Hard Substrate Habitats Research Articles

Strong site fidelity, residency and local behaviour of Atlantic cod (Gadus morhua) at two types of artificial reefs in an offshore wind farm.

Globally, biogenic temperate reefs are among the most threatened habitats. In the North Sea in particular, large shellfish reefs were lost owing to fishing activities in the 1900s. The impact of offshore wind farms (OWFs) on marine wildlife is extensive, and it offers the possibility to reintroduce new hard substrate habitats that are protected from fisheries at a large scale. In addition to the submerged structures of OWFs, marine hard substrate habitat can be further enhanced by providing extra artificial reefs. In an operational OWF along the Dutch coast, four artificial reefs (two with a scour bed and two without) were deployed in the vicinity of a wind turbine. Acoustic telemetry was used to monitor the fine-scale movement of 64 Atlantic cod (Gadus morhua). The monitoring ran from July 2021 to January 2023. Detailed information on behaviour, area utilization and attraction to the structures was determined. Results showed strong attraction (high site fidelity and residency) to the artificial reef, with no significant difference between the two tested types of reefs, and only a few individuals staying over winter. Cod spent a large proportion of their time hiding in the artificial reefs, suggesting that adding pipes for shelter has a beneficiary effect.

Mollusc Diversity of Hard Substrate Habitats of Gökçeada Island

In the present study, molluscs diversity between 0-0.5 m depths of the hard substrate habitats of Gökçeada coasts was investigated qualitatively and quantitatively, seasonally, and monthly in 2010 and 2011. In addition, in summer months of 2021, samples were taken to investigate the mucilage impact on the mollusc fauna, which occurred in the Marmara Sea in 2021 and affected Gökçeada coasts due to the currents. A total of 76 mollusc species were identified and 27 of the determined species are new records for Gökçeada. Among the found species, while Cardita calyculata (Linnaeus, 1758) was the most dominant species at the Yıldız Koy station sampled monthly, Musculus costulatus (Risso, 1826) was the dominant one at all other sampling stations. The Tepeköy station was found to has the highest number of species and individuals. As the main reason for this can be indicate the limited human access to the region and the presence of various habitats. Mollusc species diversity, which was detected in 2010 due to the pouring of sand for tourists by the municipality at Yıldız Koy station, was found to be increased greatly in 2021 after this practice was abandoned. It was also detected that the mollusc species distributed on the coasts of Gökçeada were not acutely affected by the mucilage event occurred in 2021. However, more detailed studies need to be monitoring and report the negative effects that may occur the mucilage event on the species diversity of the region in the coming years.

The Suitability of Machine-Learning Algorithms for the Automatic Acoustic Seafloor Classification of Hard Substrate Habitats in the German Bight

The automatic calculation of sediment maps from hydroacoustic data is of great importance for habitat and sediment mapping as well as monitoring tasks. For this reason, numerous papers have been published that are based on a variety of algorithms and different kinds of input data. However, the current literature lacks comparative studies that investigate the performance of different approaches in depth. Therefore, this study aims to provide recommendations for suitable approaches for the automatic classification of side-scan sonar data that can be applied by agencies and researchers. With random forests, support vector machines, and convolutional neural networks, both traditional machine-learning methods and novel deep learning techniques have been implemented to evaluate their performance regarding the classification of backscatter data from two study sites located in the Sylt Outer Reef in the German Bight. Simple statistical values, textural features, and Weyl coefficients were calculated for different patch sizes as well as levels of quantization and then utilized in the machine-learning algorithms. It is found that large image patches of 32 px size and the combined use of different feature groups lead to the best classification performances. Further, the neural network and support vector machines generated visually more appealing sediment maps than random forests, despite scoring lower overall accuracy. Based on these findings, we recommend classifying side-scan sonar data with image patches of 32 px size and 6-bit quantization either directly in neural networks or with the combined use of multiple feature groups in support vector machines.

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.

Suspension-Feeding Benthic Species’ Physiological and Microbiome Response to Salmon Farming and Associated Environmental Changes

Caged salmon farming is increasingly undertaken in water bodies with strong hydrodynamics where hard and mixed substrate habitats are more prevalent. Yet, these structurally complex and heterogeneous habitats support diverse benthic communities including several cnidarians and sponges that remain poorly characterized. This study used a combination of respirometry measurements, gas chromatography and 16S rRNA metabarcoding to define the respiration rate, stable carbon (δ13C) and nitrogen isotopes (δ15N), fatty acid (FA) and microbial profiles, and assess the impact of salmon farming on four important epibenthic suspension-feeders along the western Norwegian coast: the sponges Craniella and Weberella, the soft coral Duva florida and the anemone Hormathia digitata. Our results showed striking differences in fatty acid profiles and host microbiome communities in terms of identity, functional capabilities and genetic properties across the suspension-feeders. We found evidence of increased mortality rate in specimens located near fish farm activities and of a species-specific effect on respiration rate, with D. florida showing increased activity under the farm. Effects of fish farming on the suspension feeders were also species-specific and particularly evidenced by functional microbial turnover and by alteration of overall FA profiles in the soft coral and sea anemone. In particular, D. florida showed reduced level of FAs close to the farm (0-350 m), with significant difference in composition along a distance gradient. Only H. digitata showed evidence of incorporation of organic material from the fish farm waste via fatty acids trophic markers (FATM) and stable isotope analysis. Overall, our study demonstrates that suspension feeders have taxon-specific sensitivity towards the effect of salmon farming, and identified several potential molecular indicators that could be used as surrogate of impact gradient upon further research and validation. It also provides a wealth of ecological and physiological information on some of the most common sessile epibenthic organisms within Arctic and sub-Arctic regions, enabling us to better understand their response and evaluate their resilience to environmental changes.

Current ecological study of hard substrate habitats from coastal waters of the Romanian Black Sea area

Current ecological study of hard substrate habitats from coastal waters of the Romanian Black Sea area

Mesophotic Depth Biogenic Accumulations (“Biogenic Mounds”) Offshore the Maltese Islands, Central Mediterranean Sea

The mesophotic domain is a poorly explored part of the oceans, notably in the Mediterranean Sea. Benthic communities in these depths are not well documented and as such are under higher risk from anthropogenic impacts. Hard substrate habitats in this depth window are not common and are a key ecotope. The Malta Plateau in the central Mediterranean, which is characterized by low sedimentation rates, offers a potentially unexplored domain for these features. Bathymetric and backscatter data offshore of the eastern coast of the island of Malta were used to identify > 1,700 small structures in mesophotic depths. These structures were verified to be biogenic mounds by dives. The mounds extend from several meters to tens of meters in diameter and occur in two main depth windows −40 to 83 meters below present sea level (mbpsl) and 83–120 mbpsl—each formed probably in a different stage during the last glacial cycle. The mounds are composed of interlocking bioconstruction by encrusting organisms and are colonized by sponges and various cold water corals (most of which are protected; e.g., Madrepora oculata). This unique and important habitat is currently under grave threat by human activity, most immediately by trawling and anchoring activity.

Recent shallow water foraminifera from the Selvagens Islands (Northeast Atlantic) – Assemblage composition and biogeographic significance

We report the first survey of shallow-water foraminifera from the Selvagens Islands, an isolated archipelago located in the subtropical Northeast Atlantic. In our samples, we identified 63 species of foraminifera belonging to 43 genera in the >63 μm fraction, including eight species of symbiont-bearing Larger Benthic Foraminifera (LBFs). The assemblages consist mostly of epiphytic species and species associated with coarse grain and hard substrate habitats. Analysis of dead assemblages revealed two main clusters that mostly correspond with the two island groups within the archipelago and relates to their environmental characteristics. Our results provide illustrated baseline information for this remote and relatively pristine habitat, wherein foraminifer community assemblages show a higher degree of similarity to those of the Canary Islands, Cape Verde, and Bermuda as compared to those of Mauritania, Northwest Africa. Hence, this study suggests that geographic proximity and oceanic barriers plays a lesser role than environmental parameters when determining foraminiferal biogeography. In addition, given that all LBF species reported in Selvagens are also present across the Atlantic Ocean, our results suggest that the previously defined ‘Caribbean’ LBF realm should be extended to the subtropical North Atlantic, taking into account local conditions, particularly minimal temperatures, controlling specific LBF assemblages.

Evolution and phylogeny of glass-sponge-associated zoantharians, with a description of two new genera and three new species

AbstractHexactinellid sponges are important members of deep-sea benthic ecosystems because they provide available hard substrate habitats for filter-feeding invertebrates. However, symbioses between hexactinellid sponges and their symbionts are poorly known. Zoantharians associated with hexactinellid sponges have been reported widely from deep-sea marine ecosystems, either on the bodies or stalks of hexactinellid sponges. Despite these records, there has been a lack of research on their diversity and phylogenetic relationships. In this study, 20 specimens associated with amphidiscophoran and hexasterophoran sponges were collected from the waters of Australia and Japan in the Pacific, and from Curaçao in the southern Caribbean, and these were examined in addition to museum specimens. Based on molecular phylogenetic analyses and morphological observations, we formally describe two new genera and three new species of Zoantharia and report several previously described species. The results suggest at least two independent origins for the symbioses between hexactinellid sponges and zoantharians. Our results demonstrate that the diversity of hexactinellid sponge-associated zoantharians is much higher than has been previously thought. The new taxa described in this work further reconfirm that the deep-sea harbours high levels of undescribed zoantharian diversity.

A Substrate-Independent Benthic Sampler (SIBS) for Hard and Mixed-Bottom Marine Habitats: A Proof-of-Concept Study

Sea cage fish farms are increasingly situated over hard and mixed substrate habitats for production and waste-dispersion reasons; yet in many cases, these installations are not being effectively managed with respect to benthic impacts due to the lack of a practical sampling method. This study presents the first set of results from a newly developed Substrate Independent Benthic Sampler (SIBS) device that captures the unconsolidated organic and inorganic matter that overlies almost all substrates. The contents of the samples were analyzed using extracted environmental DNA (eDNA) followed by metabarcoding of the bacterial 16S rRNA gene. SIBS microbial assemblages reliably changed with proximity to farm and concurred with visual assessments of impact. Moreover, the approach appeared to be very sensitive with respect to the enrichment gradient, being able to discern influences at distances of 500–1500 m from the impact source. Other spatial differences, due to region and farm, were small in comparison, and the effect of the underlying substrate type was minor. The samples contained sufficient previously described bacterial bioindicator taxa from enriched sediments, such that a meaningful biotic index could be calculated, thereby placing them on a well-established benthic enrichment spectrum with established environmental thresholds. SIBS-derived bacterial data provide a powerful new approach for mapping spatial boundaries of farm effects irrespective of substrate type and topography. More importantly, the tool should also permit quantitative assessment of benthic enrichment levels irrespective of substrate type from depths of at least 100 m. It therefore has the potential to solve the hard-bottom problem that has until now prohibited effective environmental monitoring at mixed and hard-bottom locations.

The role of habitat heterogeneity and canyon processes in structuring sediment macrofaunal communities associated with hard substrate habitats in Norfolk Canyon, USA

Topographic and hydrodynamic complexity in submarine canyons promotes steep gradients in food availability and geophysical parameters which affect ecological assemblages and beta diversity. While habitat heterogeneity in submarine canyons is known to support diverse and abundant megafaunal communities, due to difficulty in sampling little is known about infaunal communities adjacent to hard substrate habitats, their contribution to canyon assemblages, and overall deep-sea diversity. Sediments were collected in three distinct habitat types: within Norfolk Canyon (western Atlantic) adjacent to hard substrate habitats including canyon walls and large boulders, along the main axis of the canyon, and on the adjacent continental slope to quantify macrofaunal (>300 μm) density, diversity and community composition, and sediment geochemical parameters including grain size, organic content, and stable isotope composition. While macrofaunal densities were similar among habitats sampled at comparable depths, diversity was higher near the hard substrate environments. Discrete communities were present in each habitat type, while annelid functional composition was similar between hard substrate adjacent and canyon axis habitats. Although diversity and abundance of hard substrate adjacent sediment communities did not change with depth, communities were driven by sediments with low organic matter content, whereas canyon and slope community assemblages were best explained by depth and higher organic content. Beta diversity among hard substrate adjacent sediments and canyon axis communities was high, with 27% of canyon taxa and 10% of regional taxa only occurring in hard substrate habitats. Given the thousands of submarine canyons worldwide, our results highlight the overall importance of substrate habitat heterogeneity within canyons in supporting deep-sea benthic diversity and suggest that both within-canyon and regional diversity are underestimated.

Offshore Wind Farm Artificial Reefs Affect Ecosystem Structure and Functioning: A Synthesis

Offshore wind farms (OWFs) are proliferating globally. The submerged parts of their structures act as artificial reefs, providing new habitats and likely affecting fisheries resources. While acknowledging that the footprints of these structures may result in loss of habitat, usually soft sediment, we focus on how the artificial reefs established by OWFs affect ecosystem structure and functioning. Structurally, the ecological response begins with high diversity and biomass in the flora and fauna that gradually colonize the complex hard substrate habitat. The species may include nonindigenous ones that are extending their spatial distributions and/or strengthening populations, locally rare species (e.g., hard substrate-associated fish), and habitat-forming species that further increase habitat complexity. Functionally, the response begins with dominant suspension feeders that filter organic matter from the water column. Their fecal deposits alter the surrounding seafloor communities by locally increasing food availability, and higher trophic levels (fish, birds, marine mammals) also profit from locally increased food availability and/or shelter. The structural and functional effects extend in space and time, impacting species differently throughout their life cycles. Effects must be assessed at those larger spatiotemporal scales.

Macro-zoobenthic biodiversity of northern Adriatic hard substrates: Ecological insights from a bibliographic survey

A bibliographic survey of macro-zoobenthic species colonizing natural and artificial subtidal formations scattered off the Venetian coasts (North Adriatic Sea, Italy) was realized to update and organize the hard to find and fragmentary information on species lists and functional groups, with the aim to contribute to the ecological analysis of the hard substrate habitat. The results of the survey carried out at 39 sites revealed high species richness especially in the broader rocky outcrops, and a total number of species per site ranging from 13 to 142. As a whole, 399 species were recorded, mainly Mollusca, Crustacea, Annelida and Porifera. Among the recorded species, 36 were commercially valuable and 9 were included in protection lists. From a functional point of view, the category “suspension feeders” was the most numerous group. Low species evenness among the various taxa was generally observed, only few sites, specifically those more distant from the coast and/or included in protected areas, showing well-structured macro-zoobenthic communities. The analysis of taxonomic distinctness highlighted altered taxonomic structure at most sites, mainly due to changes in Mollusca assemblages, suggesting that anthropogenic stressors are likely affecting the surveyed hard substrates.

Hydroacoustic Mapping of Geogenic Hard Substrates: Challenges and Review of German Approaches

Subtidal hard substrate habitats are unique habitats in the marine environment. They provide crucial ecosystem services that are socially relevant, such as water clearance or as nursery space for fishes. With increasing marine usage and changing environmental conditions, pressure on reefs is increasing. All relevant directives and conventions around Europe include sublittoral hard substrate habitats in any manner. However, detailed specifications and specific advices about acquisition or delineation of these habitats are internationally rare although the demand for single object detection for e.g., ensuring safe navigation or to understand ecosystem functioning is increasing. To figure out the needs for area wide hard substrate mapping supported by automatic detection routines this paper reviews existing delineation rules and definitions relevant for hard substrate mapping. We focus on progress reached in German approval process resulting in first hydroacoustic mapping advices. In detail, we summarize present knowledge of hard substrate occurrence in the German North Sea and Baltic Sea, describes the development of hard substrate investigations and state of the art mapping techniques as well as automated analysis routines.

Environmental drivers of rhodolith beds and epiphytes community along the South Western Atlantic coast

Environmental conditions shape the occurrence and abundance of habitat-building organisms at global scales. Rhodolith beds structure important hard substrate habitats for a large number of marine benthic organisms. These organisms can benefit local biodiversity levels, but also compete with rhodoliths for essential resources. Therefore, understanding the factors shaping the distribution of rhodoliths and their associated communities along entire distributional ranges is of much relevance for conservational biology, particularly in the scope of future environmental changes. Here we predict suitable habitat areas and identify the main environmental drivers of rhodoliths’ variability and of associated epiphytes along a large-scale latitudinal gradient. Occurrence and abundance data were collected throughout the South-western Atlantic coast (SWA) and modelled against high resolution environmental predictors extracted from Bio-Oracle. The main drivers for rhodolith occurrence were light availability and temperature at the bottom of the ocean, while abundance was explained by nitrate, temperature and current velocity. Tropical regions showed the highest abundance of rhodoliths. No latitudinal pattern was detected in the variability of epiphytes abundance. However, significant differences were found between sampled sites regarding the composition of predominant taxa. The predictors influencing such differences were temperature and nitrate. The Tropical region is abundant in species with warm-water affinities, decreasing toward warm temperate region. The expressive occurrence of tropical species not referred before for warm temperate beds indicate a plausible tropicalization event.

Low genetic connectivity in a fouling amphipod among man-made structures in the southern North Sea

Offshore environments are increasingly invaded by man-made structures that form hard-substrate habitats for many marine species. Examples include oil and gas platforms, wind turbines and shipwrecks. One of the hypothesised effects is an increased genetic connectivity among natural populations due to new populations growing on man-made structures that may act as stepping stones. However, few data are available on genetic connectivity among organisms inhabiting artificial offshore structures. Here, we present a study on the common fouling amphipod Jassa herdmani from offshore structures in the southern North Sea. Partial mitochondrial DNA sequences (cytochrome-c-oxidase 1, N = 514) were obtained from artificial structures at 17 locations in the southern North Sea, including 13 shipwrecks, 2 wind turbines and 2 platforms. Samples from these locations were significantly differentiated, meaning that strong population structure exists for this species in the area. Levels of intraspecific variation were consistent with stable population sizes. No evidence was found for isolation by distance. Using coalescent simulations, the oldest population subdivision events were estimated to date back to the time the study area was flooded following the Last Glacial Maximum. We therefore tentatively conclude that J. herdmani may have colonised man-made structures from previously existing populations on the sea floor, and that the increase in offshore installations has not led to an overall increase in genetic connectivity for this species.

Epibenthic assemblages of hard-substrate habitats in the German Bight (south-eastern North Sea) described using drift videos

Globally, marine subtidal hard-substrate habitats provide important ecological functions to a variety of sessile and mobile taxa. The southern North Sea is a marine environment with relatively low amounts of hard-substrate areas. Surveys with high taxonomic resolution on the associated hard-substrate epifauna, e.g. by diving observations, are challenging in this region because of strong tidal currents and considerable water depths. Therefore, the characteristics and functions of hard-substrate habitats in the North Sea remain poorly understood.The goal of this study was to extend the knowledge on sessile taxa assemblages in four hard-substrate areas within the German Bight (SE North Sea) using underwater video observations as a straight-forward and visual method to obtain and process detailed data on seafloor characteristics. Still images from drift videos were analyzed to determine the epifaunal assemblages attached to the stones on a presence/absence scale. A subsample of cobbles, boulders and large boulders was further investigated to derive quantitative data on the colonized area for each taxon and stone size. The four areas showed significant differences in sessile taxa richness, with higher numbers of sessile taxa in the deeper far-shore areas than in the shallower nearshore areas, despite a generally high similarity in the sessile animal taxa spectrum. Within the areas, cobbles were mainly dominated by short-lived taxa (<5 years) like Spirobranchus triqueter and ascidians, while boulder-sized stones were largely colonized by long-lived taxa (>5 years) like Metridium dianthus, Flustra foliacea and Alcyonium digitatum. Most of the stones were typically covered by epifauna by more than 50%. The differences in the sessile taxa spectrum between the areas were interpreted as a consequence of reduced abiotic stress and higher habitat complexity in the far-shore areas. The dominance of short-lived taxa on cobbles, in comparison to long-lived taxa on boulder-sized stones, possibly reflects the frequency of disturbance and the competitiveness of these taxa.

Hard-substrate habitats in the German Bight (South-Eastern North Sea) observed using drift videos

Comprehensive empirical data to inform benthic species distribution models for marine hard-substrate-dominated environments, are pivotal. However, such data are difficult to obtain. These data are crucial to the definition and demarcation of protected areas and for assessment of the ecological status and function of hard-substrate habitats.In this study, underwater video-observations of hard-substrate habitats within four target areas in the sand-dominated German Bight (SE North Sea) were investigated to obtain comprehensive information on hard-substrate distribution patterns, on the amount and sizes of stones and on the presence of sessile organisms.Based on three size classes (cobbles, boulders, large boulders) three hard-substrate distribution classes were identified: (1) widely scattered stones, (2) accumulations of stones and (3) dense stone fields. The ratios between cobbles, boulders and large boulders differed significantly between the investigated areas. Boulders and large boulders were largely colonized by sessile organisms, whereas cobbles in coastal areas were least frequently colonized. Physical disturbances of epibenthos resulting from abrasion and coverage by mobile sediments are discussed as a possible explanation for the proportional differences in the colonization of stones. Hard substrates in shallower, coastal areas appeared to be strongly influenced by sand abrasion because of higher current velocities and storm-induced waves. In deeper areas, located further offshore, disturbances caused by migrating sandy ripples mobilized by storm-events seemed to be more relevant.Habitat modelling of hard substrates and resultant ecological studies require sound information on the probability of epifaunal colonization for different substrate sizes, hard-substrate distribution patterns combined with hydrodynamic and physicochemical properties of the marine environment to produce valid results. We used a structured approach for the video-based analysis of hard-substrate habitats and present estimates of the colonization probability of differently-sized stones. Our study shows that the analysis of drift videos provides basic data at a suitable resolution to contribute to the monitoring and modelling of marine ecological processes.

Understanding Mn-nodule distribution and evaluation of related deep-sea mining impacts using AUV-based hydroacoustic and optical data

Abstract. In this study, ship- and autonomous underwater vehicle (AUV)-based multibeam data from the German ferromanganese-nodule (Mn-nodule) license area in the Clarion–Clipperton Zone (CCZ; eastern Pacific) are linked to ground-truth data from optical imaging. Photographs obtained by an AUV enable semi-quantitative assessments of nodule coverage at a spatial resolution in the range of meters. Together with high-resolution AUV bathymetry, this revealed a correlation of small-scale terrain variations (&lt; 5 m horizontally, &lt; 1 m vertically) with nodule coverage. In the presented data set, increased nodule coverage could be correlated with slopes &gt; 1.8∘ and concave terrain. On a more regional scale, factors such as the geological setting (existence of horst and graben structures, sediment thickness, outcropping basement) and influence of bottom currents seem to play an essential role for the spatial variation of nodule coverage and the related hard substrate habitat. AUV imagery was also successfully employed to map the distribution of resettled sediment following a disturbance and sediment cloud generation during a sampling deployment of an epibenthic sledge. Data from before and after the “disturbance” allow a direct assessment of the impact. Automated image processing analyzed the nodule coverage at the seafloor, revealing nodule blanketing by resettling of suspended sediment within 16 h after the disturbance. The visually detectable impact was spatially limited to a maximum of 100 m distance from the disturbance track, downstream of the bottom water current. A correlation with high-resolution AUV bathymetry reveals that the blanketing pattern varies in extent by tens of meters, strictly following the bathymetry, even in areas of only slightly undulating seafloor (&lt;1 m vertical change). These results highlight the importance of detailed terrain knowledge when engaging in resource assessment studies for nodule abundance estimates and defining mineable areas. At the same time, it shows the importance of high-resolution mapping for detailed benthic habitat studies that show a heterogeneity at scales of 10 to 100 m. Terrain knowledge is also needed to determine the scale of the impact by seafloor sediment blanketing during mining operations.

Distribution patterns of the introduced encrusting bryozoan Conopeum chesapeakensis (Osburn 1944; Banta et al. 1995) in an estuarine environment in upper San Francisco Bay

The factors shaping the distributions of nonindigenous species (NIS) are of particular interest for understanding their success and potential impacts within their invaded ranges. In the San Francisco Bay estuary, the encrusting bryozoan Conopeum chesapeakensis (Osburn, 1944; Banta et al., 1995) occurs in peak abundances in lower salinity hard substrate habitats, with lower abundances upstream and downstream; however, little is known about the factors that control its distribution. To investigate several hypotheses about what allows this broadly tolerant invader to be numerically dominant in this region, a field transplant experiment was conducted across three sites in upper San Francisco Bay estuary. Colonies settled on PVC plates in the peak abundance zone and transplanted to upstream and downstream treatment sites, or returned to the settlement site, which served as a control. Salinity, temperature, chlorophyll a levels, and the abundance of interspecific competitors varied at each site and were measured throughout the experiment. Mixed effects models incorporating these measurements compared net growth rate and zooid size observed across treatment sites. Colonies transplanted upstream experienced high barnacle settlement, a potential competitive threat, and decreased salinity, and exhibited an average net growth rate of 6.60 zooids/day. Colonies at the control site faced almost no potential interspecific competition, intermediate salinity, and had an average net growth rate of 4.96 zooids/day. At the downstream site, colonies grew an average of 4.62 zooids/day and experienced high potential competition from serpulid polychaete settlement and the highest salinity of all sites. The best-fit models indicated that overall abundance of potential competitors, especially the serpulid Ficopomatus enigmaticus (Fauvel, 1923), was negatively correlated with the net growth rate of C. chesapeakensis colonies. Zooid size was also negatively correlated with F. enigmaticus abundance and temperature, though the relationship weakened over time. Many colonies at both the upstream and downstream transplant sites experienced fast initial growth following transplantation, but then experienced partial colony loss corresponding with an increase in the abundance of potential competitors. In contrast, colonies at the control site showed slow but continuous growth throughout the study in absence of interspecific competitors. These results suggest that the numerical dominance of C. chesapeakensis in upper-estuary habitat may be partly explained by a lack of potential interspecific competitors. As San Francisco Bay and other estuaries face high invasion pressure in brackish upper-estuarine regions, understanding which factors influence the distribution of NIS can help predict impacts to resident communities.