Remineralization Of Organic Matter Research Articles

Formulation, optimization, and sensitivity of NitrOMZv1.0, a biogeochemical model of the nitrogen cycle in oceanic oxygen minimum zones

Abstract. Nitrogen (N) plays a central role in marine biogeochemistry by limiting biological productivity in the surface ocean; influencing the cycles of other nutrients, carbon, and oxygen; and controlling oceanic emissions of nitrous oxide (N2O) to the atmosphere. Multiple chemical forms of N are linked together in a dynamic N cycle that is especially active in oxygen minimum zones (OMZs), where high organic matter remineralization and low oxygen concentrations fuel aerobic and anaerobic N transformations. Biogeochemical models used to understand the oceanic N cycle and project its change often employ simple parameterizations of the network of N transformations and omit key intermediary tracers such as nitrite (NO2-) and N2O. Here we present a new model of the oceanic N cycle (Nitrogen cycling in Oxygen Minimum Zones, or NitrOMZ) that resolves N transformation occurring within OMZs and their sensitivity to environmental drivers. The model is designed to be easily coupled to current ocean biogeochemical models by representing the major forms of N as prognostic tracers and parameterizing their transformations as a function of seawater chemistry and organic matter remineralization, with minimal interference in other elemental cycles. We describe the model rationale, formulation, and numerical implementation in a one-dimensional representation of the water column that reproduces typical OMZ conditions. We further detail the optimization of uncertain model parameters against observations from the eastern tropical South Pacific OMZ and evaluate the model's ability to reproduce observed profiles of N tracers and transformation rates in this region. We conclude by describing the model's sensitivity to parameter choices and environmental factors and discussing the model's suitability for ocean biogeochemical studies.

Speciation of dissolved inorganic iodine in a coastal fjord: a time-series study from Bedford Basin, Nova Scotia, Canada

We report a long-term (4.5 year) time-series with weekly resolution of iodide and iodate measurements made at 4 depths within the Bedford Basin: a 70 m deep, seasonally stratified, coastal fjord located near Halifax, Nova Scotia, Canada. The subsurface data (60 m) reveal strong inverse correlations of both iodide and total dissolved iodine (TDI) with dissolved oxygen and indicate that there is in-situ reduction of iodate in subsurface waters (in the presence of oxygen) as well as an additional external source of iodide from the remineralization of sinking organic matter, a flux from sediments, or both. Surface water (<10 m) iodide concentrations increase gradually from spring (70 nmol L-1) through fall (120-150 nmol L-1) and are not well represented by the current empirical parameterizations used to predict surface water iodide levels globally. The vertical gradient of iodide between subsurface and surface waters increases over the summer as a result of subsurface processes and, together with diapycnal mixing, may contribute to the seasonal accumulation of iodide in surface water. Examination of a global surface water iodide data compilation reveals an inverse relationship with subsurface oxygen concentrations which suggests that subsurface remineralization and sediment-water fluxes coupled with vertical mixing may also contribute to surface water iodide variability on a global scale.

Spatial Patterns of Planktonic Fungi Indicate Their Potential Contributions to Biological Carbon Pump and Organic Matter Remineralization in the Water Column of South China Sea

Fungi have long been known to be dynamic in coastal water columns with multiple trophic modes. However, little is known about their interactions with abiotic and biotic components, contribution to the biological carbon pump (BCP), and organic matter remineralization in the oceanic water column. In this study, we investigated how fungi vary spatially and how their variations relate to that of bacteria in the water column of the South China Sea (SCS). Fungi were about three orders less prevalent than bacteria, and the main factors influencing their distribution were depth, temperature, and distance from the sites of riverine inputs. The decline in the abundance of fungi with depth was less steep than that of bacteria. Correlation tests revealed a strong positive association between the abundance of fungi and bacteria, especially in the twilight (r = 0.62) and aphotic (r = 0.70) zones. However, the co-occurrence network revealed mutual exclusion between certain members of fungi and bacteria. The majority of fungi in the water column were saprotrophs, which indicated that they were generally involved in the degradation of organic matter, particularly in twilight and aphotic zones. Similar to bacteria, the involvement of fungi in the metabolism of carbohydrates, proteins, and lipids was predicted, pointing to their participation in the turnover of organic carbon and the biogeochemical cycling of carbon, nitrogen, and sulfur. These findings suggest that fungi play a role in BCP and support their inclusion in marine microbial ecosystem models.

Spatial and seasonal distribution of particulate phosphorous and nitrogen in the Persian Gulf: Nitrogen enrichment ties to diazotroph bloom in stratified warm waters

Understanding the particulate organic and inorganic forms of nutrients and their elemental stoichiometry in suspended particulate matter of marginal seas has important implications in several areas. Here, we present the findings from two consecutive research expeditions conducted in September 2021 and January 2022 aboard the Persian Gulf Explorer research vessel, focused on the particulate P and N in the Persian Gulf and the Strait of Hormuz. The prominent features of spatial and seasonal distribution characteristics were nearshore-offshore decreasing gradients in particulate nutrients, the summer-to-winter declining trend in particulate nutrients more notably for particulate organic nitrogen, and higher biomass N:P ratio in summer than in winter with high spatial inhomogeneity. The trends were mainly governed by shallow remineralization of organic matter, resuspension of fine sediments, summer stratification/winter mixing of the water column, and inflow of the Indian Ocean Surface Waters from the Gulf of Oman. The contribution of inorganic N in the particulate pool of N (1.8% in summer and 1.3% in winter) was much less than the contribution of inorganic P in the particulate pool of P (23.0% in summer and 32.4% in winter), and hence the biomass N:P was higher than the corresponding bulk N:P by a factor of ∼1.5. Seasonal pattern in biomass N:P was similar to those observed at many sites being higher in the summer and lower in the winter. The formation of summer thermocline, characterized by high surface temperature (∼33 °C) and limited upward flux of remineralized inorganic nutrients, coincided with the proliferation of nitrogen-fixing cyanobacteria. Samples collected from the deep offshore region in summer showed significantly higher biomass N:P ratio, reaching up to 47, which is far beyond the classical Redfield ratio of 16. The central Persian Gulf experienced less seasonal variations than the deep region in the western Persian Gulf, the northwestern Arvand estuary, and the northwestern Gulf of Oman.

Habitat heterogeneity effects on microbial communities of the Gulf of Maine

Many biological and physical variables influence microbes at the sediment-water interface, whose response to those variables affects the overall fate and residence time of organic matter (OM) in the marine environment. In addition to the many ecological roles microbes play in deep-sea sediments, further studies are needed to address microbial community diversity and its influence on seafloor organic matter remineralization rates. We explored microbial diversity, distribution, and associated organic matter remineralization among contrasting habitats and following 24-h experimental phytodetrital enrichment of sediments from Canyon, Inter-Canyon, and Channel habitats of the Gulf of Maine, Northwest Atlantic. Based on multivariate analyses of community composition we found that habitat heterogeneity influenced microbial community composition but not microbial diversity. Similarly, the phytodetrital addition did not translate to significant differences in microbial diversity but altered nitrate flux in Inter-canyon sediments and phosphate flux in Channel sediments. Bacteria were dominant over Archaea and, of the environmental variables we measured, quantity and quality of organic matter best explained overall microbial community variation. Proteobacteria, Bacteroidae, Acidobacteriota, Planctomycetota, and NB1-J Deltaproteobacteria dominated all habitats, but variation in the microbial community at our study sites explained only 7% of nutrient fluxes. Our exploratory study did not suggest a strong contribution of microbial community composition to OM remineralization variation both among contrasting habitats and in response to phytodetrital addition, and thus to this aspect of deep-sea ecosystem functioning, at least over short-term incubations.

Carbon budgets of Scotia Sea mesopelagic zooplankton and micronekton communities during austral spring

Zooplankton form an integral component of epi- and mesopelagic ecosystems, and there is a need to better understand their role in ocean biogeochemistry. The export and remineralisation of particulate organic matter at depth plays an important role in controlling atmospheric CO2 concentrations. Pelagic mesozooplankton and micronekton communities may influence the fate of organic matter in a number of ways, including: the consumption of primary producers and export of this material as fast-sinking faecal pellets, and the active flux of carbon by animals undertaking diel vertical migration (DVM) into the mesopelagic. We present day and night vertical biomass profiles of mesozooplankton and micronekton communities in the upper 500 m during three visits to an ocean observatory station (P3) to the NW of South Georgia (Scotia Sea, South Atlantic) in austral spring, alongside estimates of their daily rates of ingestion and respiration throughout the water column. Day and night community biomass estimates were dominated by copepods >330 μm, including the lipid-rich species, Calanoides acutus and Rhincalanus gigas. We found little evidence of synchronised DVM, with only Metridia spp. and Salpa thompsoni showing patterns consistent with migratory behaviour. At depths below 250 m, estimated community carbon ingestion rates exceeded those of metabolic costs, supporting the understanding that food quality in the mesopelagic is relatively poor, and organisms have to consume a large amount of food in order to fulfil their nutritional requirements. By contrast, estimated community rates of ingestion and metabolic costs at shallower depths were approximately balanced, but only when we assumed that the animals were predominantly catabolising lipids (i.e. respiratory quotient = 0.7) and had relatively high absorption efficiencies. Our work demonstrates that it is possible to balance the metabolic budgets of mesopelagic animals to within observational uncertainties, but highlights the need for a better understanding of the physiology of lipid-storing animals and how it influences carbon budgeting in the pelagic.

Submarine Groundwater Discharge-Derived Nutrient Fluxes in Eckernförde Bay (Western Baltic Sea)

Excess nutrient supply by the rivers and the atmosphere are considered as the major causes for the persistently poor ecological status of the Baltic Sea. More than 97% of the Baltic Sea still suffers from eutrophication due to past and present inputs of nitrogen and phosphorus. One of the poorly quantified nutrient sources in the Baltic Sea is submarine groundwater discharge (SGD). Through seepage meter deployments and a radium mass balance model, a widespread occurrence of SGD along the coastline of Eckernförde Bay was detected. Mean SGD was 21.6 cm d−1 with a calculated freshwater fraction of 17%. Where SGD was detected, pore water sampled by a piezometer revealed a wide range of dissolved inorganic nitrogen (DIN: 0.05–1.722 µmol L−1) and phosphate (PO43−: 0.03–70.5 µmol L−1) concentrations. Mean DIN and PO43− concentrations in non-saline (salinity < 1) pore waters were 59 ± 68 µmol L−1 and 1.2 ± 1.9 µmol L−1, respectively; whereas pore water with salinities > 1 revealed higher values, 113 ± 207 µmol L−1 and 6 ± 12 µmol L−1 for DIN and PO43−, respectively. The nutrient concentrations along the salinity gradient do not suggest that land-derived groundwater is the definitive source of nutrients in the Baltic Sea. Still, SGD may contribute to a major autochthonous nutrient source, resulting from remineralization or dissolution processes of organic matter in the sediments. The DIN and PO43− fluxes derived from SGD rates through seepage meters are 7.9 ± 9.2 mmol m−2 d−1 and 0.5 ± 0.4 mmol m−2 d−1, lower by a factor of ~ 2 and ~ 5 when compared to the fluxes derived with the radium mass balance model (mean DIN: 19 ± 28 mmol m−2 d−1; mean PO43−: 1.5 ± 2.7 mmol m−2 d−1). Assuming that these mean radium-based nutrient fluxes are representative for the coastline of Eckernförde Bay, we arrive at SGD-borne nutrient fluxes of about 1 t km−1 y−1 of nitrogen and 0.2 t km−1 y−1 of phosphorous. These fluxes are lower for DIN and in the same range for phosphorus as compared to the riverine nutrient supply (DIN: 6.3 t km−1 y−1, P: 0.2 km−1 y−1) to the German Baltic Sea identifying SGD-borne nutrients as a secondary nutrient source to the Baltic Sea.

Sulfate-driven anaerobic oxidation of methane inferred from trace-element chemistry and nickel isotopes of pyrite

In marine sediments, formation of pyrite (FeS2) is promoted by both organoclastic sulfate reduction (OSR) and sulfate-driven anaerobic oxidation of methane (SD-AOM), and these two microbial pathways might yield differing patterns of sulfur and nickel isotopic fractionation and trace-element enrichment. To better understand these pathways, we analyzed the geochemistry of authigenic pyrite aggregates from the Upper Quaternary of the western Andaman Sea (International Ocean Discovery Program, Expedition 353, Site U1447A). 34S-enriched pyrites (to ∼+41‰) in Unit Ib are interpreted as products of SD-AOM, and their enrichments in Co and Ni and low δ60Ni values (−0.63‰ to −0.09‰) may represent diagnostic signatures of a reverse-methanogenesis microbial pathway. In contrast, 34S-depleted pyrites (∼–46‰ to –38‰) in both Units I and II are consistent with sulfide formation through early diagenetic remineralization of organic matter, and their relative enrichments in Cu and V and heavier δ60Ni values (to +0.41‰) may be diagnostic of this alternative microbial pathway. This study reports for the first time a fractionation of Ni isotopes linked to preferential uptake of isotopically light Ni by the enzyme Mcr (M reductase enzyme) in reverse methanogenesis and demonstrates that OSR- and SD-AOM-associated pyrites are potentially distinguishable on the basis of their δ60Ni compositions. Such geochemical signatures may prove useful in determining processes of pyrite formation in paleo-depositional systems and in identifying ancient methane emission events.

Respiration, Production, and Growth Efficiency of Marine Pelagic Fungal Isolates.

Despite recent studies suggesting that marine fungi are ubiquitous in oceanic systems and involved in organic matter degradation, their role in the carbon cycle of the oceans is still not characterized and fungal respiration and production are understudied. This study focused on determining fungal growth efficiencies and its susceptibility to temperature differences and nutrient concentration. Hence, respiration and biomass production of three fungal isolates (Rhodotorula mucilaginosa, Rhodotorula sphaerocarpa, Sakaguchia dacryoidea) were measured in laboratory experiments at two temperatures and two nutrient concentrations. We found that fungal respiration and production rates differed among species, temperature, and nutrient concentration. Fungal respiration and production were higher at higher temperatures, but higher fungal growth efficiencies were observed at lower temperatures. Nutrient concentration affected fungal respiration, production, and growth efficiency, but its influence differed among species. Altogether, this study provides the first growth efficiency estimates of pelagic fungi, providing novel insights into the role of fungi as source/sink of carbon during organic matter remineralization. Further research is now needed to unravel the role of pelagic fungi in the marine carbon cycle, a topic that gains even more importance in times of increasing CO2 concentrations and global warming.

Late Quaternary Isoëtes megaspores as a proxy for paleolimnological studies of the southeastern Amazonia

Isöetes have higher diversity in South America, with two endemic species (I. serracarajensis and I. cangae) recently discovered in the Carajás Mountain Range (CMR), southeastern Amazonia. I. serracarajensis is observed in upland oligotrophic to eutrophic swamps and ponds, while I. cangae is restricted to a single oligotrophic upland and active lake (Amendoim) the CMR since it is very sensitive to algal blooms and the eutrophication process. In this study, a species level identification and quantification of Isöetes megaspores were carried out for the first time in sediment cores of upland lakes of the CMR during the last 50k cal yr BP, and the main factors influencing population dynamics and conservation status were discussed. Generally, during millennial-scale drier hydroclimate conditions, swamps and ponds mostly dehydrated, and I. serracarajensis were restricted to active lakes, which became eutrophicated due to the increase in nutrient production from the remineralization of organic matter deposited in bottom sediments triggered by the decrease in water levels. On the other hand, the highly-sensitive population of I. cangae contracted since the Amendoim Lake become eutrophicated. During wetter climate, these species reappeared in their preferred habitats. However, the CMR is currently under climate and hydrological stress mainly due to an intense land use–land cover changes during the last four decades, with a worst-case scenario for the next 30 years, according to recent hydrological models. This may strongly impact the natural habitats of Isöetes and further increase pressure on already endangered populations.

Dissolved rare earth elements in the Northwest Pacific: Sources, water mass tracing, and cross‐shelf fluxes

In the Northwest Pacific, a key area for understanding the sources and transport of materials in the ocean, knowledge of the sources, transport, and biogeochemical cycling of trace elements is limited. Trace elements such as the rare earth elements (REEs) can trace the sources and transport of water masses. Here we present dissolved REE concentrations along a longitudinal transect (150 oE) from 13°N to 40°N in the Northwest Pacific (≤2000 m). We divided the transect into two subregions: a mixed water region (MWR; 37~40 °N, where the Oyashio and Kuroshio currents mix) and a subtropical region (13~34 °N). In the MWR, REEs were strongly positively correlated with apparent oxygen utilization in subsurface water (depth &amp;gt; the chlorophyll maximum layer, potential density &amp;lt;26.6 kg/m3), with about a 4-fold higher slope (0.15±0.06) than in the subtropical region in subsurface and intermediate waters (0.04±0.003, potential density &amp;lt;27.5 kg/m3). This suggests that REEs are released by organic matter remineralization at a higher efficiency in the MWR vs. in the subtropical region, which can be explained by different water masses and plankton community structures. In addition, we observed a lithogenic input signal of REEs from the Aleutian Islands based on the high La/Yb ratio (&amp;gt;0.35). This ratio was controlled by lateral transport and showed a good agreement with salinity, indicating that it is a useful tracer of low salinity water originating from the subarctic region. Furthermore, we estimated the cross-shelf fluxes of Nd in the Northwest Pacific. The estimated Nd fluxes from the Sea of Okhotsk, the Sea of Japan, the East China Sea, and the South China Sea into the Northwest Pacific were 29~32 t/y, 159~302 t/y, 142~616 t/y, and -298~34 t/y, respectively. This study highlights the importance of considering the cross-shelf REE fluxes in the Northwest Pacific when constructing the oceanic REE budgets.

Short-term dynamics of nano- and picoplankton production in an embayment of the southern Benguela upwelling region

In the southern Benguela ecosystem, regenerated production and high levels of organic matter remineralisation are expected to dominate during periods of relaxation. To study microbial growth and productivity under these remineralising conditions, we measured size fractionated (micro-nanoplankton: 10–200 μm and nano-picoplankton: 0.3–10 μm) net primary production, uptake rates of nitrate, ammonium, and urea, as well as nano- and picoplankton community composition and biomass over five consecutive days in autumn (March 2018). Samples were collected from three depths (1 m, 25 m, and 50 m) at a single station in St Helena Bay and abundances of nanophytoplankton, picophytoplankton and heterotrophic bacteria were determined using flow cytometry. There were differences in productivity among days but depth-differentiation was more apparent in the rates of net primary production and nitrogen uptake, with the highest rates (rate ± SD) (NPP: 3.36 ± 1.82 μmol L−1 d−1; ρNO3-: 0.30 ± 0.18 μmol L−1 d−1; ρNH4+: 2.27 ± 0.75 μmol L−1 d−1; ρUrea: 1.38 ± 0.02 μmol L−1 d−1) measured at the surface. Ammonium and urea uptake rates were two-to six-fold higher than those of nitrate, indicating that most of the biomass was produced through regenerated production with f-ratios of 0.02–0.21. Nano-picoplankton comprised 67% of the carbon biomass and were responsible for 90% of net primary production and 79–85% of total nitrogen (i.e., nitrate + ammonium + urea) uptake. Small cell size likely conferred advantages on nano-picoplankton in the nutrient-deplete euphotic zone and the low-oxygen (<89.3 μmol L−1) conditions at 25 m and 50 m. Nitrite oxidation rates were fastest at deeper depths where heterotrophic bacteria were most abundant. Heterotrophic bacteria also contributed the most (95%) carbon biomass at all depths, suggesting a major role for these microorganisms in carbon and nitrogen cycling throughout the water column of St Helena Bay.

Simultaneous sulfate and nitrate reduction in coastal sediments

The oscillating redox conditions that characterize coastal sandy sediments foster microbial communities capable of respiring oxygen and nitrate simultaneously, thereby increasing the potential for organic matter remineralization, nitrogen (N)-loss and emissions of the greenhouse gas nitrous oxide. It is unknown to what extent these conditions also lead to overlaps between dissimilatory nitrate and sulfate respiration. Here, we show that sulfate and nitrate respiration co-occur in the surface sediments of an intertidal sand flat. Furthermore, we found strong correlations between dissimilatory nitrite reduction to ammonium (DNRA) and sulfate reduction rates. Until now, the nitrogen and sulfur cycles were assumed to be mainly linked in marine sediments by the activity of nitrate-reducing sulfide oxidisers. However, transcriptomic analyses revealed that the functional marker gene for DNRA (nrfA) was more associated with microorganisms known to reduce sulfate rather than oxidise sulfide. Our results suggest that when nitrate is supplied to the sediment community upon tidal inundation, part of the sulfate reducing community may switch respiratory strategy to DNRA. Therefore increases in sulfate reduction rate in-situ may result in enhanced DNRA and reduced denitrification rates. Intriguingly, the shift from denitrification to DNRA did not influence the amount of N2O produced by the denitrifying community. Our results imply that microorganisms classically considered as sulfate reducers control the potential for DNRA within coastal sediments when redox conditions oscillate and therefore retain ammonium that would otherwise be removed by denitrification, exacerbating eutrophication.

Network analysis of 16S rRNA sequences suggests microbial keystone taxa contribute to marine N2O cycling

The mechanisms by which large-scale microbial community function emerges from complex ecological interactions between individual taxa and functional groups remain obscure. We leveraged network analyses of 16S rRNA amplicon sequences obtained over a seven-month timeseries in seasonally anoxic Saanich Inlet (Vancouver Island, Canada) to investigate relationships between microbial community structure and water column N2O cycling. Taxa separately broadly into three discrete subnetworks with contrasting environmental distributions. Oxycline subnetworks were structured around keystone aerobic heterotrophs that correlated with nitrification rates and N2O supersaturations, linking N2O production and accumulation to taxa involved in organic matter remineralization. Keystone taxa implicated in anaerobic carbon, nitrogen, and sulfur cycling in anoxic environments clustered together in a low-oxygen subnetwork that correlated positively with nitrification N2O yields and N2O production from denitrification. Close coupling between N2O producers and consumers in the anoxic basin is indicated by strong correlations between the low-oxygen subnetwork, PICRUSt2-predicted nitrous oxide reductase (nosZ) gene abundances, and N2O undersaturation. This study implicates keystone taxa affiliated with common ODZ groups as a potential control on water column N2O cycling and provides a theoretical basis for further investigations into marine microbial interaction networks.

Reactive oxygen species affect the potential for mineralization processes in permeable intertidal flats

Intertidal permeable sediments are crucial sites of organic matter remineralization. These sediments likely have a large capacity to produce reactive oxygen species (ROS) because of shifting oxic-anoxic interfaces and intense iron-sulfur cycling. Here, we show that high concentrations of the ROS hydrogen peroxide are present in intertidal sediments using microsensors, and chemiluminescent analysis on extracted porewater. We furthermore investigate the effect of ROS on potential rates of microbial degradation processes in intertidal surface sediments after transient oxygenation, using slurries that transitioned from oxic to anoxic conditions. Enzymatic removal of ROS strongly increases rates of aerobic respiration, sulfate reduction and hydrogen accumulation. We conclude that ROS are formed in sediments, and subsequently moderate microbial mineralization process rates. Although sulfate reduction is completely inhibited in the oxic period, it resumes immediately upon anoxia. This study demonstrates the strong effects of ROS and transient oxygenation on the biogeochemistry of intertidal sediments.

Annual biogeochemical cycling in intertidal sediments of a restored estuary reveals dependence of N, P, C and Si cycles to temperature and water column properties

Estuarine intertidal sediments are important centres for organic matter remineralization and nutrients recycling. Nevertheless, there is limited understanding regarding how these processes occur along the salinity gradient and their seasonality. Here, we report on the seasonal biogeochemical cycles from three types of intertidal sedimentary habitats (freshwater, brackish and marine) located in the Western Scheldt estuary (The Netherlands and Belgium). A full year of solute fluxes, porewater nutrient and sediment pigment concentrations at a monthly resolution revealed clear differences in the biogeochemistry of the three sites, indicating that environmental conditions determined the local nutrient dynamics. Temperature controlled sediment oxygen consumption rates and nutrient fluxes, but also affected pore water nutrient concentrations up to 14 cm deep. Fresh and brackish sediments had a net influx of dissolved inorganic nitrogen (DIN) (−1.62 mmol m−2 d−1 and -2.84 mmol m−2 d−1, respectively), while only the freshwater sediments showed a net influx of phosphate (−0.07 mmol m−2 d−1). We estimated that intertidal sediments remineralized a total of 10,000 t C y−1, with 97% of mineralization occurring in the brackish and marine parts. Overall, sediments removed 11% (1500 t N y−1) and 15% (∼200 t P y−1) of the total nitrogen and phosphorus entering the estuary from riverine input. Moreover, observations revealed the historical improvement of water quality resulting from water treatment policies. This spatiotemporal study of OM remineralization and early diagenesis in estuarine systems highlights the importance of intertidal sediments for estuarine systems. Our observations can be used in models to predict estuarine biogeochemistry or assess climate change scenarios.

Elevated anthropogenic CO2 invasion and stimulated carbonate dissolution in the South China Sea Basin

Great concern has been drawn in how marginal seas respond to elevated atmospheric carbon dioxide (CO2) and changes of carbonate system during absorbing anthropogenic CO2 (Cant). Decades of Cant absorption had prominently changed the carbonate system in the South China Sea (SCS). The Cant penetrated 1200–1800 m in the SCS in 2015, deeper than it in 1997. While the inventory of Cant reached 0.66 Gt C in the entire SCS, approximately 10% higher than it in 1997.With the invasion of the Cant, the average saturated depth of aragonite and calcite became shallower. The organic carbon pump and carbonate pump in the central and southern SCS Basin were less efficient than those in the northern SCS Basin especially in the intermediate waters, which were probably caused by the different biogenic compositions and residence time from north to south. The less calcite carbonate (CaCO3) load and longer residence time in the southern SCS Basin enhanced organic matter remineralization therein and thus resulted in a less efficient organic carbon pump. The decomposition of organic matter enhanced the CaCO3 dissolution while more CaCO3 dissolution enhanced the ability of buffering the atmospheric CO2 especially in the intermediate waters. In the foreseeable future, the SCS will absorb more Cant and continue decreasing the saturation depth of CaCO3. Plain language summaryThe ocean absorbs more atmospheric carbon dioxide (CO2) and changes its carbonate system under increasing emission of anthropogenic CO2 (Cant) since the industrial evolution. Decades of Cant absorption resulted in a deeper penetration depth by several hundred meters from 1997 to 2015 in the South China Sea (SCS), and enhanced the inventory of Cant in the meantime. During the invasion of the Cant, although changes of carbonate system could not be found from the dissolved oxygen, apparent oxygen utilization (AOU), normalized dissolved inorganic carbon (NDIC) and normalized total alkalinity in the upper 1000 m between 1997 and 2015 since the discrepancies were not significant enough. But the signals of acidification in the SCS became more severe, resulting in a shallower saturated depth of aragonite and calcite.The distribution of NDIC and AOU showed distinct pattern from north to south in the SCS Basin especially in the intermediate water, indicating a less efficient organic carbon pump and carbonate pump in the central and southern SCS Basin than those in the northern SCS Basin. The composition of carbonate (CaCO3) and residence time contributed to the discrepancies of these two pumps. Under the invasion of Cant, more CaCO3 dissolved and enhanced the ability of absorbing atmospheric CO2. In the foreseeable future, the SCS will absorb more Cant and continue decreasing the saturation depth of CaCO3.

Microbial Community Diversity of Coral Reef Sediments on Liuqiu Island, Southwestern Taiwan

Microbes in coral reef sediments are thought to play an important role in organic matter remineralization and nutrient recycling. Microbial communities also reflect the environmental conditions, such as nutrient status, of an ecosystem. This study investigates the relationship between microbial community diversity in the reef sediments and environmental conditions at Liuqiu Island. We sampled sediments seasonally from four sites around the island, Beauty Cave, Geban Bay, Houshi Fringing Reef, and Lobster Cave, from 2015–2020. The V5–V6 hypervariable region of 16S rRNA was amplified and sequenced using the Illumina MiSeq platform to identify the microbial communities. The results showed that the high abundance of Pseudomonadota, Planctomycetota, and Bacteroidota might reflect the eutrophic environments of the sediments on Liuqiu Island. Second, the identification of putative pathogens and human-related genera suggests that human activities have affected the marine environment of Liuqiu Island. Third, the insignificant spatial differences and the significant temporal differences in the microbial communities of Liuqiu Island indicate that annual or periodical events, such as the Kuroshio Branch Current and South China Sea Surface Current, could shape the microbial communities of Liuqiu Island. Furthermore, the abundance of human-related genera—Cutibacterium, Herbaspirillum, Corynebacterium 1, Escherichia-Shigella, and Kocuria—increased dramatically in the Lobster Cave site in September 2015 and may have been induced by a strong climate event, such as a typhoon or heavy rainfall. Our results revealed that the microbial communities of Liuqiu Island are dynamic and sensitive to adjacent environmental conditions. The sedimented microbial communities could monitor the bacteria and pathogens related to human activities and even reveal the putative events that could affect the ecological environments.

Contemporary Controls on Terrestrial Carbon Characteristics in Temperate and Sub‐Tropical Australian Wetlands

AbstractA thorough understanding of controls over terrestrial sedimentary organic carbon characteristics in both the present and the past is pivotal to better understand atmospheric CO2 pathways into depositional sinks such as peats, swamps, and lakes. We explored the relationship between wetland sediment organic matter storage, climate (precipitation, temperature) and catchment vegetation data (catchment vegetation cover in percent; leaf carbon content in g/m2) by means of multivariate statistical analyses to investigate patterns of carbon deposition in modern wetlands and to provide a more robust framework for interpreting sediment bulk organic geochemistry as a proxy for past carbon cycling. Carbon and nitrogen elemental concentration and stable isotope composition were analyzed from sub‐surface sediments at 18 wetlands in eastern Australia. The statistical analyses indicate that variability in geochemical organic matter data in wetland sediments is best explained by geographic differences in catchment vegetation cover and, by inference, the balance of terrestrial versus aquatic organic matter input to the sediment. TOC/TN of aquatic matter may be additionally driven toward higher (terrestrial) values by nitrogen limitation in the catchment and the lakes. These processes explain up to ∼40% of the total variance in the sediment geochemistry (redundancy analyses). Up to ∼10% of the total variance may be attributed to post‐depositional processes and organic matter remineralization. The remaining ∼50% of total variance in the data may be attributed to local conditions across the sites, geochemical processes that were not captured in this study, or to the different timescales covered by the sediments at each site.

Nutrients and chlorophyll-a in the Gulf of Oman: High seasonal variability in nitrate distribution

As part of comprehensive oceanographic studies on the Persian Gulf and Gulf of Oman, this article documented the seasonal and vertical distribution of nutrients, chlorophyll-a, and nitrogen deficiency in the Strait of Hormuz and northern part of Gulf of Oman. During the three consecutive research cruises of RV Kavoshgar Khalij Fars (Persian Gulf Explorer) in September–October 2018, December 2018, and May 2019, 7–18 stations were surveyed. During the southwest monsoon season, the water column is characterized by a 10–20 m shallow mixed layer and high spatial variation in nutrients and their ratio in the upper 100 m. Shallow remineralization of nitrogen-rich organic matter and lateral advection from the Arabian Sea upwelling system lead to nutrient accumulation, and enrichment of nitrate-nitrogen compared to P and Si in the euphotic zone with some sign of Si limitation. On the other hand, deepening of the mixed layer due to the cool convective mixing during the northeast monsoon, brings nutrients to the surface and stimulate the phytoplankton blooms (i.e. with surface chlorophyll-a maximum of 1.46 ± 0.57 μg l−1). So, the primary production is likely to be controlled by grazing rather than nutrient availability during the northeast monsoon. In the spring inter-monsoon, the multilayer structure of the water column with a less well-characterized mixed layer has a surface layer severely depleted in nitrite and nitrate with low yet detectable concentrations of silicate (0.77 ± 0.44 μM) and phosphate (0.29 ± 0.09 μM). The distribution of surface chlorophyll-a concentration was patchy with generally low concentrations of 0.37 ± 0.10 μg l−1 with highly variable subsurface maxima occurring at 10–25 m depth level. Primary production seemed to be potentially limited by inorganic nitrogen availability in spring inter-monsoon, as severe nitrogen deficiencies extended from the surface to the bottom layer (17.54 ± 4.39 μM at 400–1000 m depth level).