, C1 (Ex/Em = 300/340 nm) was regarded the same as D1 (Ex/Em = 300/335 nm) identified in a 180-day degradation experiments of G. lemaneiformis detritus, indicating that the cultured seaweed modulated DOM through the seasonal production of C1. In addition, the incubation experiment revealed that 0.7 % of the total carbon content of seaweed detritus could be preserved as recalcitrant dissolved organic carbon (RDOC). However, fish culture appeared to contribute to liable DOC and protein-like C2, exerting a substantial impact on DOM during winter but making a negligible contribution to carbon sequestration, while abalone culture might promote the potential export and sequestration of seaweed-derived carbon to the ocean. Our results highlight the influences of mariculture activities, especially seaweed culture, in shaping DOM pool in coastal bays. These findings can provide reference for future studies on the carbon accounting of mariculture.

Effect of organic matter on the environmental behavior of sulfur and heavy metals in mariculture sediments during the aging process

Organic matter (OM) significantly impacts the environmental behavior of sulfur and heavy metals. In this study, the effects of OM on the migration and transformation of sulfur and heavy metals in mariculture sediments were investigated. The results indicated that baiting had a strong impact on the accumulation of acid volatile sulfur (AVS) (P < 0.05) and increased the environmental risk of sulfide in sediments. The addition of bait promoted the generation of chromium (II)-reducible sulfur (CRS); however, the resistance of AVS to CRS conversion increased with increasing bait addition. The addition of bait considerably influenced Cd accumulation. The acid-soluble fractions of Cr and Cu and the oxidizable fraction of Cd were primarily affected by the bait addition (coefficient of variation>15 %). An increase in the reducible fraction promoted the conversion of AVS to CRS, which reduced the degree of sediment aging. Higher OM levels reduced the diversity and abundance of the bacterial communities. The sulfate respiration functional microbiota was particularly affected by OM.

Carbon removal and climate change mitigation by seaweed farming: A state of knowledge review

The pressing need to mitigate the effects of climate change is driving the development of novel approaches for carbon dioxide removal (CDR) from the atmosphere, with the ocean playing a central role in the portfolio of solutions. The expansion of seaweed farming is increasingly considered as one of the potential CDR avenues among government and private sectors. Yet, comprehensive assessments examining whether farming can lead to tangible climate change mitigation remain limited. Here we examine the results of over 100 publications to synthesize evidence regarding the CDR capacity of seaweed farms and review the different interventions through which an expansion of seaweed farming may contribute to climate change mitigation. We find that presently, the majority of the carbon fixed by seaweeds is stored in short-term carbon reservoirs (e.g., seaweed products) and that only a minority of the carbon ends up in long-term reservoirs that are likely to fit within existing international accounting frameworks (e.g., marine sediments). Additionally, the tiny global area cultivated to date (0.06 % of the estimated wild seaweed extent) limits the global role of seaweed farming in climate change mitigation in the present and mid-term future. A first-order estimate using the best available data suggests that, at present, even in a low emissions scenario, any carbon removal capacity provided by seaweed farms globally is likely to be offset by their emissions (median global balance net emitter: -0.11 Tg C yr-1; range -2.07-1.95 Tg C yr-1), as most of a seaweed farms' energy and materials currently depend on fossil fuels. Enhancing any potential CDR though seaweed farming will thus require decarbonizing of supply chains, directing harvested biomass to long-term carbon storage products, expanding farming outside traditional cultivation areas, and developing robust models tracing the fate of seaweed carbon. This will present novel scientific (e.g., verifying permanence of seaweed carbon), engineering (e.g., developing farms in wave exposed areas), and economic challenges (e.g., increase market demand, lower costs, decarbonize at scale), many of which are only beginning to be addressed.

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.

Spatiotemporal distributions of poorly-bound heavy metals in surface sediments of a typical subtropical eutrophic estuary and adjacent bay

The toxicity of heavy metals is dependent on their bioavailability. This study explored the relationship existing among sedimentary nutrients such as bulk nitrogen (TN) and phosphorus (TP), organic carbon (OC), water column chlorophyll-a (Chl-a) and the poorly-bound fraction of sedimentary heavy metals (Cd, Ni, Zn, Cu, Pb and Cr) in the Dafengjiang River Estuary and adjacent Sanniang bay in 2017 and 2018. Results showed that the texture of the surface sediments was dominated by coarse sand, while sedimentary organic matter was dominated by marine phytoplankton and mariculture biodeposits. Surprisingly, concentrations of poorly-bound heavy metals in sediments were relatively high. The average contents of Cd and Ni did not vary both spatially and temporally, Cu and Pb only varied spatially, Cr varied both spatially and temporally, while Zn only varied temporally. Significant positive correlations occurred between sedimentary TN, TP, and OC, including water column Chl-a and poorly-bound heavy metals in sediments. As sediments are important sources of nutrients for primary productivity, the results of this study suggest that the remobilization of sequestered poorly-bound heavy metals in surface sediments deposited in shallow eutrophic estuaries and coastal waters enriched by labile organic matter can enhance by nutrients. The relationship between the poorly-bound heavy metals and nutrients in surface sediments and water column Chl-a is concerning and requires further in-depth investigation. This is because estuaries are economically important ecosystems rich in bioresources, characterized by dynamic biogeochemical conditions.

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.

Variability of biochemical compounds in surface sediments along the eastern margin of the Arabian Sea

Different fractions of organic matter in surface sediments from three transects along the eastern margin of the Arabian Sea (AS) were quantified to determine the sources of organic matter, and also to study its impact on microbial community structure. From the extensive analyses of different biochemical parameters, it was evident that the distribution of total carbohydrate (TCHO), total neutral carbohydrate (TNCHO), proteins, lipids, and uronic acids (URA) concentrations and yield (% TCHO-C/TOC) are affected by organic matter (OM) sources and microbial degradation of sedimentary OM. Monosaccharide compositions from surface sediment was quantified to assess the sources and diagenetic fate of carbohydrates, suggesting that the deoxysugars (rhamnose plus fucose) had significant inverse relationship (r = 0.928, n = 13, p < 0.001) with hexoses (mannose plus galactose plus glucose) and positive relationship (r = 0.828, n = 13, p < 0.001) with pentoses (ribose plus arabinose plus xylose). This shows that marine microorganisms are the source of carbohydrates and there is no influence of terrestrial OM along the eastern margin of AS. During the degradation of algal material, the hexoses seem to be preferentially used by heterotrophic organisms in this region. Arabinose plus galactose (glucose free wt %) values between 28 and 64 wt% indicate that OM was derived from phytoplankton, zooplankton, and non-woody tissues. In the principal component analysis, rhamnose, fucose, and ribose form one cluster of positive loadings while glucose, galactose, and mannose form another cluster of negative loadings which suggest that during OM sinking process, hexoses were removed resulting in increase in bacterial biomass and microbial sugars. Results indicate sediment OM to be derived from marine microbial source along the eastern margin of AS.

Differences in the composition, source, and stability of suspended particulate matter and sediment organic matter in Hulun Lake, China

The occurrence, migration, transformation, and stability of sediment (SOM) and suspended particulate (SPOM) organic matters have important effects on the environmental behaviors of carbon, nitrogen, phosphorus, and other pollutants in a water environment. The content, composition, fluorescence characteristics, source, and stability of SOM and SPOM in Hulun Lake, a typical lake in cold and arid region of China, were compared by sequential extraction, three-dimensional fluorescence spectroscopy, parallel factor technique, carbon-nitrogen ratio, and stable carbon isotope. Contents of SOM and SPOM in north and west were higher than those in east and south. The average content of SPOM (24.70 ± 4.63 g/kg) was slightly higher than that of SOM (23.04 ± 10.27 g/kg), but the difference was not significant. Humin was the dominant component in SOM and SPOM, accounting for 73.7% and 61.2%, respectively. Humus was the main fluorescence component of water-extractable organic matter in SOM and SPOM, accounting for 79.9% and 70.4%, respectively, of the total fluorescence intensity. SOM and SPOM were derived from terrestrial sources with a relative contribution rate of about 70%. SPOM was more influenced by autochthonous sources and had a significantly lower humification degree and stability than SOM. Effects of climate changes on migration, transformation, stability, and bioavailability of organic matters and endogenous pollutants closely related to organic matters in lakes of cold and arid regions should be paid attention in the future.

Anthropogenic impacts on the temporal variation of heavy metals in Daya Bay (South China)

A detailed study of a sediment core from Daya Bay (South China) has revealed three stages of heavy metal deposition over the past century. Prior to the 1980s, heavy metal concentrations were low with limited influence by human activities. From the 1980s to 2000, metal pollution intensified, and anthropogenic activities, such as oil and petrochemical industries, and fuel combustion, had the greatest direct influence on Hg, Ni, Pb, and Zn concentrations, whereas atmospheric deposition and mariculture were also contributors to the continued increase in Cr, Cu, Pb, Zn, and Ni. Since the year 2000, heavy metal concentration has declined and stabilized. It is noteworthy that anthropogenic input of Cu and Pb is ongoing and may result in a moderate pollution risk. Both modified pollution index (MPI) and modified ecological risk (MRI) consistently indicate that the ecological risk in terms of heavy metals in Daya Bay has remained moderate over the past 70 years.

Ecological risks assessment of sulfur and heavy metals in sediments in a historic mariculture environment, North Yellow Sea

The environment behaviors of sulfur and heavy metals in sediments are closely related to sediment aging in mariculture area. In this study, the distributions and ecological risks of reduced inorganic sulfur (RIS) and heavy metals were investigated, along with the relationships between different occurrences of RIS and heavy metals. The results indicated that the adequate organic matter in mariculture sediments significantly enhanced the accumulation of acid volatile sulfur (AVS) compared to the control area. In shellfish farming area, biological sedimentation contributed to accumulation of AVS. The chromium (II)-reducible sulfur (CRS) was the main component of RIS in mariculture area. The environmental risks of heavy metals in mariculture area presented low levels. Principal component analysis (PCA) showed that distribution of Cu closely related to mariculture activities compared to other heavy metals. For ecological risks of heavy metals, the ratio of ∑(acid-soluble fraction (F1) + reducible fraction (F2) + oxidizable fraction (F3))/AVS was the appropriate index rather than conventional simultaneous extraction of heavy metals (SEM)/AVS, because SEM/AVS would overestimate the toxicity of heavy metals. AVS/RIS ratios significantly positively correlated with Pb (F2/(F1 + F2 + F3 + residual fraction (F4)), F2/∑F), Pb (F3/∑F), and Zn (F3/∑F), while significantly negatively correlated with Pb (F4/∑F) and Cu (F1/∑F). These results indicated that the accumulation of AVS during the mariculture process was conducive to the formation of F2 and F3 of Pb, and F3 of Zn, conversely to the formation for F4 of Pb and F1 of Cu, because it was opposite to the accumulation of CRS.

Influence of sedimentary organic matter sources on the distribution characteristics and preservation status of organic carbon, nitrogen, phosphorus, and biogenic silica in the Daya Bay, northern South China Sea

Surface sediment samples were collected from Daya Bay in October 2018, and analyzed for total organic carbon (OC), total nitrogen (TN) and their stable isotopes (δ¹³C and δ¹⁵N), total phosphorus (TP), biogenic silica (BSi), sediment textures and specific surface area (SSA). The primary objective was to evaluate the influence of mariculture/aquaculture on the distribution characteristics of organic matter (OM), and preservation status of OC, TN, TP, and BSi in sediments. The average δ¹³C and δ¹⁵N values, and OC/TN ratios were -21.27‰, 6.74‰, and 8.90, respectively. Monte Carlo simulation results revealed that mariculture/aquaculture biodeposits accounted for >40% of the buried OM at sites where the breeding rafts and cages are located, whereas marine OM increased gradually to the open sea. Terrestrial OM was generally low accounting for 17% by average. The contents and distribution characteristics of biogenic elements were more influenced by mariculture/aquaculture and primary productivity than sediment textures. Lower OC/SSA (0.3-1.2 mg OC/m²), TN/SSA (~0.05-0.18 mg TN/m²), and TP/SSA (0.02-0.04 mg TP/m²) loadings indicated that increased sequestration of labile OM in a coastal bay could contribute to significant degradation of recalcitrant OM in sediments with significant loss of P relative to OC. Nitrogen contamination in surface sediments was due to increased injection of aquaculture biodeposits, and may pose a detrimental effect on the ecological sustainability of the bay. Higher BSi/SSA loadings (0.9-1.7 mg BSi/m²) revealed that BSi was more preserved, and that BSi-based proxy could be used for paleo-productivity studies. However, such preservation may induce adverse dissolved silicate limitation in a bay perturbed by eutrophication. Fine-grained sediments (clay and silt) accounted for >77% of the sediment texture types with higher SSA, and while controlling the contents of biogenic elements under given depositional conditions were not the main determining factors of OC, TN, TP, and BSi preservation.

Factors influencing the occurrence and distribution of microplastics in coastal sediments: From source to sink

Microplastic (MP) pollution is attracting growing global attention, but little is known about the factors influencing MP occurrence and distributions in marine sediments. Here, MPs were sampled from the sediments of two semi-enclosed bays (Jinghai Bay and Laizhou Bay) and two coastal open zones (Lancelet Reserve and Solen grandis Reserve) in China. The order of MP abundance was Jinghai Bay > Laizhou Bay > Lancelet Reserve > Solen grandis Reserve. Average MP diversity indices for Laizhou Bay (1.84 ± 0.18), Lancelet Reserve (1.59 ± 0.43), S. grandis Reserve (1.58 ± 0.89), and Jinghai Bay (1.43 ± 0.14) revealed Laizhou Bay had the most complicated MP sources. A significant negative correlation between MP abundance and sediment grain size occurred in the semi-enclosed coastal zones (p = 0.004, r = -0.618) rather than in the open coastal zones (p = 0.051, r = -0.480), indicating small sediment particles can strongly enhance MP accumulation in semi-enclosed costal sediments. Although anthropogenic activities influence the MP distribution at source, the composition of regional and local sediments might impact MP occurrence in semi-enclosed coastal zones from the sink. These results help to improve our understanding of the fate and inventory of MPs in coastal sediments.

Anthropogenic impact on the organic carbon sources, transport and distribution in a subtropical semi-enclosed bay

Suspended particulate organic carbon (POC) and sedimentary total organic carbon (TOC) in coastal areas play critical roles in the global carbon cycle, yet sources and dynamics of coastal POC and TOC have been affected by various anthropogenic activities such as aquaculture, sewage discharge, dam construction and land reclamation. To better understand the anthropogenic impacts on coastal organic carbon, this study was carried out in a representative semi-enclosed bay, Dongshan Bay, Southeast China. Through analyses of stable isotopic compositions of both POC (δ¹³C_POC and δ¹⁵N_PN) and TOC (δ¹³C_TOC and δ¹⁵N_TN), the ratio of total organic carbon vs. total nitrogen (C/N), grain size, Chl-a concentrations and hydrological parameters, our study led to the following main findings: 1) During flood season, the distribution of δ¹³C_POC, δ¹³C_TOC, δ¹⁵N_PN and δ¹⁵N_TN values within the bay did not follow the conventional land-sea transition pattern. This distribution pattern indicated more terrestrial organic matter input seaward, which contrasts with the conventional organic matter distribution along the estuarine gradient. 2) Using the organic δ¹³C, δ¹⁵N and C/N signatures of different endmembers, we found that the sources of organic matter deposited in the bay were strongly related to anthropogenic activities, including municipal wastewater discharge, aquaculture, land reclamation and sluice-dyke construction. Furthermore, 3) by applying the Grain Size Trend Analysis Model and the previously-estimated residual current directions, we suggested that human activities have not only altered the sources of organic matter to the semi-enclosed bays, but also significantly modified their transportation and deposition patterns, and might influence the ultimate fate of organic matter into and out of Dongshan Bay. The conclusions of this study should be applicable to similar coastal bays around the world.

Spatiotemporal variations of biogenic elements and sources of sedimentary organic matter in the largest oyster mariculture bay (Maowei Sea), Southwest China

China is the largest mariculture producer in the world, but detailed information on the spatiotemporal variations of biogenic elements and sources of sedimentary organic matter (SOM) via mariculture is limited. The primary objective of this study was to assess the influence of mariculture on the origin of SOM in relation with biogenic elements and geochemical paramaters due to the importance of SOM as a potential source of nutrients and energy in coastal marine environments. Surface sediments from the Maowei Sea were collected in August (summer) and December (winter), 2016 for grain size, total organic carbon (TOC), total nitrogen (TN), organic phosphorus (OP), biogenic silica (BSi), δ¹³C and δ¹⁵N analyses. Significant correlation (p < 0.01) was observed between TOC and TN in summer and winter respectively, indicating that they have common source in both seasons. The spatiotemporal distributions of TOC, TN, OP and BSi were influenced by the sources and distribution of SOM, grain sizes and hydrodynamic conditions in the Maowei Sea. The overall ranges of δ¹³C (-26.86‰ to -23.01‰) and δ¹⁵N (2.54‰ to 9.82‰) and C/N ratio (5.83 to 18.67) showed that SOM is derived from mixed sources. The δ¹³C and δ¹⁵N-based three-end-member mixing model results revealed that >40% of the deposited SOM originates from terrestrial source during two seasons. The SOM from shellfish mariculture was seasonal, mainly deposited in the intensive mariculture areas, and its proportions were only higher than contributions from marine plankton in summer. Generally, this study indicates that shellfish biodepositions can significantly influence the cycle of carbon and other biogenic elements in the intensive mariculture areas. Nevertheless, the overall dominance of terrestrial and marine SOM suggests that the sources of SOM and factors influencing carbon cycling in the Maowei Sea do not exclusively depend on the intensity of mariculture activities.

Spatial and Seasonal Variations of Sedimentary Organic Matter in a Subtropical Bay: Implication for Human Interventions

Elemental (total organic carbon (TOC) and total nitrogen (TN)) and stable carbon and nitrogen isotope compositions (δ¹³C and δ¹⁵N, respectively) in the surface sediment of Zhanjiang Bay (ZJB) in spring and summer were measured to study the spatial and seasonal changes of organic matter (OM) and assess the human-induced and environment-induced changes in the area. The OM in the surface sediment of ZJB was a mixture of terrestrial and marine sources, and was dominated by marine OM (54.9% ± 15.2%). Compared to the central ZJB, the channel and coastal ZJB areas had higher δ¹³C and δ¹⁵N values, higher TOC and TN concentrations, and lower TOC/TN ratios, indicating higher primary productivity and higher percentages of marine OM in the latter two subregions. Mariculture activities, sewage inputs, and dredging were responsible for these phenomena. Clear seasonal variations in OM were observed in ZJB. The average proportions of terrestrial OM in summer increased by 10.2% in the ZJB channel and 26.0% in the coastal ZJB area compared with those in spring. Heavy rainfall brought a large amount of terrestrial OM into the channel and coastal ZJB areas, leading to the increase of the terrestrial OM fraction in these two subregions in summer. In summary, anthropogenic influences had a significant influence on the spatial and seasonal variations of sedimentary OM in ZJB.