Search

Direct underwater observations at seamounts across the western North Atlantic basin revealed the presence of the false boarfish, Neocyttus helgae (Oreosomatidae), at depths of 915 to 1829 m. These observations extend the known range of this species across the North Atlantic basin and support the hypothesis that seamounts are stepping stones for dispersal. Fishes were associated with basalt habitats featuring corals and sponges (both nearly horizontal basalt sheets and steep cliffs) as well as depressions in sheets of basalt. These features provided refuge from flow and predators as well as immediate access to zooplankton and pelagic prey delivered by rapid currents. Encounters with fishes were primarily as single individuals or pairs. Territorial behaviors, such as raised dorsal spine and lateral display, occurred when submersible vehicles approached fish, suggesting that territorial defense is a common behavioral attribute.

High resolution imaging of the seafloor has been limited to fixed sensors and/or limited field-of-view instruments. As a result, our ability to characterize large areas of benthic habitat has relied on acoustic maps and photomosaics combined with point sampling. In addition, the response of motile epibenthos and demersal fish to habitat patches, structure and disturbance has been largely inferred or based on disruptive sampling and manipulative experiments. Direct quantitative observation of large areal variation in these factors is possible through a combination of traditional video census techniques with high resolution optical imaging of the seafloor. Laser Line Scan Systems (LLSS) are capable of imaging continuous swaths of the seafloor from 3 to 15 m wide at speeds up to 5 km/h. Image quality is dependent on system performance and turbidity but can exceed limits required to distinguish benthic habitats and census undisturbed fish and epibenthos. This areal coverage rate (1 km/day) is several orders of magnitude higher than video sleds and can complement high resolution acoustic surveys (multibeam or side scan). We present results from several applications of these systems for assessment of benthic resources and disturbance in turbid estuarine environments.

Megafaunal assemblages were studied in August–September 1992 using a towed camera sled along seven cross-isobath transects on the continental slope off Cape Hatteras. A total of 20,722 megafaunal organisms were observed on 10,918 m 2 of the sea floor between the depths of 157 and 1 924 m. These data were compared with data previously collected off Cape Hatteras in 1985 and at other locations along the eastern U.S. coast between 1981 and 1987. Megafaunal populations on the upper and lower slopes off Cape Hatteras were fouond to be similar, in terms of density and species composition, to those observed at the other locations. In contrast, megafaunal abundances were found to be elevated (0.88 and 2.65 individuals per m 2 during 1985 and 1992, respectively) on the middle slope off Cape Hatteras when compared to most other slope locations ( 2 ). These elevated abundances mainly reflect dense populations of three demersal fish, two eel pouts ( Lysenchelys verrilli and Lycodes atlanticus ) and the witch flounder Glyptocephalus cynoglossus , and a large anemone ( Actinauge verrilli ). These four species dominated the megafauna off Cape Hatteras, whereas they represented only a minor component of megafaunal populations found at other slope locations. Additionally, numerous tubes of the foraminiferan Bathysiphon filiformis were observed off Cape Hatteras, but not elsewhere. The high density of demersal fish found off Cape Hatteras appears to be related to the high densities of infaunal prey reported from this area. The high densities of A. verrilli and B. fuliformis may be related to the same factors responsible for the high infaunal densities, namely enhanced nutrient inputs in the form of fine particles. Extreme patchiness also was observed in the distributions of the middle slope taxa off Cape Hatteras. This patchiness may reflect the habitat heterogeneity of this exceptionally rugged slope and the sedentary nature of the organisms inhabiting it.

Historical data from the slope off Cape Hatteras show this environment to be atypical of the rest of the Atlantic slope in terms of high rates of sedimentation and high benthic/demersal standing stocks. This paper focuses on identifying potential sources for sediments and nutrients on the Hatteras slope and on likely transport mechanisms. Sediments consist mainly of subequal mixtures of sand, silt, and clay and contain an average of 1% carbon. This pool of carbon represents weathered organic matter containing polyunsaturated fatty acids (PUFA) and sterols typical of relatively refractory shelf/estuarine sediments. The labile organic fraction is derived from phytoplankton and zooplankton as reflected in short chain fatty acids (

A study of the Monterey Canyon and fan was conducted to investigate contemporary sedimentary activity using a camera sled, Alvin dives and Sea-Beam bathymetry. Physical characteristics and plan view morphology of the walls, terraces, thalweg floor and levees of the Monterey Canyon and fan, as well as the gullied and non-gullied regions of the adjacent continental slope have been studied. The canyon floor and fan valley thalweg channel from 2900 to 3500 m exhibited properties indicative of moderate to low energy conditions (weak currents, smoothed sea-bed, deposits of disintegrated kelp, bioturbated mud substrate, fecal pellets, absence of scour). Chemosynthetic communities consisting of clams and a pogonophoran, were found in the fan valley from 3000 to 3600 m. Local areas of rockfalls and slumps from the canyon and fan valley walls were not fresh (sedimented, abundantly colonized by benthic biota). Terraces and levee crests were mud-draped and bedrock exposures on canyon walls were encrusted by benthic organisms. Freshest disturbances were found in gullies on the adjacent continental slope where bedrock was scoured clean of sediment and loose debris. A major submarine slide detached from escarpments on the lower slope, and extends across the fan. The slide surface was mud-draped, hummocky, and contained bedforms with wavelengths up to 150 m and up to 10 m relief. The survey was conducted shortly before the Loma Prieta earthquake (October 17, 1989) that caused substantial ground motion in on-shore regions of Monterey Bay. Any substantial sub-sea disturbances generated by the ensuing earthquake should be discernable from the pre-earthquake state by comparison with the deep-sea photographs and observations.

Flocculent, detrital material was photographed on the sea floor between 450 and 2400 m during two camera-sled tows conducted along a transect on the continental slope south of Georges Bank in the spring of 1985. The two tows were taken slightly less than 5 days apart (27 April and 1–2 May). Photographs were taken at automatic 15-s intervals throughout each tow. The consistency and color of the detrital material and the timing of its deposition indicate that the spring bloom was the most likely source of this material. With the exception of the upper slope, substantially more detrital material was seen at the time of the second tow. The pattern of relative amounts of detritus visible on the sea floor across the slope indicated a preferential settlement of particles on the middle slope and a downslope shift of material between the first and second tow. These results suggest that continental margin processes may play an important role in the transfer of organic material to the deep sea.

The δ13C and δ15N of the soft tissues of two Calyptogena phaseoliformis specimens sampled from the floor of Ascension Fan-Valley, off the Monterey Peninsula, California, ranged from -35.0 to -33.6% and -6.2 to -0.6%, respectively. The carbon isotope abundances are similar to those of other Calyptogena known or believed to be nutritionally dependent on sulfide-based chemoautotrophic symbionts. Calyptogena δ15N values and their geographic variation indicate an unusual source of dietary nitrogen for these animals. The δ13C and δ18O values for Calyptogena shells from the Ascension and Monterey Fan-Valleys approximate those reported for Calyptogena from three other locations in the Pacific and appear to reflect shell formation from constituents within ambient deep-ocean water.

Alvin dives and camera tows within the “meander area” of the Monterey and Ascension Fan Valleys have located nine chemosynthetic communities over depths ranging from 3000 to 3600 m over a distance of 55 km. Most of the observed communities consist largely of Calyptogena phaseoliformis, but Solemya (species unknown) and a pogonophoran (genus Polybrachia), have also been identified. The δ13C values (-35.0 to -33.6 per mil) and the presence of APS reductase and ATP sulfurylase in the C. phaseoliformis tissue is consistent with sulfur chemoautotrophy. Two reduced organic matter sources for the H2S are proposed: (1) older beds exposed by the deep erosion (up to 400 m) of the fan valleys and (2) concentrations of anaerobically decomposd organic matter buried in the valley floor.

Megafaunal assemblages at four locations on the slope south of New England were surveyed using a towed camera sled, with photographs taken at automatic 15 s intervals. A total of 284,692 organisms were seen in 94,380 m2 of sea floor viewed. Classification and ordination analysis defined four megafaunal zones that corresponded to different regions of the slope: an upper slope zone, an upper-middle slope zone, a transitional zone on the lower-middle slope, and a lower slope zone. Each of these zones could be further subdivided into shallow and deep portions of higher species similarity. Faunal densities were high in the upper and lower slope zones and low in the two middle slope zones. The boundaries between zones were frequently marked by abrupt shifts in faunal density and trophic structure. High species overlap between adjacent zones suggested a pattern of continual species turnover along the depth gradient, with zones representing regions of lesser faunal changes separated by regions of greater faunal change. Species replacement with depth was very abrupt in steep areas and gradual in flat areas. The pattern of species turnover and the depths of faunal boundaries varied among geographic locations, reflecting differences in local topography and geology. Patterns of density, and species composition, with depth were very similar at three of the locations but differed significantly at the eastern edge of Georges Bank, a site characterized by very steep topography and numerous glacial erratics and outcrops. Faunal assemblages in zones were frequently dominated by one or two very abundant species, and the boundaries between zones usually reflected the depth limits of these species. At each location, depths of major faunal boundaries coincided with relative changes in slope declivity rather than absolute angles of inclination. Environmental heterogeneity provided by different substrata may further enhance species turnover along the depth gradient. Variable relief also may allow extension of depth ranges by providing refuge from predation and/or altering food resources. Factors controlling the distribution of megafaunal assemblages on sediment-covered slopes may be related to the effect of local topography on current intensities and related differences in food availability.

The processes that generate thinning, extension, and finally rifting of continental crust are often assumed to be either “active” or “passive.” There are two major continental rifting provinces active in the world today: the East African Rift and the Baikal Rift in Siberia. Many authors suspect that the Baikal Rift formed in response to the India-Eurasia collision, which deformed the lithosphere in central Asia creating the Himalayas in the south and an extensional province in Siberia [Molnar and Tapponier, 1975; Tapponier and Molnar, 1979; Zonenshain and Savostin, 1981]. Others believe that the Baikal Rift is primarily an active phenomenon, citing updoming around Lake Baikal and the presence of Cenozoic volcanics as an example of slightly elevated partial melting in the asthenospheric material underneath the lake [Artemyev and Artyushkov, 1971; Zorin, 1971, 1981; Puzyrev et al., 1974; Logatchev et al., 1983].

Evidence from over 200 sediment cores, numerous submersible dives, and bottom photographs prove that bioturbation and bioerosion are ongoing processes affecting northeastern U.S. continental slope and rise sedimentation. Evidence of biological activity was found in greater than 95% of the cores examined. Submersible dive observations reveal that the results of biological activity often dominate sea-floor microtopography. Bioturbation can disturb sediments several centimeters deep in a matter of seconds and is in some areas the primary sediment transport mechanism. Many cores with sandy intervals were profoundly disturbed by bioturbation. Biologically camouflaged sand-rich intervals can easily be missed by visual observation.

Dense biological communities of large epifaunal taxa similar to those found along ridge crest vents at the East Pacific Rise were discovered in the abyssal Gulf of Mexico. These assemblages occur on a passive continental margin at the base of the Florida Escarpment, the interface between the relatively impermeable hemipelagic clays of the distal Mississippi Fan and the jointed Cretaceous limestone of the Florida Platform. The fauna apparently is nourished by sulfide rich hypersaline waters seeping out at near ambient temperatures onto the sea floor.

Eighty 0.25m2 box cores were collected and analyzed to determine the faunal composition, species diversity and sample similarity of the benthic communities of three sites in the eastern Equatorial Pacific. (The results are part of a baseline survey of the fauna of the manganese nodule province of the Pacific.) The bottom topography of this region is characterized by abyssal hills, with a relief of 50 to 300 meters, at a depth of 4350 to 5150 meters.

If one is concerned with the mechanisms of adaptation to environmental disturbances on the non-genetic level, hydra are particularly appropriate for several reasons. First, the problem is somewhat simplified since the behavioral repertoire of hydra is severely limited, the morphology at least on a gross and light microscopy level is reasonably simple and a spectrum of responses ranging from the cellular to the behavioral can be recorded on genetically identical individuals. As a consequence of this simplicity it ought to be possible to catalogue the response of hydra to change in such a way that we can distinguish between the rapid and slower responses and get some sense of what the implications of each of these responses might be for the further adaptive capacities of the hydra themselves. The rationale for this type of inquiry is presented in Slobodkin and Rapoport (1974 and papers cited therein.

Two major types of interactions between benthic fauna and submarine slides are addressed in this paper: (1) the contribution of biological activity to slope instability and (2) the biological consequences of submarine slope failure. Through the processes of bioturbation and bioerosion, benthic fauna can substantially alter the physical properties of their substrata. Activities that lead to structural weakening of both sediment and outcrop are: consistent reworking of sediment preventing consolidation; excavation of sediment and semi-consolidated clay resulting in the net transport of material downslope; boring of outcrops causing decrease of rock mass; and attachment to outcrops resulting in increased drag force and gravitational pull on their surfaces. Along the continental margins located off both the east and west coasts of the U.S., the activity of benthic fauna appears to play a major role in contemporary submarine erosion.

A sea bottom of middle Eocene calcareous claystone cut by downslope-trending furrows was observed during an Alvin dive to the mouth of Berkeley Canyon on the continental slope off New Jersey. The furrows are 10 to 50 m apart, 4 to 13 m deep, linear, and nearly parallel in water depths of 2,000 m. They have steep walls and flat floors 3 to 5 m wide, of fine-grained sediment. Mid-range sidescan-sonar images show that similarly furrowed surfaces are found on nearby areas of the lower continental slope, not associated with canyons. The furrows are overlain in places by Pleistocene sediments. Although they show evidence of erosional origin, they do not appear to be related to observed structures, and their straight, parallel pattern is not well understood. A general cover of flocky unconsolidated sediments implies that bottom-current erosion is not active now.