Soft-sediment Habitats Research Articles

Different approaches to estimate benthic metazoan diversity associated with free-living macroalgae (Fucus vesiculosus) on shallow soft sediments

Habitat complexity can boost biodiversity by providing a wide range of niches allowing species co-existence. Baltic Sea benthic communities are characterised by low species diversity. Thus the occurrence of the habitat forming macroalga Fucus vesiculosus may influence benthic communities and promote diversity. Here biodiversity estimates were obtained through conventional and eDNA approaches for the benthic assemblages associated with free-living Fucus and the adjacent bare-sediment habitats at six sites from the Northern Baltic Proper and the Gulf of Finland. Free-living F. vesiculosus habitats are heterogeneous with biodiversity estimates varying considerably among sites. The additional habitat complexity provided by F. vesiculosus tended to improve taxa richness as a result of additional epifauna assemblages, although macroinfaunal taxa richness and abundance was often reduced. Consequently the complex habitats provided by free-living F. vesiculosus often improve biodiversity, yet alters the composition of assemblages in soft sediment habitats and consequential ecosystem functioning. The study emphasised the disparity in biodiversity estimates achieved when employing different biodiversity approaches. Biodiversity estimates were more similar within approaches compared to between habitat types, with each approach detecting exclusive taxa. Consequently, biodiversity estimates can benefit from a multi-approach design where both conventional and eDNA approaches are employed in complement.

Utility of a modified lightweight recreational scallop dredge for surveys of estuarine ark clams (cockles)

Arcid cockles support extensive subsistence, recreational and commercial fisheries across the world. In south-eastern Australia, there is minimal published research and a lack of quantitative surveys for the primary harvested species, Sydney Cockle Anadara trapezia, with which to inform management of the resource. Here, we describe the modification and application of an off-the-shelf lightweight recreational scallop dredge for conducting scientific surveys of arcid clams in estuarine soft sediment habitats. Specifically, we look at performance of the gear for sampling medium-large (>45 mm shell length) cockles, and report on the outcomes of preliminary trials of the gear for surveying Sydney Cockle. Efficiency trials showed that the gear captured and retained ∼95% of the cockles in the tow path, however efficiency decreased in compacted sediments (∼66%). Trial surveys captured cockles between 33 and 78 mm shell length, at densities as high as 186 individuals 100 m−2. The modified scallop dredge provides a robust and portable (∼10 kg weight) fishing gear for sampling estuarine soft sediment habitats from small vessels. Both the sampling gear, and the methodology presented here, are broadly applicable for surveying arcid cockles and other bivalve species from similar soft sediment habitats elsewhere.

Mesophotic benthic communities associated with a submerged palaeoshoreline in Western Australia.

Key ecological features (KEFs) are elements of Australia's Commonwealth marine environment considered to be important for biodiversity or ecosystem function, yet many KEFs are poorly researched, which can impede effective decision-making about future development and conservation. This study investigates a KEF positioned over the Last Glacial Maximum (LGM) shoreline on the northwest shelf of Australia (known as the 'Ancient Coastline at ~125m depth contour'; AC125). Seafloor bathymetry, sedimentology and benthic habitats were characterised within five study areas using multibeam sonar, sediment samples and towed video imagery. Direct evidence for the existence of a palaeoshoreline formed during the LGM was not found, however candidate areas to find palaeoshoreline material at or just below the modern seabed were discovered. Approximately 98% of the seabed surveyed was comprised of unconsolidated soft sediment habitat (mud/sand/silt) supporting negligible epibenthic biota. The prevalence of soft sediment suggests that post-glacial sediments have infilled parts of the palaeoshoreline, with cross-shelf, probably tidal currents in the northern section of the study area responsible for some of the sediment mobilisation and southern study areas more influenced by oceanic conditions. Within study areas, total biotic cover ranged from 0.02% to 1.07%. Of the biota encountered, most comprised filter feeder organisms (including gorgonians, sponges, and whip corals) whose distribution was associated with pockets of consolidated hard substrate. Benthic community composition varied with both study area and position in relation to the predicted AC125. In general, consolidated substrate was proportionally higher in water shallower than the AC125 compared to on the AC125 or deeper than the AC125. Spatially continuous maps of predicted benthic habitat classes (pre-determined benthic communities) in each study area were developed to characterise biodiversity. Spatial modelling corroborated depth and large-scale structural complexity of the seafloor as surrogates for predicting likely habitat class. This study provides an important assessment of the AC125 and shows that if a distinct coastline exists in the areas we surveyed, it is now largely buried and as such does not provide a unique hard substrate habitat. However, much work remains to fully locate and map the ancient coastline within the vast region of the AC125 and additional surveys in shallow waters adjacent to the AC125 may identify whether some sections lie outside the currently defined KEF.

Using environmental DNA to better inform decision making around decommissioning alternatives for offshore oil and gas infrastructure

Artificial reefs are being utilised globally to aid in natural resource management, conservation, restoration or the creation of unique marine habitats. There is discussion around the optimal construction materials and designs for artificial reefs, the influences these have on biological communities, and the resulting ecological and social benefits. This discussion also includes the ecological value of repurposed marine infrastructure, such as decommissioned oil and gas platforms. Platforms often have an operational life spanning multiple decades, over which time they can develop extensive and unique community assemblages. The creation of artificial reefs by repurposing oil and gas platforms can have ecological, economic and sociological merit. However, with >12,000 platforms requiring decommissioning globally, there is the need for holistic assessment of biological communities associated with these platforms to inform the potential outcomes of different decommissioning options. We use environmental DNA metabarcoding (eDNA) of water, bio-foul and sediment samples to census broad eukaryotic diversity at eight platforms in the Gulf of Thailand (GoT) and five nearby soft sediment habitat locations. We sampled three target depths at sites (shallow, mid, deep) and detected 430 taxa at platforms, with higher diversity in shallow (near-surface) samples (313 taxa), compared to mid (30 m collection depth; 261 taxa) and deep (50 m; 273 taxa). Three percent of taxa were shared among all depths at platforms with distinct assembles at each depth. Introduced species are an ongoing risk for platforms, however the eDNA detected no known introduced species. While the eDNA data provide broad taxon coverage and significant assemblage patterns, ongoing sampling innovation, assay design and local reference material still require development to obtain the maximum benefit of the technique. This study highlights the versatility and scalability of eDNA metabarcoding to holistically census marine infrastructure and inform the management and potential conservation of extant communities.

Temporal and Spatial Trends in Benthic Infauna and Potential Drivers, in a Highly Tidal Estuary in Atlantic Canada

Infaunal invertebrate communities of coastal marine sediments are often impacted by human activities, particularly in harbours and estuaries. However, while many studies have attempted to identify the key factors affecting benthic infauna, few have done so for highly energetic tidal estuaries. Samples were collected over a decade (2011–2020) from a series of reference sites in Saint John Harbour (45.25° N, 66.05° W), a highly tidal estuary in the Bay of Fundy, Canada. These data were used to examine spatial and temporal trends in infaunal invertebrate communities and sediment properties and to determine the extent to which the biological patterns were driven by measured physical and chemical variables. There were substantial differences among sites in infaunal invertebrate abundance (median ranging from 688 to 13,700 individuals per square meter), infaunal species richness (median ranging from 8 to 22), and Shannon diversity (median ranging from 1.26 to 2.34); multivariate analysis also revealed variation in species composition among sites. Sediment contaminant concentrations also varied among sites, but differences tended to be smaller (e.g. median chromium concentrations ranging from 21.6 to 27.6 mg/kg). Sample contaminant concentrations were all below probable effect levels, and almost all below threshold effect levels (Canadian interim sediment quality guidelines), but relationships with biological data were still detectable. However, physical variables (depth, sediment characteristics) were better predictors of biological variables and community composition. These results confirm the importance of physical factors in shaping infaunal communities in soft-sediment habitats in tidally influenced coastal waters.

A comparative analysis of sampling methodologies for assessing abundance and distribution of young-of-the-year groundfishes in nearshore soft-sediment habitats

Groundfishes are important species for commercial, recreational, and tribal fisheries along the Pacific Coast of the United States. Understanding the abundance and distribution of these fishes is necessary for successful fisheries management. Many groundfish species spend at least part of their life cycle in nearshore habitats. However, assessing groundfish species assemblages in nearshore habitats can be limited by the relatively small size of young-of-the-year and subadult life history stages, which may not be quantitatively retained with traditional survey gears. In addition, portions of the innermost nearshore (<55 m water depth) can be inaccessible to vessels employed in trawl surveys. To help improve the fishery-independent data informing the current understanding of nearshore assemblages, we compared catches resulting from two different but spatially overlapping surveys. One is the National Oceanic and Atmospheric Administration’s (NOAA) West Coast Groundfish Bottom Trawl Survey (WCGBTS), which is representative of a typical groundfish survey in USA and EU continental shelf habitats. The other is an ad-hoc beam trawl survey along the Newport Hydrographic (NH) Line – an oceanographically well-monitored sampling line off the Pacific Coast. Our objectives were to evaluate the representativeness of nearshore fish communities in the WCGBTS and to assess if additional nearshore sampling would enhance the WCGBTS. For the purposes of this study, “nearshore” is defined as the continental shelf shoreward of 200 m depth. Examining fish catches and environmental variables from both surveys, we found significant differences in fish assemblages, oxygen, temperature, and salinity based on depth, season, year, and survey type. The WCGBTS is designed and well-suited for sampling subadult and adult groundfishes in trawlable habitat. However, year-round sampling of the beam trawl survey allows it to capture ephemeral environmental conditions better than the WCGBTS. The gear type and sampling location make the beam trawl survey better suited than the WCGBTS for sampling smaller individuals and species that live at shallower depths. The beam trawl survey caught 69 unique species or taxa groups within the comparative data subset, whereas the WCGBTS caught 83. The three most commonly occurring species in the beam trawl survey were Pacific sanddab (Citharichthys sordidus), English sole (Parophrys vetulus), and speckled sanddab (Citharichthys stigmaeus), while the WCGBTS most commonly caught rex sole (Glyptocephalus zachirus), Dover sole (Microstomus pacificus), and spotted ratfish (Hydrolagus colliei). Of the fish with measured lengths, the catch from the beam trawl survey had a mode standard length of 2 cm, contrasted with the mode length (total length, fork length, anal length, and natural length combined) of 22 cm for the WCGBTS. We found that there is a transition in species assemblages right where the WCGBTS ends (about 55 m). Thus, while the surveys overlap in some capabilities, this analysis shows that the beam trawl survey or a similar nearshore survey would complement the WCGBTS, providing an enhanced characterization of groundfish assemblages.

Small coral reef fishes with large ecological footprints

Seascape connectivity can facilitate key ecosystem functions in complex ecosystems like coral reefs. Many reef fishes move across the seascape, bridging different ecosystems. However, their role in shaping important functions, such as biomass production and nutrient cycling, is still poorly understood. This study, therefore, assesses the extent of natural movements of cardinalfishes (Apogonidae), one of the major contributors to nocturnal fish biomass production with the potential for a major role in transferring energy and nutrients between sandy ecosystems and adjacent coral reefs. Consistent with previous work, showing their ability to move distances of 100 s to 1000 s of metres when displaced, we reveal that these small fishes undergo major voluntary nocturnal foraging forays extending up to at least 145 m from reef structures. Their estimated daily movement distances are at least 430 times greater than expected based solely on body size-home range expectations. Given their large travel distances and strong homing abilities, apogonids may provide a major conduit for material transfer between lagoonal soft sediment habitats and adjacent coral reefs. These results highlight the potential importance of apogonids in the cross-system or cross-habitat transport of energy and nutrients on coral reefs.

The functional diversity of fish assemblages in the vicinity of oil and gas pipelines compared to nearby natural reef and soft sediment habitats

As the offshore hydrocarbon industry matures and decommissioning activities are expected to increase, there is a requirement to assess the environmental consequences of different pipeline decommissioning options. Previous research on fish and other ecological components associated with pipelines has focused on examining species richness, abundance and biomass surrounding structures. The extent to which subsea pipelines mimic or alter ecosystem function compared with nearby natural habitats is unknown. We analyse differences in fish assemblage biological trait composition and the functional diversity at exposed shallow-water subsea pipelines, nearby natural reef and soft sediment habitats, using mini stereo-video remotely operated vehicles (ROV). Habitats significantly differed in assemblage trait composition. The pipeline and reef habitats shared a more similar functional composition and had the presence of key functional groups required for the development and maintenance of healthy coral reef systems. The reef habitat had the greatest functional diversity, followed by the pipeline habitat and soft sediment habitat respectively.

Fish size spectra from imaging sonar reveal variation in habitat use across nearshore coastal ecosystems

Structured coastal habitats provide foraging opportunities and refuge from predation for fish species of varying size and function. However, comprehensive assessments of fish communities among ecosystems are challenging because of biases arising across traditional sampling methods that target subsets of the community in different habitats (e.g. traps, seines, trawls, or visual census). Here, we used dual-frequency identification sonar (DIDSON) to examine shallow, nearshore habitat use by fishes in multiple structured habitats (seagrass, coral reefs, oyster reefs, rocky reefs, mangroves, woody debris, and docks) relative to soft-sediment habitat across 4 study locations ranging from tropical to temperate: Bocas del Toro, Panama, and Florida, Maryland, and California, USA. We then examined the distribution of individual fish sizes using size-spectra analysis. For temperate docks (Maryland) and eelgrass beds (California), size-spectra slopes were less steep than for soft-sediment habitats, indicating that larger fish associated with these structured habitats. No differences in slopes were identified for (sub)tropical Florida or Panama, although spectra intercepts from docks were higher in each location relative to soft sediment, denoting higher total abundance. Our results suggest geographically stratified habitat use with a tendency towards comparatively greater importance of structure in determining fish size distribution at higher latitudes, and greater importance of structured habitat in governing total abundance at lower latitudes. This study also demonstrates the potential of imaging sonar as a new tool for revealing variation in fish communities among habitats at local to continental scales.

Piers are hotspots for benthic marine debris in an urbanised estuary.

Records of anthropogenic marine debris and the threats it poses are increasing worldwide, yet we know relatively little about the distribution of benthic debris. The seafloor is the final destination for a large proportion of debris due to the degradation and sinking of items. A more detailed understanding of debris distributions in hotspots such as urbanised estuaries can help decision makers target management and remediation activities. We selected sites frequented by fishers and boaters in Sydney Harbour, an urbanised estuary, to investigate the impacts of recreational activities on debris abundance. The aim of this study was to examine variation in macro debris (>5mm in diameter) type and abundance at two habitat types (piers and non-piers). We chose five locations at various distances from the estuary mouth. In each location SCUBA teams performed fixed transects at two sites, one under a pier and one over nearby soft-sediment habitat. Debris was recovered by the divers and brought to the surface for classification and disposal. Surveys were repeated multiple times at each location between November 2019 and February 2020, recording a total of 2803 debris items over 36 survey events. Overall, piers had more than ten times the debris abundance of soft-sediment sites, and much higher proportion of debris types related to recreational fishing. Over half of the debris items in this study were plastic (65%), and approximately 70% of the total debris was classified as related to recreational fishing. This trait was most prominent in debris at sites closest to the estuary mouth, likely reflecting increased fishing activity in this area. This study indicates that policy makers and community groups in urbanised estuaries should focus monitoring, reduction, and remediation efforts near artificial structures such as piers, and that public awareness campaigns should target the behaviour of recreational users of these structures.

Trait-based assessment of polychaete assemblages distinguishes macrofaunal community structure among four distinct benthic eco-regions on a shallow arctic shelf

In a rapidly changing Arctic, effects of taxonomic and functional composition of infaunal benthic communities on ecosystem function are poorly characterized for many areas of the continental shelf environment. Infaunal macrobenthos perform essential ecosystem services in soft-sediment habitats, including sediment oxygenation, organic-matter burial, and nutrient regeneration. Changes in the distribution and composition of infauna are likely to impact these important processes. We investigated spatial patterns in macrofauna on the northern Bering and southern Chukchi Sea continental shelves in conjunction with a suite of environmental variables as part of the Arctic Shelf Growth, Advection, Respiration, and Deposition (ASGARD) project. Depth-stratified macrofauna samples (>500 μm) were collected from multi-cores in June 2017 and 2018. Abundance and biomass were recorded for broad taxonomic categories (phylum or class). Polychaetes were identified to family to capture well-described functional-trait information. Functional guilds were assigned to polychaete families, reflecting feeding mode, motility, and feeding structures. Macrofauna exhibited no significant spatial patterns when examined at higher taxonomic levels; however, analysis of polychaete families yielded three distinct station groups, and polychaete functional guilds yielded four distinct groups. These groups were aligned with variations in key habitat characteristics and suggest variation in ecosystem function across the study area. At sandy stations, tube-dwelling, particle-selective suspension and surface deposit feeders were relatively abundant, reflecting an advective system with moderate current speeds and low deposition. Sites with gravelly substrate and moderate levels of organic matter in deeper sediment layers likely experience sporadic organic-matter deposition and disturbance from scouring and were inhabited by larger-bodied species, including bivalves and subsurface deposit-feeding or carnivorous polychaetes. Muddy, offshore stations in the southern Chukchi Sea also contained large bivalves and large carnivorous polychaetes, with evidence of high bioturbation. At more coastal Chukchi Sea stations, macrofauna were dominated by small, motile polychaetes concentrated in the 1-cm surface layer, with indications of anaerobic conditions below. Overall, polychaete functional traits and vertical distribution reflected the quality and quantity of organic matter input and the depositional environment inferred from grain size and current speed, with implications for biogeochemical cycling in sediments.

The Effects of Long-Term Nitrogen Enrichment on Estuarine Benthic-Pelagic Coupling

Biogeochemical cycling in the marine coastal zone regulates the availability of nitrogen and carbon within soft sediment habitats. However, these pathways are being fundamentally altered by anthropogenic increases in nutrient delivery. Few studies have incorporated long-term enrichment and ecological complexity (in situ experiments), restricting our ability to manage effectively and prevent ecological shifts. This study investigates the influence of sediment nutrient availability (at 3 levels, across 2 seasons) on biogeochemical cycling over a 20-month period in 4 estuaries. Overall, net denitrification rates were highly variable, ranging between 4 and 208 µmol N m−2 h−1. However, no increases were observed with increasing enrichment highlighting the limited capacity for nitrogen removal in response to large increases in bioavailable nitrogen. Additionally, macrofaunal communities and sediment trophic status were shown to have important influences on nitrogen processing. Overall, alterations to ecosystem relationships and the appearance of non-linear responses to increasing nutrient enrichment reveal the vulnerability of estuaries to increasing stressor loads owing to the increased likelihood of reaching a tipping point.

The interplay of temperature and algal enrichment intensifies bioturbation of the intertidal amphipod Corophium volutator

Bioturbation is a central transport process for ecosystem functioning, especially in large soft sediment habitats like the Wadden Sea. The amphipod C. volutator is a dominant bioturbator in the Wadden Sea, due to its great abundance and almost continuous particle movement. Expedition or loss of its bioturbation activity could thus hold ramifications for ecosystem functioning within sediments, like carbon sequestration and nutrient recycling. Here we test the effect that temperature and organic enrichment have on the bioturbation of C. volutator; two prevalent abiotic factors in the Corophiid's habitat that have fluctuated over recent decades, and are expected to change in the future. In-situ experiments were conducted under 8 and 15 °C, with varying levels (0 g, 0.1 g, and 0.2 g) of powdered Ulva compressa enriching cores containing C. volutator. We found a significant interaction effect of temperature and organic enrichment on the bioturbation rate of the amphipod, with bioturbation only increasing with added organic enrichment at 15 °C. Further, a threshold within our experiments was also reached under 15 °C, where the amphipod ceased to expedite bioturbation under higher organic enrichment. This upper limit on this dominant bioturbation imposed with organic enrichment emphasizes the sensitivity of C. volutator. Our findings reveal bioturbation can be limited by temperature in colder months, and opposingly, limited by organic enrichment under warmer conditions. In future Wadden Sea scenarios where temperature is predicted to be warmer and winters milder, enhanced bioturbation activity by C. volutator could prove crucial in continued ecosystem functions.

Can sea snakes slither through seascape structure? Comparative phylogeography and population genetics of Hydrophis group sea snakes in Australia and Southeast Asia

Pleistocene sea level changes substantially shaped the biogeography of northern Australia and the Indo-Malayan Archipelago (IMA). For co-distributed species, their phylogeographic and population genetic patterns are expected to be concomitant with geological transformations of the Pleistocene. However, species-specific ecologies and life history traits may also be influential in generating patterns which depart from simple expectations arising from biogeographic features. Thus, comparative population genetic studies, which use taxa that reduces variation in taxonomy and geography, may refine our understanding of how biogeographic elements shape the populations of co-occurring species. Here, we sampled two sea snake species, Hydrophis curtus and H. elegans, throughout their known ranges in the IMA and northern Australia. These sea snakes have similar life history strategies and ecologies as well as overlapping distributions across the Torres Strait, a well-known biogeographic feature. We analysed two mitochondrial DNA (mtDNA) fragments and 10 microsatellite loci using traditional population genetic approaches and used Bayesian clustering methods to examine species- specific phylogenetic relationships, genetic diversities, and population genetic structures. For both species, we found a consistent lack of significant genetic variation among sampling sites across the Gulf of Carpentaria (GOC) and the Great Barrier Reef (GBR). Similarly, Bayesian clustering showed no to weak genetic partitioning across the historical Torres Strait land bridge. Both species sampled in Australia displayed population expansion signatures in tests using mtDNA and microsatellite markers. We conclude that the phylogeographic and population genetic patterns of these sea snake species do not align with the Torres Strait land bridge. This lack of population genetic structure departs from previous findings on Aipysurus sea snakes and may be linked to the association of Hydrophis species to soft sediment habitats typically found across northern Australia. These divergent patterns between the sea snake groups present the importance of considering taxon-specific attributes in formulating conservation strategies.

Ecosystem function responses to nutrient enrichment mediated by mud content in soft sediment habitats

ABSTRACT Estuaries are threatened with increasing sedimentation and nutrients, however research is limited on the interaction of these stressors on ecosystem functions. A field experiment adding nitrogen (N) fertiliser at high (600 g N m−2) and medium (150 g N m−2) concentrations and control plots was established in two sandy and two muddy sites (mud content 0–15%) to investigate context dependency of nitrogen enrichment on nutrient cycling and primary production. There was a significant interaction of mud x treatment on ammonium (NH4 +) fluxes. High N-addition plots had 133x greater dark NH4 + efflux than controls in muddy sediments, compared to 17x greater dark NH4 + efflux in sandy sediments. Net primary production increased with mud content, while photosynthetic efficiency of the microphytobenthos (GPPchl-a ) decreased above 4% mud. Mud content also affected the bivalve, Austrovenus stutchburyi (>30 mm), macrofaunal abundance and functional groups, and their influence on nutrient cycling. Thus minor increases in mud can alter the ecosystem response to nutrient enrichment directly through changes in sediment properties and indirectly by affecting the bivalves and macrofauna that influence these processes. This study highlights how multiple stressors interact to predispose estuaries to tipping points.

Estuaries as coastal reactors: importance of shallow seafloor habitats for primary productivity and nutrient transformation, and impacts of sea level rise

ABSTRACT Estuaries are hotspots of primary productivity and nutrient transformation that contribute to food webs and ecosystem functioning locally and in adjacent ecosystems. The depth-dependence of nutrient transformation and primary productivity rates in the water column and the seafloor were investigated, and the lateral transport of solutes and materials across the estuary mouth were quantified. Using an estuary dominated by shallow soft-sediment habitats as a case study, the effects of sea level rise (SLR) on productivity and nutrient transformation processes were projected. The estuary was a net importer of dissolved nutrients from the coast, and a net exporter of suspended sediments and chlorophyll a, supporting the notion that estuaries are important nutrient transformation reactors. A significant depth (and light) effect on productivity indicates that increasing stressors associated with climate change that reduce light at the seafloor (SLR and increased turbidity) will negatively impact estuarine productivity. Intertidal and shallow subtidal benthic habitats were responsible for most of the productivity of the estuary and this is likely to be consistent for other shallow estuaries globally. SLR and anthropogenic intervention that prevents landward migration (i.e. seawalls and armouring), will result in the loss of these habitats and their significant contributions to adjacent coastal ecosystems.

Habitat formed by the invasive macroalga Caulerpa filiformis (Suhr) Hering (Caulerpales, Chlorophyta) alters benthic macroinvertebrate assemblages in Peru

The green macroalga Caulerpa filiformis has been spreading on shallow soft sediment habitats along the Peruvian coast, colonizing previously unvegetated sediments to create monospecific meadows. We examined the nature of the impact of C. filiformis meadows on the density, taxonomic richness and assemblage structure of epifaunal and infaunal benthic macroinvertebrates. Specifically, we tested whether the spread of C. filiformis has resulted in different macroinvertebrate assemblages than those formed by the dominant native macroalgae (i.e., Rhodymenia spp.) and unvegetated sediments. Surveys were undertaken in two bays in each of two locations, in central and southern Peru, during winter 2017 and summer 2018. In general, our results show that macroinvertebrate assemblages were similar across all three habitats, although there were some differences, related to location and time, but with no clear patterns observed. Taxonomic richness and density was generally higher in the vegetated habitats than the unvegetated habitat, and where there were differences between the two vegetated habitats there was no consistent pattern of which habitat supported the highest richness or density. Given invading C. filiformis is primarily colonizing unvegetated habitats it would appear that this species is creating a new niche which supports similar assemblages, but higher taxonomic richness and density than unvegetated habitats. While our study suggests that C. filiformis is having a limited ecological impact we recommend that actions be put in place to limit the spread of this invasive species at the same time as increasing monitoring of the ecological impacts of this species as lags in the ecological impacts of invasive species are common.

Estuarine Infauna Within Incidentally Retained Sediment in Artificial Rockpools

Artificial coastal structures (ACSs) are primarily designed to provide services for human use, such as flood defence or shipping, and are generally poor for marine biodiversity. Consequently, there has been significant research effort to enhance these hard structures to increase biodiversity and habitat availability via eco-engineering. On seawalls and breakwaters, this has included the creation of habitats for benthic species found on natural rocky shores, including the provision of cracks, crevices and water retaining features, such as artificial rockpools. When sediment retention in these features has occurred, it has often been deemed detrimental to the overarching aim of the intervention. Yet, it is soft sediment habitat that is impacted the most through coastal construction. As ecological enhancement of a flood defence scheme, nine concrete retrofit rockpools were installed at three different tidal elevations between mean high water neap tide and mean tide level on steel sheet piling on the Arun Estuary in Littlehampton Harbour, United Kingdom, which naturally filled with mud 1 year after installation. To explore how analogous the faunal assemblages and sediment profile of rockpool mud were to two local mudflats, core samples were taken and analysed for species richness, abundance, biomass, assemblage structure, median grain size, and organic matter content. More benthic species were observed in the artificial rockpool than in the local mudflats. Although the rockpools were placed at higher tidal levels than the lower shore mudflat, their assemblage structure and species richness were more similar to the lower shore mudflat at the base of the sheet piling than the upper shore mudflat. This study demonstrates that retained sediment within eco-engineered features on hard ACSs can create habitat for benthic assemblages. Providing sediment-retentive features on ACSs has the potential to provide a novel eco-engineering option that may be appropriate for some heavily modified waterbodies on sheltered, depositional coasts.

Megafaunal Ecology of the Western Clarion Clipperton Zone

The Clarion Clipperton Zone (CCZ) is a vast area of the central Pacific Ocean where the abyssal seabed is a focus for future polymetallic nodule mining. Broad-scale environmental gradients occur east-to-west across the CCZ seabed, including organic matter supply and nodule abundance, factors that influence benthic faunal community structure and function. A network of protected areas across the CCZ, called Areas of Particular Environmental Interest (APEIs), has been designated to cover this variation. Most previous studies of the benthic environment and megafaunal communities have focussed on the eastern CCZ, leaving the impact of these large-scale gradients unexamined and the network design untested. Seamounts are a further source of heterogeneity in the region. We examined the benthic megafaunal ecology of three APEIs in the western CCZ, spanning a range of environmental conditions. We used a combination of seabed photography and direct sampling to assess the environment and megafauna on the soft sediment habitats on the abyssal plain in three APEIs, and seamounts in two of those APEIs. We found that environmental conditions on abyssal plains differed between the three APEIs in terms of water depth, nodule abundance and coverage, sediment particle size distribution, and estimated organic matter flux. Megafauna were low density and high diversity, with few common morphotypes between sites and many morphotypes being observed only once. Xenophyophores dominated the assemblages. The density and diversity of invertebrates were greater at the sites with lower organic matter inputs and greater nodule abundance. Seamounts in the same APEIs were nodule-free and had coarser sediments than on the plain. Invertebrate megafaunal diversity was lower on the seamounts than on the plains, and most morphotypes recorded on the seamounts were only found on seamounts. Low morphotype overlap also suggests little connectivity between APEIs, and between seamounts and adjacent abyssal plains. Our results provide the first evaluation of the seabed habitats and megafaunal ecology in the western CCZ, highlighting environmental gradients that influence benthic communities, and are important for evaluating the design of the network of protected areas.

An updated inventory of sea slugs from Koh Tao, Thailand, with notes on their ecology and a dramatic biodiversity increase for Thai waters.

Improved access to field survey infrastructure throughout South-East Asia has allowed for a greater intensity of biodiversity surveys than ever before. The rocky bottoms and coral reef habitats across the region have been shown to support some of the highest sea slug biodiversity on the planet, with ever increasing records. During the past ten years, intensive SCUBA surveys have been carried out at Koh Tao, in the Gulf of Thailand, which have yielded remarkable findings in sea slug biology and ecology. In this work a brief history of sea slug biodiversity research from Thailand is covered and a complete inventory of sea slugs from Koh Tao, Gulf of Thailand is provided. This inventory is based on surveys from 2012 to 2020, with previously unreported findings since 2016. Habitat specificity and species-specific ecology are reported where available with a focused comparison of coral reef habitats and deeper soft-sediment habitats. The findings contribute 90 new species records for Thai waters (92 for the Gulf of Thailand) and report a remarkable consistency in the proportional diversity found to be exclusive to one habitat type or another. Additionally, taxonomic remarks are provided for species documented from Koh Tao that have not been discussed in past literature from Thailand, and a summary of previous records in the Indo-West Pacific is given.