Particulate Organic Matter Research Articles

Food web structures of irrigated rice fields estimated from carbon and nitrogen stable isotopes: Special reference to the role of filamentous green algae as a food resource of aquatic consumers

AbstractRice fields are characterized by high algal production and dense mats of filamentous green algae, and they function as habitats for various aquatic organisms. However, how algal production supports the secondary production of aquatic consumers in rice fields has not been assessed. Herein, aquatic food web structures in two adjacent rice fields in northeastern Japan were examined using carbon and nitrogen stable isotope analyses. Filamentous algae often showed higher δ13C values (mean, −23.1‰ to −21.4‰) than other organic matter sources, including particulate organic matter, benthic microalgae, and soil organic matter (−30.1‰ to −25.4‰). A δ13C‐based isotopic mixing model estimated that among 19 taxa of herbivores and omnivores, most taxa depended on filamentous algae‐derived carbon (ca. >30%). Particularly, Peltodytes intermedius larvae, a specialist feeder for filamentous algae, showed the highest reliance (mean, 66.7%–80.0%). The microcrustacean, Ostracoda sp. also utilized a diet consisting of a substantial proportion of filamentous algae‐derived carbon (57.6%). Seven out of 10 carnivorous taxa showed reliance on filamentous algae‐derived carbon by >30%. Particularly, larvae of beetles (Berosus sp., Enochrus sp., and Hydrochara affinis) showed the highest reliance (40.2%–44.3%). They could assimilate filamentous algae‐based carbon by consuming herbivores, such as P. intermedius. Because fresh filamentous green algae were eaten less by aquatic consumers, the herbivores could consume decomposed filamentous algae rather than fresh ones. Our results suggest that filamentous algae are one of the major organic matter sources supporting secondary production in rice field food webs.

Consumer isoscapes reveal heterogeneous food webs in deep-sea submarine canyons and adjacent slopes

The deep sea is the largest biome on earth, but one of the least studied despite its critical role in global carbon cycling and climate buffering. Deep-sea organisms largely rely on particulate organic matter from the surface ocean for energy – these organisms in turn play critical roles in energy transport, transformation, storage, and sequestration of carbon. Within the deep sea, submarine canyons are amongst the most complex and dynamic environments in our oceans, where varied morphology, powerful currents, and variable nutrient conditions influence the distribution of species and transport of organic material throughout the water column and the seafloor. Significant habitat heterogeneity provides ideal substrates for cold-water corals, making submarine canyons of interest to conservation and management. However, how these and other topographic features in the deep ocean influence energy flow and trophic pathways is poorly known. Thus, submarine canyons serve as model systems to track variability in organic material flux and consequential utilization and assimilation by the benthos. In this study, we used an extensive stable isotope dataset to examine food-web structure in Baltimore and Norfolk submarine canyons and compared them to their adjacent slopes located along the U.S. Atlantic margin. Linear models were used to construct geospatially-explicit consumer isoscapes that predicted variation in carbon and nitrogen isotopes across the canyon-slope seascape, providing a predictive map from which to test hypotheses on the distribution and flow of energy resources, relevant to understanding whole community function. Communities were composed of isotopically diverse feeding groups with photosynthetically-derived organic carbon providing the basal food resource. Canyon communities were distinct from the slope, with canyon consumers significantly 13C-depleted, indicating a greater supply and/or utilization of fresh organic matter compared to the slope. Isoscapes for benthic and suspension feeders were distinct, possibly due to the consumption of different quality organic matter sources (fresh = suspension feeders, old = benthic feeders), each with distinct isotope composition. To our knowledge, our modeled isoscapes represent the first spatially extensive isotopic maps of deep-sea consumers, providing insights into regional-scale variation in stable carbon and nitrogen isotopes for different consumer groups. They provide a baseline for tracking climate-change induced fluctuations in the quality and availability of surface primary production and the consequential impact to benthic communities, which play critical roles in carbon cycling in our world’s oceans.

Exploring contribution of phytoplankton cell death to settleable particulate organic carbon in the East China Sea in spring

Phytoplankton's death contributes to marine settleable particulate organic matter (POM). In this study, we used laboratory cultivation of different algal species to identify a positive correlation between the cumulative number of dead algal cells and POC>75 (carbon content of the settleable POM). The contribution coefficient of cell death to POC>75 varied among different algal species. Additionally, the field survey and incubation experiment were conducted in the East China Sea (ECS) to explore the spatial-temporal correlation between phytoplankton death and POC>75. The results concluded that phytoplankton death was the main factor controlling POC>75. In the ECS, the relationship between the surface cumulative mass of POC>75 and the cumulative number of dead cells followed: Cumulative mass of POC>75(mg) = 0.487 × Cumulative number of dead cells (/104) + 0.069. This study provided a methodology to quantitatively explain the relationship between phytoplankton death and settleable POM.

Application of Fine Bubbles in Biofloc Aquaculture: Towards Environmental Sustainability

Biofloc Technology (BFT) is specifically designed to tackle critical challenges in aquaculture, including the reduction of excessive water usage, minimizing effluent discharge, optimizing nutrient utilization from feed, and strengthening overall biosecurity on farms. This innovative approach utilizes clusters of bacteria, algae, or protozoa within a matrix rich in particulate organic matter to enhance water quality, improve waste management, and control diseases. Given the system loading rates, there is a heightened need for elevated dissolved oxygen levels and optimal flow rates. Acknowledging the limitations of traditional aeration systems, this review hypothesizes employing fine bubbles as a panacea. The article, therefore, condenses information on fine bubble impacts in biofloc with a special focus on faster biofloc establishment, favorable microbial diversity, improved respiratory health, accelerated growth rates, optimized metabolism, improved feed conversion ratios, reducing costs, and enhanced overall aquatic health. The suitability of fine bubbles in diverse aquaculture environments is also explored with highlights on areas for further research to optimize and scale up fine bubble-fueled biofloc as an environmentally friendly aquaculture.

Surface active functional groups enhanced PET non‐woven fabric for the filtering of harmful substances in smoke

AbstractHarmful components in cigarette smoke, such as nicotine, tar, and organic particulate matter, are the primary culprits behind lung diseases. While conventional filter materials based on cellulose, carbon, and molecular sieves exhibit commendable filtration capabilities, their high cost restricts their widespread applications. Based on this, the authors aim to prepare PET‐based filter materials with good adsorption properties through a simple surface functionalisation strategy. The adsorption performance of the PET‐based non‐woven fabric was enhanced by the introduction of sodium alginate (SA). The gas adsorption experiments results reveal that SA‐modified PET (SA‐PET) exhibits significantly improved filtration efficiency for nicotine, tar, and total particulate matter—increasing by 27.1%, 26.2%, and 21.3%, respectively. Moreover, SA‐PET exhibits more odour control ability than traditional activated carbon‐based filter materials. These results prove that surface‐functionalised SA‐PET has better filtration performance for harmful substances in smoke and provides a new strategy for the design of high‐performance filtration materials.

Holocene organic matter composition in relation to relative sea level stands and associated morphometric changes of a lagoonal system

Garzón Lagoon is a temperate shallow coastal lagoon influenced by sea level variation. A study on the isotopic composition of Holocene sedimentary organic matter was performed. Although the system was subject to significant changes in sea level, leading to important changes in the lagoon morphometry during the Holocene, the composition of the organic matter was rather surprisingly constant, being dominated by particulate organic matter of marine origin, and to a lesser extent of terrestrial origin. By ∼5000 yr BP, the sea level was +5 m amsl, and the lagoon displayed the largest morphometric dimensions. Once the sea level decreased towards 3000 yr BP and present times, the lagoon area progressively decreased. Even though we inferred significant changes in morphometry, the composition of the organic matter remained relatively constant, as the lagoon was always connected to the ocean, a condition that remains today, at least intermittently during storms or heavy rains. Only after the 1960s, a slight change in the organic matter composition was detected, consisting of a decrease in particulate organic matter of terrestrial origin.

Trophic relationship between mussels and scale worms under various seepage intensities in the haima cold seep: Insights from stable isotopes (δ13C and δ15N) and C:N:P stoichiometry

The deep-sea mussels Gigantidas haimaensis (Mytilidae: Bathymodiolinae) usually contain one scale worm Branchipolynoe pettiboneae in their mantle cavity in the Haima cold seep, South China Sea. To explore their environmental adaptation and coexistence mechanisms, the stable isotopes (δ13C and δ15N) and C:N:P ecological stoichiometry of G. haimaensis and the associated B. pettiboneae were investigated under different methane seepage intensities. In the presence of seepage, most mussels harbored one scale worm in their mantle cavity. However, under seepage cease, the physiological status of mussels looked unhealthy, and no scale worm appeared in their bodies. The variation in δ13C values was great among different mussel tissues, ranging from −49.7‰ to −57.8‰. The δ13C values of mussel tissues followed the order of foot > gill > mantle under active seepage, while no regular trend was found under seepage cease. The δ13C and δ15N of scale worms were averagely enriched by 2.9 ‰ and 3.2 ‰ relative to their mussel hosts, and the trophic niche separation between scale worms and mussels was more significant under active seepage. The δ13C value of mussel foot was significantly higher under active seepage (average −50.8‰) than under seepage cease (average −55.1‰), indicating that mussels might ingest and assimilate more 13C-rich suspended particulate organic matter (POM) under active seepage. In mantle tissues, the high C:N ratio and low δ13C value should be attributed to the high content of energy storage substances. The C:N ratio of mantle dramatically declined with the exhaustion of energy storage materials under seepage cease. The variation of P content was most significant in the gill, which might be regulated by the abundance of symbiotic bacteria. It can be speculated that the dramatic decline of P content in the gill was attributed to the loss of high-P bacterial symbionts under seepage cease. Both the C:P and N:P ratios of gill tissues significantly increased under seepage cease, which suggested the decline of food quality for scale worms. Mussel-dwelling scale worms might detect the change of food quality and abandon their starving mussel host. Our results help to better understand the environmental adaptation of chemoautotrophic mussels and scale worms under unstable seepage intensity in cold seep ecosystems.

Distribution and stable isotope ratio characteristics of Japanese eel leptocephali in relation to hydrographic structure of their Pacific Ocean spawning area

AbstractTo understand the larval distribution, size variation, and stable isotope ratios of Japanese eel leptocephali in relation to the salinity front and their feeding ecology, larvae from 7 research cruises (2002–2013) in the North Equatorial Current (NEC) spawning area were examined. The smallest early‐stage larvae were distributed south of or near the salinity front, confirming that the salinity front is an important oceanic feature to understand spawning locations of the Japanese eel. Larger size larvae tended to distribute into higher latitudes. Transport to northern latitudes with their growth would facilitate transport into the Kuroshio region, but retention in the Subtropical Countercurrent (STCC) might be detrimental. Preleptocephalus isotope ratios reflected maternal ratios, but feeding‐stage leptocephali (8–56 mm) tended to have higher ∂15N values at lower latitudes typically in areas south of a salinity front. Feeding larvae quickly assimilate isotope ratios from the NEC after spawning and early growth. Large differences of ∂13C values of larvae between the NEC and STCC might vary with spatial baselines in the two currents. However, diel vertical migrations should be considered, because the isotope ratios in particulate organic matter distinctly depend on the depth. Comparisons among Japanese eel larvae and other taxa of leptocephali in the NEC illustrate the need for further studies on the trophic ecology of leptocephali.

Trophodynamics and potential health risk assessment of heavy metals in the mangrove food web in Yanpu Bay, China

Mangroves are the cradle of coastal water biodiversity and are susceptible to heavy metal pollution. However, the trophic transfer mechanism of heavy metals in the mangrove food web and the resulting human health risks are not fully understood. Heavy metal concentration (Cr, Ni, Cu, Zn, As, Cd, Pb, V, Co) and stable isotope ratios of carbon and nitrogen (δ13C and δ15N) were evaluated in sediments and particulate organic matter, litter, and aquatic organisms (plankton, arthropods, mollusks, omnivorous fish, and carnivorous fish) from the Yanpu Bay mangroves. The results revealed that heavy metals exhibited different trophic transfer patterns. As and Hg were efficiently biomagnified, with trophic magnification factors of 1.17 and 1.42, respectively; while Cr, Ni, Cu, Cd, Pb, V, and Co were efficiently biodiluted. Zn exhibited a trophic magnification factor > 1 and was not significantly correlated with δ15N (p > 0.05), suggesting no biomagnification or biodilution. The heavy metals in the important fishery species (omnivorous fish and carnivorous fish) were below the permissible limits, except for Zn in Ophichthus apicalis. The assessment of probabilistic health risks revealed that fish consumption in adults and children posed an acceptable risk (total target hazard quotient <1).

Ensemble analysis and forecast of ecosystem indicators in the North Atlantic using ocean colour observations and prior statistics from a stochastic NEMO–PISCES simulator

Abstract. This study is anchored in the H2020 SEAMLESS project (https://www.seamlessproject.org, last access: 29 January 2024), which aims to develop ensemble assimilation methods to be implemented in Copernicus Marine Service monitoring and forecasting systems, in order to operationally estimate a set of targeted ecosystem indicators in various regions, including uncertainty estimates. In this paper, a simplified approach is introduced to perform a 4D (space–time) ensemble analysis describing the evolution of the ocean ecosystem. An example application is provided, which covers a limited time period in a limited subregion of the North Atlantic (between 31 and 21∘ W, between 44 and 50.5∘ N, between 15 March and 15 June 2019, at a 1/4∘ and a 1 d resolution). The ensemble analysis is based on prior ensemble statistics from a stochastic NEMO (Nucleus for European Modelling of the Ocean)–PISCES simulator. Ocean colour observations are used as constraints to condition the 4D prior probability distribution. As compared to classic data assimilation, the simplification comes from the decoupling between the forward simulation using the complex modelling system and the update of the 4D ensemble to account for the observation constraint. The shortcomings and possible advantages of this approach for biogeochemical applications are discussed in the paper. The results show that it is possible to produce a multivariate ensemble analysis continuous in time and consistent with the observations. Furthermore, we study how the method can be used to extrapolate analyses calculated from past observations into the future. The resulting 4D ensemble statistical forecast is shown to contain valuable information about the evolution of the ecosystem for a few days after the last observation. However, as a result of the short decorrelation timescale in the prior ensemble, the spread of the ensemble forecast increases quickly with time. Throughout the paper, a special emphasis is given to discussing the statistical reliability of the solution. Two different methods have been applied to perform this 4D statistical analysis and forecast: the analysis step of the ensemble transform Kalman filter (with domain localization) and a Monte Carlo Markov chain (MCMC) sampler (with covariance localization), both enhanced by the application of anamorphosis to the original variables. Despite being very different, the two algorithms produce very similar results, thus providing support to each other's estimates. As shown in the paper, the decoupling of the statistical analysis from the dynamical model allows us to restrict the analysis to a few selected variables and, at the same time, to produce estimates of additional ecological indicators (in our example: phenology, trophic efficiency, downward flux of particulate organic matter). This approach can easily be appended to existing operational systems to focus on dedicated users' requirements, at a small additional cost, as long as a reliable prior ensemble simulation is available. It can also serve as a baseline to compare with the dynamical ensemble forecast and as a possible substitute whenever useful.

Jet aircraft lubrication oil droplets as contrail ice-forming particles

Abstract. The radiative characteristics and lifetimes of contrails are dependent on the number concentration of ice-forming particles in the engine exhaust plume. Aircraft gas turbine engines produce a variety of particles, yet it is understood that non-volatile black carbon aggregates are the dominant source of ice-forming particles with typical, fossil-derived jet fuel. However, with cleaner combustion technologies and the adoption of alternative fuels (e.g. hydrogen or synthetic aviation fuel), non-volatile black carbon particle emissions are expected to decrease or even be eliminated. Under these conditions, contrail properties will depend upon the concentration and characteristics of particles other than black carbon. Ultrafine (&lt; 100 nm) jet lubrication oil droplets constitute a significant fraction of the total organic particulate matter released by aircraft; however, their ability to form contrail ice crystals has hitherto been unexplored. In this work, we experimentally investigate the activation and freezing behaviour of lubrication oil droplets using an expansion chamber, assessing their potential as ice-forming particles. We generate lubrication oil droplets with a geometric mean mobility diameter of (100.9 ± 0.6) nm and show that these activate to form water droplets, which subsequently freeze when the temperature is below ∼ 235 K. We find that nucleation on lubrication oil droplets should be considered in future computational studies – particularly under soot-poor conditions – and that these studies would benefit from particle size distribution measurements at cruise altitude. Overall, taking steps to reduce lubrication oil number emissions would help reduce the climate impact of contrail cirrus.

Reassessing the indicator value of the EPT group in karst rivers under hydromorphological pressure

Abstract Hydromorphological degradation is one of the most common stressors to freshwater ecosystems nowadays. Rivers lose riparian vegetation, habitat heterogeneity, natural flow velocity, etc., due to hydromorphological alterations. We analyzed macroinvertebrate communities in a wide range of hydromorphological conditions – from near natural sites to significantly altered water bodies, focusing on Ephemeroptera, Plecoptera, and Trichoptera (EPT). Considering that the EPT group is a quite sensitive and generally stenovalent group, we wanted to examine which of the hydromorphological pressures affects them the most. We also wanted to identify indicator taxa for different levels of degradation: minor, moderate, and severe. We collected samples from 84 karst rivers sites in Croatia. We found 52 taxa of EPT (Ephemeroptera – 21, Plecoptera – 11, Trichoptera – 20). Changes in river morphology proved to be the most important stressor affecting the distribution of the EPT group. Hydrological regulation did not show significance toward the EPT community, possibly due to the karst nature of the rivers studied. The most sensitive EPT taxa were those with the greatest preference for macrophytes and lithal habitats. More tolerant EPT taxa were those with a wide range of habitat preferences and/or taxa that feed on particulate organic matter.

The chemical characteristics and mixing behaviors of particulate organic matter from small subtropical rivers in coastal Gulf of Mexico

Rivers transport a substantial amount of dissolved and particulate organic matter (DOM and POM) to the oceans. In estuaries, the transition between DOM and POM due to dissolution or flocculation and their chemical compositions are key in determining their remineralization potential and thus influence the biogeochemical cycling of carbon and nitrogen. While much effort has been focused on trace metals, minerals, and nutrients in estuaries, the mixing behaviors of POM have been understudied despite their biogeochemical and ecological importance. In this work, water from several rivers from the northwestern Gulf of Mexico was mixed with seawater in different proportions, and the amount as well as the chemical characteristics of POM in the mixtures was measured. POM was characterized by organic carbon and nitrogen contents, carbon and nitrogen stable isotopes (δ13C and δ15N), and total hydrolyzable amino acids (THAA). The results showed that POM in most cases behaved conservatively after the mixing of river water with seawater. However, rapid POM formation was observed in one of the seven experiments, which was conducted in 2021 with Mission River water, a relatively pristine river. The amount of particulate organic carbon formed in this experiment (41 μM) was 12% of the dissolved organic carbon at a salinity of 6. The formed POM was more depleted in 13C and had a higher POC/PON ratio than expected for conservative mixing, suggesting that terrestrial DOM, likely sourced from lignins and/or lipids, may have contributed to the flocculates. The short duration of these laboratory experiments under dim or dark conditions excludes the potential effects from biotic or photochemical processes, and thus the observations should be attributed to the rapid physical transformation of DOM to POM or flocculation, particularly when the riverine OM is contributed more from terrestrial sources than in situ production. Overall, although POM mostly behaved conservatively, substantial POM can be produced from DOM under right conditions, and these results shed new light on the biogeochemical cycling of organic matter in subtropical estuaries.

Soil responses to inclusion of corn, soybean, and cover crops under rainfed conditions in the northern Great Plains

Crop rotations in the northern Great Plains of North America increasingly include corn ( Zea mays L.) and soybean ( Glycine max (L.) Merr.). Use of cover crops, while less extensive, is also increasing given their purported agronomic and environmental benefits. To date, soil responses to the inclusion of corn, soybean, and cover crops in rainfed cropping systems have not been well documented in the region. Therefore, soil properties were evaluated 6 years after establishment of three crop rotations (spring wheat ( Triticum aestivum L.)–soybean (SW–S), spring wheat–corn–soybean (SW–C–S), and spring wheat–corn–cover crop (SW–C–cc)) each split by no and minimum tillage on a Dark Brown Chernozem near Mandan, ND, USA. Soil responses to treatments were subtle and exclusive to the 0–7.6 cm depth. Soil pH was lower in SW–S than SW–C–cc (5.28 vs. 5.48; P = 0.05), SO4-S was greater under SW–C–cc than SW–C–S (13.4 vs. 11.6 g S kg−1; P = 0.03), exchangeable K was greater under SW–C–S and SW–C–cc than SW–S (0.83 cmol kg−1 vs. 0.52 cmol kg−1; P = 0.05), and water-stable aggregates were greater in SW–S than SW–C–S (26% vs. 19%; P = 0.08). Soil organic carbon (SOC) and total N did not differ among crop rotations or between tillage treatments, while particulate organic matter N was greater under no tillage compared to minimum tillage ( P = 0.08). Between 2012 and 2018, soil pH decreased and SOC increased under SW–C–S. Frequent monitoring of near-surface soil conditions in rotations with soybean every other year is recommended. Furthermore, innovative management practices are needed to enhance soil C and N fractions in rotations with full-season cover crops.

Precipitation increment reinforced warming-induced increases in soil mineral-associated and particulate organic matter under agricultural ecosystem

Exploring the dynamics of soil carbon (C) response to multi-faceted global climate change plays a vital role in facilitating the estimation of carbon-climate feedback. However, the direction and extent of the warming effect on soil carbon pool within different soil fractions considered simultaneously in increased precipitation and decreased precipitation system are not well understood. Here we established a 2-year field manipulation experiment based on open top chamber (OTC) to assess the effects of warming and altering precipitation regimes (40 % reduction, normal, and 40 % increment) on soil physical fractions formation and distribution. Our results showed that in the precipitation increment, warming significantly increased particulate organic carbon (POC) and mineral-associated carbon (MAOC) by an average of 48.10 % and 21.28 % in the 0–5 cm soil layer. And the formation mechanism of POC and MAOC were different. POC was directly affected by the coupling between plant input and C-degradation fungal functional genes adjusted by dissolved organic carbon (DOC) and total nitrogen (TN); and MAOC was affected by increasing labile carbon inputs (such as DOC) of maize residue and belowground biomass. However, precipitation reduction neutralized the beneficial effects of warming, mainly through reducing plant inputs and residue decomposition, thereby suppressing C-degradation fungal functional genes and labile carbon supply. Overall, our results provide new insights into the potential mechanisms through which interaction of warming and altered precipitation controls soil C dynamics.

Using Stable Isotopes and Spectral Properties of Particulate and Dissolved Organic Matter to Quantify Typhoon‐Induced Organic Matter Decomposition

AbstractTyphoons exert a profound impact on marine biogeochemical processes, with organic matter (OM) decomposition serving as a key biogeochemical process in coastal waters after typhoons. Despite its significance, the specific contributions of particulate organic matter (POM) and dissolved organic matter (DOM) to OM decomposition triggered by typhoons remains unclear. In this study, stable isotopes of POM and spectral properties of DOM were investigated before and after Typhoon “Barija" in Zhanjiang Bay, northwestern South China Sea. The typhoon‐induced intrusion of high‐salinity seawater from the lower bay to the upper bay, driven by the external clockwise wind stress, led to the formation of an ocean front in the middle bay during the typhoon. Notably, the POM decomposition induced by the typhoon in the upper bay (72%) significantly surpassed that in the lower bay (5%), attributed to the barrier effect of the ocean front and increased vertical mixing in the upper bay. In contrast, the decomposition removed only 29% DOM in the upper bay, and a net addition of DOM occurred in the lower bay due to phytoplankton growth and POM decomposition. More importantly, despite the overall larger DOM in the water column, the POM inventory in the upper bay removed by typhoon‐induced decomposition (20.19 g m−2) exceeded that of DOM (16.08 g m−2). Overall, our study suggests that POM decomposition is more critical than DOM decomposition after typhoons, primarily influenced by the ocean front and vertical mixing. These findings contribute to a clearer understanding of the post‐typhoon biogeochemical dynamics in coastal waters.

Elevated atmospheric CO2 drives decreases in stable soil organic carbon in arid ecosystems: Evidence from a physical fractionation and organic compound analysis.

The increasing concentration of CO2 in the atmosphere is perturbing the global carbon (C) cycle, altering stocks of organic C, including soil organic matter (SOM). The effect of this disturbance on soils in arid ecosystems may differ from other ecosystems due to water limitation. In this study, we conducted a density fractionation on soils previously harvested from the Nevada Desert FACE Facility (NDFF) to understand how elevated atmospheric CO2 (eCO2 ) affects SOM stability. Soils from beneath the perennial shrub, Larrea tridentata, and from unvegetated interspace were subjected to a sodium polytungstate density fractionation to separate light, particulate organic matter (POM, <1.85 g/cm3 ) from heavier, mineral associated organic matter (MAOM, >1.85 g/cm3 ). These fractions were analyzed for organic C, total N, δ13 C and δ15 N, to understand the mechanisms behind changes. The heavy fraction was further analyzed by pyrolysis GC/MS to assess changes in organic compound composition. Elevated CO2 decreased POM-C and MAOM-C in soils beneath L. tridentata while interspace soils exhibited only a small increase in MAOM-N. Analysis of δ13 C revealed incorporation of new C into both POM and MAOM pools indicating eCO2 stimulated rapid turnover of both POM and MAOM. The largest losses of POM-C and MAOM-C observed under eCO2 occurred in soils 20-40 cm in depth, highlighting that belowground C inputs may be a significant driver of SOM decomposition in this ecosystem. Pyrolysis GC/MS analysis revealed a decrease in organic compound diversity in the MAOM fraction of L. tridentata soils, becoming more similar to interspace soils under eCO2 . These results provide further evidence that MAOM stability may be compromised under disturbance and that SOC stocks in arid ecosystems are vulnerable under continued climate change.

Aggregating Synechococcus contributes to particle organic carbon export in coastal estuarine waters: Its lineage features and assembly processes

The release and deposition of phytoplankton-derived particulate organic matter is crucial in marine carbon export, yet the roles of picoplankton in these processes were seldom considered. Therefore, this study aimed to shed light on the matter by investigating the aggregating (AG) lifestyle of Synechococcus, a main group of picoplankton, in the coastal waters of the Yellow River Estuary with ample sediments acting as ballast minerals. We revealed that AG Synechococcus constituted a substantial portion, maximally reaching up to 85.4 %, of the total Synechococcus population. Pearson correlations and random forest (RF) regression analyses found significant connections (p < 0.01) between AG Synechococcus and the content of particulate organic carbon (POC), which emphasized its underlying role in facilitating POC export in this region. Furthermore, by employing high-throughput sequencing of the RNA polymerase gene (rpoC1), it was demonstrated that S5.1 clade I exhibited a significantly higher proportion in the AG fraction than in the free-living (FL) fraction (p < 0.05). This suggests distinct inclinations in the phylogenetic preference for different Synechococcus lineages between different lifestyles in the studied area. Finally, we ascertained “small-world” and higher robustness attributes of aggregates formed through the co-occurrence construction between Synechococcus and heterotrophic bacteria, likely facilitated by the reciprocal exchange of carbon and nitrogen elements. Overall, these findings have implications for our understanding of the role of Synechococcus in the ecology and biogeochemistry of marine ecosystems, and they are significant for more accurately evaluating the contribution of picophytoplankton in ocean carbon export.

Mariculture may intensify eutrophication but lower N/P ratios: a case study based on nutrients and dual nitrate isotope measurements in Sansha Bay, southeastern China

The mariculture industry has grown rapidly worldwide over the past few decades. The industry helps meet growing food demands and may provide an effective means of carbon sequestration; however, it may harm the marine ecological environment, and the extent of its impact depends on the type of mariculture. Here we focus on the impact of mariculture on the nutrient status and eutrophication in Sansha Bay, which is a typical aquaculture harbor in southeastern China that employs a combination of shellfish and seaweed farming. Nutrient concentrations and dual nitrate isotopes were measured in Sansha Bay during the winter of 2021. The average concentrations of nitrate and phosphate were 31.3 ± 10.5 and 2.26 ± 0.84 µM, respectively, indicating that the water was in a eutrophic state. However, the N/P ratios were relatively low (14.3 ± 2.2). Nitrate isotope measurements were 8.8‰–11.9‰ for δ15N-NO3− and 2.2‰–6.0‰ for δ18O-NO3−. Source analysis based on the nitrate isotope measurements indicates that nitrate in Sansha Bay is derived mainly from the excretion of organisms and sewage discharge from mariculture. The isotopic fractionation model of nitrate assimilation by organisms indicates that surface waters in Sansha Bay experience strong biological uptake of nitrate, which is likely related to seaweed farming in winter. The low N/P ratios may be attributed to excessive nitrogen uptake (relative to phosphorus) during shellfish and seaweed farming, as well as nitrogen removal through sediment denitrification, which is fueled by the sinking of particulate organic matter from mariculture. Overall, our study shows that mariculture activities dominated by shellfish and seaweed cultivation in Sansha Bay may exacerbate eutrophication but reduce N/P ratios in the water column in aquaculture areas.

Trophic structure of fish assemblages in two offshore islands (Ulleungdo and Dokdo) of Korea revealed using stable isotope analysis

The objective of this study was to characterize the trophic structure of fish assemblages on the coasts of offshore islands and the eastern mainland of the Korean Peninsula. We compared the seasonal variability in the trophic structure of fish assemblages between the coasts of two island sites (Ulleungdo and Dokdo) and one mainland site (Hupo), which are on a similar latitude. We analyzed the stable carbon and nitrogen isotope ratios (δ13C and δ15N) of fish assemblages during spring (April) and summer (August) 2021. No temporal differences in the isotope values of fish and basal resources (i.e., suspended particulate organic matter (SPOM)) were found over the sampling period at the Hupo site. In contrast, at the Ulleungdo and Dokdo sites, the fishes and SPOM showed seasonal differences in the δ13C and δ15N values between the two seasons. In particular, the fish δ15N values at the island sites were relatively higher in summer compared to those in spring, suggesting the seasonal variation in the food chains and/or trophic status between consumers and their dietary sources. These regional isotopic variations also result in differences in the seasonal tendencies of the isotopic niche parameters of fish assemblages between the mainland and island coasts. Such differences in the seasonal isotopic patterns of fish assemblages suggest a relatively substantial shift in the dietary resources available to fish consumers on island coasts compared to those on the mainland coast. Overall, our results suggest that fish assemblages in offshore island coasts have distinct seasonal variability in trophic characteristics in response to changing environmental conditions, including basal resources, compared with fish food webs on the mainland coast at similar latitudes.