Sediment Oxygen Consumption Research Articles

Rapid effects of plastic pollution on coastal sediment metabolism in nature

While extensive research has explored the effects of plastic pollution, ecosystem responses remain poorly quantified, especially in field experiments. In this study, we investigated the impact of polyester pollution, a prevalent plastic type, on coastal sediment ecosystem function. Strips of polyester netting were buried into intertidal sediments, and effects on sediment oxygen consumption and polyester additive concentrations were monitored over 72-days. Our results revealed a rapid reduction in the magnitude and variability of sediment oxygen consumption, a crucial ecosystem process, potentially attributed to the loss of the additive di(2-ethylhexyl) phthalate (DEHP) from the polyester material. DEHP concentrations declined by 89% within the first seven days of deployment. However, effects on SOC dissipated after 22 days, indicating a short-term impact and a quick recovery by the ecosystem. Our study provides critical insights into the immediate consequences of plastic pollution on ecosystem metabolism in coastal sediments, contributing to a nuanced understanding of the temporal variation of plastic pollution’s multifaceted impacts. Additionally, our research sheds light on the urgent need for comprehensive mitigation strategies to preserve marine ecosystem functionality from plastic pollution impacts.

Hydrodynamic and biochemical impacts on the development of hypoxia in the Louisiana–Texas shelf – Part 1: roles of nutrient limitation and plankton community

Abstract. A three-dimensional coupled hydrodynamic–biogeochemical model with multiple nutrient and plankton functional groups was developed and adapted to the Gulf of Mexico to investigate the role of nutrients and the complexity of plankton community in dissolved oxygen (DO) dynamics. A 15-year hindcast was achieved covering the period of 2006–2020. Extensive model validation against in situ data demonstrates that the model was capable of reproducing vertical distributions of DO and spatial distributions of bottom DO concentration, as well as their interannual variations. The study demonstrates that bottom DO dynamics and hypoxia evolution are significantly influenced by both physical processes and local biochemistry, with sedimentary oxygen consumption and vertical diffusion identified as key contributors. Summer hydrodynamics play a critical role in nutrient distribution and limitation: a notable expansion of Si limitation was simulated when coastal currents shifted eastward or northward. This effect, especially pronounced on the western part of the Louisiana–Texas shelf, underscores the importance of nutrient limitation in shaping DO dynamics. The model identifies a bi-peak primary production pattern in spring and early summer, aligned with satellite chlorophyll a variations, attributed to the complexity of the plankton community and interactions among different plankton groups. Our findings emphasize the necessity of integrating sophisticated plankton community dynamics into biogeochemical models to understand primary production variability and its impact on bottom hypoxia.

Overlooked shelf sediment reductive sinks of dissolved rhenium and uranium in the modern ocean

Rhenium (Re) and uranium (U) are essential proxies in reconstructing past oceanic oxygenation evolution. However, their removal in continental shelf sediments, hotspots of early diagenesis, were previously treated as quantitatively unimportant sinks in the ocean. Here we examine the sedimentary reductive removal of Re and U and their coupling with organic carbon decomposition, utilizing the 224Ra/228Th disequilibria within the East China Sea shelf. We identified positive correlations between their removal fluxes and the rates of sediment oxygen consumption or organic carbon decomposition. These correlations enable an evaluation of global shelf reductive sinks that are comparable to (for Re) or higher than (~4-fold for U) previously established suboxic/anoxic sinks. These findings suggest potential imbalances in the modern budgets of Re and U, or perhaps a substantial underestimation of their sources. Our study thus highlights shelf sedimentary reductive removal as critical yet overlooked sinks for Re and U in the modern ocean.

Raised water temperature enhances benthopelagic links via intensified bioturbation and benthos-mediated nutrient cycling

Sediment reworking by benthic infauna, namely bioturbation, is of pivotal importance in expansive soft-sediment environments such as the Wadden Sea. Bioturbating fauna facilitate ecosystem functions such as bentho-pelagic coupling and sediment nutrient remineralization capacities. Yet, these benthic fauna are expected to be profoundly affected by current observed rising sea temperatures. In order to predict future changes in ecosystem functioning in soft-sediment environments like the Wadden Sea, knowledge on the underlying processes such as sediment reworking, is crucial. Here, we tested how temperature affects bioturbation and its associated ecosystem processes, such as benthic nutrient fluxes and sediment oxygen consumption, using luminophore tracers and sediment incubation cores. We used a controlled mesocosm experiment set-up with key Wadden Sea benthos species: the burrowing polychaetes Arenicola marina and Hediste diversicolor, the bivalve Cerastoderma edule, and the tube-building polychaete Lanice conchilega. The highest bioturbation rates were observed from A. marina, reaching up to 375 cm2yr−1; followed by H. diversicolor, with 124 cm2yr−1 being the peak bioturbation rate for the ragworm. Additionally, the sediment reworking activity of A. marina facilitated nearly double the amount of silicate efflux compared to any other species. Arenicola marina and H. diversicolor accordingly facilitated stronger nutrient effluxes under a warmer temperature than L. conchilega and C. edule. The oxygen uptake of A. marina and H. diversicolor within the sediment incubation cores was correspondingly enhanced with a higher temperature. Thus, increases in sea temperatures may initially be beneficial to ecosystem functioning in the Wadden Sea as faunal bioturbation is definitely expedited, leading to a tighter coupling between the sediment and overlying water column. The enhanced bioturbation activity, oxygen consumption, and facilitated nutrient effluxes from these invertebrates themselves, will aid in the ongoing high levels of primary productivity and organic matter production.

Quantifying sedimentary oxygen consumption contribution to total oxygen depletion in shallow waters via the oxygen isotope approach

To understand dissolved oxygen depletion in estuaries and coasts, it is essential to distinguish the contributions of diffusion-mediated sedimentary oxygen respiration (SOR) and water column respiration (WCR). For the present study, near-bottom waters were sampled in May, July, and October 2019 in Dinghai Bay, China, to reveal the oxygen depletion mechanism in shallow nearshore waters (3-20 m). Field observations of oxygen isotopes and endmember incubations were conducted. Dissolved oxygen concentration ([O2]) and δ18O ranged from 157 to 241 μM and -1.34 to 3.41‰, respectively. Except for one outlier value, -1.34‰ (site TT1), there was a strong negative relationship between [O2] and δ18O among all 3 seasons (r2 = 0.97, p < 0.001, n = 10), suggesting the prevalence of fractionation in the process of O2 consumption. Given the contrasting fractionation effect during SOR (fractionation factor, ε = -3.57‰) and WCR (ε = -18.58‰), the contributions of SOR and WCR to total oxygen depletion were quantified via a respiration-only model or photosynthesis-respiration model (TT1), respectively. The SOR% range was 18-96% (mean 73%), with the remaining contributed by WCR%. There was no clear relationship between SOR% and temperature or depth or [O2] in the near-bottom waters, but [O2] was the best indicator for SOR% when data was pooled with results from deeper waters (20-50 m) in the East China Sea. For shallow waters <20 m in Dinghai Bay, [O2] was an essential control of SOR% and hence should be considered in the water column oxygen budget in coastal ecosystem modeling and management.

Effects of seasonal deposition-erosion cycle on sedimentary organic carbon remineralization and oxygen consumption in a large-river delta-front estuary

Seasonal sediment deposition-erosion events are dominant drivers of particle-solute dynamics in large-river delta-front estuaries (LDEs), but their influence on elemental cycles is not yet fully understood. To better constrain the role of deposition-erosion events on elemental cycling in LDEs, benthic fluxes of dissolved inorganic carbon (DIC), oxygen, and pore-water solute profiles were measured over different seasons in the Changjiang LDE. Benthic DIC efflux (23.4 ± 6.0 mmol C m−2 d−1) was greater than oxygen influx (7.5 ± 2.0 mmol O2 m−2 d−1) in summer but less in winter (7.7 ± 1.2 mmol C m−2 d−1 and 10.1 ± 1.5 mmol O2 m−2 d−1, respectively). The additional oxygen consumption in sediments in winter was likely due to the oxidation of inorganic diagenetic reductive products (IDRP) (e.g., NH4+, Fe2+, and Mn2+) in deeper sediments exposed by erosion, which resulted in the development of an “oxygen debt”. Sedimentary oxygen respiration accounted for at least 48 % of total oxygen consumption (oxygen consumption in both water column and sediment) in winter and was significantly greater than in summer (∼15 %); this highlighted the importance of winter sediment erosion in oxygen depletion. In addition to IDRP oxidation, the remineralization of resuspended sedimentary organic carbon in water column also contributed to the oxygen consumption. The global dataset on benthic DIC and oxygen fluxes provides evidence that the “oxygen debt” is likely to be widespread in LDEs, exerting a significant impact on global carbon and oxygen cycling.

The impact of mariculture biofilters on the distribution of benthic nutrient fluxes, organic matters and bacterial community in a mariculture wastewater treatment system

Constructed-wetlands, biofilms, and sedimentation are among biological filters used in mariculture wastewater treatments, however, their impacts on the distribution of benthic microbial community and inorganic-nutrient fluxes have not been fully explored. This study applied 16 S rRNA high-throughput sequencing technology to investigate the microbial community distribution and their impacts on nutrient fluxes in mariculture biofilters system. Results showed that bacterial community compositions were significantly different in the constructed-wetland and biofilm treatments (p < 0.05) relative to sedimentation. The composition of the 16 S rRNA genes among the treatments were enriched with Proteobacteria (73%), Bacteroidetes (69%), Firmicutes (62%), and Flavobacteria (61%) in Biofilm compared to Proteobacteria (53%), Bacteroidetes (39%), Firmicutes (32%), and Flavobacteria (21%) in constructed wetlands. NMDS analysis showed that bacterial composition in constructed-wetland and biofilms clustered separately compared to sedimentation treatment. Functional-Annotation-of-Prokaryotic-Taxa analysis indicated that the proportions of sediment-microbial-functional groups (aerobic-chemoheterotrophy, chemoheterotrophy, and nitrate-ammonification) were 47% in the constructed-wetland, 32% in biofilm and 13% in sedimentation system. Benthic-nutrient fluxes for phosphate, ammonium, nitrite, nitrate and sediment oxygen consumption differed markedly among the treatments (p < 0.05). Canonical correspondence analysis indicated constructed-wetland had the strongest association between biogeochemical contents and the bacterial community relative to other treatments. This study suggests that the mariculture wastewater biofilters promoted microbial community distributions, sediment bacterial functional-groups including chemoheterotrophy, aerobic-chemoheterotrophy, denitrification, and nitrification and interactions with nutrient fluxes which was more pronounced in the constructed-wetland system.

Ecosystem function modified by an invasive species: Density and distance dependent changes in sediment characteristics, fluxes, and benthic communities

Coastal marine benthic ecosystems are considered hotspots for biodiversity and productivity. However, the introduction of bio-engineering non-indigenous species can alter the delivery of key functions and services provided by these systems. Here, we investigated the influence of Sabella spallanzanii, a large structure-forming invasive polychaete, on a marine soft-sediment system in New Zealand, examining effects on sediment characteristics (organic matter and chlorophyll a content), solute fluxes, and benthic communities near and far from low-, medium-, and high-density clumps of Sabella. Fluxes of dissolved oxygen and inorganic nitrogen near (<10 cm) Sabella clusters were significantly lower to those 1 m away, and fluxes differed between low-, medium-, and high-density clumps. Larger densities of Sabella and clumps with greater overall biomass were associated with increased sediment oxygen consumption, particularly in areas 1 m distant from the Sabella clumps. Similarly, ammonium production (efflux from the sediment) increased with increasing Sabella biomass at the 1 m ‘distal’ sites (from 0 μ to 40 μmol/m2/h), but NOx efflux decreased (from 5 to 0 μmol/m2/h) with distance. The flux results were consistent with findings of higher organic matter content and finer grain size sediment in ‘distal’ plots to Sabella clusters. We also showed how benthic diversity and abundance increased with increasing Sabella clump biomass. Our investigation suggests that the building of biogenic structures and the filter feeding activity by Sabella may alter near-bed flows and concentrations of organic rich biodeposits in surrounding sediments, with ramifications for organic matter distribution and nutrient recycling rates. The continuous expansion and colonisation of Sabella could reshape local benthic communities and increase oxygen consumption rates in affected marine sediments.

Sediment oxygen consumption in Antarctic subglacial environments

AbstractOxygen consumption in aquatic sediments is an indicator of overall biological activity of the ecosystem. As such, rates of sedimentary oxygen utilization are well documented for much of the open oceans and freshwater lakes. However, there are few direct measurements of sedimentary oxygen consumption from Antarctic subglacial aquatic sediments. We report the first microsensor oxygen profiles and derived sedimentary oxygen consumption rates from beneath the Ross Ice Shelf and a subglacial lake beneath the West Antarctic Ice Sheet. Rates of oxygen consumption in these two environments are relatively low, but comparable to those reported from ice‐free polar oceans and oligotrophic Arctic lakes. Our study demonstrates the presence of oxygen within Antarctic subglacial aquatic sediments and its importance for oxygen‐consuming microorganisms living in these ecosystems.

Application of Drifted Pumice Stone as a Sand-Capping Material

Large amounts of pumice stone generated by the submarine volcanic eruption at Fukutoku Okanoba on 13 August 2021 drifted ashore, affecting ship navigation and fishery operations and posing challenges for disposal and a risk to benthic sea-life. As a new approach to managing ejected pumice from submarine eruptions, we investigated the possibility of using pumice as a sand-capping material for eutrophic sediments through laboratory experiments. Crushed pumice as a sand cover material effectively reduced the sedimentary oxygen consumption rate. Nutrient release from sediment showed a similar trend, with ~25% and 82% reductions in NH4-N and PO4-P release rates, respectively. Furthermore, bivalve exposure experiments using crushed pumice suspended in seawater showed no adverse effects specific to pumice and lowered bivalve mortality to a greater extent than did using kaolin at the same concentration. This could be related to the differences in gill accumulation and blockage owing to the particle size variation of suspended particles. These results suggest that crushed pumice is effective for sand-capping and suitably suppresses oxygen consumption and nutrient release from 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.

Changes in macrofauna bioturbation during repeated heatwaves mediate changes in biogeochemical cycling of nutrients

The increasing frequency and intensity of marine heatwaves (MHWs) observed worldwide entails changes in the structure and functioning of ecological communities. While severe and extreme heatwaves often have more destructive effects, the more subtle effects of moderate and strong heatwaves may nevertheless affect ecosystem functioning through complex, context-dependent linkages between different processes. Here we conducted a laboratory experiment to study the effects of repeated short-term, strong MHWs on macrofauna bioturbation and associated solute fluxes as a measure of ecosystem functioning using natural soft-sediment communities from the Baltic Sea. Our results showed changes in both bioturbation and biogeochemical cycling of nutrients following short-term, strong heatwaves, which seemed to contribute to an enhanced degradation of organic matter in the seafloor and an enhanced exchange of solutes across the sediment-water interface as well as increased sediment oxygen consumption. Following changes in these processes, the relative contribution of macrofauna and the environmental context to ecosystem functioning was altered. Our results highlight the potential of even shorter-term, strong MHWs of having system-wide impacts due to changes in the mechanistic process of bioturbation underpinning the biogeochemical cycling of nutrients. This study also highlights the need to measure a wide range of variables for a comprehensive understanding of the changes in functioning under disturbances, such as MHWs.

Labile and refractory fractions of sedimentary organic carbon off the Changjiang Estuary and its implications for sedimentary oxygen consumption

Estuarine and coastal area is dynamic zone for both marine carbon and oxygen cycles. There is limited evaluation of the labile and refractory fraction of sedimentary organic carbon and further a lack of attention to the corresponding potential in dissolved oxygen utilization. In this study, we collected sediment samples from the Changjiang estuary and adjacent coastal areas in the East China Sea in June 2019 to reveal the distribution, main controlling factor, and implications of four operational fractions of sediment organic carbon that range from labile to refraction. In the laboratory, we processed and defined the four operational fractions of sedimentary organic carbon, includes 1) pure water extractable carbon (WEC), 2) salt (K2SO4 solution) extractable carbon (SEC), 3) Microbial biomass carbon (MBC; chloroform extractable carbon), and 4) refractory organic carbon (ROC; the acid-processed fraction). We revealed that off the Changjiang Estuary, the bulk sedimentary organic carbon (SOC), WEC, SEC, MBC, and ROC were 9.43 ± 3.07 g/kg, 0.12 ± 0.10 g/kg, 0.16 ± 0.07 g/kg, 1.58 ± 0.45 g/kg, and 4.38 ± 1.95 g/kg, respectively. Sediment grain size played a vital role in constraining the LOC fractions, and samples from aquaculture areas showed extra-higher levels of LOC when compared to samples from the non-aquaculture area. The LOC and MBC fraction showed clear negative relation with dissolved oxygen (DO) in the near bottom waters under high DO background (DO &amp;gt; 5.5 mg/L), whereas in a low DO background (DO &amp;lt; 3-4 mg/L) no clear relation can be found. Based on our observed LOC content and DO concentration in the near bottom waters, the depth of hypoxia in the near bottom waters, which would be caused by a total decomposition of 4 mm of LOC in the top sediment, ranged between 0.3 m to 4.7 m with an average depth of 2 m. This value (2 m) means that sedimentary oxygen consumption would contribute 10% in the occurrence of hypoxia, with the rest contributing from water column respiration. Overall, we used the LOC fraction of sedimentary organic carbon to address the dissolved oxygen consumption in the near bottom waters and further study is needed to evaluate this approach.

Role of phosphorus in the seasonal deoxygenation of the East China Sea shelf

Abstract. The Changjiang is the largest river in Asia and the main terrestrial source of freshwater and nutrients to the East China Sea (ECS). Nutrient concentrations have long been increasing in the Changjiang, especially after 1960 with urbanization, the development of industrial animal production, and fertilizer application in agriculture, resulting in coastal eutrophication and recurring summer hypoxia. The supply of anthropogenic nitrogen (N) exceeds that of phosphorus (P) relative to the Redfield ratio. This results in seasonal P limitation in the Changjiang plume. P limitation and its effects on primary production, respiration, and hypoxia in the ECS have not been studied systematically, although such knowledge is needed to understand bloom dynamics in the region, to assess the consequences of altered nutrient loads, and to implement nutrient reduction strategies that mitigate hypoxia. Using a coupled physical–biogeochemical model of the ECS that was run with and without P limitation, we quantify the distribution and effects of P limitation. The model shows that P limitation develops eastward of the Changjiang Estuary and on the Yangtze Bank but rarely southward along the Zhejiang coast. P limitation modifies oxygen sinks over a large area of the shelf by partly relocating primary production and respiration offshore, away from the locations prone to hypoxia near the Changjiang Estuary. This relocation drastically reduces sediment oxygen consumption nearshore and dilutes the riverine-driven primary production and respiration over a large area offshore. Our results suggest that the hypoxic zone would be 48 % larger in its horizontal extent, on average, if P limitation was not occurring. Results are summarized in a conceptual model of P limitation on the ECS shelf that is also applicable to other systems. Then we carried out nutrient reduction simulations which indicate that, despite the effect of P limitation on hypoxia, reducing only P inputs as a nutrient reduction strategy would not be effective. A dual N + P nutrient reduction strategy would best mitigate hypoxia. The model results suggest that decreasing the size of the hypoxic zone by 50 % and 80 % would require reductions in N + P load of 28 % and 44 %, respectively.

An effective process-based modeling approach for predicting hypoxia and blue tide in Tokyo Bay

ABSTRACT Hypoxia and blue tide are the most significant environmental issues in Tokyo Bay as they have been damaging fisheries for a long time. Although studies on modeling these two associated phenomena have been conducted for decades, the scarcity of relevant observational datasets has greatly hindered the progress, and no study has successfully reproduced the entire processes of blue tide or predicted the time and place of its outbreak. To address the problems from limited data, this study proposed a relatively conventional benthic flux model and developed a novel method that converts the total organic carbon content into the fluxes of sediment oxygen consumption and sulfide release to represent the spatial differences in benthic fluxes. A pelagic sulfur model with only three key chemical reactions of blue tide that includes the disproportionation of elemental sulfur was proposed. The method significantly improved the results of dissolved oxygen in bottom water, as seen by the root mean square error decreasing by 15.9% and 18.9% in two simulations with largely different forcings. The sulfur model also accurately predicted the outbreaks of blue tides in each simulation, which is significant to the stakeholders as it facilitates the forecast of blue tides in Tokyo Bay.

Equilibrium simulation and driving factors of dissolved oxygen in a shallow eutrophic Inner Mongolian lake (UL) during open-water period

Abstract A water quality monitoring buoy installed in the center of the eutrophic shallow lake Ulansuhai was used to explore the dissolved oxygen (DO) balance. A revised DO model for shallow eutrophic lakes was applied to identify trends in the lake's DO content during the non-frozen period and determine the equilibrium relationship of DO in the water body. The coefficient of determination and the Nash efficiency of the model proved the feasibility of the model. The main drivers affecting the DO balance of the lake were photosynthesis, aeration and the lateral movements of oxygen-rich water, which accounted for 49.28, 14.72 and 36%, respectively, whereas respiration and sediment oxygen consumption, on the other hand, accounted for 1.56 and 98.44%, respectively. These findings suggest that photosynthesis and sediment oxygen consumption dominate the DO balance in eutrophic shallow lakes. A trend analysis of the average oxygen production and consumption rates indicated a maximum of 0.22 mg/L·h for photosynthesis and 0.20 mg/L·h for sediment oxygen consumption. A correlation analysis showed that water temperature was involved in changing the DO level of the lake mainly by affecting the oxygen consumption process.

Trawling effects on biogeochemical processes are mediated by fauna in high-energy biogenic-reef-inhabited coastal sediments

Abstract. Dynamic, sandy environments are generally less vulnerable to mechanical stress compared to silty, low-energy habitats. Biogenic reef communities, however, may provide an exception to this. This study explores the physical, biological, and biogeochemical effects of bottom trawl fishing on a coastal ecosystem dominated by the tube-building polychaete Lanice conchilega. Two specific gear types, both used to exploit North Sea sole (Solea solea), were compared: electric pulse trawls and tickler chain-rigged beam trawls. We detected a ∼1 cm bathymetric deepening after trawling associated with significant losses in benthic chlorophyll a caused by both fishing gears. Tickler chain trawls significantly reduced sediment oxygen consumption (57 %), total organic matter mineralization (56 %), denitrification (61 %), nitrification (60 %), and total benthos densities (52 %), while pulse trawls had no statistically significant impact on these parameters. Before trawling, significant relationships could be found between L. conchilega and very fine sand fractions, oxygen and nitrate fluxes, macrobenthos densities, and species richness; however, the trawl disturbances from both gears disrupted these connections. Our results suggest a reduced average effect for pulse compared to tickler chain beam trawls for several ecological and biogeochemical characteristics, though their impact was still significant for L. conchilega and associated species. This study also suggests that faunal-mediated ecosystem functions in habitats dominated by L. conchilega may be sensitive to relatively shallow sediment penetration from trawl gears and should be considered when assessing habitat vulnerability.

Weak Hypoxia Enhanced Denitrification in a Dissimilatory Nitrate Reduction to Ammonium (DNRA)-Dominated Shallow and Eutrophic Coastal Waterbody, Jinhae Bay, South Korea

The effects of seasonal hypoxia on sediment-water interface nitrogen (N) transformations in Jinhae Bay were examined from 2015 to 2019. The rates of benthic denitrification, anaerobic ammonium oxidation (anammox), dissimilatory nitrate reduction to ammonium (DNRA), nutrient exchange, and sediment oxygen consumption were measured seasonally. The oxygen (O2) and hydrogen sulfide (H2S) depth profiles were measured using microelectrodes. Neither penetration nor consumption of oxygen decreased during hypoxia. Denitrification, anammox, and DNRA ranged from 0 to 0.73, 0.13, and 1.09 mmol N m-2 day-1, respectively. Denitrification, the dominant N removal pathway, increased by 75% while anammox ceased, which led to an overall increase of 55% in the total N2 gas production during hypoxia relative to that during normoxia. Enhanced denitrification is the result of increased coupled nitrification–denitrification due to the intermittent supply of oxygen during bottom water hypoxia (“weak hypoxia”). In the hypoxic period, DNRA decreased by 62%, and the relative contribution of DNRA to the total nitrogen reduction process decreased from 81 to 58%, but it still outperformed denitrification as the main nitrate reduction pathway. Sediments were strong sources of ammonium for the water column, both under normoxia and hypoxia, whereas they were a sink of nitrate from the water column during hypoxia. Bioturbation may be important for maintaining oxygen penetration and consumption in sediments. The dominance of DNRA was mainly due to the relatively high content of sulfide and organic-rich sediments. The repressed macrofaunal activity and increased coupling of nitrification and denitrification during hypoxia may have contributed to enhanced denitrification. Taken together, the overall dominance of DNRA might contribute to the development and maintenance of eutrophication and seasonal hypoxia in this system. However, in contrast to the previous results, denitrification was enhanced during “weak hypoxia,” which might be helpful in alleviating eutrophication.

Variations in benthic fluxes of sediments near pier pilings and natural rocky reefs

Marine artificial structures such as pilings are replacing natural habitats, and modifying surrounding areas, often resulting in local decreases in species diversity and facilitation of bioinvasion. Most research on the impacts of artificial structures in marine ecosystems has primarily focused on rocky bottom habitats and biodiversity, overlooking the effects of these structures on the functioning of nearby sedimentary habitats. Here we compared, for the first time, benthic metabolism (O2 fluxes) and sediment-water nutrient (inorganic nitrogen, phosphate, and dissolved organic nitrogen) fluxes in shallow water sediments adjacent to pilings and natural reefs. We also measured sediment properties (grain size, total organic carbon, total nitrogen, C:N ratio and chlorophyll-a content). We found that sediments near pilings were generally finer with greater C:N ratios than those near reefs, while differences in other sediment properties between types of habitats were dependent on the site. We found significant differences in the oxygen consumption, primary productivity, and net ecosystem metabolism in sediments around pilings compared to sediments near natural reefs, but these patterns differed by site. Net nutrient fluxes were similar in sediments near pilings and reefs at both sites. This study showed that although pilings can be associated with changes in the functioning of sedimentary habitats, patterns and the direction of change seem to vary depending on local conditions.

Water Oxygen Consumption Rather Than Sediment Oxygen Consumption Drives the Variation of Hypoxia on the East China Sea Shelf

AbstractSediment oxygen consumption (SOC) is important in modulating the oxygen budget in the East China Sea where seasonal hypoxia occurs. Porewater advection, molecular diffusion and bioturbation supply oxygen for sedimentary organic matter degradation. A pelagic‐benthic coupled model was applied to quantify the SOC. A comparison with observations showed good model performance in reproducing the hydrographic and ecological environments, particularly for the interannual variation in the hypoxic zone. Simulation results show that porewater‐advection‐induced flux is the predominant component of the SOC in sandy areas on the Changjiang bank and outer shelves, while the bioturbation‐induced flux is predominant at mud depocenters. By comparing SOC to the water oxygen consumption (WOC) below the pycnocline, the contribution of SOC is generally below ∼40% in the hypoxic zone. The spatial distribution of SOC in summer is relatively steady from year to year, while the high WOC patches explain more about the interannual variation in the hypoxic zone. WOC rather than SOC drives the variation of hypoxia. Particularly on the Changjiang bank, milder hydrodynamics are favorable for both the higher WOC and bioturbation‐induced benthic oxygen flux but substantially suppress the porewater advective flux, which results in the net lower contribution of SOC to hypoxia. This finding may shed light on other pelagic‐benthic coupling processes in coastal shelf seas where hypoxia occurs on permeable sediments.