Hard-bottom Sites Research Articles

Sea water temperature and light intensity at high-Arctic subtidal shallows – 16 years perspective

Manifestations of climate change in the Arctic include an increase in water temperatures and massive loss of sea ice enabling more light penetration. Yet to understand tempo and scale of these parameters change over time, constant monitoring is needed. We present 16-yr long-term datasets of sea water temperature and relative light intensity at two depth strata (8 and 14 ± 1 m) of two hard-bottom sites in southern Isfjorden proper (Spitsbergen, 78°N). The high temporal resolution of the datasets (every 30 min, between 2006–2022) makes them suitable for studying changes at a local scale, correlating environmental variability with observed processes in benthic assemblages, and serving as ground-truth for comparison with, for example, remotely sensed or mooring data. These datasets serve as baseline for long-term investigations in the shallows of a high-Arctic fjord undergoing severe environmental changes.

An exploration of Lake Ontario’s offshore epibenthic cladoceran community

Pelagic cladocerans are relatively well studied within the Great Lakes. However, little is known about cladocerans that more closely interact with benthic substrate. In 2018, 26 sites in Lake Ontario were sampled with benthic grabs to collect meiofauna. The stations ranged in depth from 0.1 to 184 m with sand, silt or hard bottom substrate. Epibenthic cladocerans were recovered from all 12 sites shallower than 20 m, five of eight sites between 20 and 40 m and one (63 m deep) of six sites deeper than 40 m. The density of epibenthic cladocerans were at most 12% of the density of harpacticoid copepods (shallow hard bottom sites). The Lake Ontario epibenthic cladoceran community was represented by 16 species from 8 different genera although one of these species, Illyocryptus cf. sordidus, is likely represented by two cryptic species. The most widespread species was identified as Alona sibirica (Sinev, 2020), a newly described species from Russia formerly of the Alona cf. affinis (Leydig, 1860) species complex. The Great Lakes specimens were 98.3% similar genetically to sequences assigned to A. cf. sibirica. Among substrates, sand had the highest densities (1662 m2 in 0–20 m) and species richness (10) of epibenthic Cladocera. After cladocerans were identified, the cytochrome oxidase I (COI) gene of 37 specimens were successfully sequenced through the Barcode of Life (BOLD). Six specimens were assigned to three previously existing Barcode Identification Numbers (BINs), the remaining 31 specimens were assigned to eight new BINs.

Response of Fish and Invertebrate Larvae to Backreef Sounds at Varying Distances: Implications for Habitat Restoration

Underwater sound is used by many marine larvae to orient to coastal habitats including backreef, sponge-dominated hardbottom habitat in the Florida Keys (FL, United States)—a particularly “noisy” coastal habitat. However, the distance over which acoustic cues are attractive to settlement-stage larvae is generally unknown. We examined this phenomenon in a region of the Florida Keys where mass sponge die-offs have diminished both underwater soundscapes and larval settlement. The absence of pronounced hardbottom-associated sound over such a large area allowed us to experimentally test in situ the response of fish and invertebrate larvae to broadcasted sounds at different distances from their source. We first measured the signal-to-noise ratio of healthy hardbottom habitat soundscapes broadcast from an underwater speaker at seven distances to determine the maximum range of the signal. Based on those results, larval collectors were then deployed at 10, 100, 500, and 1,000 m from speakers broadcasting sounds recorded at either degraded or healthy hardbottom sites for five consecutive nights during each of three new and full moon periods in summer/fall 2019. Larval settlement onto those collectors was affected by lunar phase and soundscape type, but varied among species. In most cases, the effect was small and not likely to be ecologically significant. The absence of a strong larval settlement response to a sound cue lies in contrast to results from other studies. We suspect that the small (<500 m) radius of the broadcasted soundscapes may have limited the magnitude of the larval response to locally available larvae whose abundance may have been low because the experiment was conducted within a large, relatively quiet seascape. If true, it is possible that planktonic larvae may require a series of acoustic “sign-posts,” perhaps in combination with other cues (e.g., chemical), to successfully orient to distant nursery habitats. Although habitat restoration efforts may be able to restore healthy soundscapes, the typically small size and number of restoration sites may limit the range of the acoustic cue and thus larval attraction to restored habitats.

Examining the ecological function of structure: species assemblages at casitas and coral heads in the Lower Florida Keys

Casitas are artificial structures used widely in the Caribbean to facilitate the harvest of spiny lobsters by divers. Casitas are often used by many non-target species, and therefore have the potential to increase local species diversity and mitigate the loss of natural habitats. Their ecological impact, however, is largely unknown. We examined the distribution of benthic habitat and the abundance and diversity of fish and motile invertebrates at casitas (artificial structures), coral heads (natural habitat with vertical structure >15 cm high), and low-relief hardbottom (natural habitat with vertical structure <15 cm high). Casitas and coral heads exhibited similar habitat features and fish and invertebrate species assemblages. Low-relief hardbottom sites had significantly less abundant and diverse fish and invertebrate species assemblages than at either structure. The only significant difference observed between casitas and coral heads was the abundance of lobsters. Lobsters were found in significantly greater numbers at casitas than at either coral heads or on low-relief hardbottom. Determining how an increased concentration of lobsters at casitas affects the local ecology is complex, but our data suggest that the presence of lobsters in such large aggregations at casitas did not alter the surrounding flora and fauna. In regions with limited shelter, species composition and ecological function at casitas are similar to those at coral heads. This study shows that casitas may be efficient dual-purpose devices; that is, they function not only as lobster fishing gear, but also as the ecological equivalent of coral heads in shelter-limited habitat.

Deep Lake Explorer: A web application for crowdsourcing the classification of benthic underwater video from the Laurentian Great Lakes

Underwater video is increasingly used to study aspects of the Great Lakes benthos including the abundance of round goby and dreissenid mussels. The introduction of these species has resulted in major ecological shifts in the Great Lakes, but the abundance and impacts of these species have heretofore been underassessed due to limitations of monitoring methods. Underwater video (UVID) can “sample” hard bottom sites where grab samplers cannot. Efficient use of UVID data requires affordable and accurate classification and analysis tools. Deep Lake Explorer (DLE) is a web application developed to support crowdsourced classification of UVID collected in the Great Lakes. Volunteers (i.e., the crowd) used DLE to classify 199 videos collected in the Niagara River, Lake Huron, and Lake Ontario for the presence of round gobies, dreissenid mussels, or aquatic vegetation, and for dominant substrate type. We compared DLE classification results to expert classification of the same videos to evaluate accuracy. DLE had the lowest agreement with expert classification for hard substrate (77%), and highest agreement for vegetation presence (90%), with intermediate agreement for round goby and mussel presence (89% and 79%, respectively). Video quality in the application, video processing, abundance of species of interest, volunteer experience, and task complexity may have affected accuracy. We provide recommendations for future crowdsourcing projects like DLE, which can increase timeliness and decrease costs for classification but may come with tradeoffs in accuracy and completeness.

Spatial Patterns in Relative Abundance and Habitat Use of Adult Gray Snapper off the Southeastern Coast of the United States

AbstractGray Snapper Lutjanus griseus is an economically and ecologically important species in the estuarine and coastal environments of the southeastern United States. Previous research has focused primarily on juvenile Gray Snapper due to their accessible inshore distribution and ecological importance, while adults, which often occur offshore and are the main focus of fishing pressure, remain poorly understood. Seven years of baited underwater video data (2011–2017; N = 8,379 videos; ~14,000 h of video) were collected along the continental shelf between North Carolina and Florida (~100,000 km2) to better understand the ways in which the relative abundance of Gray Snapper varied by space, time, habitats, and environmental conditions. Adult Gray Snapper were observed on 6.9% of the videos overall, but they were much more commonly observed in Florida (16.9% of the videos) compared with the states that are north of Florida (1.4% of the videos). We used delta‐generalized additive models to determine that adult Gray Snapper primarily occurred in high‐relief hardbottom sites south of St. Augustine, Florida, in warm water less than 50 m deep, after accounting for imperfect detection on video. Temporal variability was relatively minor despite relatively high precision (the mean annual coefficient of variation = 24%). Fifteen large aggregations of Gray Snapper (i.e., >20 individuals counted on a single frame) were observed on video, but it is unclear whether these aggregations indicated potential spawning aggregation sites. This work provides greater insight into the ecology of Gray Snapper during their important coastal‐ocean adult life stage, which will improve their management and conservation.

Biodegradation of some aquaculture chemotherapeutants weathered in flocculent samples collected at hard-bottom sites in Newfoundland (Canada)

The impact of aquaculture chemotherapeutants (antibiotics, and anti-lice parasiticides) on non-target organisms is dependent on their dissipation and degradation in marine sediments. Abiotic and biotic weathering may change their composition and toxicity, and influence their fate. In this study, biodegradation of a few aquaculture chemotherapeutants was investigated over a 96 days period at 4 °C in a marine flocculent sample collected at an active site. The detected antibiotics ([sulfadiazine, amoxicillin], and one parasiticide [deltamethrin]) would have been bound to flocculent organic matter for periods exceeding a year while another anti-lice compound (emamectin benzoate) for approximately 3 months. Our results show that organic matter degradation was not accompanied by a decrease in concentrations of the antibiotics and parasiticides. This preliminary study suggests longer half-lives than documented in literature for two antibiotics and highlights the difficulty in using half-lives of chemicals determined in laboratory settings without taking into account weathering.

Occurrence and habitat use of European eel (Anguilla anguilla) in running waters: lessons for improved monitoring, habitat restoration and stocking

To improve the management of the European eel (Anguilla anguilla) in freshwater, it is essential to define important lotic habitats. Electrofishing data from 289 wadeable, hard-bottom sites in 69 Swedish coastal rivers and streams, originally surveyed for salmonid monitoring, were used to evaluate the effects of sampling- and habitat-related factors on eel occurrence. Probability of eel occurrence, as influenced by sampling procedure (sampled area, number of consecutive runs and ambient water temperature) and habitat characteristics (size of catchment, dominating bottom substrate, shade, water velocity, mean depth), was evaluated for small (total length ≤ 150 mm) and large (> 150 mm) yellow eels. Data were analysed in a mixed presence/absence generalized linear model with dispersal (distance to mouth from sampled site), habitat and sampling-related variables as covariates. The two models explained variation in occurrence to 81.5% for small eel and 76.2% for large eel. Probability of eel occurrence decreased with distance from the river mouth, and increased with sampled area, number of runs, water temperature, coarser substrate and size of river. We suggest that future eel habitat restoration should focus on lower reaches of larger rivers with suitable coarse bottom habitats. Stocking of young eel should be carried out in comparable accessible habitats in the upper reaches where eel densities are low. The results also strongly indicate that eel may be sampled together with young salmonids with DC electrofishing in wadeable habitats.

Linking the presence of visual indicators of aquaculture deposition to changes in epibenthic richness at finfish sites installed over hard bottom substrates.

Effects of finfish aquaculture on benthic communities at hard bottom sites can be assessed using visual indicators of organic enrichment, namely bacterial mats, opportunistic polychaetes, and/or barren substrates (i.e., with no visible epifauna). Under some regulatory frameworks, the presence of visual indicators beyond a certain threshold proportion (e.g., > 70%) of mandatory sampling stations at a site signals an unacceptable degree of benthic organic loading. However, relationships between visual indicator presence and other biological characters such as epibenthic taxon richness are unclear, and should be examined to advise on the validity of existing legislative frameworks. Here, we used video data collected for regulatory purposes before and after aquaculture production to document (1) change in epibenthic taxon richness (TR) and its discriminatory power in determining aquaculture impact and (2) the association between TR change and the presence of visual indicators. Despite low richness values overall, our results show a significant post-production decrease in TR in the near-cage area, which was predicted to be affected by aquaculture. Decreases in TR were associated with visual indicator presence, validating the use of a suite of visual indicators to detect organic deposition. Importantly, visual indicators should be considered together in the context of regulation, given that relationships between TR and indicators were not linear when the latter are considered individually.

Spatial and temporal changes in epibenthic communities at deep, hard bottom aquaculture sites in Newfoundland

This study explores spatio-temporal changes in epibenthic communities due to salmon aquaculture at deep (>30m), hard-bottom dominated sites in Newfoundland (Canada). Using a drop-camera approach, we followed changes with production at two aquaculture sites, as well as potential recovery processes at two fallow sites, comparing their epibenthic composition with nearby non-production sites. Multivariate analysis revealed that aquaculture production led to rapid changes in epibenthic communities, as evidenced primarily by the presence of two visual indicators: bacterial mats and opportunistic polychaetes. Due to low taxon richness and abundances, as well as variability among sites, no clear intermediate successional stages were apparent. Beggiatoa-like mats and opportunistic polychaetes appeared rapidly once aquaculture production began; these indicators did not increase in spatial extent during the survey and were typically observed up to 100m, and occasionally up to 160m from cages. After 15 months of fallowing, Beggiatoa-like mats and opportunistic polychaetes remained present at sites and were occasionally accompanied by few other taxa, suggesting incomplete recovery.

Epibenthic community assessments indicate high spatial and temporal variability among continental shelf hard bottom sites in a marine transition zone

We studied 19 hard bottom sites within Onslow Bay, North Carolina during 2007–2010 using photoquadrat analyses to investigate the role of temperature, depth, and fish community variables in the structure of the epibenthic (macroalgae and sessile invertebrates) community. Although significant variation in community structure was found both by site and year, depth was the most important factor in structuring these epibenthic communities with significant differences found among five depth categories: 18–20 m; 24–29.2 m; 31 m; 32.5–37 m; 38.5–42 m. The largest community difference was found between depths ⩽31 m and ⩾32.5 m, resulting in a shift from macroalgae dominance at the shallower sites to shared macroalgae and sessile invertebrate dominance at deeper sites. Depth is a complicated variable as it relates to structuring the epibenthic community in this region because it reflects varying winter bottom water temperatures, light levels, and periodicity of nutrient influxes. The location of North Carolina marine habitats at the transition from cold-temperate to warm-temperate/tropical zones, and the spatial compression of this transition zone along inshore to offshore transects make this an ideal area for tracking climate change related shifts in marine communities. However, a better understanding of the relationship among variables such as depth, light, temperature and nutrients and the epibenthic community, as well as seasonal and short annual community variation, is needed before climate related shifts can be determined.

Stress resistance in two colonial ascidians from the Irish Sea: The recent invader Didemnum vexillum is more tolerant to low salinity than the cosmopolitan Diplosoma listerianum

It is widely believed that non-native species possess a high tolerance toward abiotic stress that allows them to survive transport and to colonize new habitats. However, to date empirical support for this concept is scarce. We therefore tested whether Didemnum vexillum, which recently invaded the Irish Sea, is more tolerant to fluctuations in salinity than the closely related Diplosoma listerianum, a cosmopolitan species of wide ranging distribution. The two fouling organisms exhibit similar habitat requirements and co-occur in many hard-bottom sites along the coasts of Ireland and Wales. We collected adult specimen of both taxa in Holyhead Marina, Wales, UK, and exposed them to different levels of reduced salinity ranging from 10 to 27 in short-term laboratory experiments. In separate trials, we mimicked chronic (2 weeks) as well as episodic (2 h) low-salinity stress to account for the natural temporal variability in osmotic fluctuations. We compared the ascidians' performances in the face of stress and found that the invasive D. vexillum showed higher growth and survival under low salinities than the cosmopolitan D. listerianum. This indicates that D. vexillum is generally better in adapting to fluctuating ion concentrations. This competitive advantage may facilitate the establishment and spread of D. vexillum, especially if osmotic conditions in coastal habitats will be altered by future climate-change induced shifts in local precipitation regimes.

Habitat-based intraguild predation by Caribbean reef octopus Octopus briareus on juvenile Caribbean spiny lobster Panulirus argus

Intraguild predation occurs when species simultaneously compete for resources and interact as predator and prey, which describes the interaction between juvenile Caribbean spiny lob- ster Panulirus argus and Caribbean reef octopus Octopus briareus in the Florida Keys, USA. Octo- puses are notorious predators of decapod crustaceans, and their use of crevice shelters suggests that they may also compete for shelter with their lobster prey. Lobsters use mainly chemical cues to detect and avoid octopus, so we hypothesized that the negative association between these species may be as much the consequence of avoidance of a superior competitor as it is of direct predation. Surveys of lobsters and octopuses occupying artificial shelters at 19 hard-bottom sites confirmed that lobsters do not share dens with octopuses, and also show that lobster and octopus abundances are negatively correlated. Tethering experiments on a subset of those sites revealed that predation on lobster was indeed higher on sites with more octopuses. Results from mesocosm studies indicated that although juvenile lobsters do not attain a size refuge from octopus predation, the presence of alternative prey and lobster conspecifics reduces the risk of predation on lobster by octopus. Mesocosm experiments also showed that octopuses were the competitive dominants when shelter was limited. Thus, the neg- ative association between lobster and octopus in the field appears to be driven by both predation and avoidance of octopus-rich sites by lobsters, rather than competition per se. However, crevice shelters suitable for juvenile lobster are limited in many hard-bottom areas in the Florida Keys, so areas where octopuses are abundant may further limit the local accessibility of shelters for juvenile spiny lobsters even if the direct effects of predation by octopus are minimal.

Spatial characterization of a foraging area for immature hawksbill turtles (Eretmochelys imbricata) in Yucatan, Mexico

Abstract The submarine habitats in the Rio Lagartos Sea Turtle Sanctuary, Mexico, are an important feeding and development area for juvenile Hawksbill turtles (Eretmochelys imbricata). The characterization of these critical habitats is an important and urgent issue to attend for the conservation of this species in Mexico. The objective of this study is to identify, locate and describe the marine benthic habitats in this area, and explain the distribution of the juvenile Hawksbill turtles in the study area. We used submarine techniques such as videotransects and spot checks to characterize the bottom types in the area, and geostatistic techniques to elaborate thematic maps of the different benthic components, integrating all of them into a GIS. We obtained the bottom type map with eight submarine habitats at different depth ranges. We found juvenile Hawksbill turtles distributed mainly on hard bottom sites covered by octocorals, such as Pseudopterogorgia, and sponges of the genera Chondrilla and Spheciospongia. We estimated habitat ranges for the captured turtles and the bottom types occupied by them, then we calculated the distribution densities of juvenile Hawksbill turtles by bottom type. We concluded that the combination of spatial techniques and multivariate statistics is efficient for mapping the marine bottom types in the area, and recommend the generation of specific conservation strategies to protect this area because of the natural marine resources and process ocurring in it.

Stable isotope analysis of production and trophic relationships in a tropical marine hard-bottom community

Seagrasses produce much of the organic carbon in the shallow waters of the Caribbean and it has long been assumed that a substantial portion of this carbon is exported to nearby habitats, contributing substantially to their food webs. In the shallow coastal waters of the Florida Keys (USA), seagrass intersperses with hard-bottom habitat where bushy, red macroalgae are the most prominent primary producers. However, the relative importance of seagrass-derived carbon versus autochthonous algal production or phytoplankton in supporting higher trophic levels within hard-bottom communities has never been investigated. We compared the carbon and nitrogen isotopic values of potential primary producers and representative higher trophic level taxa from hard-bottom sites on the bay-side and ocean-side of the Florida Keys. We also included in our study a set of bay-side sites that experienced significant ecological disturbances over the past decade (e.g., cyanobacteria blooms, seagrass die-off, and sponge die-offs) that may have altered trophic relationships in those regions. We did not detect any differences among regions in the trophic status of hard-bottom taxa that might be associated with ecosystem disturbance. However, our results suggest that autochthonous production of algal detritus is an important source of secondary production in these hard-bottom communities, with seagrass and phytoplankton contributing smaller fractions.

Utilizing gamma isotope tracers to determine sediment source at reef sites near the Charleston Ocean Dredged Material Disposal Site

The Charleston, South Carolina Ocean Dredged Material Disposal Site (ODMDS) has been heavily utilized as a disposal site for dredged material resulting from maintenance and channel deepening in the Charleston Harbor. Continuous monitoring by the South Carolina Department of Natural Resources at the ODMDS has indicated the presence of fine-grained sediment within the monitoring zones. However, since the Charleston Harbor is formed by the conjunction of three rivers, it has been suggested that some of the fine-grained sediment surrounding the ODMDS could be due to river transport rather than solely by disposal activities. In order to trace the outflow of sediment from the harbor, natural and man-made isotopes were utilized. 7Be (natural cosmogenic isotope) and 137Cs (man-made isotope) are often associated with estuarine sediments. Both isotopes were used as tracers in an attempt to determine the extent of density driven sediment flow from the Charleston Harbor. 7Be was detected in many of the offshore sampling stations indicating a direct correlation to the harbor. 137Cs was only found in one sediment trap sample offshore, but none the less indicated some transport from the harbor. Further study for utilizing isotopic tracers in determining offshore sediment transport is still being conducted at the disposal site. It is anticipated that further 7Be and 137Cs isotopic monitoring offshore Charleston will aid in determining the role that tidal and density driven sediments play in the sediment budgets at the hard bottom reef sites.

Impacts of lost fishing gear on coral reef sessile invertebrates in the Florida Keys National Marine Sanctuary

The Florida Keys coral reef ecosystem supports multimillion-dollar commercial and recreational fisheries. The ecological effects caused by fishing gear that is lost when cut or broken after snagging on the bottom is a growing concern to managers and scientists. Few data exist, however, to assess the impacts of lost fishing gear to benthic organisms and habitat structure. In this study, 63 offshore coral reef and hard-bottom sites were surveyed during 2001 to quantify the impacts of lost fishing gear to coral reef sessile invertebrates. Lost hook-and-line fishing gear accounted for 87% of all debris (N=298 incidences) encountered and was responsible for 84% of the 321 documented impacts to sponges and benthic cnidarians, predominantly consisting of tissue abrasion causing partial individual or colony mortality. Branching gorgonians (Octocorallia) were the most frequently affected (56%), followed by milleporid hydrocorals (19%) and sponges (13%). Factors affecting the impacts of lost fishing gear include sessile invertebrate density, the density of lost fishing gear, and gear length. While lost hook-and-line fishing gear is ubiquitous in the Florida Keys, less than 0.2% of the available milleporid hydrocorals, stony corals, and gorgonians in the habitats studied are adversely affected in terms of colony abrasions and partial mortality.

The influence of bottom type and shelf position on biodiversity of tropical fish inside a recently enlarged marine reserve

Abstract A necessary component of implementing a successful marine reserve is the quantification of the biological resources that fall under its protection. Without such an initial assessment, the future effects of the reserve on the local habitat and biotic community cannot be quantified and will remain the subject of debate. This study provides such a baseline assessment of fish diversity and habitat types within a recently enlarged marine reserve. Buck Island Reef National Monument, US Virgin Islands, was recently enlarged from approximately 4 km2 to over 76 km2. Areas of sand, seagrass, and hard‐bottom under protection were increased from 0.29 km2, 0.47 km2, and 1.96 km2 to 2.70 km2, 2.89 km2, and 18.30 km2 respectively when the Monument was expanded. A 53 km2 area of pelagic/deep‐water habitat with unknown bottom type is now also protected by the Monument. Visual counts of fish within 25×4 m2 transects conducted during the day were used to assess fish community structure and habitat utilization patterns. Species richness, diversity, assemblage structure, and fish density were evaluated and compared among sand, seagrass, and hard‐bottom habitats. Hard‐bottom sites had over twice the mean species richness and diversity as sand and seagrass sites, and several times greater mean fish density. Quantification of the fish community in pelagic and deep‐water habitats within the reserve is recommended to provide a more comprehensive assessment of the offshore areas of the reserve. Fish numbers, size, and diversity outside the reserve boundaries must also be evaluated to allow quantification of the effects of the marine reserve on the adjacent fish communities. Copyright © 2004 John Wiley & Sons, Ltd.

Multi-scale Data Used to Analyze the Spatial Distribution of French Grunts, Haemulon Flavolineatum, Relative to Hard and Soft Bottom in a Benthic Landscape

We evaluated the day-time distribution of juvenile and adult French grunts, Haemulon flavolineatum, relative to the spatial configuration of hard and soft bottom areas in a benthic landscape. Probability of juvenile presence on hard bottom sites was inversely correlated with distance to soft bottom. Adults presence at hard bottom sites showed no significant relationship with distance to soft bottom. A significant and positive relationship was found between presence of juveniles on hard bottom sites and area of soft bottom within 100 m, but no significant relationship was found for area of soft bottom within 500 m. Adults exhibited no significant relationship with area of soft bottom for either distance tested. These distributions are suspected to be the result of the combined influence of larval settlement patterns and foraging behaviors associated with hard and soft bottom. This study indicates that data collected at very fine scales can be analyzed in the context of the broad-scale mosaic of habitats in the benthic landscape to predict patterns of fish distribution. Such spatially explicit conclusions are not possible through analysis of fine-scale or broad-scale data alone.

Biological-geological interactions: storm effects on macroalgal communities mediated by sediment characteristics and distribution

The North Carolina mid-continental shelf is characterized by a complex sequence of rock suostrates, or hardbottoms, which support rich communities of invertebrates and benthic macroalgae. Three hardbottom sites in southern Onslow Bay have been studied over 3 years, including studies of sedimentology and dynamics of the surficial sand bodies and the benthic community structure. Between 1991 and 1993 at two of the sites, there were large differences in sediment characteristics and depth at several of the zones sampled. These differences coincided with considerable variability in the abundance and species composition of the macroalgal community. In sampling zones where there was consistently low sediment coverage, algal biomass was uniformly high. There was only sparse, if any, macroalgal growth where gravelly sand or fine sand covered the hard substrate. Based on hindcasting of bottom currents generated by the March 1993 ‘storm of the century’, it appears that this episodic event cleared fine sand from upper flat hardbottoms, resulting in considerable colonization and growth of macroalgae between the summers of 1992 and 1993. Macroalgae continued to be excluded from areas of gravelly sand, even after storm passage. Furthermore, the storm did not have any significant effects on scarps and ramp areas where sediment cover was already low. Therefore, distribution and characteristics of sediments influenced where the storm was to have biological consequences. Based on the wide areal extent of the upper flat hardbottoms in Onslow Bay, the newly formed macroalgal meadows represent a significant increase in benthic biomass and habitat complexity on a regional scale with potentially important ecological consequences.