Sediment Organic Matter Content Research Articles

To change or not to change: macrozoobenthic community variation versus persistence in Tortolì lagoon (Sardinia, Italy)

In the framework of the complexity of transitional ecosystem dynamics, which strongly drive ecological processes and their effects on communities and ecosystem functioning, the seasonal changes in 2021 of macrobenthic community were analysed at four sites of the Tortolì lagoon (Sardinian, Italy), which were characterised in terms of temperature, salinity, organic matter content of sediment and dissolved oxygen. Two macrobenthic community conditions separated by 20 years, between 2003 and 2021, were also compared. Both the composition of the community in terms of species and the main functional traits, related to the relationship with the sediment, feeding mode and development type, were investigated. The objective was to identify which aspects (species composition/functional species traits) of the benthic community are most responsible for changes on an annual and multi-year scale, and which persist two decades later. The benthic assemblage was extraordinarily rich in species (86 species in 2021 and 124 species in total over the two years). Most of species were typical of coastal sheltered waters and properly marine, while opportunistic and brackish/halolimnobic species were scarce; two alien Isopods (Paracerceis sculpta, Mesanthura sp.) and the new record of Enteropneusta (Glossobalanus minutus) were also reported. Differences between seasons and sampling sites resulted significant sources of variation and marine species mainly contributed to discriminate sites closer to the sea, while the others were especially distributed in the confined sites. The comparison between 2003 and 2021 highlighted that the species richness remained high in the two periods, but significant changes in the species composition of the benthic community, explained in terms of a 50 % species turnover rate. This was attributed to different dispersal modes, which proved that new colonisers arrived in the lagoon from the adjacent marine area by active (via larvae) and passive (adult rafting, human-mediated activities) means. The persistence in terms of functional groups composition and diversity of the macrobenthic community after twenty years was also evidenced. The considerable diversification of functional groups in the benthic food web, together with the high level of species richness and taxonomic diversity, is assumed to be indicative of an efficient degree of ecosystem functioning of the Tortolì lagoon 20 years later.

Water Column Nitrogen Removal During Storms in a Low‐Order Watershed

AbstractWater column removal in streams is a nitrogen (N) cycling pathway that has been historically overlooked. Studies filling this knowledge gap have focused on the role of water column N removal in mid‐to‐large‐order rivers with consistently high suspended sediment concentrations. However, smaller streams may provide comparable suspended sediment concentrations during and after storm events, creating favorable conditions for water column N removal. To assess the presence, magnitude, and control of water column N removal during storms in low‐order watersheds, we measured water column denitrification and heterotrophic assimilatory N uptake rates at three locations in a Mid‐Atlantic watershed during five storm events of different magnitude, sediment loads, and nutrient availability. We found large variations in water column denitrification (0–5.56 mg N g−1 d−1) and assimilatory uptake (0.003–1.67 mg N g−1 d−1). Higher rates of N removal occurred during flow recession, with a correlation between suspended sediment organic matter content and denitrification. On average, denitrification rates in the water column were higher when flashy responses to storm events occurred. In contrast to denitrification, water column N removal rates (as both denitrification and heterotrophic assimilation) during storm events were comparable to those measured at baseflow in larger rivers. However, water column denitrification could only account for less than 10% of potential reach‐scale N removal during most of the storm events. Our findings provide insight into the ecological relevance of small stream water columns and suggest that more research is needed to understand the magnitude of stream water column processing on watershed‐scale N removal.

Characterization of sediment organic matter in the outer Yangtze River Estuary using stable isotopes, optical techniques, and FT-ICR-MS: Implications for the carbon burial mechanism

The burial of sediment organic matter (SOM) in the estuary and shelf plays an important role in the global carbon cycle. However, it is challenging to determine the source, composition, and burial of SOM in the coastal sea, especially at the molecular level. This was explored in the coastal area outside the largest Yangtze River of China with multiple techniques including elemental and stable isotopic analysis, absorption spectroscopy, fluorescence excitation-emission matrices coupled with parallel factor analysis (EEMs-PARAFAC), and ultra-high resolution Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR-MS). The end-member mixing analysis based on δ13C and δ15N showed a dominance of marine contribution (up to 70%) at most stations while the terrestrial contribution increased to >55% nearshore in summer at a high fluvial sediment flux. This was consistent with the offshore decreasing humic-like C1 and C2, humification index (HIX), %lignin-like compounds, and %CHO but increasing tryptophan-like C3, biological index (BIX), %protein-like compounds, and %CHOS from EEMs-PARAFAC and FT-ICR-MS analysis. The %clay correlated positively with SOM content, HIX, %lignin-like compounds, O/C, and modified aromaticity index (AImod) but correlated negatively with %C3, H/C, and the relative abundance of labile formulas (MLBL), while %silt showed contrasting correlations. These results indicated the fine clay sediments adsorbed more humified, aromatic, oxygenated, and terrestrial compounds that were probably more resistant to biodegradation and thus had a higher burial efficiency than those on the silty sediments. Principal component analysis based on SOM indices further revealed different characteristics of SOM in the nearshore, northern offshore, and southern offshore regions, which were probably dependent on the delivery by local current systems. Overall, these findings contributed to unraveling the source and molecular composition of SOM associated with different grain size sediments and local current delivery, which are fundamental for understanding the factors underlying carbon burial in the complex coastal environment.

Effects of variations in hydrological connectivity on the macrobenthic community structure in reclaimed wetlands

The Nanhui Dongtan Wetland is the most extensively reclaimed part of the Yangtze River Estuary wetland. In recent decades, urbanization has led to the extensive reclamation of the intertidal wetlands of Nanhui Dongtan. Macrobenthos are crucial as secondary production groups in the food web. However, there is a lack of in-depth research on the response mechanisms of macrobenthic communities to environmental disturbances in reclaimed wetlands. This study investigated the impact of hydrological connectivity changes caused by land reclamation on the macrobenthic community based on the macrobenthic community composition in preserved tidal flats and closed and open reclamation areas in Nanhui Dongtan. The results showed that the macrobenthos species richness in the closed reclamation area was significantly lower than that in the other areas. After dividing the functional groups of macrobenthos, structural equation modeling revealed a negative correlation between the salinity and the functional group composition. Analysis of the food sources revealed significant positive correlations between predatory and sediment-feeding populations and sediment organic matter content, between detritivorous group and environmental chlorophyll-a content, and between herbivorous group and suspended organic matter content in water. Therefore, variations in hydrological connectivity in different reclamation areas caused differences in food source distribution, which led to different compositions of macrobenthic functional groups. The results of this study provide a theoretical reference for the study of intertidal wetland habitat restoration.

Lateral migration differs between phytolith morphotypes on sand dune surfaces

AbstractArid and semi‐arid lands are exceptionally sensitive to climate change. However, the application of phytolith analysis to these environments is hindered by the potential for lateral migration of phytoliths during wind erosion, which may affect the reliability of phytolith‐based paleoenvironmental reconstructions. Moreover, there is a lack of quantitative studies of the dispersion and deposition of phytoliths by wind erosion. Here we apply Sutton's equation and theoretical models from the field of blown sand physics and engineering to quantify the lateral migration of various phytolith morphotypes in the surface soil of sand dunes in the Horqin Sandy Land in China. Phytolith morphotypes and concentrations were determined in addition to sedimentary organic matter content and grain size. Combined with the analysis of plant communities, these measurements were used to quantify the lateral migration of phytolith morphotypes, and the results were compared with theoretical models. We found that phytolith concentrations decreased exponentially under an annual average wind speed with distance from the surface source; specifically, a large proportion of lateral phytolith migration occurred within the distance of ~3–5 m. There were significant linear correlations between the phytolith concentration and other environmental factors. A comprehensive form of Sutton's equation was used to estimate that a relatively large proportion (8.35%) of short‐cell phytoliths may migrate laterally on dunes that are vulnerable to wind erosion. However, large phytoliths are deposited almost in situ, and relatively limited lateral migration of wind‐transported phytoliths occurs in the Horqin Sandy Land overall. Our results provide a theoretical model and practice template for the application of phytolith analysis to soil and sediments, especially as a proxy of past vegetation and ecological change in the Horqin Sandy Land, and other areas affected by wind erosion. Additionally, short‐cell phytoliths in palaeoenvironmental contexts satisfy the criteria necessary to investigate the extent with frequent aeolian activity.

Evolving multispectral sensor configurations using genetic programming for estuary health monitoring

ABSTRACT Assessing ecosystem health on a large scale is crucial for a wide range of management and regulatory decisions. Technologies such as hyperspectral imaging allow noninvasive and rapid estimation of key attributes based on observed reflectance. However, these images are high-dimensional and real-world applications require models based on fewer wavelengths. This paper proposes a new wavelength selection and feature extraction method for hyperspectral image analysis based on genetic programming to automatically select key wavelength regions and informative image features. A dataset of hyperspectral images of sediment in the field was collected and paired with ground-truth measurements of the sediment porosity and organic matter content. Two new program structures were proposed to construct feature extraction trees from either the mean reflectance spectra (spectra-based) or full hyperspectral images (image-based). SVR models were constructed to predict attributes based on the extracted features. Various regression models were used to predict the porosity and organic matter content. Full-wavelength models were constructed to reliably predict the organic matter content. The proposed spectra-based genetic programming solutions show competitive results compared to common wavelength selection methods, such as SPA, CARS, and RC. Finally, the best-evolved solution was applied to predict sediment organic matter content across all collected images.

Distribution and risk assessment of nutrients and heavy metals from sediments in the world-class water transfer projects

South-to-North Water Diversion Project, a globally renowned engineering feat, aims to address water supply issues. The sediments within reservoirs play a pivotal role in natural ecosystems, not only as habitats for diverse biota but also as repositories of heavy metals, organic matter, and other contaminants. These sediments serve as a critical interface between sediment and water bodies. This comprehensive analysis focused on the spatial patterns of nutrients, organic matter, and heavy metals in the surface layer and profiles of reservoirs, exploring their interconnectedness. Leveraging the integrated pollution index, organic pollution index, potential ecological risk index, and geo-accumulation index, an ecological risk assessment was performed. The key findings are as follows: (i) Along the Middle Route of the South-to-North Diversion Project, the nutrient and organic matter contents in sediments tends to rise with distance from the Danjiangkou Reservoir, with the TN and TC contents increasing by 2.35- and 3.05-fold, respectively. (ii) As the sediment depth increases, the carbon, nitrogen, phosphorus, and organic matter contents exhibit varying degrees of decline, with average decreases of 62.38%, 67.47%, 17.56%, and 41.83% for TN, TC, TP, and OM, respectively. (iii) Among the eight heavy metals, only manganese (Mn) and zinc (Zn) in the Yahekou Reservoir showed moderate pollution levels, according to the geo-accumulation index. The Mn content within the surface sediments of the six reservoirs ranges from 550 to 1837 mg/kg (average, 1019.5 ± 548.3 mg/kg), whereas the Zn content ranges from 89 to 360 mg/kg (average, 156.5 ± 101.6 mg/kg). (vi) Total phosphorus (TP) and total nitrogen (TN) emerged as the primary pollutants in surface sediments. Comprehensive nitrogen and phosphorus pollution assessment revealed that the surface sediment of Danjiangkou Reservoir is mildly polluted, while Baiguishan and Jiangang Reservoirs are moderately polluted, and the rest are heavily contaminated. For the Yahekou and Chaohe Reservoirs, the average pollutant content indicates moderate pollution, while the remaining reservoirs show mild pollution levels.Graphical

Seasonal Variability in the Influence of Coastal Aquaculture Operation on Benthic–Pelagic Coupling Processes in Shallow Aquatic Ecosystems

Coastal shellfish aquaculture can influence benthic–pelagic-coupled systems because cultured species consume phytoplankton in the water column and return the captured organic matter and nutrients to the environment as biodeposits, which fall to the seafloor, affecting local sediment characteristics and the benthic community. In 2023, we conducted monthly field surveys to characterize the relationships between shellfish aquaculture and the surrounding environment by examining a range of physical and biological variables along the benthic–pelagic gradient at multiple sampling locations in relation to their distances from the aquaculture facilities in Onagawa Bay, Japan. The abundances of benthic macrofauna were dominated by polychaetes (86.3%), followed by gastropods (4.7%), malacostracans (2.7%), ophiuroids (2.1%), and bivalves (1.5%). Both benthic biomass and biodiversity were markedly higher, but the chlorophyll-a concentration of the water column and the sediment organic matter content were significantly lower at the closest proximity to the aquaculture facilities. Although the physical presence of shellfish aquaculture may effectively enhance pelagic–benthic energy fluxes, such processes may also pose a new challenge under the influence of recent global warming, causing widespread hypoxic conditions due to increased stratification in the water column accompanied by excess organic inputs from the aquaculture.

Heterogeneous marine response during the Toarcian Oceanic Anoxic Event (TOAE): The potential role of storminess

The Toarcian Oceanic Anoxic Event (TOAE; ∼183 Ma) represents an important hyperthermal and deoxygenation event in the Early Jurassic. However, TOAE marine records are spatially heterogeneous with regard to nutrient levels, primary productivity, redox conditions and organic enrichment. This non-uniform response to global hyperwarming is not readily accounted for by local variations in paleogeography, climate, or water depth. Largely overlooked to date is the intensified storm activity that characterized the TOAE, and the role that this may have played in controlling marine responses to that event. A review of TOAE studies from multiple marine environments suggests that storm intensity covaried with paleoceanographic conditions, such as nutrient availability, primary productivity, redox conditions, and organic-rich sedimentation. At mid-paleolatitude sites, relatively weak storm activity during the TOAE induced short-term watermass oxygenation, and marine settings were mainly characterized by enhanced anoxia (even euxinia), water-column stratification, increased primary productivity (fueled by terrestrial runoff and P regeneration in euxinic settings), and organic-rich sedimentation. At low-paleolatitude sites, TOAE storm activity was relatively strong, and contributed to marine environments characterized by oxic to suboxic conditions, reduced water-column stratification, decreased primary productivity (possibly due to limited P regeneration and upwelling), low sedimentary organic content, and locally high oolite abundance. TOAE marine sites at all paleolatitudes exhibit: i) sea-level rise and enhanced continental weathering fluxes linked to an intensified hydrological cycle; ii) reduced dinoflagellate and increased cyanobacterial activity; and iii) low δ15N values (mainly −1‰ to +3‰) linked to enhanced diazotrophic nitrogen fixation. The spatial heterogeneity of the response of TOAE marine systems is difficult to reconcile with scenarios linking increased terrestrial flux to marine eutrophication, primary productivity increase and organic-rich sedimentation. Consequently, we hypothesize that the intensity of storm activity influenced TOAE marine systems, and that this factor can, at least partially, account for heterogeneous patterns of environmental changes at middle versus low paleolatitudes and open versus restricted marine settings. Importantly, increased storm activity can induce pycnocline deepening via vertical water-column mixing, thereby promoting: i) enhanced aerobic degradation of organic matter (low sediment organic matter content) due to a reduced oxygen-minimum zone; ii) less nutrient upwelling from deep waters into the photic zone (nutrient-depleted upper ocean), and iii) blooms of nitrogen-fixing cyanobacteria (low δ15N) and calcification (ooid formation). Thus, the interaction between storminess and pycnocline depth is a potentially important factor affecting marine environmental changes during TOAE. These findings have implications for modern oceans now experiencing climatic warming and intensified tropical storm activity.

Contamination of depressional wetlands in the Mpumalanga Lake District of South Africa near a global emission hotspot

The Mpumalanga Lake District (MLD) of South Africa hosts a regionally unique cluster of water bodies of great importance for wetland biodiversity. It is also located close to a global hotspot for coal-fired power station emissions but the local impacts from these sources of pollution are poorly understood. Sediment cores from three contrasting wetlands were 210Pb dated and analysed for a range of contaminants linked to fossil fuel combustion, including trace elements, Hg, sulphur and spheroidal carbonaceous fly-ash particles (SCPs). At the two sites with pre-industrial (1900) baseline sediments, Pb, Zn and especially Cr concentrations and fluxes showed significant increases in the impact period (post-1975). Mercury showed the greatest proportional increase in flux (>4-fold) of all trace metals. Mercury and sulphur concentrations and fluxes showed highly significant correlations with emissions over the corresponding periods, while SCPs in sediments also closely tracked emissions. In a global context, levels of sediment contamination are relatively minor compared with other heavily industrialised regions, with only Cr exceeding the sediment Probable Effects Concentration for biological impact post-1975. Despite the relatively large increases in Hg, concentrations do not reach the Threshold Effects Concentration. The unexpectedly low levels of contamination may be due to i) low levels of many trace contaminants in South African coals compared to global averages, ii) prevailing recirculation patterns which transport pollution away from the study area during the wet season, minimising wet deposition, and iii) pollutant remobilisation through desiccation of wetlands or volatilization. The effects of hydrology and sediment accumulation rates lead to differential transport and preservation of organic-associated and more volatile contaminants (e.g. Hg, S) relative to non-volatile trace elements in wetlands of the MLD. The greatest fluxes of Hg and S are recorded in the site with the highest catchment: lake area ratio, lowest salinity and greatest sediment organic matter content.

Polyester microfiber impacts on coastal sediment organic matter consumption

As plastic pollution continues to accumulate at the seafloor, concerns around benthic ecosystem functionality heightens. This research demonstrates the systematic effects of polyester microfibers on seafloor organic matter consumption rates, an important benthic ecosystem function connected to multiple reactions and processes. We used a field-based assay to measure the loss of organic matter, both with and without polyester microfiber contamination. We identified sediment organic matter content, mud content, and mean grain size as the main drivers of organic matter consumption, however, polyester microfiber contamination decoupled ecosystem relationships and altered observed organic matter cycling dynamics. Organic matter consumption rates varied across horizontal and vertical spaces, highlighting that consumption and associated plastic effects are dependent on environmental heterogeneity at both small (within sites) and larger (between sites) scales. Our results emphasize the important role habitat heterogeneity plays in seafloor organic matter consumption and the associated effects of plastic pollution on ecosystem function.

THE RELATIONSHIP BETWEEN ORGANIC MATERIAL CONTENT IN SEDIMENTS AND THE ABUNDANCE OF EPIPELIC DIATOMS IN THE INTERTIDAL ZONE OF THE WATERS OF DUMAI CITY, RIAU PROVINCE

This research was conducted in March 2023, located in Koneng Beach, Dumai City. The purpose of this study was to determine the organic matter content and abundance of epipelic diatoms vertically and to analyze the relationship between the organic matter content in sediments and the abundance of epipelic diatoms vertically in the intertidal area of Dumai City Waters. The method used is the survey method. The sampling point is determined using a purposive sampling method. Sampling of epipelic diatoms was carried out using a modified corer, which was then brought to the laboratory to identify and calculate the abundance of diatoms and the sediment samples to calculate the content of organic matter and sediment fractions. Water quality measurements accompanied sampling with a brightness value of 0.57 m, temperature of 33 °C, current speed of 0.33 m/s, salinity of 28 ppt, and pH of 8.13. The results of the analysis of organic matter are 2.55 – 13.38%. The results of the sediment fraction analysis show that the research location is dominated by sandy silt. Nineteen genera of epipelic diatoms were found in the waters of Koneng Beach, Dumai City, with abundance values between 5,291–39,983 ind/cm2. The one-way ANOVA test results obtained an average value of 0.000, meaning there is a significant difference in the abundance of epipelic diatoms between the sediment layers. The results of the linear regression test showed that the effect of sediment organic matter on the abundance of epipelic diatoms was 50.85%.

GASTROPOD DIVERSITY AS A BIO-INDICATOR OF AQUATIC POLLUTION IN SIRONJONG GADANG ISLAND PESISIR SELATAN DISTRICT

This study aims to determine the diversity of gastropods and water conditions on Sironjong Gadang Island, Pesisir Selatan Regency. This research was conducted in August 2022 in the waters of Sironjong Gadang Island and the Marine Biology Laboratory and Marine Chemistry Laboratory of the Department of Marine Science, Universitas Riau. The method used in this research is the survey method, which is direct observation of the research area and sampling and measurement of water quality parameters in the field. The results of the study found gastropod species consisting of 4 (four) families, 4 (four) genera, and 8 (eight) species. The diversity value (H') was at a moderate level of diversity. The highest gastropod density was found at Station I, with a density value of 16.33 ind/m², and the lowest density was found at Station III, with a density value of 13.89 ind/m². The highest relative density was Monodonta labia species, with a relative density value of 27.6%, while the lowest relative density was Haustrum-scoring species, with a relative density value of 3.3%. Temperature 28-29˚C; salinity 25-27 ppt; pH 7.91-7.95; brightness 1.55-2.65 m. The sediment type at Station I was dominated by muddy gravel, Station II was dominated by sandy gravel, and Station III was defeated by sandy gravel with a total sediment organic matter content of 6.10-10.05%.

Spatial and seasonal pattern of microbial nitrate reduction in coastal sediments in the Vistula River plume area, Gulf of Gdańsk

Estuaries can remove and/or retain land-derived nitrogen (N) and act as filters buffering N loads to the open sea. The N coastal filter can be seasonally variable depending on water temperature and transported loads, two factors acting in synergy and strongly influenced by climate change. The capacity of sediments to mitigate riverine N loads was investigated at four sites in the Vistula River plume area (Gulf of Gdańsk, Southern Baltic Sea). Samplings were carried out in two contrasting seasons: spring and summer, characterized by different water temperatures and nitrate (NO3-) levels. Inorganic N fluxes, and rates of denitrification and dissimilatory nitrate reduction to ammonium (DNRA) were measured in intact sediment cores by means of dark incubations and 15N-nitrate concentration-series experiments. Sampling sites were selected along a gradient of depth (5 to 24 m), that was also a gradient of sediment organic matter content. In both seasons, denitrification rates increased along with depth and from spring (6.5 ± 7.0 µmol m-2 h-1) to summer (20.4 ± 15.4 µmol m-2 h-1), despite lower NO3- concentrations in summer. In spring, at higher NO3- loading, denitrification was likely limited by low water temperature, and elevated sediment oxygen penetration. Coupled denitrification-nitrification prevailed over denitrification of water column NO3- across all sites and seasons, contributing to over 80% of the total denitrification. Notably, no anammox was detected at the sampling sites. DNRA exhibited low to undetectable rates in spring, especially at the shallowest sites. However, during summer, N recycling via DNRA increased and ranged from 0.7 to 14.9 µmol m-2 h-1. The denitrification efficiency (DE), calculated as the ratio between molecular nitrogen (N2) flux and dissolved inorganic N effluxes from sediments, ranged from 0 to 37% in spring, whereas in summer DE did not exceed 16%. Despite the dominance of denitrification over DNRA, the analyzed sediments acted as weak N buffers under in situ dark conditions. However, concentration-series experiments suggested high potential denitrification capacity, exceeding 400 µmol m-2 h-1, in response to short-term, large riverine inputs of NO3-.

Benthic foraminiferal assemblages in the coastal zone of the yellow river delta: Environmental significance and characteristics of microbial diversity

Unicellular eukaryotes known as benthic foraminifera have sophisticated survival mechanisms and ecological environmental indicators. Ten surface samples from the Yellow River Delta's coastal region were taken for this study to analyze the distribution of foraminifera in relation to environmental factors and to examine their microbial diversity. We looked at the physicochemical aspects of the environment at each sampling site (such as pH, TN, TOC, EC, δ13C, δ15N, PAHs, etc.), and we employed 16S rRNA high-throughput sequencing analysis to look into the interactions between microbial communities and foraminiferal species. The findings demonstrated that the Yellow River Delta's advantage benthic foraminiferal species were Quinqueloculina complanata, Ammonia beccarii, and Ammonia aomoriensis, and that TOC and TN were significant determinants of the distribution of benthic foraminiferal communities. PAH enrichment in the coastal zone effects on microbial communities and benthic foraminifera are not yet readily apparent. Sediment organic carbon and nitrogen isotope analysis revealed that marine plankton may be the dominant source of organic matter in the coastal zone sediments in the research area, which was made up of both land-based and marine organic matter. It implies that foraminifera have some environmental indicators, especially when combined with the distributional traits of the benthic foraminiferal assemblages and their considerable association with environmental parameters. The findings revealed that Q. complanata was found in the Yellow River water-affected estuarine coastal zone, A. aomoriensis was significantly influenced by sediment organic matter content and reflected the estuarine and nearshore environments and the combination of Ammonia beccarii-Ammonia aomoriensis-Elphidium excavatum assemblage indicated a semi-open intertidal shallow marine environment. Additionally, benthic foraminifera showed a responsive association with microorganisms, indicating that microbial diversity may be one of the driving forces behind benthic foraminifera's ability to adjust to environmental changes. The findings of this study will open up new avenues for research into the environmental importance of coastal zone ecosystems and for understanding how benthic foraminiferal communities survive.

Bubbles dominated the significant spatiotemporal variability and accumulation of methane concentrations in an ice-covered reservoir

Climate-sensitive ice-covered reservoirs are critical components of methane (CH4) release. However, the mechanisms that influence CH4 dynamics during ice-covered periods remain poorly studied. To investigate the effects of bubbles on CH4 dynamics, we conducted intensive field and incubation experiments in an ice-covered reservoir (ice growth, stability, and melt period) in Northeast China. We found that the mean dissolved CH4 concentrations in the ice (625.9 ± 2419.7 nmol L−1) and underlying water (1218.9 ± 2678.9 nmol L−1) were high, making them atmosphere CH4 sources. The visible bubble bands (bubble area) in the riverine zone and the vertical profile of the CH4 concentration in the ice reflect the distribution of trapped bubbles. The mean CH4 concentration in the ice of the bubble area (1674.8 ± 3926.8 nmol L−1) was 2 orders of magnitude higher than that of no-bubble area (53.7 ± 9.2 nmol L−1). Moreover, a large amount of CH4 accumulated under the ice in the bubble area. These findings suggest that bubbles determine the CH4 storage in ice and CH4 accumulation in the underlying water. Ice growth increases CH4 storage in ice and the underlying water because of the entrapment and re-dissolution of CH4 bubbles. However, ice melting releases the CH4 accumulated in the ice and underlying water. A comparison of the field and incubation experiments indicated that the deep-water environment of the reservoir had a CH4 burial effect. Stepwise regression analysis revealed that higher sediment organic matter content, median particle size, and porosity increased the production and release of CH4 bubbles, trapping more CH4 bubbles in ice. Overall, this study improves the mechanistic understanding of CH4 dynamics and predictability of CH4 emissions during ice-covered periods.

Fungal Abundance and Diversity in the Mariana Trench, the Deepest Ecosystem on Earth.

Hadal trenches host abundant and diversified benthic prokaryotic assemblages, but information on benthic fungi is still extremely limited. We investigated the fungal abundance and diversity in the Challenger Deep (at ca. 11,000 m depth) and the slope of the Mariana Trench in comparison with three sites of the adjacent abyssal plain. Our results indicate that trench sediments are a hotspot of fungal abundance in terms of the 18S rRNA gene copy number. The fungal diversity (as the number of amplicon sequence variants, ASVs) was relatively low at all sites (10-31 ASVs) but showed a high turnover diversity among stations due to the presence of exclusive fungal taxa belonging to Aspergillaceae, Trichosphaeriaceae, and Nectriaceae. Fungal abundance and diversity were closely linked to sediment organic matter content and composition (i.e., phytopigments and carbohydrates), suggesting a specialization of different fungal taxa for the exploitation of available resources. Overall, these findings provide new insights into the diversity of deep-sea fungi and the potential ecological role in trench sediments and pave the way for a better understanding of their relevance in one of the most extreme ecosystems on Earth.

Bioturbation and faunal-mediated ecosystem functioning in a deep-sea benthic community recovering from a severe seabed disturbance

Kaikōura Canyon, offshore Aotearoa/New Zealand, is a hotspot for deep-sea benthic biology with globally high faunal abundance. The Mw7.8 Kaikōura earthquake in 2016 triggered a severe disturbance that reshaped the canyon, evacuating an estimated 850 metric megatonnes of sedimentary material down canyon. The Kaikōura Canyon habitat is now recovering from this removal of both seafloor substrate and associated organisms.We measured post-event benthic macrofauna density together with biomass and sediment properties and related these to infaunal bioturbation activity, sediment community oxygen consumption (SCOC) and associated benthic macronutrient biogeochemical fluxes. Three legacy sites were used as reference along a depth transect of the canyon axis at 917, 1249, and 1480 m water depths. We found distinct differences in benthic functioning between sites.While many parameters measured followed the expected relationships of decreasing with depth some benthic biological activity metrics did not. Our data suggests that the sites differ based on the relative impact and type of disturbance experienced during the earthquake-triggered, canyon-flushing event. We find SCOC was linked to bioturbatory behaviour and sediment organic matter content, rather than taxa density or biomass. While the greatest bioturbation activity was measured at the shallowest site, the greatest SCOC was measured at the middle depth site due to elevated sediment organic matter content.These observations provide a wider benchmark for infaunal bioturbation and faunal-mediated biogeochemical processes in deep-sea canyon environments and provide rare insights of recovery trajectories for deep-sea benthic communities and ecosystem function after large seafloor disturbances.

Nutrient fluxes from recent deposits of fine-grained, organic-rich sediments in a Florida estuary

Nutrient fluxes from fine-grained, organic-rich sediments in estuaries can hasten the onset and progression of eutrophication and harmful algal blooms. Targeted efforts to manage degraded sediments and improve estuarine water quality require a better understanding of physicochemical controls and the relative importance of benthic fluxes. Toward that end, we determined fluxes from organic-rich, high porosity sediments deposited during the past 5-6 decades along 60 km of the Indian River Lagoon, a barrier island lagoon in Florida, USA. Highly bioavailable ammonium and phosphate were the predominant chemical forms of interstitial nitrogen and phosphorus in these highly-reducing sediments. Median fluxes of ammonium and phosphate were 320 µmol m-2 h-1 and 11 µmol m-2 h-1, respectively. Fluxes were 3-10 times greater when sediment temperatures were >28°C and interstitial sulfide concentrations were >1 mM. Temperature-compensated fluxes of ammonium and phosphate were significantly correlated with sediment organic matter content; total organic carbon averaged 5.3 ± 2.4% and the maximum was 12.4% for the sediments studied. Fine-scale physical probing, plus lidar data, showed that these organic-rich sediments covered <10% of our study area; however, fluxes from these sediments were estimated to supply 20-40% of internal + external annual loads of nitrogen and phosphorus. Furthermore, 60% of nitrogen and phosphorus fluxes from sediments in the northern Indian River Lagoon came from just 22% of the total surface area. Lagoon segments with high benthic fluxes overlapped in part with areas prone to harmful algal blooms. Effective strategies to manage degraded sediments in the Indian River Lagoon depend on knowing the relative magnitude of internal loading of nutrients as well as appropriate techniques to mitigate sediment fluxes.

Environmental heterogeneity of cold seep by biological trait analysis of marine nematodes at Site F cold seep in South China Sea

Cold seeps provide high environmental heterogeneity for marine benthos. Site F is one of the active cold seeps in the South China Sea. In this study, free-living marine nematode communities were investigated at Site F and the adjacent deep-sea area. A total of 67 genera and 32 families were identified. The mean density at cold seep sites ranged from 13.6 to 181.8 ind./10 cm2, and that at the adjacent deep-sea sites ranged from 36.9 to 301.4 ind./10 cm2. At cold seep sites, the most dominant nematode genera were Desmoscolex, Pierrickia, Sabatieria, Halalaimus, and Dorylaimopsis while at deep-sea sites, the most dominant genera were Retrotheristus, Thalassomonhystera, Desmoscolex, Cobbia, and Halalaimus. Deposit feeders of nematodes were dominant at all sites. Results of biological trait analysis showed that there was high environmental heterogeneity for nematodes at Site F. Water depth, sediment organic matter content, and sand proportion had important influences on nematode communities.