Low Primary Productivity Research Articles

Environmental changes affect the microbial release of hydrogen sulfide and methane from sediments at Boknis Eck (SW Baltic Sea).

Anthropogenic activities are modifying the oceanic environment rapidly and are causing ocean warming and deoxygenation, affecting biodiversity, productivity, and biogeochemical cycling. In coastal sediments, anaerobic organic matter degradation essentially fuels the production of hydrogen sulfide and methane. The release of these compounds from sediments is detrimental for the (local) environment and entails socio-economic consequences. Therefore, it is vital to understand which microbes catalyze the re-oxidation of these compounds under environmental dynamics, thereby mitigating their release to the water column. Here we use the seasonally dynamic Boknis Eck study site (SW Baltic Sea), where bottom waters annually fall hypoxic or anoxic after the summer months, to extrapolate how the microbial community and its activity reflects rising temperatures and deoxygenation. During October 2018, hallmarked by warmer bottom water and following a hypoxic event, modeled sulfide and methane production and consumption rates are higher than in March at lower temperatures and under fully oxic bottom water conditions. The microbial populations catalyzing sulfide and methane metabolisms are found in shallower sediment zones in October 2018 than in March 2019. DNA-and RNA profiling of sediments indicate a shift from primarily organotrophic to (autotrophic) sulfide oxidizing Bacteria, respectively. Previous studies using data collected over decades demonstrate rising temperatures, decreasing eutrophication, lower primary production and thus less fresh organic matter transported to the Boknis Eck sediments. Elevated temperatures are known to stimulate methanogenesis, anaerobic oxidation of methane, sulfate reduction and essentially microbial sulfide consumption, likely explaining the shift to a phylogenetically more diverse sulfide oxidizing community based on RNA.

Hotspots and drivers of compound marine heatwaves and low net primary production extremes

Abstract. Extreme events can severely impact marine organisms and ecosystems. Of particular concern are multivariate compound events, namely when conditions are simultaneously extreme for multiple ocean ecosystem stressors. In 2013–2015 for example, an extensive marine heatwave (MHW), known as the Blob, co-occurred locally with extremely low net primary productivity (NPPX) and negatively impacted marine life in the northeast Pacific. Yet, little is known about the characteristics and drivers of such multivariate compound MHW–NPPX events. Using five different satellite-derived net primary productivity (NPP) estimates and large-ensemble-simulation output of two widely used and comprehensive Earth system models, the Geophysical Fluid Dynamics Laboratory (GFDL) ESM2M-LE and Community Earth System Model version 2 (CESM2-LE), we assess the present-day distribution of compound MHW–NPPX events and investigate their potential drivers on the global scale. The satellite-based estimates and both models reveal hotspots of frequent compound events in the center of the equatorial Pacific and in the subtropical Indian Ocean, where their occurrence is at least 3 times higher (more than 10 d yr−1) than if MHWs (temperature above the seasonally varying 90th-percentile threshold) and NPPX events (NPP below the seasonally varying 10th-percentile threshold) were to occur independently. However, the models show disparities in the northern high latitudes, where compound events are rare in the satellite-based estimates and GFDL ESM2M-LE (less than 3 d yr−1) but relatively frequent in CESM2-LE. In the Southern Ocean south of 60∘ S, low agreement between the observation-based estimates makes it difficult to determine which of the two models better simulates MHW–NPPX events. The frequency patterns can be explained by the drivers of compound events, which vary among the two models and phytoplankton types. In the low latitudes, MHWs are associated with enhanced nutrient limitation on phytoplankton growth, which results in frequent compound MHW–NPPX events in both models. In the high latitudes, NPPX events in GFDL ESM2M-LE are driven by enhanced light limitation, which rarely co-occurs with MHWs, resulting in rare compound events. In contrast, in CESM2-LE, NPPX events in the high latitudes are driven by reduced nutrient supply that often co-occurs with MHWs, moderates phytoplankton growth, and causes biomass to decrease. Compound MHW–NPPX events are associated with a relative shift towards larger phytoplankton in most regions, except in the eastern equatorial Pacific in both models, as well as in the northern high latitudes and between 35 and 50∘ S in CESM2-LE, where the models suggest a shift towards smaller phytoplankton, with potential repercussions on marine ecosystems. Overall, our analysis reveals that the likelihood of compound MHW–NPPX events is contingent on model representation of the factors limiting phytoplankton production. This identifies an important need for improved process understanding in Earth system models used for predicting and projecting compound MHW–NPPX events and their impacts.

Controls on organic matter accumulation on the Late-Ordovician shales in Awati Sag in northwestern Tarim Basin, NW China

The black shales of the Upper Ordovician Yingan Formation (Fm.) in the northwestern margin of the Tarim Basin was previously considered to be an important source rock. However, the total organic carbon (TOC) values of Yingan black shales was low, averaging 0.51%. Therefore, it remains debated whether they can be served as high-quality source rock. Here, we investigated the sedimentary environment (including terrigenous input, chemical weathering intensity, paleoproductivity, paleoredox conditions) of the Yingan shales and the factors affecting organic matter (OM) enrichment through organic and inorganic geochemical methods. The proxies of terrigenous flux (e.g., Ti/Al and Zr/Al) and sedimentation rate flux [e.g., (La/Yb)P] shows that the terrigenous inputs has no effect on OM enrichment. The low primary productivity (e.g., low values of P/Al, Ba/Al and Ni/Al) of the Yingan shales also exhibits a little effect on OM enrichment. However, the paleoredox proxies (e.g., U/Al, V/Al, U/Th and V/Cr) show that the black shale was deposited in a partial oxidation environment of bottom waters, which is unfavorable for the preservation and enrichment of OM evidenced. The oxidation environment was caused by the enhanced thermohaline circulation and ventilation of bottom waters due to the increasing cooling global climate during Late Ordovician. Therefore, the Yingan shales cannot be served as a source rock or shale gas reservoir. Additionally, the poor OM enrichment model provides new insights for the evaluation of shales linked to cold climatic conditions.

Archean to early Paleoproterozoic iron formations document a transition in iron oxidation mechanisms

It is generally accepted that photosynthetic marine planktonic bacteria were responsible for the oxidation of dissolved ferrous iron (Fe(II)) and the subsequent deposition of iron formations (IFs) throughout the Archean and early Paleoproterozoic. However, the relative roles of the different biological Fe oxidation mechanisms in driving IF deposition—such as anoxygenic photosynthesis (photoferrotrophs) and oxygenic photosynthesis (cyanobacteria)—remain poorly resolved. Here, we present coupled bulk-rock Fe isotope and manganese (Mn) versus Fe ratios from Archean to early Paleoproterozoic IFs in order to provide a new perspective on Earth’s early redox history and processes leading to IF deposition. Based on this updated IF geochemical record, we bolster the case that the partial oxidation of Fe(II) to Fe(III) was central to IF genesis, arguing against extensive water column Fe(II) silicate formation as the main process driving IF deposition. The geochemistry of IFs deposited prior to the Great Oxidation Event (GOE) shows that partial Fe(II) oxidation was a common feature in either anoxic or low oxygen (O2) conditions, where metabolic Fe(II) oxidation by photoferrotrophs is likely to have prevailed over ambient Fe(II) oxidation by O2 produced by cyanobacteria. Assuming that cyanobacteria evolved in the Archean, the presence of partial Fe(II) oxidation suggests that O2 production was relatively muted during this time. This points to a model for Archean surface redox conditions, whereby oxygen oases were relatively limited in extent, likely due to low primary productivity of cyanobacteria and high Fe fluxes. We further demonstrate a gradual displacement of metabolic Fe(II) oxidation in the Archean by quantitative O2-driven Fe(II) oxidation during the GOE by ca. 2.31 Ga.

Variations in the reproductive cycle of Bornean montane tree species along elevational gradients on ultrabasic and non‐ultrabasic soils

AbstractAlthough lowland tree species in the ever‐wet regions of Southeast Asia are characterized by the supra‐annual cycle of reproduction, the reproductive phenology of montane tree species remains poorly understood. In this study, we investigated the reproductive phenology of montane tree species using litter samples that were collected every 2 weeks from six rainforest sites, consisting of three elevations (1700, 2700, and 3100 m), on Mount Kinabalu, Borneo. At each elevation, one site was on infertile ultrabasic soil and one was on relatively fertile non‐ultrabasic soil. We used a composite sample from 10 or 20 litter traps per site and sorted it by species. Therefore, the obtained data captured reproductive phenology in the population of each species rather than in an individual tree. Ten‐year time series of flower and fruit litterfall were obtained for 30 and 39 tree species, respectively. Fourier analysis was used to identify the dominant cycle of each time series. The most abundant cycle across species was supra‐annual, followed by sub‐annual, and annual cycles. Many species at higher elevations showed supra‐annual cycles of flower litterfall, whereas species in the 1700 m sites often showed annual or sub‐annual cycles regardless of soil types. No systematic differences were found among sites for fruit litterfall. Mechanisms underlying these elevational patterns in reproductive cycle remain unclear but may include more severe El Niño droughts, lower primary productivity, lower soil fertility, and the absence of some sub‐annually or annually reproducing families at higher elevations.

Assessing marine communities and carbon cycling that sustained shallow-marine ecosystems on Silurian, reef-rimmed carbonate platforms

A detailed middle Silurian to Early Devonian biomarker stratigraphic record was obtained from thermally well-preserved strata, sampled from a > 400 m long drill-core 25 from Podillya, Ukraine, deposited in the Podillyan peri-continental basin. These lipid biomarker records provide useful insights into the temporal changes in microbial ecology and ocean chemistry for this ancient, tropical shallow-marine reefal carbonate platform. The paleoenvironmental conditions favored sustenance of bacteria over algae (as indicated by elevated hopane/sterane ratios (average: 10.7, maximum value of 36.9); and appreciable 2α-methylhopanes and 3β-methylhopanes) in this nutrient-depleted, marine reefal habitat. The setting behind the reef tract in a lagoon and muted nutrient supply from the open ocean likely contributed to the persistently low primary productivity and low net biomass over millions of years of deposition. The most prolific algal primary producers were green algae consistent with a C29 sterane dominance. No detectable contributions of 24-n-propylcholestane (24-npc) and/or other C30 regular sterane compounds were found with GC–MRM-MS. Elevated 3β-methylhopane index values (3-MeHI; range: 3–13%, mean: 6.4%) were observed throughout the core interval, which is a characteristic associated with source contributions from methanotrophic bacteria and indicative of an enhanced marine methane cycle. This extends the previous findings of consistently high abundances of 3-methylhopanes for Ordovician marine environments to include a tropical, marine shelf setting persisting through the Silurian Period.

Towards process-oriented management of tropical reefs in the anthropocene

Tropical reefs and the fish relying on them are under increasing pressure. Shallow-reef fish provide important ecological information in addition to sustaining fisheries, tourism and more. Although empirical metrics of fish biomass are widely used in fisheries management, metrics of biomass production—how much new biomass is produced over time—are rarely estimated even though such production informs potential fisheries yields. Here we estimate fish standing biomass (B), biomass production (P, the rate of biomass accumulation) and biomass turnover (P/B ratio, the rate of biomass replacement) for 1,979 tropical reef sites spanning 39 tropical countries. On the basis of fish standing biomass and biomass turnover, we propose a conceptual framework that splits reefs into three classes to visualize ecological and socio-economic risk and help guide spatial management interventions (for example, marine protected areas) to optimize returns on conservation efforts. At large scales, high turnover was associated with high human pressure and low primary productivity, whereas high biomass was associated with low human pressure and high primary productivity. Going beyond standing fish biomass to consider dynamic ecological processes can better guide regional coral reef conservation and sustainable fisheries management.

COVID-19 Lockdowns Promoted Recovery of the Yangtze River’s Aquatic Ecosystem

The impacts of COVID-19 lockdowns on human life, air quality, and river water quality around the world have received significant attention. In comparison, assessments of the implications for freshwater ecosystems are relatively rare. This study explored the impact of COVID-19 lockdowns on aquatic ecosystems in the Yangtze River by comparing river water quality, phytoplankton, zooplankton, and fish data collected at the site in the middle reach of the Yangtze River in 2018 and 2020. The results show that during COVID-19 lockdowns, the reduction in industrial and domestic effluent discharge led to a reduction in organic pollution and industrial plant nutrient pollution in rivers. Among them, PO43−−P, CODMn, and TP were significantly decreased (p < 0.05). During lockdowns, nutrient supplies such as TN and TP were reduced, which led to inhibition of algae growth and decreased phytoplankton abundance. Phytoplankton affects the abundance of zooplankton through a bottom-up effect, and a decrease in phytoplankton density leads to a decrease in zooplankton density. The decrease in plankton density led to lower primary productivity in rivers, reduced fish feed supplies, intensified competition among fish populations, with increases in population dominated by high trophic level carnivorous fish. In addition, the decrease in fishing intensity has contributed to an increase in the number of rivers–sea migratory fish; the fish community was earlier mainly dominated by small-sized species with a short life cycle, and the number of supplementary populations has now increased. As a consequence, the fish community structure shows a tendency toward high complexity and high fish diversity. Overall, these observations demonstrate that the rapid revival of the retrogressive Yangtze River ecosystem is possible through limitation of anthropic interferences.

Variations of Primary Productivity in the Northwestern Arabian Sea During the Last 23,000 Years and Their Paleoclimatological Implications

AbstractThe Arabian Sea (AS) is one of the most productive oceanic regions in the world due to several monsoon‐related processes that can increase nutrients contents in the euphotic zone. Previous studies of the imprint of oceanic primary productivity (PP) in AS sediments yielded diverse results depending on the studied area and the chosen proxies, with unprecise paleoclimatic interpretations. Here, we provide multi‐decennial PP and coastal upwelling dynamic records off northern Oman over the last 23 Kyr, based on the analysis of coccoliths from sediment core MD00‐2354. Our results have been compared with previous paleoenvironmental records as well as new modeling data to get precise paleoclimatic interpretations. We document higher PP and weaker coastal upwelling during the Last Glacial Maximum relative to the Holocene, and significant millennial‐scale variations over the last deglaciation corresponding to the fluctuations of the Atlantic Meridional Overturning Circulation strength. Higher PP and weaker upwelling are found during the cold stadials, while lower PP and stronger upwelling during the warm interstadial. We propose that the increases of PP were driven by increased bioavailable nutrient content in surface waters under both stronger winter monsoon conditions that strengthened the convective mixing, and higher aeolian inputs. Over the Holocene, stronger upwelling and slightly lower PP are found during the Early‐Mid Holocene, when higher summer insolation triggered stronger summer monsoon. At that time, the lower PP was probably the result of restricted advection of eutrophic summer upwelling seawater under negative wind stress curls and less aeolian inputs.

Nitrogen Isotopes from the Neoproterozoic Liulaobei Formation, North China: Implications for Nitrogen Cycling and Eukaryotic Evolution

The nitrogen isotope compositions (δ15N) of sedimentary rocks can provide information about the nutrient N cycling and redox conditions that may have played important roles in biological evolution in Earth’s history. Although considerable δ15N data for the Precambrian have been published, there is a large gap during the early Neoproterozoic that restrains our understanding of the linkages among N cycling, ocean redox changes and biological evolution during this key period. Here, we report bulk δ15N and organic carbon isotope (δ13Corg) compositions as well as the total nitrogen (TN) and total organic carbon (TOC) contents from the Tonian fossiliferous Liulaobei Formation in the southern part of the North China Platform. The δ15N in the study section is dominated by very stable values centering around +4.3‰, which is moderately lower than in modern sediments (~ +6‰). These positive δ15N values were attributed to partial denitrification under low primary productivity (scenario 1) and/or denitrification coupled with dissimilatory nitrate reduction to ammonium (DNRA) (scenario 2). In either case, the availability of fixed nitrogen may have provided the nutrient N required to facilitate facilitated eukaryotic growth. Our study highlights the pivotal role of nutrient N in the evolution of eukaryotes.

Paleoenvironmental changes during the early Toarcian Oceanic Anoxic Event: Insights into organic carbon distribution and controlling mechanisms in the eastern Tethys

• Unlike N Europe, TOC contents are significantly varied in lower Toarcian sediments of the eastern Tethys . • Anoxic, but non-sulfidic conditions prevailing during the T-OAE. • Humid climates promoted moderate continental weathering during the T-OAE. • Marine productivity was commonly high during the T-OAE in the eastern Tethyssouthern Qiangtang Basin. • Organic carbon burial and oil shale accumulation sustained by increased primary productivity. The Toarcian Oceanic Anoxic Event (T-OAE; Early Jurassic, ca. 183 Ma) represents an intense Mesozoic organic carbon burial event that was associated with the formation of oxygen-deficient conditions at paleo-shelf seas, as well as in lacustrine settings. Global scale increase in organic carbon accumulation occurred in connection with profound changes in climate conditions and perturbations of biogeochemical cycles. This study presents geochemical data for lower Toarcian and Bajocian-Bathonian sediments exposed in the Bilong Co section, located in the southern Qiangtang Basin of central Tibet. During Early and Middle Jurassic times, the study site was located in the north-eastern marine of the Tethys Ocean. A total of 83 rock samples, spanning the Quse (lower Toarcian) and Sewa formations (upper Bajocian-Bathonian), were analyzed for their total organic carbon (TOC) content, carbon isotopes of bulk carbonate and kerogen, as well as major and trace elements. Data allowed to assess changes in the environmental conditions in the eastern Tethys Ocean and further allowed to reconstruct changes in paleo-weathering conditions, marine primary productivity, and redox regimes that occurred in association with the T-OAE. In addition, factors that controlled the accumulation of organic material in the sediment are discussed. Enrichment factors (EF) of redox-sensitive trace elements, their relationship with TOC, and C org /P total molar ratio attest to predominantly anoxic pore and bottom waters in southern Qiangtang Basin during the T-OAE. On the contrary, oxic conditions prevailed during pre- and post-T-OAE times. During the T-OAE, the development of anoxic conditions were most likely linked to an increase in marine primary productivity, as revealed by C org /P total molar ratio, as well as Al-normalized Ba, and Zn concentrations, and their relationship with TOC. Data further attest to a moderate to low marine primary productivity during pre- and post-TOAE times. During the T-OAE, an increase in the marine primary productivity was linked to intensified continental weathering, facilitated by semi-humid climate conditions. Compilation of TOC data from various depositional settings in the eastern Tethys Ocean revealed a high spatial variability, highlighting the importance of local intrabasinal processes on organic matter accumulation.

Paleoenvironmental Conditions and Factors Controlling Organic Carbon Accumulation during the Jurassic–Early Cretaceous, Egypt: Organic and Inorganic Geochemical Approach

The Jurassic–Early Cretaceous was a time of variable organic carbon burial associated with fluctuations of marine primary productivity, weathering intensity, and redox conditions in the pore and bottom water at paleo-shelf areas in north Egypt. This time interval characterized the deposition of, from old to young, the Bahrein, Khatatba, Masajid, and Alam El Bueib Formations in the north Western Desert. Although several studies have been devoted to the excellent source rock units, such as the Khatatba and Alam El Bueib Formations, studies on paleoenvironmental changes in redox conditions, paleoproductivity, and continental weathering and their impact on organic carbon exports and their preservation for this interval are lacking. This study presents organic and inorganic geochemical data for the Jurassic–Lower Cretaceous sediments from the Almaz-1 well in the Shushan Basin, north Western Desert. A total of 32 cuttings samples were analyzed for their major and trace elements, carbonates, and total organic carbon (TOC) contents. Data allowed the reconstruction of paleoenvironmental conditions in the southern Tethys Ocean and assessment of the changes in paleo-redox, paleo-weathering, and marine primary productivity, and the role of sediment supply. Additionally, factors that governed the accumulation of organic matter in the sediment were interpreted. Results showed that the Khatatba Formation was deposited during a phase of enhanced marine primary productivity under prevalent anoxia, which triggered enhanced organic matter production and preservation. During the deposition of the Khatatba Formation, significant terrigenous sediment supply and continental weathering were followed by a limited contribution of coarse clastic sediment fluxes due to weak continental weathering and enhanced carbonate production. The Bahrein, Masajid, and Alam El Bueib Formations were deposited during low marine primary productivity and prevalent oxygenation conditions that led to poor organic matter production and preservation, respectively. A strong terrigenous sediment supply and continental weathering predominated during the deposition of the Bahrein Formation and the lower part of the Alam El Bueib Formation compared to the limited coarse clastic supply and continental weathering during the deposition of the carbonate Masajid Formation and the upper part of the Alam El Bueib Formation. Such conditions resulted in the enhanced dilution and decomposition of labile organic matter, and, thus, organic carbon-lean accumulation in these sediments.

Export of particulate organic carbon (POC) in the eddy region of the tropical northwest Pacific

We examined particulate organic carbon (POC) export using 238U–234Th disequilibrium in the tropical northwest Pacific Ocean, where numerous eddies are present. We obtained data from an anticyclonic eddy in 2019 and from both anticyclonic and cyclonic eddies in 2020. In 2019, excess 234Th and higher POC concentrations were observed in the upper 100 m layer inside the anticyclonic eddy compared with the outer area of the eddy (the reference site). We speculate that the peculiar feature of excess 234Th in the surface layer was caused by horizontal transport of POC into the eddy and consequent POC degradation and release of particulate 234Th to a dissolved form. However, in 2020, lower POC concentrations with 234Th deficiency were observed in both cyclonic and anticyclonic eddies relative to the reference site. In both years, POC export was lower in the cores of the anticyclonic and cyclonic eddies relative to the reference site. We propose that severe nutrient depletion in the upper 150 m layer hindered nutrient supply by vertical water movement in the eddies. Despite the low POC export at 100 m depth, POC export at 500 m depth was comparable to values observed at 500 m depth at Station Papa in the more productive northeastern Pacific region. Our results imply that POC export into the deep ocean interior in this region may not be as low as expected from the low primary productivity in the euphotic zone.

Climate change controls on extreme organic matter enrichment in Late Permian marine–terrestrial transitional shales in Guizhou, south China

Black organic-rich shale is an important target for unconventional natural gas exploration. However, compared with marine shale, the mechanism of organic matter enrichment in shale deposited in the marine–continental transitional environment is controversial because of the strong heterogeneity. In this study, an integrated sedimentological and elemental geochemical investigation was conducted for the dark shales in the Lopingian coal-bearing strata of western Guizhou, South China. This study aims to clarify the detailed control mechanism of major geological events on the organic matter accumulation in the transitional facies. Sedimentological observations show that the Lopingian in the Liupanshui area is mainly composed of coal, tight sandstones, and dark mudstones, which were deposited in the tidal flat-lagoon and fluvial delta. Petrological and geochemical evidence suggests the provenance was mainly resulted from the Emeishan basalt weathering in the west of the study area. In addition, the geochemical proxies indicate that the Longtan shales are characterized by low primary productivity, oxic–suboxic bottom water, and a relatively low depositional rate. The paleoclimate index indicates that the climate was predominantly warm and humid with intense chemical weathering during the Lopingian period, and changed to an extremely hot climate in the Early Triassic. Combined with the previous research, this study proposes that the Large Igneous Province (LIP) played a critical role on the organic matter enrichment of the Lopingian across the western Yangtze Block. Firstly, the onset of Emeishan LIP created an essential tectonic background and paleogeographic pattern for massive terrigenous debris to gather here from the west. Secondly, the climate was significantly affected due to the continuous weathering of the Emeishan basalt plateau and the greenhouse gases emission from the Siberian LIP. During the Lopingian, the plants flourished under the tropical rainforest climate and supplied the terrigenous organic matter. Accompanying chemical weathering increased, massive reworked coal fragments were brought into the brackish tidal flat-lagoon environment. Particularly, global warming related to the Siberian LIP and regional transgression occurred in the late Lopingian, forming the black shales with the highest organic matter content (up to 26.39%) of member Ш of the Longtan Formation. However, both the production and supplementation of organic matter significantly decreased because of the end-Permian mass extinction, thus stopped the organic matter enrichment in the Early Triassic Feixianguan Formation.

The upper ocean silicon cycle of the subarctic Pacific during the EXPORTS field campaign

Diatoms are major contributors to marine primary productivity and carbon export due to their rapid growth in high-nutrient environments and their heavy silica ballast. Their contributions are highly modified in high-nutrient low-chlorophyll regions due to the decoupling of upper-ocean silicon and carbon cycling caused by low iron (Fe). The Si cycle and the role of diatoms in the biological carbon pump was examined at Ocean Station Papa (OSP) in the HNLC region of the northeastern subarctic Pacific during the NASA EXport Processes in the Ocean from RemoTe Sensing (EXPORTS) field study. Sampling occurred during the annual minimum in surface silicic acid (Si(OH)4) concentration. Biogenic silica (bSi) concentrations were low, being in the tens of nanomolar range, despite high Si(OH)4 concentrations of about 15 μM. On average, the >5.0-µm particle size fraction dominated Si dynamics, accounting for 65% of bSi stocks and 81% of Si uptake compared to the small fraction (0.6–5.0 μm). Limitation of Si uptake was detected in the small, but not the large, size fraction. Growth rate in small diatoms was limited by Fe, while their Si uptake was restricted by Si(OH)4 concentration, whereas larger diatoms were only growth-limited by Fe. About a third of bSi production was exported out of the upper 100 m. The contribution of diatoms to carbon export (9–13%) was about twice their contribution to primary productivity (3–7%). The combination of low bSi production, low diatom primary productivity and high bSi export efficiency at OSP was more similar to the dynamics in the subtropical gyres than to other high-nutrient low-chlorophyll regions.

Organic matter enrichment of the Late Triassic Bagong Formation (Qiangtang Basin, Tibet) driven by paleoenvironment: Insights from elemental geochemistry and mineralogy

The Upper Triassic dark mudstones in the Qiangtang Basin are key source rocks for petroleum exploration. However, the factors controlling organic matter enrichment in these dark mudstones remain unclear, especially for pro-deltaic core mudstones, which inhibit source rock distribution predictions. Well QZ-7 contains several Upper Triassic dark mudstone intervals (marked by A to G) with total organic carbon (TOC) contents of 0.57–2.73% (avg. of 1.13%). The TOC contents in intervals A, C, E, and G are generally higher than those in intervals B, D, and F. Here, detailed mineralogical and geochemical analyses of these dark mudstones are analyzed to address the organic matter enrichment mechanism. Paleosalinity, redox, detrital influx, sedimentation rate, and productivity proxies suggest that these dark mudstones were deposited in a brackish-salt and anoxic-dysoxic water environment with low detrital influx, sedimentation rate, and primary productivity. Organic-rich intervals of A, C, E, and G generally show higher salinity, lower oxygen levels, detrital influx, and sedimentation rates than the organic-poor intervals of B, D, and F. The significant positive correlations of TOC with paleosalinity proxies and redox indices indicate that anoxic and stratified high-salinity water columns primarily controlled organic matter preservation. The significant negative correlations between TOC and detrital influx proxies and moderate negative correlation between TOC and (La/Yb)N suggest that increasing detrital influxes diluted organic matter, while a low sedimentation rate promoted organic matter preservation. The effect of primary productivity on organic matter enrichment was limited, as evidenced by the weak correlation between TOC and P/Ti.

A Multidisciplinary Approach for A Better Knowledge of the Benthic Habitat and Community Distribution in the Central and Western English Channel

About 80% of the seabed of the English Channel (EC) is covered by coarse sediment, from coarse sand to pebbles. Quantitative data on the benthic macrofauna in these types of sediment remains are rare due to the difficulty of using grab corers in such hard substrates. The deepest central part of the EC (45–101 m depth) was prospected during two VIDEOCHARM surveys in June 2010 and June 2011 to increase knowledge of such sublittoral coarse sediment benthic habitats. Sampling focussed on a longitudinal transect in the deepest part of the EC (13 boxes), extending from the western approach to the Greenwich meridian. Both indirect (side scan sonar, Remote Operated Vehicule) and direct (grab sampling with benthos determination, and grain-size analyses) approaches were used and combined, permitting description of the benthic habitats and communities using seven methods. Five benthic EUNIS habitats (European Nature Information System) were reported: MC3215, MD3211, MC4, MC3212 and MC4215, of which two extended main habitats (MC3211 and M23212) corresponded to an eastern/western gradient from sandy gravel to sandy gravel and pebbles sediment. Three other spatially discrete habitats were associated with poor coarse sand and gravel habitats as well as sandy gravel and pebbles with the presence of the brittle star Ophiothrix fragilis. Taxonomic richness of both extended habitats was on the same order of magnitude as the coarse sand habitat reported elsewhere in the EC, whilst the abundances were among the lowest in deeper areas with low nutrient input and low primary production. The epifauna appeared relatively homogenous in this type of sediment at the scale of the sampling area and was not determined to assign a EUNIS habitat/class. ROV footage illustrated the presence of large epifauna and provided valuable information to ground truth in other sampling methods such as side scan sonar mosaic. Grab photos showing surface sediment was relevant to determine the sediment type, whilst granulometric analyses gave additional information on fine particles content (typically very low).

Phytoplankton size-class contributions to new and regenerated production during the EXPORTS Northeast Pacific Ocean field deployment

The NASA EXport Processes in the Ocean from RemoTe Sensing (EXPORTS) program was established to better quantify the pathways of the biological carbon pump in order to gain a more comprehensive understanding of global carbon export efficiency. The summer 2018 field campaign in the vicinity of Ocean Station Papa (Station P; 50°N, 145°W) in the Northeast Pacific Ocean yielded evidence of low phytoplankton biomass and primary productivity dominated by small cells (<5 µm) that are reliant on recycled nutrients. Using combined 13C/15N stable isotope incubations, we calculated an average depth-integrated dissolved inorganic carbon uptake (net primary production) rate of 23.1 mmol C m–2 d–1 throughout the euphotic zone with small cells contributing 88.9% of the total daily DIC uptake. Average depth-integrated NO3– uptake rates were 1.5 mmol N m–2 d–1 with small cells contributing 73.4% of the total daily NO3– uptake. Estimates of new and regenerated production fluctuated, with small cells continuing to dominate both forms of production. The daily mixed-layer f-ratio ranged from 0.17 to 0.38 for the whole community, consistent with previous studies, which indicates a predominance of regenerated production in this region, with small and large cells (≥5 μm) having average f-ratios of 0.28 and 0.82, respectively. Peak phytoplankton biomass, total primary productivity and new production occurred between Julian Days 238 and 242 of our observation period, driven primarily by an increase in carbon and nitrate assimilation rates without apparent substantial shifts in the phytoplankton size-class structure. Our findings demonstrate the importance of small cells in performing the majority of net primary production and new production and the modest productivity fluctuations that occur in this iron-limited region of the Northeast Pacific Ocean, driven by ephemeral increases in new production, which could have significant ramifications for carbon export over broad timescales.

Greater functional diversity and redundancy of coral endolithic microbiomes align with lower coral bleaching susceptibility

The skeleton of reef-building coral harbors diverse microbial communities that could compensate for metabolic deficiencies caused by the loss of algal endosymbionts, i.e., coral bleaching. However, it is unknown to what extent endolith taxonomic diversity and functional potential might contribute to thermal resilience. Here we exposed Goniastrea edwardsi and Porites lutea, two common reef‐building corals from the central Red Sea to a 17-day long heat stress. Using hyperspectral imaging, marker gene/metagenomic sequencing, and NanoSIMS, we characterized their endolithic microbiomes together with 15N and 13C assimilation of two skeletal compartments: the endolithic band directly below the coral tissue and the deep skeleton. The bleaching-resistant G. edwardsi was associated with endolithic microbiomes of greater functional diversity and redundancy that exhibited lower N and C assimilation than endoliths in the bleaching-sensitive P. lutea. We propose that the lower endolithic primary productivity in G. edwardsi can be attributed to the dominance of chemolithotrophs. Lower primary production within the skeleton may prevent unbalanced nutrient fluxes to coral tissues under heat stress, thereby preserving nutrient-limiting conditions characteristic of a stable coral-algal symbiosis. Our findings link coral endolithic microbiome structure and function to bleaching susceptibility, providing new avenues for understanding and eventually mitigating reef loss.

Integrated environmental influences quantification of pilot-scale constructed wetlands based on modified ecological footprint assessment

Constructed wetlands (CWs) are widely used for non-point source pollution control and water environmental quality improvement. Though it is effective in water quality improvement under most conditions, the overall impacts on the ecological environment in terms of greenhouse gases (GHGs) emissions is a growing concern. Besides, large area requirement has limited further applications of the technology in urban areas. A novel assessment tool of integrating grey water footprint into the ecological footprint framework is established for the assessment of pilot-scale CWs. Findings are compared with a natural riparian wetland adjacent to the researched CWs which were monitored simultaneously. Results demonstrated the CWs had relatively good water quality polishing performance, especially for nitrogen removal. Nonetheless, a large amount of CO2 and some CH4 and N2O emissions were recorded. Meanwhile, a substantial amount of CO2 was also sequestrated by wetland plants via photosynthesis. The strong reducing environment of the CWs inhibited CO2 and N2O generation to a great extent. Calculation of all gaseous emissions and sequestration in CO2 equivalents demonstrated that CWs are an efficient carbon sink. By contrast, the natural wetland was a carbon source because of the high emission of CO2 and N2O under its weak reducing environment conditions and low gross primary production. The carbon footprints of the constructed and natural wetlands were −24.24 and 12.99 gha respectively. Modified ecological footprint values were determined by integrating the carbon footprint, water footprint and build-up lands footprint, and a value of −24.36 gha was obtained for the CWs and 12.99 gha for the natural wetlands. The results indicated that the CWs had substantial beneficial impacts on the ecological environment. On account of the multifunctional service values provided by the CWs, a typical paradigm for water pollution remediation and carbon sequestration was presented for ecological and environmental governance, especially for riparian areas.