The influence of ice scour on benthic communities at three contrasting sites at Adelaide Island,Antarctica

Abstract Ice scouring is a key structuring force acting on high latitude shallow benthic communities. Despite its importance, detailed studies of scoured communities are still rare. Here we report the ecological effects of 12 iceberg impacts, across three contrasting study sites, at Adelaide Island, West Antarctic Peninsula. Grounded icebergs were marked with GPS and the newly formed scours (at 10–17 m depth) were sampled within 20 days of formation. Comparisons between scoured and adjacent unscoured assemblages were made using measures of abundance, biomass, taxon richness and the relative abundance of secondary consumers. Ice scouring was catastrophic at all sites, despite differences in substratum type, exposure and background community. Compared with undisturbed areas, scour assemblages were 95% lower in mean macrofaunal abundance and 75.9% lower in species richness. There was no general trend across all sites of ice scouring selecting for secondary consumers. The echinoid Sterechinus neumayeri and bivalve mollusc Mysella charcoti were highly abundant in undisturbed areas and were the biggest contributors to the observed differences between scours and undisturbed areas.     

The influence of depth,site exposure and season on the intensity of iceberg scouring in nearshore Antarctic waters

Ice scour disturbance has a significant effect on the physical and biological characteristics of polar benthos. A series of grids, each consisting of 25 markers, were deployed along depth transects and replicated at two contrasting study sites at Adelaide Island, West Antarctic Peninsula. Markers were surveyed and replaced every 3 months for 2 years in order to assess the frequency and intensity of iceberg impacts. Depth, site, season and year were all highly significant factors influencing ice scouring frequency. We observed a high variation in the duration of winter fast ice between sites and years, which had a marked effect on ice scouring frequency. The ecological effects of the disturbance regime are likely to include depth zonation of benthic assemblages, patchiness of communities at varying stages of recovery and the near denudation of sessile fauna in the shallow subtidal.

Iceberg killing fields limit huge potential for benthic blue carbon in Antarctic shallows

Climate‐forced ice losses are increasing potential for iceberg‐seabed collisions, termed ice scour. At Ryder Bay, West Antarctic Peninsula (WAP) sea ice, oceanography, phytoplankton and encrusting zoobenthos have been monitored since 1998. In 2003, grids of seabed markers, covering 225 m², were established, surveyed and replaced annually to measure ice scour frequency. Disturbance history has been recorded for each m² of seabed monitored at 5–25 m for ~13 years. Encrusting fauna, collected from impacted and nonimpacted metres each year, show coincident benthos responses in growth, mortality and mass of benthic immobilized carbon. Encrusting benthic growth was mainly determined by microalgal bloom duration; each day, nanophytoplankton exceeded 200 μg L^?1 produced ~0.05 mm radial growth of bryozoans, and sea temperature >0 °C added 0.002 mm day^?1. Mortality and persistence of growth, as benthic carbon immobilization, were mainly influenced by ice scour. Nearly 30% of monitored seabed was hit each year, and just 7% of shallows were not hit. Hits in deeper water were more deadly, but less frequent, so mortality decreased with depth. Five‐year recovery time doubled benthic carbon stocks. Scour‐driven mortality varied annually, with two‐thirds of all monitored fauna killed in a single year (2009). Reduced fast ice after 2006 ramped iceberg scouring, killing half the encrusting benthos each year in following years. Ice scour coupled with low phytoplankton biomass drove a phase shift to high mortality and depressed zoobenthic immobilized carbon stocks, which has persevered for 10 years since. Stocks of immobilized benthic carbon averaged nearly 15 g m^?2. WAP ice scouring may be recycling 80 000 tonnes of carbon yr^?1. Without scouring, such carbon would remain immobilized and the 2.3% of shelf which are shallows could be as productive as all the remaining continental shelf. The region's future, when glaciers reach grounding lines and iceberg production diminishes, is as a major global sink of carbon storage.     

Meiofauna response to iceberg disturbance on the Antarctic continental shelf at Kapp Norvegia (Weddell Sea)

The impact of iceberg scouring on meiofauna communities, especially nematodes, was studied on the Kapp Norvegia shelf in the Weddell Sea, Antarctica. Three stations with different stages of recolonisation following scour were selected on the basis of seafloor video images, sediment characteristics and faunal occurrences. These stations comprised a fresh scour, an older scour, and an undisturbed control site where a sponge spicule mat covered the sediment with dense epifauna. Meiofaunal abundance and taxonomic diversity of meiofauna groups were significantly reduced in the fresh scour. The highest abundance and diversity were found in the older scour as compared with the undisturbed site. The abundance and diversity of nematodes also decreased due to scouring. The abundance in the older scour recovered to the level of the undisturbed site whereas the diversity remained low. Scouring also changed the nematode community structure, with the suborders Desmoscolecina and Leptolaimina as the most sensitive groups. In addition, scouring resulted in the decrease of selective deposit feeders and the Maturity Index. The low diversity and the change in nematode generic composition in the older scour compared with the undisturbed site, despite the complete recovery in terms of abundance, suggest that the deep continental shelf nematode community in this area is sensitive to iceberg disturbance.     

Soft bottom species richness and diversity as a function of depth and iceberg scour in Arctic glacial Kongsfjorden (Svalbard)

Macrozoobenthic soft-sediment communities of central Arctic Kongsfjorden inhabiting six depth zones between 5 and 30 m were sampled using SCUBA-diving during June–August 2003 and analysed comparatively. About 63 taxa were found, nine of which had not been reported for Kongsfjorden and four for Svalbard. Suspension feeding or surface and sub-surface detritivorous polychaetes and deposit-feeding amphipods were dominant. Only 11 of the 63 taxa (45 species and additional 18 families not further identified) inhabited the complete depth range. Biomass ranged from 3.5 to 25.0 g ash free dry mass m⁻² and mean Shannon diversity (Log e) was 2.06. Similarity clustering from abundance and biomass data showed a significant difference between the shallow station (5 m) and the rest. The latter formed two sub-groups (10–20 and 25–30 m). Depth is irrevocably correlated with ice-scouring. Thus the differences in diversity together with the predicted iceberg scour intensity support the ‘intermediate disturbance hypothesis’ indicating that habitats impacted by moderate iceberg scouring enable higher diversity. In contrast, biotopes frequently affected only host pioneer communities, while mature, less diverse assemblages dominate depths of low impact.     

Polychaete assemblages of the northwestern Ross Sea shelf: worming out the environmental drivers of Antarctic macrobenthic assemblage composition

We have limited understanding of which environmental factors structure the distribution patterns and composition of Antarctic macrobenthos assemblages, and the spatial scales on which such factors operate. In 2004, the “BioRoss Survey” was conducted on the northwestern Ross Sea shelf between Cape Adare and Cape Hallett in depths of 50–750 m to describe and quantify the assemblage patterns of benthic macroinvertebrates. In order to determine the influence of primary productivity, disturbance and habitat heterogeneity on the distribution and composition of the macrofaunal assemblages, polychaete data derived from 52 grab samples were analysed. Although differences in the composition of polychaete assemblages among different sampling transects and depth strata were not particularly pronounced (yet statistically significant), the results suggested that large-scale differences in both primary productivity and iceberg disturbance influence distribution patterns. The combination of sediment chl a content, sorting coefficient, sponge spicule content and distance to the nearest iceberg scour best explained polychaete assemblage patterns. This finding supports previous contentions that multiple environmental drivers working at varying scales influence Antarctic shelf assemblages. The results do not supply support for a pronounced decoupling of pelagic and benthic systems, as has been suggested by another study of deeper water benthic assemblages on the Ross Sea shelf.     

Long-term variability of meiobenthos: value,synopsis, hypothesis generation and predictive modelling

Few long-term data sets exist for meiofauna. Such data sets are expensive to collect, sort and identify; continuous meiofauna data for a period of greater than two years are limited to one site in Belgium (7 yrs) and two sites (one mud, one sand) in South Carolina, USA (11 yrs). The Belgian study concentrates on benthic copepod abundances whereas data from South Carolina includes major taxa and benthic copepods as well as 4 years of concurrent macrofauna abundance and 3 years of nematode species abundances. In South Carolina, the variance associated with meiofaunal abundance had 6 or 12 month recurrent cycles. Similar analyses on 4 years of macrofauna from the same 2 sites indicated the same cyclicity: one year. Seasonality of the South Carolina major taxa and the 6 most abundant mud copepod species was pronounced at the mud site, but absent or less pronounced at the sand site. Similar results were also found for the nematode species over three years. Variability in meiofaunal abundance was greater year-to-year than within a year. Many such long-term data sets are analysed and abandoned. Herewith, I use our long-term results to hypothesize the causes of the high temporal variance in mud and the lower temporal variance in sand. Is it because the mud fauna is controlled by seasonal inputs of natant predators while at the hydrodynamically active sand site temporal variability is homogenized by constant physical activity? By appropriate statistical modelling long-term data sets can also be used to assess the appropriateness of the sampling schedule (spatial and temporal) and as a predictor of future trends.     

Depth Distribution and Abundance of the Common Bully, Gobiomorphus cotidianus (Eleotridae), in Three Oligotrophic New Zealand Lakes, One of which is Turbid

The depth distribution of the common bully, Gobiomorphus cotidianus, a small benthic forage fish, was measured by trapping at set depths from 0–70m in three large oligotrophic lakes, including one where inorganic sediment from a glacially-fed river produces turbid conditions. Bullies occurred at all depths from 0.5–70m in the clear lakes, but none were present below 25m in the turbid lake. Two groups of bullies were present in the clear lakes; a high-density, littoral stock at depths of 0.5–25m, and a low-density, profundal stock at depths of 30–70m. These groups were further distinguished by differing buoyancy requirements and feeding habits. The swimbladders of littoral bullies contained gas, but those of the profundal bullies, which fed more than littoral bullies by both day and night, did not. The variation in mean CPUE with depth within the littoral zone was not related to water temperature, oxygen concentration, or conductivity. Nor was it related to a reduction in light levels or to reduced water transparency caused by increased turbidity. It may therefore be controlled by biotic factors. The absence of a profundal stock below the littoral zone in the turbid lake indicates that the settlement of fines from turbid inputs may affect bully abundance in deeper waters. As conventional measures of the abundance of benthic fish in lakes are often restricted to littoral habitats, and do not reflect changes in abundance with depth, an index of overall abundance based on depth distribution was developed to allow comparisons between lakes.     

Links between the structure of an Antarctic shallow-water community and ice-scour frequency

Ice is a major structuring force in marine and freshwater environments at high latitudes. Although recovery from scouring has been quantified in time, the frequency of scouring in the Antarctic has not. We placed grids of markers at 9–17 m depth at two sites, to study ice-scouring over 2 years at Adelaide island (Antarctic Peninsula). We quantified the time scale of scour frequencies, and linked this to community mortality, age and diversity. Markers were hit from zero to at least three times in 2 years. At the least disturbed site (South Cove) 24% of markers were destroyed per year, whereas in North Cove 60% of markers were destroyed. There were significant differences in scouring frequency between our two sites: a given area in North Cove was on average hit twice as often as one in South Cove. Compared with near shore environments elsewhere, faunas of both sites were characteristic of high disturbance regimes, exhibiting low percent cover, diversity, ages and a high proportion of pioneers. Aspects of the encrusting communities studied reflected the differences between site disturbance regimes. North Cove was scoured twice as often, and bryozoan communities there had half the number of species, two-thirds the space occupation and twice the mortality level of those in South Cove. Maximum age in North Cove bryozoans was also half that in South Cove. Although there are natural disturbance events that rival ice-scouring in either frequency or catastrophic power at lower latitudes, none do both nor across such a wide depth range. We suggest that ice scour effects on polar benthos are even more significant than the same magnitude of disturbance at lower latitudes as recovery rates of high latitude communities are very slow. Climate warming seems likely to increase iceloading of near shore polar waters, so that some of the worlds most intensely disturbed faunas may soon suffer even more disturbance.     

Ecological determinants of methylmercury bioaccumulation in benthic invertebrates of polar desert lakes

We investigated concentrations of monomethylmercury (MMHg) at the base of benthic food webs in six lakes from polar desert (biologically poor and low annual precipitation) on Cornwallis Island (Nunavut, Canada, ~75°N latitude). Anthropogenic mercury emissions reach the Arctic by long-range atmospheric transport, and information is lacking on processes controlling MMHg entry into these simple lake food webs, despite their importance in determining transfer to lake-dwelling Arctic char. We examined the influences of diet (using carbon and nitrogen stable isotopes), water depth, and taxonomic composition on MMHg bioaccumulation in benthic invertebrates (Chironomidae and Trichoptera). We also estimated MMHg biomagnification between benthic algae and invertebrates. Similar MMHg concentrations of chironomid larvae in nearshore and offshore zones suggest that benthic MMHg exposure was homogeneous within the lakes. Chironomid δ¹³C values were also similar in both depth zones, suggesting that diet items with highly negative δ¹³C, specifically methanogenic bacteria and planktonic organic matter, were not important food (and therefore mercury) sources for profundal larvae. MMHg concentrations were significantly different among two subfamilies of chironomids (Diamesinae, Chironominae) and Trichoptera. Higher MMHg concentrations in Diamesinae were likely related to predation on other chironomids. We found high MMHg biomagnification between benthic algae and chironomid larvae compared with literature estimates for aquatic ecosystems at lower latitudes; thus, benthic processes may affect the sensitivity of polar desert lakes to mercury. Information on benthic MMHg exposure is important for evaluating and tracking impacts of atmospheric mercury deposition and environmental change in this remote High Arctic environment.     

Benthic diatom distribution and grazing by Sarotherodon mossambicus in Lake Sibaya, South Africa

SUMMARY. Distributions of benthic diatoms and benthic floc in Lake Sibaya were examined to determine if preferential grazing by juvenile Sarotherodon mossambicus on benthic detrital aggregate in shallow water maximized diatom concentrations in the diet of the fish. Between 0.1 and 12.0 m depth, the range of diatom densities was 1–60 × 10⁴ cells cm⁻² with high but extremely variable densities above 3 m and consistently low densities at greater depths. Light intensity and burial by wave action appear to be primary factors affecting diatom abundance, but the effect of unidentified factors causing local patchiness is also evident. Substrate stabilization by diatoms was observed between 0.3 and 1.8 m depth. Weight of benthic detrital aggregate generally increased with depth such that diatom concentrations were variable above 3 m but were extremely low in benthic detrital aggregate at greater depths. Comparison of diatom concentrations in fish stomachs and benthic detrital aggregate shows that within the shallow waters, fish do not selectively graze those areas richest in diatoms.     

Ecological traits of benthic assemblages in shallow Antarctic waters: does ice scour disturbance select for small, mobile, secondary consumers with high dispersal potential?

Benthic communities in nearshore habitats around Antarctica are strongly influenced by ice disturbance. It has been suggested that where ice scour disturbance is severe, the relative importance of certain ecological groups is elevated. I examined the relative contributions of mobility, size, feeding strategy and development mode groups to total faunal abundance and species richness in relation to ice disturbance at Adelaide Island, West Antarctic Peninsula. The contributions of ecological groups were assessed along a depth/disturbance gradient from 5 to 25 m depth at two sites. At one site, the relative abundance of the low mobility group was significantly greater at low disturbance levels, whilst the relative abundance of the high dispersal group (taxa with pelagic larvae) was elevated at high disturbance levels. At the other site, the relative abundance of secondary consumers was greater at high disturbance levels. Even over small spatial scales, certain ecological traits seem advantageous to a fauna shaped by intense, catastrophic ice scour.     

Feeding behavior and diet of the eight-banded butterflyfish Chaetodon octofasciatus in the Thousand Islands,Indonesia

Marine fishes reach their highest diversity in coral reef ecosystems, which they utilize as territory, place of refuge and reproduction, and source of food. One type of predation among reef fishes is feeding on coral polyps, and a single, easily identifiable family of fishes, Chaetodontidae, contains the majority of obligate corallivore species. Multiple studies have examined the behaviour and ecology of Chaetodontidae and their relationship with the benthic habitat. However, many questions remain about their feeding ecology and food specifity at the species level. The present study is the first attempt to systematically decipher the feeding behavior of the obligate corallivore, Chaetodon octofasciatus. Field data were collected from four sites in the Thousand Islands, Indonesia, during two sampling campaigns in 2006, covering two seasons and two different depths. A high abundance of C. octofasciatus was positively related to hard coral cover. Out of a total of 57 scleractinian coral genera observed during the study period, 24 were utilized by C. octofasciatus. All fish collected during the study (n?=?36) had >86 % nematocysts in their guts, supporting their classification as obligate corallivores. Based on the Strauss electivity index, C. octofasciatus displayed a preference for the coral genus Acropora at all depths, while preference for Fungia became more marked at greater depth. Both Acropora and Fungia were observed at high density at all sites, with Acropora density decreasing markedly with depth and Fungia density increasing. Bite rates showed an asymptotic relationship to Acropora density, levelling off between 15 and 20 bites/5 min. The existence of a strong relationship with live coral cover and the preference for specific genera such as Acropora designate C. octofasciatus as a potential bioindicator in the Thousand Islands reefs, while making it particular susceptible to the ongoing degradation of reefs in the region.     

Larval habitat preference of the endemic Hawaiian midge,Telmatogeton torrenticola Terry (Telmatogetoninae)

Telmatogeton torrenticola Terry is a large endemic chironomid (lastinstar >20 mm) commonly found in high gradient Hawaiian streams on smoothrock surfaces with torrential, shallow flow and in the splash zones ofwaterfalls. We have quantified benthic water flow in larval habitat in a 50m segment of Kinihapai Stream, Maui using a thermistor-based microcurrentmeter. Under base flow conditions at sites suitable for larval attachment,depth was measured and bottom water velocity measurements were made 2 mmabove populations. Larval densities ranged from 386.9–1178m^–2, habitat bottom water velocities from 13.4–64.2 cms^–1, and water depths from 1.5–50 cm. Bottom velocitiesof sites with zero larvae ranged from 20.8–21.8 cm s^–1with depths from 50 to >160 cm. Larval densities were greatest inareas with high bottom water velocities and shallow depths. Stepwisemultiple regression analyses showed that density could be confidentlypredicted best by Froude number (r=0.81; p=0.008). In the absence of Froudenumber as a regression term, the best variable to predict larval density wasbottom velocity ratio: relative depth ratio (r=0.75; p=0.019). In addition,the torrential habitat of the larvae was always characterized by aperiphyton community that appeared to be the primary food resource for thelarvae. These data suggest that torrential flows over appropriate substratesare important factors regulating habitat availability for T. torrenticolaand that reduced discharge (e.g. affected by water diversions) couldsignificantly reduce the amount of available habitat for this organism andother flow sensitive stream fauna.     

An eddy,a wake and a plume: controls on bathyal foraminifera around Tobago,western tropical Atlantic Ocean

Oceanic islands in the paths of currents induce the development of wakes and stationary eddies. The situation to the lee of Tobago, western tropical Atlantic Ocean, is further complicated by the occurrence of the seasonally variable, hypopycnal Orinoco plume. Here we investigate the impact of the combined plume, wake and eddy on bathyal benthic foraminifera to the NW of Tobago. Three surface sediment samples were recovered from around each of five well-sites to the NW of Tobago, three of the sites (Warap-A, Cassra-A and Cassra-CC) being at upper bathyal depths and two (Bene-1, Sancoche-1) at middle bathyal depths. Warap-A, Cassra-A, Cassra-CC and Bene-1 form a transect along the northern side of the leeward wake, while the other two sites are in the vicinity of the stationary eddy. The samples obtained around Sancoche-1 were taken north of the wake. These were supplemented by samples from four 80-cm piston cores from upper bathyal and outer neritic depths sampled at ~ 10 cm intervals. Benthic foraminifera reveal different biofacies at upper (Warap-A, Cassra-A, Cassra-CC) and middle (Bene-1, Sancoche-1) bathyal depths. The upper bathyal biofacies is dominated by Cassidulina curvata and the middle bathyal biofacies contains abundant Uvigerina hispidocostata, both of which are indicative of a high nutrient flux. The presence of Martinottiella communis and M. pallida at Warap-A indicate that pore waters are low in dissolved oxygen in the immediate lee of the island. Percentages of the fauna as serial tests indicated decreasing current velocities with increasing depth, as confirmed by the high abundance of Cibicides ex gr. aknerianus in the shallowest water core. Upper bathyal bottom-current strength was at its lowest in the immediate lee of the island. Species indicative of a perennial nutrient flux were more abundant to the NW, where the interaction of the plume and eddy appears to concentrate nutrients. The short cores, each from a different biofacies, indicate that these environmental conditions have been in place for at least the later Holocene. The most northerly, upper bathyal core presented a stable community structure with low assemblage turnover, while two cores taken farther south (upper bathyal and outer neritic) had an expansive structure with high assemblage turnover. These data raise the possibility of using benthic foraminifera to track the positions of the plume, core and eddy throughout the later Neogene.     

Assessment of site specific benthic impact of floating cage farming in the eastern Hios island, Eastern Aegean Sea, Greece

The site specific impact on the benthic fauna was assessed within a 4.64 km² area of intensive aquaculture situated on the eastern side of Hios island (Aegean Sea, Greece) at 11 stations seasonally between November 2000 and October 2001. The benthic fauna showed marked changes in species numbers, diversity and faunal abundance between farm and control sites. The polychaetes Nereis diversicolor, Scolelepis fuliginosa and Capitella capitata were the most dominant species at the farm sites (35% of the total abundance), whilst the most dominant species at the control sites were the polychaete Hyalinoecia brementi and the sipunculid Aspidosiphon muelleri (23% of the total abundance). Species richness, diversity and evenness were higher at the control sites whereas numerical abundance was higher at the farm sites. K-dominance curves suggest a minor impact on the benthic community at the farm sites and temporal changes on macrobenthic assemblages.     

The composition,structure, and distribution of benthic ostracod fauna (Ostracoda,Myodocopa) in Antarctic waters

The benthic fauna of ostracods of the order Myodocopida of Antarctic waters is characterized by high diversity, relative species abundance, and a complicated taxonomic and ecological structure, with a simplified biogeographical structure. This fauna, which is distinguished by a high level of endemicity, although at a low taxonomic rank, includes a great share of deep-sea and subtidal elements. Ostracod populations of High and Low-Antarctic subzones differ qualitatively and quantitatively. A distinct impoverishment of fauna is observed in the region of the Antarctic divergence compared to the more northern areas. The number of species increases with depth to reach its maximum in the lower subtidal zone and on the upper continental slope at depths of 200–500 m. The number of species decreases with increasing depth. Myodocopida have not been yet found in the Antarctic waters deeper than 5000 m.     

Benthic foraminiferal extinctions linked to late Pliocene–Pleistocene deep-sea circulation changes in the northern Indian Ocean (ODP Sites 722 and 758)

During the late Pliocene–middle Pleistocene, 63 species of elongate, bathyal–upper abyssal benthic foraminifera (Extinction Group = Stilostomellidae, Pleurostomellidae, some Nodosariidae) declined in abundance and finally disappeared in the northern Indian Ocean (ODP Sites 722, 758), as part of the global extinction of at least 88 related species at this time. The detailed record of withdrawal of these species differs by depth and geography in the Indian Ocean. In northwest Indian Ocean Site 722 (2045 m), the Extinction Group of 54 species comprised 2–15% of the benthic foraminiferal fauna in the earliest Pleistocene, but declined dramatically during the onset of the mid-Pleistocene Transition (MPT) at 1.2–1.1 Ma, with all but three species disappearing by the end of the MPT (∼0.6 Ma). In northeast Indian Ocean Site 758 (2925 m), the Extinction Group of 44 species comprised 1–5% of the benthic foraminiferal fauna at ∼3.3–2.6 Ma, but declined in abundance and diversity in three steps, at ∼2.5, 1.7, and 1.2 Ma, with all but one species disappearing by the end of the MPT. At both sites there are strong positive correlations between the accumulation rate of the Extinction Group and proxies indicating low-oxygen conditions with a high organic carbon input. In both sites, there was a pulsed decline in Extinction Group abundance and species richness, especially in glacial periods, with some partial recoveries in interglacials. We infer that the glacial declines at the deeper Site 758 were a result of increased production of colder, well-ventilated Antarctic Bottom Water (AABW), particularly in the late Pliocene and during the MPT. The Extinction Group at shallower water depths (Site 722) were not impacted by the deeper water mass changes until the onset of the MPT, when cold, well-ventilated Glacial North Atlantic Intermediate Water (GNAIW) production increased and may have spread into the Indian Ocean. Increased chemical ventilation at various water depths since late Pliocene, particularly in glacial periods, possibly in association with decreased or more fluctuating organic carbon flux, might be responsible for the pulsed global decline and extinction of this rather specialised group of benthic foraminifera.     

Vertical zonation of megabenthic taxa on a deep photosynthetic reef (50–140 m) in the Au’au Channel,Hawaii

This study surveyed several locations at depths between 50 and 140 m within the Au’au Channel, Hawaii to characterize the deep reef habitat and determine the depth distribution and relative abundance of the dominant, habitat forming megabenthic taxa. In the Au’au Channel, the depth distribution of megabenthic taxa exhibited a pattern of vertical zonation with relatively few taxa dominating each zone. Macroalgae particularly Halimeda spp. and to a lesser extent scleractinian corals Leptoseris spp. were dominant between 50 and 80 m; Leptoseris spp. were dominant between 80 and 90 m as macroalgae decreased in abundance; the invasive octocoral Carijoa riisei was dominant between 90 and 100 m primarily on rugose features; Antipathes spp. and Leptoseris spp. were dominant between 100 and 120 m on exposed fossil reef; and small wire corals were dominant between 120 and 140 m. In general, the percentage of live benthic cover decreased with depth, particularly below 90 m where a large majority of the area was uncolonized, soft substrata. The gradient of downwelling light intensity appears to play a major role in regulating the depth distribution of photosynthetic organisms, skilophilous organisms, and other benthic fauna, which compete for space with dominant photosynthetic species. The depth of the seasonal thermocline also appears to play an important role in limiting the distribution of tropical benthic species.