Meiofaunal Community Structure Research Articles

Experimental seabed disturbance effects on Chatham Rise deep-sea meiofaunal communities, Southwest Pacific

ABSTRACT Growing global demand for deep-sea resources may lead to increased pressure on benthic ecosystems. Here we examined changes in meiofaunal communities following an in situ physical disturbance experiment. A significant change in meiofaunal community structure in surface (0-1 cm) and subsurface (1-5 cm) sediments was observed immediately following the disturbance at both the directly and indirectly disturbed sites and adjacent potentially undisturbed/indirectly disturbed sites, reflecting a decrease in the abundance of several meiofaunal taxa. Abundance, taxon richness and diversity also decreased immediately following disturbance across all sites. Surface community parameters returned to pre-disturbance values one year post-disturbance, but only a partial recovery was observed for the subsurface community over the same period. The accumulation of recently resuspended and deposited sediment could have led to lower meiofaunal density and shifts in community structure. Passive dispersal and recolonisation from outside the study area may explain why the surface community recovered more fully than the subsurface community. We show that meiofauna were negatively affected by a relatively minor disturbance compared to proposed commercial seabed mining operations, but the finding that communities had largely recovered one year after disturbance has implications for resilience of fauna to much more pronounced impacts of potential large-scale mining.

Diversity of Cryptofaunal Nematode Assemblages along the Jardines de La Reina Coral Reef, Southern Cuba

Cryptofaunal nematodes, those living on the hard substrate of the coral reefs, are largely unknown in terms of diversity and assemblage structure. We described nematode assemblages at seven sites spanning about 200 km along Jardines de La Reina, a well-preserved reef system in the Greater Antilles. We identified about 3000 nematodes, mostly of the families Desmodoridae and Chromadoridae; the most abundant species were Croconema cinctum, Desmodora communis, and Euchromadora vulgaris. The regional richness was moderate with 70 observed species (0.95 confidence interval: 65–75 species) and 75 extrapolated species (0.95 CI: 68–83 species). This richness was lower than in other reef biotopes, maybe reflecting evolutionary constraints due to interactions with stony corals. The local expected richness at 100 individuals was similar among sites, with a median of 26 species (0.95 CI: 20–34 species), and likely caused by diversification rate and evolutionary time acting at the same pace on populations. The taxonomic β-diversity was high and without differences among sites (median: 0.85; 0.95 CI: 0.33–1), probably due to significant substrate heterogeneity at 10-cm scale. The prevalence of replacement over richness difference suggests that local processes (e.g., environmental filtering and competition) contribute more to β-diversity than niche availability, which would be largely similar across the reef terraces. Contrary to our expectations, no gradient of assemblage structure occurred, nor significant effect of benthic cover on nematodes. However, nematode functional structure showed a conservative set of biological traits reflecting adaptations to hydrodynamic regime: armed oral cavity/intermediate colonizing capability/ornamented cuticles/conical tail. Our results provide insights about the taxonomic and functional diversity of nematodes and highlight the vast knowledge gaps about the processes ruling the meiofauna community structure in coral reefs.

Deciphering environmental forcings in the distribution of meiofauna and nematodes in mangroves of the Atlantic-Caribbean-East Pacific and Indo-West Pacific regions

Mangroves develop under environmental conditions and anthropogenic pressures whose impact on benthic meiofauna remains poorly understood. It is unclear how meiofauna communities are structured according to local sedimentary conditions. This study was designed to characterize the community structure of meiofauna and nematodes (dominant taxa) and the associated environmental forcings in intertidal mangrove sediments from Mayotte (Indo-West-Pacific), Martinique and Guadeloupe (Caribbean). Sediment cores were sampled at the end of the dry season at low tide on adult mangrove stands with similar immersion time. In each sediment layer, we analyzed redox potential, pH, porewater salinity, grain size, organic matter, metals, organic contaminants, prokaryotes and meiofauna. Our results show that sediments far from cities and agricultural fields trapped site-specific contaminants due to local water transport processes. Some metals, PAHs or pesticides exceeded toxicity thresholds in most of the studied stations, thus being harmful to benthic fauna. The sedimentary environment acts as a filter selecting specific meiofauna communities at station scale only in the Caribbean. In Mayotte, horizontal homogeneity contrasts with vertical heterogeneity of the sedimentary environment and the meiofauna. Nematode genera showed particular distribution patterns horizontally and vertically, suggesting the presence of sediment patches suitable for a restricted pool of genera on each island. Results in the Caribbean are consistent with nested diversity patterns due to environmental filtering. Conversely, horizontal homogeneity at Mayotte would reflect greater dispersal between stations or more spatially homogeneous anthropogenic pressures. The nematode genera present at depth may not be the most specialized, but the most versatile, capable of thriving in different conditions. Terschellingia and Daptonema showed contrasted responses to environmental forcing, likely due to their versatility, while Desmodora showed uniform responses between study areas, except when toxicity thresholds were exceeded. Our results emphasize that a given genus of nematode may respond differently to sedimentary conditions depending on sites.

Meiofauna community structure in the Yellow Sea Cold Water Mass and non‐Yellow Sea Cold Water Mass

This study investigated meiofaunal community structure over a large area of the Yellow Sea and East China Sea for 45 stations from 2021 to 2022. Meiofaunal densities were 34 ± 15–1610 ± 378 ind./10 cm2. Nematodes (67.3%) were the most dominant taxa, followed by benthic foraminifera (12.0%) and benthic harpacticoids (8.8%). The study was conducted in various sedimentary environments. Median grain size, water depth, and environmental characteristics (TOC, TN, Hg, Sr) were identified as influencing meiofauna community structure. The Yellow Sea and East China Sea, where this study was conducted, contain a variety of currents and the Yellow Sea Cold Water Mass (YSCWM) in the central waters. YSCWM and non-YSCWM areas differed in their meiofauna community structure. Even within the YSCWM region, we found differences in benthic community structure due to a variety of environmental factors. The meiofaunal taxon most affected by Sediment temperature was benthic foraminifera. Identifying foraminifera in paleo-oceanographic environments can indicate past expansion and contraction of YSCWM. Meiofauna have been used to monitor changes in marine ecosystems due to their sensitivity to changes in the surrounding environment. Meiofauna index (N/C ratio, MPI) were identified as appropriate for use in the study area.

Structure of meiofaunal communities in an urban tropical sandy beach in Pernambuco, Brazil

Sandy beaches are among the most extensive coastal ecosystems in Brazil and constitute important buffer zones between terrestrial and marine environments. The present study aimed to analyze the distribution and spatiotemporal variation of the meiofaunal community of an urban tropical sandy beach in northeastern Brazil with a particular focus on nematofauna. We set up 4 transects during 2 sampling periods to evaluate different beach zones. The meiofauna comprised 8 taxa, and the nematofauna consisted of 5 orders, 16 families, and 45 genera. The meiofaunal communities followed the qualitative-quantitative standards of those of other tropical sandy beaches in northeastern Brazil. The nematofaunal community showed variation in its composition and trophic structure between dry and rainy months, which were not significantly correlated with granulometric characteristics. This suggests that other variables may influence the structure of nematofaunal communities in Gaibu Beach. Further research on the biodiversity of the meiofaunal communities on sandy beaches in northeastern Brazil is urgently needed given the lack of available information of these environments and the extreme stress they are currently under.

Meiofauna in the southeastern Bering Sea: community composition and structuring environmental factors

The Bering Sea is the second largest marginal sea in the North Pacific and is one of the areas with highest biological productivity in high-latitude waters. The continental shelf of the Bering Sea hosts large populations of marine mammals and fishery resources. However, the smaller organisms in benthic ecosystems, including meiofauna, have been largely overlooked in this area, despite their potential importance in ecosystem functioning and the resultant biogeochemical cycles. This study analyzed spatial differences in the total abundance and community structure of the metazoan meiofauna at five stations around the Bering Canyon, located at the southeastern margin of the Bering Sea. Their association with environmental factors in sediments was also studied. The results confirmed that the investigated stations had meiofaunal standing stocks that were comparable to those of other Arctic seas. Among the investigated sediment biological and geochemical parameters (total organic carbon, median grain size, prokaryotic cell numbers, etc.), multivariate analyses showed that the C/N of organic matter in sediments was the main factor associated with meiofaunal community structure.

Ecological Responses of Meiofauna to a Saltier World—A Case Study in the Van Uc River Continuum (Vietnam) in the Dry Season

Increasing intensity of storms, typhoons, and sea level rise in conjunction with high water demand, especially for agriculture, in dry seasons in the Red River Delta may have led to seawater intruding deeper into the rivers’ estuaries. Given that losses of agricultural productivity and shortages of freshwater resources are projected, a reliable early warning of salinity invasion is, therefore, crucially needed. To evaluate the impact of salinity variations on riverine ecosystems, distribution patterns of meiofauna were examined at 20 stations along the Van Uc River continuum in the dry season. Meiofaunal richness indices were higher in the estuary and slightly decreased upriver. Nematoda was the most dominant taxon in salty stations, while Rotifera was more abundant in the less salty ones. A multiple variate analysis showed a strong interplay among salinity, nutrients, and pore water conductivity, which shaped the meiofaunal distribution. The inclusion of pore water salinity, nutrients, and meiofaunal community structure indicated a greater extent of the saline ecosystem in the estuary, posing a greater risk of freshwater salinization. Our results highlight the potential role of meiofauna as bioindicators but also call for a reformation of salinity assessment for better freshwater conservation and management.

Infaunal community structure, diversity, and function in Pacific-Arctic shelf sediments: a comparison of meiofaunal- and macrofaunal-sized nematodes

Meiofauna, and nematodes in particular, perform essential roles in benthic ecosystems and serve as bioindicators of disturbance and environmental change. In the Pacific Arctic, a region experiencing rapid environmental change, this component of the fauna has received little attention, and the role of meiofauna in ecosystem processes is poorly understood. We collected multi-core samples at 10 stations in the northern Bering and southern Chukchi Seas in June 2018 and characterized the sedimentary environment. We also assessed meiofauna community structure and abundance at higher taxonomic levels and evaluated genus-level nematode composition in meiofaunal (63-500 µm) and macrofaunal (>500 µm) size fractions. Nematodes were classified by feeding type and life-history strategies. Total meiofauna abundance and biomass varied greatly, with 1449-12875 ind. 10 cm-2 and 373-2325 µg dry weight 10 cm-2 for the upper 5 cm of sediment. Estimated production of meiofaunal-sized nematodes was 5-28 g C m-2 yr-1. Four distinct communities of meiofaunal-sized nematodes were identified in different sub-regions reflecting food availability and substrate type. The meiofaunal- and macrofaunal-sized nematodes represented 2 distinct communities. The unique taxonomic composition and large standing stock of the macrofaunal-sized nematodes (22 ± 15% of total nematode biomass) suggest they are critical components of the infauna and merit further research to assess their role in critical ecosystem functions. This study provides the first genus-level characterization of nematodes and some of the first measurements of meiofauna standing stock in the region, contributing important data for assessing ecosystem function in a rapidly changing Arctic.

Response mechanism of meiofaunal communities to multi-type of artificial reef habitats from the perspective of high-throughput sequencing technology

Multiple types of artificial reefs have been widely deployed in the coast of northern Yellow Sea, which can enhance fishery resources, restore coastal habitats and improve the marine environment. Meiofauna plays important ecological roles in marine ecosystem, but the response mechanism of meiofaunal community to different types of artificial reef is still poorly understood. In this study, we characterized the meiofaunal communities of concrete artificial reef habitat (CAR), rocky artificial reef habitat (RAR), ship artificial reef habitat (SAR) and adjacent natural habitat (NH) using 18S rRNA gene high-throughput sequencing technology, and explored the relationship of community-environment. The results showed that the diversity and community structure of meiofauna differed significantly on both spatial and temporal scales. Spatial differences were mainly contributed to the flow field effects and biological effects generated by artificial habitats, while temporal differences were driven by temperature (T) and dissolved oxygen (DO). The dominant taxa of meiofauna included arthropods, annelids, platyhelminths and nematodes. Platyhelminths were mainly positively influenced by artificial habitats but annelids were the opposite. Co-occurrence network analysis revealed that NH was more sensitive to environmental change than artificial habitat, while the performance of CAR and SAR were more stable. These results indicated that meiofauna can respond accordingly to different types of artificial habitats, and could be superimposed over the normal seasonal effects. The current study could provide fundamental data for understanding the response mechanism of meiofaunal community to different types of artificial habitats and a reference for assessments of the impact of artificial reefs on the marine environment.

Assessment of vertical and horizontal distribution of meiobenthos along a salinity gradient in the Tana and Sabaki Estuaries, north coast Kenya

Meiobenthos respond to variability in salinity gradients in estuarine habitats and are thus suitable organisms for ecological studies. The vertical and horizontal distribution of the meiofauna community structure of two major estuaries (Sabaki and Tana) on the north coast of Kenya were compared. The aim was to draw a meiofaunal dataset inventory of the two estuaries and to examine how salinity gradient, seasonality and sediment characteristics influence their structure. Replicate samples were collected from three sampling stations along the salinity gradient of each river estuary. A total of 3,556 individuals belonging to 26 taxa were recorded. Based on seasons and across stations, the upper surface (0-5 cm) layer recorded the highest meiobenthic density (90 ± 42 ind.10 cm-2), followed by 46 ± 23 ind.10 cm-2 (5-10 cm) and 30 ± 8 ind.10 cm-2 in the deepest sediment layer (10-15 cm) studied. The southeast monsoon season recorded the highest mean density (160 ± 22 ind.10 cm-2) compared to the northeast monsoon season (22 ± 12 ind.10 cm-2) for both estuaries. Results of the non-Metric Multidimensional Scaling technique revealed distinct seasonal composition in meiobenthos but not between the estuaries. Results of the 2-way ANOSIM test confirmed no significant differences in meiobenthic composition between the estuaries (p = 0.712). However, seasonal difference was significant (p = 0.001) with higher densities for nematoda (166 ± 99 ind.10 cm-2 and 56 ± 29 ind.10 cm-2) recorded in Tana and Sabaki, respectively during the southeast monsoon season. At least 7 taxa out of a total of 26 were present in both estuaries. Salinity gradient, season and sediment depth were found to influence the meiobenthic densities and taxa composition.

Community structure of deep-sea benthic metazoan meiofauna in the polymetallic nodule fields in the eastern Clarion-Clipperton Fracture Zone, Pacific Ocean

The abyssal sea floor of the Clarion-Clipperton Fracture Zone (CCFZ) in the Pacific Ocean is expected to become a commercially important nodule mining area in the near future. Hence, it is crucial to understand the community structure of biological communities there, so that nodule harvesting activities can be designed to minimize deleterious impacts on marine life. Meiofauna is an important component of the abyssal infaunal community but relatively little is known on their biodiversity, abundance, and community structure in the CCFZ. We provide here the first quantitative observations of metazoan benthic animals ≥40 μm in a 30 × 30 km area survey stratum in the Ocean Mineral Singapore contract area at the eastern end of the CCFZ. A total of 88 867 individuals, identified to 23 animal groups, were collected from 12 randomly sampled stations using a multiple corer. Even in our small surveyed area, the meiofaunal community structure appeared different at our stations, confirming that the CCFZ deep sea floor is very diverse. From the top 0–5 cm sediment layer, unsurprisingly, the three most abundant animal groups were the Nematoda (87.9 ± 2.9%), the Nauplii (5.0 ± 1.3%) and the Copepoda (4.3 ± 1.1%). The majority of the meiobenthos (76.6%) were in the top 0–2 cm sediment layer. Our results suggested that substratum shear strength was negatively and significantly correlated to meiofaunal abundances among the 12 stations. Nodule cover and nodule volume did not appear to affect the meiofaunal community structure at the major taxon level. The mean meiofaunal abundance (235.7 ± 26.4 ind./10 cm2) was relatively high at our site in comparison with previous studies elsewhere in the CCFZ, further confirming the westward decrease in meiofaunal abundance across the CCFZ.

Alive and dead foraminiferal assemblages at sandy beaches of Karachi coast-diversity, ecological distribution and responses to seasonal fluctuations

Foraminifera are eukaryotic protists and play a vital role in marine community structure. However, how physicochemical environment shape meiofaunal community structure at highly dynamic exposed sandy beaches of Arabian sea is inadequately known. This is the first study about spatiotemporal distribution of alive and dead benthic foraminiferal (BF) species from coastal regions of Pakistan based on 48 core samples collected monthly with the help of hand corer (diameter = 3 cm) from four stations at two sandy beaches of Karachi coast i.e., Sandspit and Hawksbay. Sampling was done throughout the year from March 2013 to April 2014 to observe seasonal variability in southwest and northeast monsoon season. Physicochemical parameters (temperature, salinity, pH, dissolved oxygen and nutrients i.e. Ammonia, nitrate and phosphorus) and sediment characteristics (grain size, moisture content, organic content) was analysed using standard methods.The top 0–2 cm sediment slice was analysed for both living and dead foraminiferal assemblages. A sum of 6615 individuals (ind.) cm−2 of BF consisting of 3451 ind. cm−2 empty tests and 3164 ind. cm−2 living specimens belonged to 66 species were recorded, where 47% species are Miliolina, 42% Rotaliida, 9% Textulariina and 2% are Spirillinina. The similarity percentage breakdown analysis (SIMPER) showed 18.51% average dissimilarity between foraminifera abundance at Sandspit and Hawksbay but stations within sites showed similarity (82.83% at Sandspit and 84.65% at Hawksbay). The highest abundance of alive BF was recorded in northeast monsoon season (3232 ind. cm −2) associated with high levels of nutrients. Alive and dead foraminifera assemblages at Karachi coast do not seem to be measured by a single physicochemical and environmental factor but multiple factors are controlling spatiotemporal distribution of BF at these beaches. The most significant and governing factor for benthic abundance and diversity at these beaches is the monsoon season.

First insights into the meiofauna community of a maerl bed in the Bay of Brest (Brittany)

Maerl beds, particularly those of Brittany, are important, structurally complex biogenic coastal habitats that form a unique ecosystem with high benthic biodiversity. Although they are relatively well studied throughout Europe, several faunal groups of maerl beds, such as those belonging to the meiofauna, have received little attention. We investigated the meiofaunal abundance, distribution and community structure, with a focus on nematode biomass and diversity, in a maerl area in the Bay of Brest, Brittany, compared with that on a sandy beach (Anse de Dinan, Brittany). Meiofauna was five times more abundant on the maerl bed than on the sandy beach, and 1.5 times more diversified (12 vs. 8 taxa, respectively). Nematode diversity was more than three times higher on the maerl bed than on the sandy beach and showed a distinctive nematode community that was absent from the sandy beach. Maerl beds create more heterogeneous microhabitats and promote a higher diversification of meiofauna and nematode communities than sandy beaches.

The Paradox of an Unpolluted Coastal Site Facing a Chronically Contaminated Industrial Area

Present and past industrial activities in coastal areas have left us a legacy of contamination and habitat degradation with potential implications for human health. Here, we investigated a coastal marine area enclosed in a Site of National Interest (SNI) of the central-western Adriatic (Mediterranean Sea), where priority actions of environmental remediation are required by governmental laws due the high environmental and human risk, and that is off-limits to any human activity since 2002. In particular, our investigation was focused on an area located in front of a chemical industry dismissed more than 3 decades ago. We report that the concentrations of heavy-metal and organic contaminants in the investigated sediments were generally lower than those expected to induce detrimental biological effects. Meiofaunal abundance, biomass and community structure changed among stations, but regardless of the distance from the abandoned industrial plant. Taxa richness within the SNI did not change significantly compared to the controls and the lack of some taxa in the SNI transects was not due to the contamination of the SNI area. The results of this study suggest a natural recovery of the marine area over 2 decades of restrictions on human activities, including fishing and shipping bans. If the hypothesis of the natural recovery of this SNI will be further confirmed by other studies, the plans for the identification and monitoring of the most polluted areas in Italy should necessarily be redefined also in the light of the Water Framework, the Marine Strategy Framework and the Environmental Quality Standard Directives.

Benthic Meiofaunal Diversity in Four Zones of Pichavaram Mangrove Forest, India

ABSTRACTThe community structure of meiofauna was assessed relative to environmental parameters in four zones (non-mangrove, Avicennia zone, mixed zone, and Rhizophora zone) of Pichavaram Mangrove Forest, which is located along the southeast coast of India. Field sampling was carried out from June 2016–May 2017. The meiofaunal data were evaluated using univariate and multivariate statistics. The highest abundance of meiofauna (434 ind/10 cm−2) was recorded in the Rhizophora zone and the lowest (270 ind/10 cm−2) in the non-mangrove zone. A total of 62 species representing four meiofaunal groups were recorded, including 38 species of foraminifera, 12 species of nematodes, seven species of ostracods, and five species of harpacticoids. Diversity (Shannon H′) mostly ranged from 3–4, and was highest (3.86±0.26) in the Avicennia zone and averaged ∼3.3 in the other three zones. The Margalef richness index was ∼6 in the Avicennia and Rhizophora zones and ∼5 in the non-mangrove and mixed zones. The BIO-ENV and CCA analyses identified sediment texture and organic matter as key variables influencing the distribution of meiofauna, while % calcium carbonate, salinity, and sediment pH were also important. Monitoring studies of meiofaunal taxa may help elucidate their roles in coastal blue-carbon cycling and will also contribute to understanding how mangrove-associated habitats influence meiofaunal distributions.

Allelopathic interactions between phytobenthos and meiofaunal community in an Adriatic benthic ecosystem: Understanding the role of aldehydes and macroalgal structural complexity

Macroalgae produce several allelopathic substances, including polyunsaturated aldehydes (PUAs), which may inhibit photosynthesis and growth rates of other algal species, and grazing. Additionally, macroalgal structural complexity is an important factor in determining abundance patterns and size structure of epiphytic organisms.In this study the PUAs production of two Mediterranean macroalgae, Dictyopteris polypodioides, (DP, Phaeophyceae, Dictyotales) and Cystoseira compressa (CC, Phaeophyceae, Fucales), was characterized to clarify the relationships between the meiobenthic and microphytobenthic communities. Results showed a higher PUAs production and a diverse qualitative profile for DP, which reported long-chain compounds (i.e. C14-C16) as main aldehydes, than CC, with the short-chain C6:2 as the main compound, as well as variability among sampling times. A clear separation of the meiofauna and microphytobenthos assemblages was found for the macroalgae, but with different temporal trends. Dissimilarities were due to five microalgal orders, namely Naviculales, Lyrellales, Gonyaulacales (i.e. Ostreopsis), Bacillariales, and Licmophorales, and to the meiofaunal groups nematodes, copepods, and copepod nauplii, which were more abundant on DP than on CC. Results indicate that macroalgal complexity is a major determinant of the meiofaunal community structure (accounting for 26% of the variation), rather than PUAs production itself (17%). PUAs effects seem species-specific, thus affecting some grazers instead of the entire community. Conversely, microphytobenthos affected the meiofauna assemblages, particularly harpacticoids, confirming the role of these organisms as the primary food source of all marine food chain producers. Since PUAs are produced also by several epiphytic diatoms, the understanding of their effects on the community structure and on the relationships among taxa in the field is complicated and requires further in-depth investigations in simplified systems (i.e. microcosms).

The network structure of intertidal meiofaunal communities from environmental DNA metabarcoding surveys in Northwest Iberia

The network structure of intertidal meiofaunal communities from environmental DNA metabarcoding surveys in Northwest Iberia

Distribution Pattern of the Benthic Meiofaunal Community Along the Depth Gradient of the Western Indian Continental Margin, Including the OMZ and Abyssal Plain

The continental margin harbors a variety of habitats that support incredible biodiversity and the function of their oceans' ecosystems. The meiofauna is considered a significant component of the benthic faunal community from the polar to the tropical regions. The meiofaunal community in the deep Indian Ocean, especially along the depth gradient, is poorly investigated. The present study aims to explore the benthic meiofaunal community structure along the depth gradients and its associated environment in the western Indian continental margin (WICM) and abyssal plain in the eastern Arabian Sea. Sediment samples were collected from seven different depths (111–3,918 m) along the WICM including the oxygen minimum zone (OMZ) and abyssal plain. A total of 22 taxa (groups) were encountered along the WICM. The nematodes (85%) were the most dominant taxa in all the depths, followed by copepods (11%), nauplii (5%), and polychaetes (1.36%). Our results suggest that (a) the organic matter has accumulated in OMZ sites; (b) a high amount of total organic carbon did not influence the meiofaunal density or biomass; (c) oxygen and depth gradients were significant drivers of the meiofaunal community, low levels of oxygen contributed to lower taxa diversity and density at 485 and 724 m depths; (d) a significant relationship of meiofaunal density and biomass with chloroplastic pigment equivalent (CPE) values indicates pelagic-benthic coupling. Copepods, nauplii, tanaidaceans, isopods, kinorhynchs, and cumaceans were affected by the low-oxygen conditions at the OMZ sites. Enhanced meiofaunal diversity, density, and biomass at deeper sites (non-OMZ-D) was attributed to increased abundance of copepods, nauplii, tanaidaceans, isopods, kinorhynchs, and cumaceans and were mostly concentrated on the surface sediment (0–4 cm) triggered by enhanced bottom-water oxygen and freshness of available food outside the OMZ except 3,918 m. Therefore, the present study showed the meiofaunal community pattern with respect to depth gradients and their related environmental changes, including the OMZ along the WICM and abyssal plain in the Arabian Sea. Based on this study, the future extent of these underestimated marine ecosystems must be considered a priority to understand its functional consequences.

Recreational activities trigger changes in meiofauna and free-living nematodes on Amazonian macrotidal sandy beaches

Sandy beaches are key economic assets worldwide, however, recreational activities are an important source of disturbance in these ecosystems. Intensive human use of beaches has a negative effect on benthic communities as it alters species abundance, diversity, and/or distribution. Here, we report the impacts of recreational activities (trampling and vehicles) on meiobenthic communities and nematode assemblages on Amazonian sandy beaches for the first time. For this purpose, samples were carried out in four consecutive months encompassing a period of high visitation peak (one month Before Vacation, During Vacation, and two months After Vacation) on three contrasting beaches regarding disturbances (Urban: Atalaia; Intermediary: Farol-Velho; and Protected: Corvinas) at seven equidistant sampling stations (50 m apart) from the high tide water mark to the swash zone. At each sampling station, three biological samples and sediments samples were collected haphazardly. Also, in each station, the sediment compaction was determined using a manual penetrometer. Overall, physical sediment variables (grain size, sorting) were constant over time in all beaches, whereas differences in intensity of recreational activities and sediment compaction were found between beaches and months. Sediment compaction was considered the most important factor for the differences observed in meiofauna community structure in the study areas, and it was related to intensity of recreational activities. Variations in density and changes in richness between periods were observed in Atalaia and Farol-Velho beaches. On the other hand, Corvinas beach remained the same throughout the study. In the first month after vacation, density and richness were similar to those in the period before vacation, thus indicating that the beaches had recovered in one month. Furthermore, vulnerabilities of Tardigrada and Copepoda, as well as of the Nematoda genera Daptonema, and Chromadorita, indicate that they might be potential indicators of recreational activities.

Organic Contaminant Mixture Significantly Changes Microbenthic Community Structure and Increases the Expression of PAH Degradation Genes

Studying the effects of chemical contaminants on the structure and function of microbial and meiofauna communities have traditionally focused on specific effects of single contaminants on single species. This has left the complex interactions between mixtures of contaminants and its non-specific toxicity effects on the functions and structure of sediment microbial communities mostly overlooked. In an effort to improve our insights on such questions, we performed an experiment where Baltic Sea sediments were spiked with an ecologically relevant mixture of seven organic contaminants below specific toxicity levels. We then used 16S and 18S rRNA metabarcoding from RNA extracts to monitor changes in active microbial and meiofauna diversity and community structure in the spiked treatment compared to controls. In addition, we investigated the effects of exposure to this contaminant mixture on potential nitrification rates and on the expression of key-genes in the microbial nitrification and PAH degradation pathways with qPCR. There were significant differences in both eukaryotic and microbial community structures in sediments spiked with a mixture of organic pollutants. Nematoda showed a significant increase in overall relative abundance to the added contaminants (5.5±.1.1% higher in spiked), particularly taxa of the genus Leptolaimus (increased from 10.2±.5.4% in the controls to 32.5±.10.2% in the spiked treatment). Conversely, a significant decrease in relative abundance from 18.2±.5.6% in control to 7±.3.4% in of the genus Paraplectonemus was also detected. Additionally, while the abundance of active PAH degraders was significantly higher in spiked sediments than in the controls, no significant effect of our organic mixture was found on nitrification rates or the expression of AmoA (bacterial ammonia oxidizer gene). Our data indicate that mixtures of organic pollutants can have significant effects on microbenthic community structure even when its individual components are present at concentrations below its specific toxicity. In addition, we suggest that eRNA-based metabarcoding can offer important insights in microbenthic community structure and activities and further empathizes the potential of meiofauna as bio-indicators of chemical contamination in benthic ecosystems.