Surface Geology and Biology at the Head of Kaoping Canyon off Southwestern Taiwan

A preliminary geological and biological suney of Kaoping Canyon oft' southwestern Taiwan was conducted with a remotely operated vehicle (ROV). Twenty-one dives (25-160 m depth) were conducted at 12 stations on the canyon walls aild adjacent continental shelf. Total bottom time from this series of dives was 17.0 hours. Surface sediments, microtopography, and fau­ nal assemblages were generally similar along· the series of stations across the canyon although local variability was apparent. However, features obsened on canyon stations differed significantly from those of the adjacent shelf. Sur­ face sediments on the canyon-shelf break were dominated by fine sands, silts and clays. Outcrop sediments along the canyon walls appeared to be com­ posed of clay and were laminated and burrowed. Ma�-wasting processes probably eroded the canyon walls and resulted in the deposition of gravel and shell fragments at the foot of the outcrops. Erosional feature, typically in the form of stepped terraces, were most pronounced at the lower parts of the canyon walls. Shelf areas adjacent to the canyon were relatively flat and covered by fine-grained sediments. A small number of taxa included soft corals, crabs, shrimps, crinoids and fishes were observed in the canyon. The diversity of macro-faunal organisms appeared to be greater at the shelf break than in the canyon. No systematic changes in sediment types with associated faunal assemblages could be grouped into distinct canyon habitats.


INTRODUCTION
Kaoping Canyon, located at about 22°27'N, 120°21 'E, is a major physiographic feature on the continental margin off southwestern Taiwan (Figure 1 ).The canyon was thought to end about 40 km from coastline due to limited deep-water soundings and was described as a glacially eroded submerged marine valley (Ma, 1947(Ma, , 1948(Ma, , 1963)).Studies related to submarine canyons around Taiwan, with few exceptions, have primarily used regional bathymetric survey data (Boggs et al., 1979;Chen, 1989).The recent discovery of the 240 km course of the Kaoping Canyon extending from the mouth of the Kaoping river down to the lower continental slope at a depth of about 3,000 m (Liu et al., 1991) has generated renewed interest Yu et al. (1991) described the morphology and hydroacoustic characteristics of the head of Kaoping Canyon using bathymetric profiles and 3.5 kHz echograms.The stratigraphy, structure, sediment dispersal and accumulation, and biological activity related to the fonnation and development of Kaoping Canyon have received little attention.
The curr ent study was initiated as a part of a cooperative program of undersea research between NOAA's National Undersea Research Center, The University of Connecticut at Avery Point, Groton, U.S.A. and the Institute of Oceanography, National Taiwan University, Taiwan, ROC.Herein we repon on the surface geology and biology at the head of Kaoping Canyon, and adjacent shelf regions, from direct underwater observations using a remotely operated vehicle (ROV).The objectives of this study were to: (1) describe surface sedimentary features and the microtopography of the head of Kaoping Canyon, (2) detennine the distribution of canyon fauna and associated habitat features, and (3) detennine if mass-wasting movements, bioerosion and other processes contribute to the geological and biological variations in the canyon.

Canyon Setting
Three submarine canyons have developed within a distance of 50 km along the conti nental margin off southwestern Taiwan (Figure 1).These canyons (i.e., Kaohsiung Canyon, Kaoping Canyon, and Fangliao Canyon) can be categorized into two types: the river exten sion type and the fault-related type.Kaoping Canyon belongs to the first type which runs across the continental shelf and is well aligned with the Kaoping river on land.Kaohsiung and Fangliao Canyons are assoc iated with the fault-related type (Yu andWen, 1991, Yu et al, 1992).Kaoping Canyon is the largest of these three canyons, extending over 240 Km from the mouth of the Kaoping river across the continental shelf and slope to the northern extension of the Manila Trench (Yu et al., 1991, Liu et al, 1992, Yu, 1992).The head of Kaoping Canyon, about 20 Km from the shoreline, is characterized by high and steep walls.Canyon relief exceeds (i()() m and cross-sectional morphology varies from V-shaped to broadly U-shaped (Yu et al, 1991).

l. MATERIALS AND METHODS
A MiniRover MK-II ROV was used to conduct survey dives in Kaoping Canyon from 2 to 8 May, 1991.Twenty-one daytime dives were made at 12 stations on the east and west canyon walls, and adjacent shelf (Figure 2).Observations were made on �e shelf at depths from 25-39 m, and on the eastern and western walls of the canyon from 45-161 m and 64-132 m, respectively.Station locations are given in Table 1.Total bottom time for the ROV was 17 .0hours.At each station, the support ship was anchored and the ROV surveyed an area within a 30-50 m radius from the downweight (Figure 3).Strong currents and topography limited the ability of the ROV to conduct continuous transects in the area, hence all observations were related to depth.Video imagery was recorded with an 8 mm videotape recorder.A 35 mm still camera was used to take high-resolution images of specific .,. :.: geologic features and organisms for species identifi cation.Image data was used to determine surface sediment types, canyon topography, habitat types and related geological and biological information.

General• Observations
The steep sides of the east wall consists of a 8eries of laminated clay outcrops   The diversity and • density of mobile macro-fauna was very low.Habitat complexity was also low and consisted of low relief sedimentary structures (e.g., burrows, depressions) and coral debris.Some species of mobile fauna were assoc iated with specific features (Appendix 1).For example: rockfish (Scorpaena sp.) occurred in burr ows or shallow depressions, a moray eel (Ophichthus sp.) was observed in• a small circular bmrow, gobies (primarily Am blyeleott is fasciata) were in and around burr ows and shallow depressions, and portunid crabs excavated burro ws and depressions around coral debris.Other taxa such as pleuronectid flounders (primarily Plagiopsetta glossa) and penaeid prawns were not assoc iated with any specifi c structure.The diversity of epifauna was also low.Coral debris was the only substrate for attach ment in this area, especially around the shelf break along the east wall.Crinoids, sea whips and soft corals were common taxa 1be steep outcrops along the canyon walls were heavily burr owed in some places.
Burrows were round to ovoid and generally 1-3 cm diameter.Most burr ows had distinct edges and no signs of collapse.However, no observations were made of any taxa which occupied.these features.

Specific Site Observations
Continental Shelf Northeast Of Canyon (25-42 m) -The sea floor surface at station 2 was mostly covered with cohesive silts and clays.Furr ows from the doors of trawl gear were comm on features.Scattered burrows were present but were less abundant relative to deeper areas in the canyon.Vrrtually no habitat structure (e.g., live coral, dead coral skeletons) was present in this area.
The area at station 3 was covered by sands, silts and clays with a cohesive appear ance.Trawl door furr ows were more extensive.than at station 2, indicating extensive fishing activities.A galatheid(?) crab was observed in a concave cone shaped burrow.A portu nid crab excavated a depression and horizontal burrow under debris.A burrowing shrimp (unidentified) was observed feeding at the entrance of a burrow 4-5 cm in diameter.
Eastern Canyon Wall (60-158 m) -Sediments along the shelf break and upper wall (stations 4-6; 60-80 m) generally consisted of sands, silts and clay.Sea whips, sea fans, soft corals, crinoids and hydroids were attached to biogenic debris and gravels.Shallow excavations, probably formed by crabs, were common around the attached fauna and piles of debris.Several extensive excavations (ca. 1 m in diameter) were also observed.Crabs (portunids and galatheids) and numerous small fishes (generally Amplyeleotris sp.) were assoc iated with these features.There were also small-scale sedimentary differences between stations.The sediments at station 5 were more cohesive and vertical burro ws 2-4 cm in diameter were common.Sand and gravel were more extensive at station 6. No-burro ws were observed at this station.
Between 80 and 158 m (stations 4, 7 and 8), the canyon wall consisted of a terraced slope with occasional slump structures.Clay outcrops were exposed along the vertical walls between terraces and coarse-grained materials such as sands, shell fragments and cobble were deposited at the base of these vertical segments.A sedimentary overhang was observed on dive 12 at 106 m above a 1.5 m vertical layered clay outcrop.The walls were heavily excavated in some sections with horizontal burr ows.No organisms were observed at the entrance of any of these sttuctures.
WC$tem Canyon Wall (58-135 m) -The slope was covered by silts and clays (stations 10-13).A prominent vertical clay outcrop of nearly 4 m high at 101 m was observed on dive 15 at station 10.Spur and groove (i.e., deeply incised vertical outcrops) were observed on the wall near 107 m depth on dive 15.A sedimentary overhang, like the one observed on the eastern wall, was observed above a 6 m vertical layered clay outcrop at 128 m on dive 18 at station 12.
Species of penaeid shrimps and pleuronectid flounders (generally Plagiopsetta glossa) were observed along the slope but were not assoc iated with any particular bottom structure.Large and dense aggregations of myctophid fishes were also observed in the water column and along the canyon wall.These fishes reacted strongly to the lights on the ROV so no useful observations regarding depth distribution and aggregation structure could be made.
On the upper slope (58-69 m), dense trawl door furr ows were partially filled by clays and silts.Sparse clusters of burrows were observed in this area but no mobile fauna was present.

DISCUSSION
The shelf areas we observed adjacent to Kaoping Canyon were covered by fine sands, silts and clays similar to those as described by Boggs et al. (1979).Shelf sediments also predominated along the eastern canyon wall.Localized coarse sands, gravel and shell fragments occurr ed on the upper east wall and •at the bottom of laminated clay outcrops (84-158 m).Deposits of coarse-grained material at the base of outcrops are probably due to mass -wasting movements along the steep faces.
Swface sediments on the western wall were similar to those on the eastern wall.How ever, coarse-grained gravel and shell fragments were not found at the bottom of outcrops.The lack of coarse-grained materials on the western wall could be related to the more gen tle slopes hence reducing downslope transport.Alternatively, cross-shelf transport processes (e.g., curr ents along the canyon axis) deposited coarse-grained material only on the eastern side of the canyon.
Based on the plane view of the bathymetry of the Kaoping Canyon head (Figure 2), the part of canyon cross ed by the the ROV transect can be represented by a meander bend (The Kaoping Canyon belongs to the river-extension type as mentioned previouly).The classical sedimentation model (Leopold and Wolman, 1960) of meander bends suggests that the inner bank (western canyon wall) is the site of deposition while the outer bank (eastern canyon wall) is the place of erosion.Lateral deposition of the western wall may result in a gentle slope while the erosion on the eastern canyon wall may result in a steep one.The sediments eroded from the eastern canyon wall fell into the base of the wall or canyon floor where finer sediments were winnowed to give a lag of coarse-grained malerials.
Distribution of sediments from the shelves across the canyon did not show any sys tematic changes in composition or assoc iated fauna.However, the predominance of coarse materials (i.e., gravel, shell, debris) at the shelf break along the eastern wall was probably responsible for the relatively high density of soft corals and other sessile fauna found there.
Topographic differences between the stations were most pronounced on the lower canyon walls where terr aced slopes or steep surfaces , interrupted with vertical outcrops, occurred.These are primarily erosional features resulting from slope failure of the canyon walls due to oversteepness and partly from the bioerosion of burr owing.organisms (e.g., Twichell et aL 1985).The microtopography of the canyon walls clearly demonstrates that canyon wall morphology is primarily .shaped by downslope processes and bic>erosional activity; factors which were not recognized by the previous studies using large-scale survey techniques such as bathymetric profiles and 3.5 kHz echograms (Yu et al . ,1991, Yu, 1992).It is noted that the Liuchu Hsu island, a carbonate buildup, is in the vicinity of eastern canyon wall.We postulated that the strata of eastern canyon wall are probably composed of carbonates which tend to resist erosion and maintain the cohesive strength to form the overhangs on the eastern canyon wall.Tax.a observed on the canyon floor, with the exception of penaeid shrimp and pleuronec tifonn flounders, were generally found utilizing some forms of shelter.The shelters were either distinct burrows in the silt-clay bottom, excavations around and under debris, or burial into the surface sediments.Change in faunal diversity and abundance with types of substrate in submarine canyons and continental shelves has bee n documented previously (Coo per et aL 1987,1988; Langton and Uzmann 1989; Auster et aL 1991; Stein et al. 1992).However, due to the extremely low densities of organisms and small sample sizes for each tax.a, it was not possible to establish statistically significant faunal-habitat relationships in the study area.
High densities of burrows were present in outcrops and along steep slopes on the canyon walls.The openings of most burrows had distinct edges as if recently bee n used, rather than rounded and partially collapsed as if abandoned.However, no burr ow occupants .were obsereved possibly because all dives were conducted during the day and the occupants of these structures may be active nocturnally.
The extensive trawling activities, which are apparent from trawl door furro ws in the sediments; could have reduced spatial complexity on the shelf.The relatively dense assem blage of emergent fauna (e.g., sea whips, sea fans, soft coral) at the shelf break on east wall may represent a refuge area for these species which had previously occupied a larger region of the shelf.
The information obtained from these exploratory dives suggests several avenues for future study: 1. Determine rates of erosion (e.g .•:from outcrops) and deposition (e.g., comparisons be tween the eastern and western slopes) l.n relation to specific canyon features.Each of the four areas outlined above would benefit from or require the use of direct underwater observation capabilities.Subjects 1 and 2 could be accomplished using a time lapse camera system to determine rates.A mobile underwater vehicle (i.e., large ROV or mann ed submersible) would be required for placement of the time-lapse system and measuring rods or sediment traps.Subjects 3 and 4 require a ROV or submersible to obtain the census data.
This study demonstrates the advantage of direct underwater observations for comple menting other larger scale surface based sampling teclmologies (e.g .. bathymetric profiling, grab samples).Additionally, fine-scale observations provide a perspective which can generate unique areas for future investigation.

Fig. 3 .
Fig. 3. Conceptual representation of a typical ROV deployment The support ship is anchored at the edge of the continental shelf and drifted back over the steep canyon wall.A downweight is used to reduce the effects of cmrent on the tether.A 50 m segment of tether connects the ROV to the down weight

Figure 4 .
Vertical structure and sedimentary features observed on six typical dives are summ arized in Appendix 2.

Fig. 4 .
Fig. 4. A summ ary of surface sediment distribution, topography, and sedimentary .featuresacross the canyon and on the adjacent continental shelf.
Fauna! diversity and abundance were .extremelylow at all stations.Estimates of faunal densities were not possible due to the steep topography of the canyon walls and the difficulty of conducting transects.Species-time census techniques (Michalopoul9s et al. 1992) could potentially be used if a survey design could take iµto account the long elapsed time (e.g., 20 minutes) between encounters with individual organisms.
2. Detennine bow and by what oragnisms were the burr ows formed.3. Compare shelf and slope faunas using species-time video survey techniques (e.g., Michalopoulos et al. 1992).4. Determine the effects of trawling on benthic habitats and living marine resources.

Table 1 .
Summ ary of dive locations.