Intertidal Sediments Research Articles

Foraging in heterogeneous landscapes: variation in movement patterns of a tropical sand-bubbler crab

Abstract Acquiring food in heterogeneous landscapes presents a challenge to many foragers, as searching for food in an optimal manner is difficult in spatially and temporally variable environments. Investigating individual foraging patterns can elucidate how environmental variations at different scales constrain or select for energy-optimizing movements, which can inform conservation and management strategies by identifying spatio-temporal variations in species’ habitat use. To test how such movements vary with environmental conditions, we investigated foraging patterns of the deposit-feeding sand-bubbler crab, Scopimera intermedia Balss, 1934 at multiple spatial and temporal scales on soft sediment shores in Hong Kong. On a broad, annual, scale the crabs produced foraging tracks of different length and foraged over different areas around their burrows between hot and cool seasons. Although foraging movements of the crabs were slower and more restricted during the cool season, probably due to low environmental temperatures, foraging areas during the hot season were larger but limited by increasing conspecific interactions. Over a smaller scale at which the crabs make movement decisions, parameters such as turning angle and speed were variable, even within individual foraging excursions. Such variations appeared to be responses to small-scale variations in sediment food patches, which resulted in the crabs employing multiple movement modes. This context-dependent foraging strategy enables the crabs to feed for a longer time in food-rich patches compared with a fixed strategy and is, therefore, critical for species living in environments such as intertidal sediments, where food distribution is heterogeneous and foraging time is constrained by the tide.

Spatial and temporal variability of microplastic abundance in estuarine intertidal sediments: Implications for sampling frequency

Microplastics (<5 mm) are well documented across shorelines worldwide; however, high variability in microplastic abundance is often observed within and among field studies. The majority of microplastic surveys to date consist of single sampling events that do not consider spatiotemporal variability as a potential confounding factor in the interpretation of their results. Therefore, these surveys may not accurately capture or reflect levels of microplastic contamination in the environment. Here, we provide the first investigation of small-scale spatial and temporal variability of microplastic abundance, distribution, and composition in the intertidal zone of an urbanized US estuary to better understand the short-term, daily spatiotemporal variability of microplastics in dynamic coastal environments. Intertidal sediment was collected from both the low and high intertidal zones of a sandy estuarine beach located in South Carolina, southeastern US every 1 to 2 days at low tide over 17 days (12 sampling events; total n = 72). Study-wide, microplastic abundance ranged from 44 to 912 microplastics/m2 and consisted primarily of polyethylene, nylon, polyester, and tire (or tyre) wear particles. High temporal variability was observed, with microplastic abundance differing significantly among sampling events (p = 0.00025), as well as among some consecutive tidal cycles occurring within 12 h of each other (p = 0.007). By contrast, low spatial variability was observed throughout the study with no significant differences in microplastic abundance detected between the low and high intertidal zones (p = 0.76). Of the environmental factors investigated, wind direction on the day of sampling had the greatest effect on temporal microplastic variability. Our results demonstrate that there can be significant temporal variability of microplastic abundance in estuarine intertidal sediments and are important for informing the methods and interpretation of future microplastic surveys in dynamic coastal environments worldwide.

Biosorption of cadmium and cobalt by intertidal multicomponent biofilms

Intertidal sediments are sinks for heavy metals due to the anoxic condition, fine grain size, high sulphide and organic matter contents. However with changing water pH, particularly in view of the acidifying ocean water, metals are released in the overlying water making them bioavailable. The motivation of this research was to identify a natural barrier to immobilize the released metals in the sediments and thus prevent bioaccumulation. Multi-species intertidal biofilms have higher level of stress tolerance compared to single species due to their extended exposure to the dynamic environmental conditions facilitating in-situ sequestration of contaminants. The present study focuses on evaluating the potential of naturally-occurring intertidal phototropic biofilms cultured in controlled environment to absorb and adsorb Cd and Co along with the estimation of the biological parameters, such as, chlorophyll a, biomass, extracellular polymeric substances (EPS), and cell vitality. Biofilms withstood up to 1 ppm of Cd and 5 ppm of Co (threshold concentrations) beyond which they failed to proliferate. This suggests the potential application of these biofilms as bio-indicators of metal pollution. The overall percent adsorption, ranging between 5 and 17 % was insignificant compared to percent absorption, ranging between 15 and 90 %. The intracellular absorption was 66 % for Cd and 58 % for Co at threshold concentrations. With increasing metal concentration, decreasing chlorophyll a synthesis and increasing EPS production was observed demonstrating that the biofilms combat heavy metal pollution by secreting EPS as an adaptive mechanism. We found intertidal multicomponent biofilms are capable of sequestering heavy metals from the water. In the natural environment, the biofilms would get buried in the sediments with time thus sequestering the metals.

Assessment of the vertical characteristics and contamination levels of toxic metals in sediment cores from typical Chinese intertidal zones

The heavy metal (As, Cd, Cr, Cu, Hg, Pb, and Zn) contents of 24 sediment cores were obtained and analyzed from 12 typical Chinese intertidal zones. The results revealed a gradual improvement in the environmental quality of the intertidal zone. Enrichment factor and geoaccumulation index analyses demonstrated a generally good environmental quality of intertidal sediment, with some areas of serious contamination, such as the Xiamen Jiulong Estuary, the Yangtze River Estuary, and the Pearl River Delta. Relative to the guidelines for sediment quality, the studied intertidal zones were moderately impacted, with a risk of biotoxic impacts. This research reveals the status of toxic metal pollution in the intertidal zone of China and provides a reference for coastal area development.

Gross nitrogen transformations in intertidal sediments of the Yangtze Estuary: Distribution patterns and environmental controls

Nitrogen (N) transformations in estuarine and coastal ecosystems play a crucial role in the global biogeochemical cycles. However, simultaneous estimation of gross N transformations and the underlying drivers across estuarine and intertidal wetlands remain poorly understood. Here, the spatial changes in gross N transformation rates and related functional gene abundances were explored in varying habitats of the intertidal zone in the Yangtze Estuary via an optimized 15N tracing model and quantitative PCR. The results indicate that the gross N transformation rates changed remarkably in estuarine and intertidal wetlands, with comparatively higher gross N transformation rates at the upper freshwater and low-salinity sites relative to high-salinity sites. Meanwhile, the functional gene abundances (amoB, UreC, hzo, nirS and nrfA) exhibited a similar spatial distribution pattern to corresponding N transformation processes. Sedimentary total organic carbon (TOC), nitrite (NO2–), ferrous iron (Fe(II)), and microbial functional gene abundances jointly mediated the gross N transformations. This study carried out the first simultaneous estimation of the gross N transformation pattern and driving factors in estuarine and intertidal ecosystems based on the optimized 15N tracing model, providing a better understanding of internal mechanisms on the N cycling and strategy for estuarine and coastal N management.

New records of Branchiostoma californiense (Amphioxiformes: Branchiostomatidae) in El Salvador, Central America

Introduction: Cephalochordates are benthic organisms that represent an important link in the evolution of chordates. Coastal human communities use them as food in some Asian countries. Objective: To register the presence of B. californiense in El Salvador. Methods: We collected specimens during different benthic surveys at Jiquilisco and La Union Bays in April 2018 and November 2019, with a Petite Ponar grab and core in subtidal as well as intertidal sediments, respectively. Results: Ten specimens were collected from sandy-muddy sediments at Jiquilisco and La Union bays. Conclusion: B. californiense inhabits estuarine areas in El Salvador.

Abundance, composition, and distribution of microplastics in intertidal sediment and soft tissues of four species of Bivalvia from Southeast Brazilian urban beaches

In coastal areas, microplastics (MPs) can deposit in sediment, allowing it to be ingested by benthic organisms, like mussels, thus creating a possible transfer to humans. The aim of this study is to evaluate MPs pollution in sediment as a function of shoreline elevation in two urbanized beaches and to evaluate the abundance/frequency of MPs in 4 different species of bivalves commonly used in the human diet, such as the oyster Crassostrea brasiliana, the mussels Mytella strigata and Perna perna and the clam Tivela mactroides, and identify the polymers via μ-FTIR technique. A total of 3337 MPs were found in this study, of which 1488 were found in the sediment at the five sites analyzed, and 1849 in the bivalve tissues at the two sampling sites. MPs contamination was observed in all sediment samples and species of the pool and in each of the 10 specimens of the four species. Thus, the frequency of contamination by MPs reached 100 % for the analyzed samples. The number of filaments is higher than fragments in sediment samples and in each bivalve species. Regarding types and colors, the blue were greater than fragment-type in sediments and samples. In an effort to classify the polymers via μ-FTIR, our study was able to identify polypropylene, polyethylene and polyethylene terephthalate, besides a great number of cellulose fibers.

Microplastics: Global occurrence, impact, characteristics and sorting

Stressors like microplastics (MPs) cause proliferating environmental pollution globally. Since plastics are continuously introduced into water bodies through numerous paths, novel solutions are required to segregate as well as decline their quantity in various environmental sectors. Numerous techniques have been used and proposed in the last 10 years to screen and enumerate MPs, define the particle’s properties, for instance form, color, or size, and recognize the polymer material. This critical review aims to provide an overview of advanced procedures in MP investigation, provides illustrations of probable routes forward and lingering challenges, and categorizes present approaches as per their underlying research question. Methods presently employed for MP sampling, extraction, identification, characterization, and quantification were evaluated. Studies proposing use of precursors for removal of MPs from water via the sol–gel process were reviewed. Research on microfluidics systems finds application in environmental and industrial fields and has gained momentum in concentrating, sorting, classifying, focusing, and desegregating MPs. This review briefly discusses active and passive label-free microfluidic methods that are efficient in executing the desired particle separation and are gaining momentum in the ecological analysis of MPs. Although some sets of preliminary data of MPs at selected regions across the globe have been studied and obtained, the degree of MP contamination in most important rivers, nearshore inland areas, and air is yet to be understood completely. Along the Charleston Harbor Estuary, the MP concentration in intertidal sediment was found to be 0 to 652 MPs/m2. In Asia, at the South Korean region, western Pacific Ocean, a high plastic concentration of 15–9,400 particles/m3 was reported. In India, the MP concentration was identified as 288 pieces/m3 in the Netravati River. In Turkey, ingestion of MPs was reported to be found in 458 out of 1,337 fish samples, indicating the polluted situation of the Mediterranean Sea. Despite the rapid development in MP analysis, no standardized technique for sampling along with separation has been approved. Therefore, for attaining a more inclusive picture of MPs’ fate and abundance, this study highlights the importance of a standardized procedure for MP research that can be used globally and adequately enables comparisons around the world.

Alteration of Dissimilatory Nitrate Reduction Pathways in the Intertidal Sediment during Macroalgae Blooms

To elucidate the effect of macroalgae blooms on dissimilatory nitrate reduction pathways (denitrification, anammox, and DNRA) in sediments of the hypereutrophic Yatsu tidal flat, eastern Japan, sediment denitrification, anammox, and DNRA rates were measured using a 15N tracer technique at two sites affected and unaffected by macroalgae (Ulva) blooms and in incubation experiments with and without Ulva. Anammox was insignificant at both sites and in both experiments. The denitrification rate was consistently higher than the DNRA rate, and its contributions to the total dissimilatory nitrate reduction were 82% and 85% at sites affected and unaffected by Ulva, respectively. In a sediment incubation experiment with Ulva, the contribution of DNRA had increased to approximately 30% on day 7, which is when the sulfide concentration was the highest. Sulfide produced by sulfate reduction during macroalgae blooms inhibited denitrification and did not change the DNRA, and consequently increased the DNRA contribution. On day 21, after reaching the peak sulfide concentration during the late macroalgae collapse, the DNRA contribution decreased to 15%. These results indicated that the DNRA contribution was greater during the macroalgae blooms than at the collapse, although denitrification dominated DNRA regardless of the macroalgal status. Therefore, vigorous macroalgae cover and sulfide production under the macroalgae cover had an important impact on the nitrogen dynamics.

Dynamic process and mechanism of crude oil release from silty intertidal sediment under different influencing factors

Under tidal scouring, residual petroleum in the intertidal sediment after oil spills could release again, causing secondary pollution in the marine ecosystem. The current study aimed to investigate the dynamic process and principles of crude oil release from silty intertidal sediment under different influencing factors and screened for the key factors. In this paper, the fitting equations and correlation between the release amount and various factors were explored through the single-factor and orthogonal experiments. Then, the key influencing factors were selected for multi-factor fitting of the release amount. The results showed that the oil release amount rose with the increase in oil concentration, oscillation frequency, and release time, but decreased with an increase in salinity. As the pH decreased, the oil release amount increased. The relationship between release amount and concentration/oscillation frequency can be equipped by the polynomial equation, and the average R2 was 0.95 and 0.84, respectively. The release amount can be fitted by the Lagergren pseudo-second-order kinetic equation with time, with the average R2 0.89. The pH was negatively correlated with the release amount in the fresh contaminated sediment but positively correlated with the weathered one. The correlation between each factor and oil release amount was ranked (from large to small) as oil concentration, oscillation frequency, salinity, time, and pH. At last, a polynomial equation can be fitted between the key influencing factors (oil concentration and oscillation frequency) and the release amount. The results can provide a theoretical basis for predicting the secondary pollution owing to the oil re-release from intertidal sediment.

Denitrification in intertidal sediments of a tropical estuary subject to increasing development pressures

Darwin Harbour is a working port and the most populated city in the Northern Territory of Australia. This macrotidal estuary is located in the wet–dry tropics of Northern Australia and notwithstanding mounting development pressures in the region, is largely unmodified. The prevailing oligotrophic condition of estuarine waters suggest that biogeochemical cycling in sediments remain active, buffering the influence of anthropogenic inputs. We tested the hypothesis that nutrient hotspots exist in depositional low-velocity zones, with a gradient of high to low nitrogen processing from the upper to outer reaches of the estuary. A number of factors were examined for their influence on the effectiveness of denitrification in these depositional zones, a putative key process driving nitrogen removal, with particular emphasis on carbon-loading extremes in tidal creeks, spatial gradients along the estuary and the influence of seasonality. There were significant differences in process rates between hypereutrophic/eutrophic tidal creeks that receive the largest proportion of treated sewage loads in the region and the mesotrophic/oligotrophic tidal creeks that were comparatively undisturbed. Net benthic nutrient fluxes and dinitrogen efflux rates were more than an order of magnitude higher and lower, respectively, in degraded (hypereutrophic/eutrophic) tidal creek systems where denitrification efficiency (DE%) was < 40%. Denitrification (Dinitrogen efflux) rates in tidal creeks (mesotrophic/oligotrophic) and broader estuarine sites were high (~ 8 mmol N m−2 day−1) and denitrification efficiency remained > 65%, particularly during the wet season. On a whole-of-estuary basis, denitrification in conjunction with mechanisms such as burial could feasibly make a substantial impact, abating the influence of anthropogenic inputs. Although considerable variability was encountered, particularly across seasons, the hypothesis of elevated denitrification rates as nutrient hotspots in depositional zones along the estuary was not convincing. More influential are tidal creeks as potential ‘reactors’ for N cycling and removal, but their capacity can be degraded by overloading with nutrients.

Distribution pattern of N-damo bacteria along an anthropogenic nitrogen input gradient from the coastal mangrove wetland to the South China sea sediments

Microbial nitrite-dependent anaerobic methane oxidation (n-damo) process is important for mitigating methane emission and anthropogenic nitrogen inputs in the marine environment. However, the distribution pattern of n-damo bacteria along an anthropogenic N-input gradient from the coastal wetland to the pristine South China Sea is poorly understood. This study investigated the diversity and abundance of n-damo bacteria in samples collected along a N-input gradient from Mai Po (MP) mangrove wetland sediments of the Pearl River Estuary (PRE) to the deep ocean sediments of the South China Sea (SCS). Retrieved 16S rDNA sequences showed a shift of n-damo community composition of complex structures with both freshwater and marine n-damo lineages in MP intertidal sediments to marine dominated characteristic in SCS sediments. The observed variation of Shannon and Chao1 indexes of n-damo bacteria shared a similar trend of a decrease at first followed by an increase along the targeting gradient with previously investigated methanogens, anaerobic methanotrophic archaea, ammonia-oxidizing archaea and ammonia-oxidizing bacteria, but had a reverse pattern with anammox bacteria. The community structure of pmoA gene sequences contained freshwater lineages only in SCS continental shelf sediments closer to the PRE, and turned to group with other marine samples in deeper and pristine sediments. Results suggested that n-damo bacteria might be a major contributor to anaerobic denitrification in the SCS sediments because their abundances were much higher than previously studied anammox bacteria in the same sample set. The distribution pattern of n-damo bacterial diversity, richness and abundance along the anthropogenic N-input gradient implies that they could be used as a bio-indicator for monitoring the anthropogenic/terrestrial inputs in marine environments.

Plutonium mobilization from contaminated estuarine sediments, Esk Estuary (UK)

Since 1952, liquid radioactive effluent containing238-242Pu, 241Am, 237Np, 137Cs, and 99Tc has been released with authorization from the Sellafield nuclear complex (UK) into the Irish Sea. This represents the largest source of plutonium (Pu) discharged in all western Europe, with 276 kg having been released. In the Eastern Irish Sea, the majority of the transuranic activity has settled into an area of sediments (Mudpatch) located off the Cumbrian coast. Radionuclides from the Mudpatch have been re-dispersed via particulate transport in fine-grained estuarine and intertidal sediments to the North-East Irish Sea, including the intertidal saltmarsh located at the mouth of the Esk Estuary. Saltmarshes are highly dynamic systems which are vulnerable to external agents (sea level change, erosion, sediment supply, and freshwater inputs), and their stability remains uncertain under current sea level rise projections and possible increases in storm activity. In this work, we examined factors affecting Pu mobility in contaminated sediments collected from the Esk Estuary by conducting leaching experiments under both anoxic and oxic conditions. Leaching experiments were conducted over a 9-month period and were periodically sampled to determine solution phase Pu via multicollector-inductively coupled plasma-mass spectrometry (MC-ICP-MS), and to measure redox indicators (Eh, pH and extractable Fe(II)). Microbial community composition was also characterized in the sediments, and at the beginning and end of the anoxic/oxic experiments. Results show that: 1) Pu leaching is about three times greater in solutions leached under anoxic conditions compared to oxic conditions, 2) the sediment slurry microbial communities shift as conditions change from anoxic to oxic, 3) Pu leaching is enhanced in the shallow sediments (0–10 cm depth), and 4) the magnitude of Pu leached from sediments is not correlated with total Pu, indicating that the biogeochemistry of sediment-associated Pu is spatially heterogeneous. These findings provide constraints on the stability of redox sensitive Pu in biogeochemically dynamic/transient environments on a timescale of months and suggests that anoxic conditions can enhance Pu mobility in estuarine systems.

Ammonia-oxidizing archaea and ammonia-oxidizing bacteria communities respond differently in oxy-gen-limited habitats

Ammonia-oxidizing archaea (AOA) and bacteria (AOB) are the most important ammonia oxidation functional community, while the coastal environment just provides a different oxygen environment for the ammonia oxidation process. However, few surveys concentrated on the influence of oxygen concentration on the niche specialization of AOA and AOB in the ocean intertidal zones. Here, high-throughput sequencing by Illumina MiSeq and qPCR were applied to detect the change of abundance, diversity as well as community structure of both AOA and AOB with 0–60 cm sediments depth in the intertidal zone in Qingdao, China. Results showed that the AOA/AOB amoA gene copy numbers and AOA/AOB OTU numbers rate increased as sediment depth went more profound, which indicated that AOA was more adaptive to oxygen-limited niches compared to AOB. Oxygen indeed led to the niche specialization of AOA and AOB in intertidal sediments. The dominant AOA and AOB were the clusters of Nitrosopumilus and Nitrosospira, respectively, which indicated ecological success in the intertidal zone. A significant and positive correlation (p &amp;lt; 0.01) between AOB abundance/AOB OTU numbers and Oxidation-reduction potential (ORP) was observed. In addition, both total nitrogen (TN) (p &amp;lt; 0.01) and pH (p &amp;lt; 0.05) were significantly negatively correlated to AOB abundance. TN was also significantly negatively correlated to AOB OTU numbers (p &amp;lt; 0.05). Hence, oxygen led to niche specialization of AOA and AOB, especially under anoxic conditions, AOA played a dominant role in the process of ammonia oxidation. The Nitrosopumilus and Nitrosospira clusters were the dominant AOA and AOB, respectively, representing an ecological success in the intertidal zone.

Deposit-feeding worms control subsurface ecosystem functioning in intertidal sediment with strong physical forcing.

Intertidal sands are global hotspots of terrestrial and marine carbon cycling with strong hydrodynamic forcing by waves and tides and high macrofaunal activity. Yet, the relative importance of hydrodynamics and macrofauna in controlling these ecosystems remains unclear. Here, we compare geochemical gradients and bacterial, archaeal, and eukaryotic gene sequences in intertidal sands dominated by subsurface deposit-feeding worms (Abarenicola pacifica) to adjacent worm-free areas. We show that hydrodynamic forcing controls organismal assemblages in surface sediments, while in deeper layers selective feeding by worms on fine, algae-rich particles strongly decreases the abundance and richness of all three domains. In these deeper layers, bacterial and eukaryotic network connectivity decreases, while percentages of clades involved in degradation of refractory organic matter, oxidative nitrogen, and sulfur cycling increase. Our findings reveal macrofaunal activity as the key driver of biological community structure and functioning, that in turn influence carbon cycling in intertidal sands below the mainly physically controlled surface layer.

Application of environmental DNA as a tool for detecting intertidal habitat use by forage fish

Many coastal forage fish species spend most of their life history using the water column, but some species such as Pacific Sand Lance (Ammodytes personatus) and Surf Smelt (Hypomesus pretiosus) have a requirement for intertidal sediments for spawning and or burying. Detection of suitable but typically uncommon intertidal habitats for spawning has historically relied on extensive visual surveys for eggs from large sand samples. In the present study, we developed and validated two new non-destructive qPCR-based tools for detecting Sand Lance and Surf Smelt eDNA from small sand samples. A total of 279 composite transect sand samples were collected from 101 beaches and were tested for eDNA sample integrity using the IntegritE-DNATM test. Initial testing showed that all but 42 of 46 samples passed after an inhibitor clean-up step. Of the 101 beaches sampled 275 times, there were 111 detections of Sand Lance on 52 beaches, and there were 13 detections of Surf Smelt on 10 beaches. One hundred and eighty-one samples were paired with visual egg counts and 76 (42%) of these tested positive for Sand Lance while 9 of 132 (7%) samples tested positive for Surf Smelt. For the 27 paired samples with two or more Sand Lance eggs visually detected there was a significant positive correlation (r = 0.694, p = 0.0001) with eDNA copies per liter suggesting that a larger Sand Lance presence is easily and reliably detected using eDNA. The methods and approaches described herein will serve as a foundation for broad application of eDNA approaches for forage fish species that utilize intertidal benthic habitats.

Metal contamination of intertidal sediment and macroalgae in an area impacted by paint from abandoned boats

Metals commonly employed in boat paint (Ba, Cr, Cu, Pb, Sn and Zn) have been determined in 63-μm-fractionated intertidal sediments and in Ulva lactuca and Fucus vesiculosus sampled in the vicinity of abandoned vessels. Metal concentrations in sediment were elevated but highly variable, both between sites and amongst replicates from the same site (e.g., mean Cu ~ 100 to 1200 mg kg−1; mean Pb ~ 130 to 6900 mg kg−1) due to heterogeneous contamination by metal-rich boat paint particles. Concentrations of all metals except Zn were higher in U. lactuca than F. vesiculosus but in both species metal levels were also elevated and variable. These observations were attributed to contamination by sediment particles and physical and chemical interactions between fine, suspended or deposited paint particles and the algal surface. The latter interactions act as a means by which boat paint metals may enter the foodchain.

Zonations and oscillations via heterotrophic processes in tidal unvegetated aquifers

AbstractGlobal‐change drivers that alter tidal amplitude or sea level could directly impact coastal aquifers' seawater–microbe interactions. The semidiurnal or diurnal tide can have consequences for chemical redistribution, because it deepens oxygen and nutrients penetration depth, thereby increasing O2 depletion via microbial respiration in sediments during groundwater circulation. However, chemical and microbial distributions in tidal aquifers remain unclear, in deference to their link with depth and lateral location. By combining the microbes‐accumulated equation with the traditional groundwater flow model in coastal aquifers, we explicitly unravelled the heterotrophic zonation, the salinity zonation, and the oxic zonation of tidal aquifers. According to the model estimates, salt penetrates deeper than reactive chemicals, and the heterotrophic high biomass zone is presumably 2–3 m deep. Such depth of heterotrophic distribution is 2–3 orders of magnitude higher than phototrophic distribution. Additionally, heterotrophic biomass can exhibit faint oscillations, even under the tide forces, whereas dissolved chemicals fluctuate significantly with tidal pumps. Due to the transport of nutrients by tidal penetration, intertidal sediments have a higher respiration rate than subtidal sediments, converting more organic carbon from seawater to atmospheric CO2 via microbes. We demonstrate that heterotrophic biomass may play the dominant role in the chemical reactions of nearshore sediments. Our results confirm the efficacy of unvegetated tidal sediments that rely on ubiquitous heterotrophs as a driver of oxygen consumption and DOC transformation. These steps above will allow for better field investigations of coastal aquifers within a framework of zonations and oscillations.

Microphytobenthos as a source of labile organic matter for denitrifying microbes

Nitrogen loads in natural waters remain elevated in populated and agricultural areas with serious impact on estuarine and coastal ecosystems. Intertidal sediments can play a significant role in attenuating the high nitrogen levels in water via microbial nitrate reduction, in general dominated by denitrification. These heterotrophic processes are heavily mediated by both the quantity and quality of organic matter available. Benthic microalgae were experimentally investigated as organic carbon source for denitrifying microbes in intertidal mudflat sediments from the Seine Estuary (France). Dry microphytobenthos (including algae and their extracellular polymeric substances) were added to sediments and nitrate reduction rates were monitored over a two-week period using anoxic controlled flow-through reactor approach. Our results show that microphytobenthos addition resulted in significantly higher nitrate reduction (67–332% increase), highly related to the added amount of microphytobenthos. Moreover, increase of the low molecular weight carbohydrates consumption (11–39%) highlight the measurable contribution of extracellular polymeric substances to the carbon consumption during nitrate reduction. The addition of microphytobenthos increased the abundance of nitrite reductase genes, especially those encoding the nirS gene (43–152% increase) while nitrous oxide reductase genes (nosZ gene) remained constant. Microphytobenthos appeared to favor complete denitrification as suggested by an increase in nirS and a decrease in clade II nosZ gene copy numbers. This study confirms experimentally the assumption that microbes use microalgae and particularly labile extracellular polymeric substances as a carbon substrate for nitrate reduction. These results reinforce the impact played by microphytobenthos in intertidal mudflats by highlighting their role on denitrifying microbes and nitrate removal from water.

Microplastics distribution in different habitats of Ximen Island and the trapping effect of blue carbon habitats on microplastics

Sediments are considered to be important sinks of microplastics, but the enrichment process of microplastics by blue carbon ecosystems is poorly studied. This study analyzed the spatial distribution and temporal changes, assessed the polymer types and morphological characteristics of microplastics in sediments of five ecosystems, i.e. forests, paddy fields, mangroves, saltmarshes and bare beaches on Ximen Island, Yueqing Bay, China. The trapping effect of blue carbon (mangrove and saltmarsh) sediments on microplastic was further explored. Temporal trends in microplastic abundance showed a significant increase over the last 20 years, with the enrichment of microplastics in mangrove and saltmarsh sediments being 1.7 times as high as that in bare beach, exhibiting blue carbon vegetations have strong enrichment effect on microplastics. The dominant color, shape, size, and polymer type of microplastics in sediments were transparent, fibers and fragments, <1 mm, and polyethylene, respectively. Significant differences in the abundance and characteristics of microplastics between intertidal sediments and terrestrial soils reveal that runoff input is the main source of microplastics. This study provided the evidence of blue carbon habitats as traps of microplastics.