Bank: Sidescan Sonar and Sediment Data from a Deep-Water Coral Reef Habitat off
Kathryn M. Scanlon1
Open-File Report 99-10
1U.S. Geological Survey, Woods Hole, MA 02543
2 National Marine Fisheries Service, Panama City, FL 32408
The Experimental Oculina Research Reserve (EORR) is located along the shelf edge off east-central Florida in water depths of about 60 to 100 meters. It is about 7.5 km wide and 43 km long and encompasses numerous high-relief rocky pinnacles where Oculina varicosa, a fragile deep-water coral, grows. These coral reefs have historically been the sites of prolific grouper spawning aggregations and have supported a large variety of other reef fish (Gilmore and Jones, 1992). Serious decline of the fishery in the area prompted the establishment of the EORR.
The data presented in this open-file report were collected as part of a cooperative project between the U.S. Geological Survey (USGS) Coastal and Marine Geology Program and the National Marine Fisheries Service (NMFS) of the National Oceanographic and Atmospheric Administration (NOAA). The projects goal was to provide reconnaissance geologic maps of the Experimental Oculina Research Reserve and an unprotected control area north of the reserve to support the NMFS studies of grouper spawning aggregations. To accomplish this, we collected sidescan sonar data and sediment samples throughout both study areas and used video and observations from a manned submersible at selected sites. This report includes digital mosaics of the sidescan sonar data, tabulated sediment data, and interpretative maps of the seafloor geology. The video and submersible observations are not included in this report, but were used in the interpretation of the sidescan data.
History of the Oculina Reserve
In 1984, a 322 km2 portion of the Oculina Bank was established by the South Atlantic Fishery Management Council (SAFMC) as a Habitat Area of Particular Concern (HAPC). This area (east and west boundaries, 79°56W and 80°00W, north and south boundaries, 27°53N and 27°30N) was closed to mobile fishing gear such as trawls and dredges. In 1994, the same area was declared the Experimental Oculina Research Reserve and was closed to all bottom fishing for a trial period of 10 years, and in 1995 to all anchoring in perpetuity. The impetus for the additional restriction was the observation of severely altered demographics in gag and scamp grouper populations (Koenig and other, in press; Coleman and others, 1996) resulting from aggregation fishing and the observation of such aggregations in the Oculina HAPC (Gilmore and Jones, 1992). Trolling (fishing for coastal pelagics) is presently permitted within the Oculina reserve, but closure is pending.
For our study, we designated a 180 km2 control area (where fishing was still allowed) of similar habitat about 20 km north of the EORR. The east and west boundaries of the Control Area are 79° 59' W and 80° 02 W; north and south boundaries are 28° 22 N and 28° 04 N. Our original study objectives included mapping of habitat features, monitoring demographics of grouper spawning aggregations, and monitoring fish community structure and abundance. However, the extensive Oculina habitat damage observed by Reed (1980) was found to be much worse in 1995 (Koenig et al. unpublished data), so we added the goal of Oculina habitat restoration to our list of objectives. This report is concerned with mapping of habitat features. More information on the other aspects of the project can be found in Appendix I, the information handout (download ~14MB), and in Koenig and others (in press).
To assess the extent of various types of seafloor habitat in the EORR and the control area we undertook to map the area using sidescan sonar. In 1995, using the NOAA ship R/V Chapman, sidescan-sonar data were collected with continuous coverage over the study areas. The data were logged digitally using the SIS-1000 sidescan instrument with the ISIS data logging system, set to yield a 375-meter range (total swath width of 750 m). The data were decimated to a 0.4-m pixel size using a median filtering routine developed by Malinverno and others (1990). They were then processed using procedures developed by Danforth and others (1991) and Danforth (1997) which include corrections to the slant range (to remove the water column artifact and convert slant-range distance to true ground distance), destriping (to correct minor striping noise or dropouts), and beam angle (to correct variations in beam intensity). Further processing using the routines developed by Chavez (1986) as modified by Paskevich (1992) for application to high-frequency sidescan-sonar imagery was performed to remove additional noise and to orient each sidescan line in space. Digital mosaicking was accomplished using the PCI1 Remote Sensing software package as described by Paskevich (1996). This dataset was mapped at a resolution of 2m/pixel in a UTM zone 17 projection with the WGS84 ellipsoid. Darker tones on the sidescan-sonar images represent areas of relatively low acoustic-backscatter intensity and lighter tones, areas of high backscatter.
Lines were spaced about 625 meters apart, giving sufficient overlap of the adjacent 375-meter swaths for digital mosaicking. The fish was towed at approximately 3.5 to 4.0 kts. Chirp subbottom data were also collected simultaneously with the sidescan sonar data.
Ship navigation for the sidescan tracklines was by Global Positioning System (GPS), using a military p-code descrambler for increased accuracy. It is estimated that the ships position was known to within 10 to 20 meters. The offset between the ship's position and the position of the sidescan towfish was estimated based on the length of cable out and by correlation of distinctiver seafloor features in overlapping swaths. The digital navigation data is included in the navigation files.
1 Use of tradenames is for description purposes only and does not
imply endorsement by the federal government.
In 1995, four sediment samples were collected in the EORR during dives of the Clelia, a research submarine operated by Harbor Branch Oceanographic Institution. In 1996 and 1997, 65 sediment samples were collected during cruises of the NOAA Research Vessel Chapman using a van Veen grab sampler. In all, 52 of the samples are from the EORR and 17 from the unprotected control area north of the EORR.
All samples (except those that were made up of chunks of coral or coral rubble) were analysed for particle size and carbonate content in the sedimentology laboratory of the U.S. Geological Survey at Woods Hole, Massachusetts. Texture terminology used in this report is according to Folk (1974). The percent of calcium carbonate material was determined by weight loss of 15 grams of bulk material after digestion with 10 percent hydrochloric acid. Details of the laboratory techniques can be found in Appendix II: Sediment Texture Analysis Techniques. The data are presented in Appendix III: Table of Sediment Analyses; (also in *.xls, Excel format)
Previous studies of the physiography of the East-Florida shelf edge in this region (Avent and others, 1977; Thompson and Gilliland, 1980; Thompson and others, 1978) describe steep bathymetric highs (called "pinnacles" because of their appearance on vertically exaggerated echo-sounder records) along parts of the shelf edge off eastern Florida. A deep-water, ahermatypic coral, Oculina varicosa, commonly called the "ivory tree coral", grows on these bathymetric highs (Reed, 1980). The pinnacles, which are clustered near the 80 meter isobath, are up to 30 meters in height above the surrounding sea floor and vary in diameter from 10s of meters to several 100s of meters. Rock dredge samples obtained by Macintyre and Milliman (1970) from the pinnacles were mainly oolitic limestones and some algal limestones, and had radiocarbon ages from late Pleistocene to early Holocene. The presence of Callianassa sp. shrimp burrows and relict hermatypic coral heads suggest a shallow water origin. Macintyre and Milliman (1970) interpret the pinnacles to be oolitic dunes that were deposited and lithified in a marine environment during the Holocene transgression. Subsequent erosion by the strong Gulf Stream currents and growth of ahermatypic corals has produced the irregular high-relief pinnacles seen in the study area today.
The study area lies near the western edge of the Gulf Stream, a major northward-flowing current. During our research cruises in the region, surface currents of up to 4 kts (200 cm/sec) were observed. The direction of the current was always within a few degrees of due north. This surface current created significant problems during sidescan, sampling, submersible and Remotely Operated Vehicle (ROV) field operations.
Sidescan data were affected because to maintain a constant 4 kt speed over the bottom, the ships speed through the water varied from nearly 0 kts to nearly 8 kts (400 cm/sec), depending on whether we were travelling with or against the current. The sidescan tow fish is designed to be most stable at speeds of about 4 kts (200cm/sec) through the water; excessively high and low speeds cause the tow fish to pitch, yaw, and crab. Since we did not have the ability to locate the sidescan fish precisely, but relied on the position of the ship to approximate the position of the tow fish, these motions of the tow fish made mosaicking of the image data more difficult.
The effect of the surface current on the ROV, submersible, and sampling operations was mainly an increase in the difficulty of holding station. Many dive days were lost because the combination of moderate winds and the strong current produced "confused" seas, making launch and recovery of the manned submersible dangerous.
Bottom currents in the Oculina Bank area are not as strong as the surface currents. In June, 1995, the U.S. Navy (T. Szlyk, personal commun.) measured current velocity near the surface and at depth across the shelf at latitude 27°27, about three nautical miles south of the southern boundary of the EORR. Currents near the bottom (about 15 m above the bottom), at depths similar to those in our study areas, were between 1 and 1.5 kts (50 to 75 cm/sec) and due north. Currents near the surface were also due north and between 3.25 and 3.75 kts (162 to 175 cm/sec). These findings were consistent with observations made during submersible and ROV deployments in the study areas (Koenig, personal commun.).
Currents at the sediment-water interface are undoubtedly less than those observed 15 meters above the seafloor. Extrapolation of the unpublished U.S. Navy current profiles (Szlyk, personal commun.) suggests that currents at the seafloor could be 0.5 kts (25 cm/sec). That velocity would be enough to erode silt and sand (e.g., Hollister and Heezen, 1972; Reineck and Singh, 1980). Indeed, our video data taken near the pinnacles at the northern end of the EORR showed lag deposits of finger-sized pieces of broken Oculina lying on top of finer-grained unconsolidated sediment.
The sediment samples were collected between 59 and 110 meters water depth. (The pinnacles are based near the 80-meter isobath, but extend up to 30 meters above the surrounding sea floor.) The deeper samples (water depth greater than 90 m) generally have a higher percentage of silt (that is, they are finer grained) than the shallower samples (graph silt vs. depth). Samples taken near pinnacles and in scoured areas are generally sands and gravels and contain less than 20% silt. A few have high silt contents in the 20% to 30% range, similar to the deeper samples, and one sample stands out with more than 70% silt. The sediments from the pinnacle and scoured areas (60 to 90 m) have been winnowed by the strong bottom currents, leaving behind only the coarser sands and gravels. The few samples with the higher silt content probably came from pockets of fine sediment trapped in depressions or in the lee of bathymetric highs. In general, the finer-grained sediments have lower percentages of calcium carbonate than the sands and gravels (graph of silt vs. CaCO3).
Calcium carbonate content (Appendix III) also shows a general correlation with water depth (graph of %CaCO3 vs. Depth). All of the samples collected from water depths greater than 100 m have less than 60% calcium carbonate content, whereas most of the samples from water depths between 70 and 90 meters (the depth range surrounding the pinnacles) are between 80% and 100% calcium carbonate. Those from sites shallower than 70 m have a wider range of calcium carbonate contents (from 55% to 95%). No samples were collected from water depths between 90m and 100m. This distribution of calcium carbonate is not surprising, because visual inspection of the coarse fraction of our sediment samples reveals that the corals on the pinnacles are a major source of the carbonate in the sediments.
On the basis of the sidescan-sonar data, analyses of sediment samples, direct observations from submersibles, and video collected using an ROV, we divided the seafloor of the EORR and Control Area into three types. They can be described as high relief/high backscatter, low relief/high backscatter, and low relief/low backscatter and are shown in the bottom-types map.
High relief/high backscatter (HR/HB)
The HR/HB areas are concentrated along the 80-meter isobath. They are often isolated peaks, rising a few to 30 meters above the surrounding seafloor. Two large, elongate areas of multiple peaks, ledges, and outcrops are mapped in the northern portion of the EORR and the northern portion of the Control Area. The HR/HB terrain is very rough and rocky and frequently supports a variety of echinoderms, molluscs, and sponges. Oculina rubble is ubiquitous on and around the outcrop. Living Oculina was seen occasionally. These high relief areas are preferred by numerous species of reef fish for spawning and feeding (Koenig and others, in press) and are the only areas where large Oculina thickets are known to grow in the region (Reed, 1980). This seafloor type constitutes about 3% of the total area enclosed in the EORR.
Low relief/high backscatter (LR/HB)
LR/HB areas generally surround HR/HB areas and range from 70 m to 90 m in depth. These areas may contain some rocky hardbottom, but with less than a meter of relief. Much of the area is covered with gravelly sand, with carbonate contents greater than 90 percent. In some areas the sediment cover is thin (a few centimeters) over a rocky hardbottom. The sidescan data show approximately north-south streaks in the LR/HB areas, which we interpret to be caused by winnowing and scour by the strong bottom currents. Echinoderms, mollusks, and sponges may be present in these areas, but Oculina colonies are small (Reed, 1980) and we observed few demersal fish.
Low relief/low backscatter (LR/LB)
Seaward and landward of the high relief areas and between the isolated peaks, the seafloor appears to be smooth and is covered by sands and muddy sands, which have a low backscatter signature on the sidescan image. The samples collected from deeper than 90 meters, on the uppermost slope, contain markedly less calcium carbonate (45% to 60%) than other samples. The samples from areas landward of the pinnacles and between the pinnacles have textures similar to the deepest samples but vary in calcium carbonate content from about 50% to about 95%.
We have mapped the extent of three types of seafloor: High relief carbonate outcrops, low relief areas with high acoustic backscatter (carbonate gravel or hard bottom, with or without a thin carbonate sand cover), and areas of finer-grained sediment (sand) of variable carbonate content. The rocky, high relief habitat accounts for about 3% of the total area encompassed by the EORR. Near the high relief pinnacles, the seafloor may be covered with gravel or gravelly sand with CaCO3 contents approaching 100%. Seaward of the pinnacles, the seafloor is covered by sand and muddy sand with calcium carbonate contents below 60%. Landward of the pinnacles, the sediments are sands and muddy sands with a wide range of calcium carbonate contents.
Control Area Links
Chavez, P.S., Jr., 1986, Processing techniques for digital sonar images from GLORIA: Photogrammetric Engineering and Remote Sensing, v52, no. 8, p.1133-1145.
Coleman, F.C., C.C. Koenig, and Collins, L.A., 1996. Reproductive styles of shallow-water grouper (Pisces: Serranidae) in the eastern Gulf of Mexico and the consequences of fishing spawning aggregations. Environmental Biology of Fishes 47:129-141.
Danforth, W.W., O'Brien, T.F., and Schwab, W.C., 1991. Near real-time mosaics from high-resolution sidescan sonar - an image processing technique to produce hard-copy mosaics 'on site' proved successful during USGS survey, Sea Technology, v.32, no.1, pp54-59.
Danforth, W.W, 1997. Xsonar/ShowImage: A complete system for rapid sidescan sonar processing and display, USGS Open-File Report 97-686, pp. 77.
Folk, R.L. (1974). Petrology of sedimentary rocks. Hemphill Publishing Co., Austin, TX, 182 p.
Gilmore, R. G., and Jones, R.J., 1992. Color variation and associated behavior in the epinepheline groupers, Mycteroperca microlepis (Goode and Bean) and M. phenax Jordan and Swain. Bull. Mar. Sci. 51:83-103.
Hollister, C.D. and Heezen, B.C, 1972, Geological Effects of Ocean Bottom Currents, in : Studies in Physical Oceanography, v.2 pp. 37-66, A.L. Gordon, ed., New York, Gordon & Breach.
Koenig, C. C. and Coleman, F.C., 1998. Absolute abundance and survival of juvenile gags in seagrass beds of the northeastern Gulf of Mexico. Trans. Amer. Fish. Soc. 127: 44 55.
Koenig, C.C., Coleman, F.C., Fitzhugh, G.R., Gledhill, C.T., Scanlon, K.M., Grace, M., and Grimes, C.B., in press. Networks of marine reserves for the conservation of warm temperate reef systems of the southeastern United States, Bulletin of Marine Science.
Malinverno, A., Edwards, M., and Ryan, W.B.F., 1990, Processing of SeaMARC swath sonar data: IEEE Journal of Oceanic Engineering, vol. 15, p. 14-23.
Macintyre, I. G. and Milliman, J.D., 1970, Physiographic features on the outer shelf and upper slope, Atlantic Continental Margin, southeastern United States, Geol. Soc. of America Bull., v. 81, p.2577-2598.
Paskevich, V., 1992, Digital mapping of side-scan sonar data with the Woods Hole Image Processing System software: U.S. Geological Survey Open-File Report 92-536, 87p.
Paskevich, V., 1996, MAPIT: An improved method for mapping digital sidescan sonar data using the Woods Hole Image Processing System (WHIPS) Software: U.S. Geological Survey Open-File Report 96-281, 73p.
Poppe, L. J., Eliason, A. H., and Fredericks, J. J., 1985, APSAS: An automated particle-size analysis system: U.S. Geological Survey Circular 963, 77 p.
Reed, J. K., 1980. Distribution and structure of deep-water Oculina varicosa coral reefs off central eastern Florida. Bulletin of Marine Science 30:667-677.
Reineck, H.-E. and Singh, I.B., 1980, Depositional Sedimentary Environments, Springer-Verlag, New York, 549p.Shideler, G.L., 1976, A comparison of electronic particle counting and pipette techniques in routine mud analysis: Journal of Sedimentary Petrology, v. 42, p. 122-134.
Thompson, M.J. and Gilliland, L.E., 1980, Topographic mapping of shelf edge prominences off southeastern Florida. Southeastern Geology, v.21, no.2, p. 155-164.
Thompson, Gilliland, L.E. and Mendlein, J.E., 1978, Bathymetric mapping of three selected areas on the southeastern Florida Continental Shelf. Harbor Branch Foundation, Inc. Technical Report No. 027, 54p.Virden, W.T., Berggren, T.L., Niichel, T.A., and Holcombe, T.L., (1996) Bathymetry of the Shelf-edge Banks, Florida East Coast, unpublished report, National Geophysical Data Center, Boulder, CO).
Oculina varicosa, ivory tree coral, forms the primary habitat structure of Oculina reefs. These reefs grow in the form of 1 to 2 m diameter spherical coral heads that resemble bushes and form a thicket-like habitat. Coral thickets grow only on high-relief (5 - 20 m) shelfedge (70 100 m water depth) pinnacles which are scattered throughout the reserve and to the north of the reserve.
In our submersible and ROV studies, conducted annually from 1995 to 1998, we observed extensive damage to the Oculina habitat, ranging from toppled and broken coral heads to completely destroyed habitat. The only intact Oculina reef we observed was "Jeffs Reef", a small double pinnacle reef only about 4 hectares in area located at the southern-most end of the reserve. Although we cannot prove that Oculina habitat damage is the result of trawling, it is highly likely. We deduce trawl damage from the fact that the non-living bases of the coral heads are missing (or pulverized) from the areas of greatest damage and the rubble in these areas is of a uniform size, as if repeatedly sieved. Considering that the non-living bases of the coral heads were probably hundreds of years old, it is unlikely that they would be lost from the areas of greatest damage without some form of mechanical removal (or crushing). If they had been removed by either storms or high currents, then the living intact Oculina on "Jeffs Reef" and the toppled and broken Oculina coral heads elsewhere would have been removed as well. The fact that "Jeffs Reef" is untouched may be because it is rather small and isolated from other pinnacles. Additional support for the contention that trawling is the cause of Oculina habitat destruction is the public testimony of some rock shrimpers, who stated that they attempt to trawl "close" to the coral because rock shrimp are most abundant there.
Transplantation and recruitment experiments are underway to evaluate the restoration potential of the damaged habitat areas, and the reproductive biology of the coral is being studied. We need to understand why Oculina has not recolonized the destroyed areas. Is there insufficient or inappropriate substrate? Is larval transport limiting? Are there localized environmental influences that preclude establishment of viable colonies? Our field experiments are designed to answer these questions. The growth rate of Oculina is too slow to provide unaided restructuring of Oculina habitat in the foreseeable future; however, if stable settlement or transplant substrate that would simulate the non-living portion of the coral head could be put in place, then recovery of the habitat--and repopulation of the associated fish community--might occur much more rapidly. At a growth rate of 1 2 cm per year, one meter diameter colonies could be expected in 30 years from transplants. Restoration of such Oculina habitat is similar in time-frame to the restoration of a forest in a clear-cut area.
Data gathered on manned submersibles (Harbor Branch Oceanographic Institute, HBOI, research submersibles) before and after a 15 year period of intensive fishing in the reserve area (spring 1980 compared to spring 1995) indicate a dramatic decline in abundance of grouper, snapper, and amberjack species. In the 1980 videos, taken primarily on "Jeff's" (~4 hectares) and "Chapman's reefs" (~70 hectares) in the southern portion of the reserve, species such as snowy grouper, speckled hind, black sea bass, gag, scamp, Warsaw grouper, blackfin snapper, greater amberjack, little tunny, and red porgy were abundant. Our 1995 videos indicate that they are now greatly reduced or absent. In addition, the gag spawning aggregations documented in 1980 have been fished to extinction, and the scamp aggregations have been reduced from hundreds of large (>50 cm total length) to fewer that 10 small (<35 cm total length) individuals. Analysis of the 1980 video indicated that the historic proportion of males (~ 20 %) was present in these unfished aggregations.
Proof of continued bottom fishing within the reserve comes from our Oculina transplant studies. In that work we used large (~ 1m2) concrete block structures to transplant the Oculina colonies. Each block had a subsurface float attached about 5 m above the block for exact echosounder positioning of the transplant blocks. In 1997 we examined these structures in situ and found that they were covered with fishing line and one even had a planer snagged on its side. We also observed a fishing boat bottom drift fishing over "Jeffs reef" while we were conducting ROV operations there. Clearly, fishermen were continuing to bottom fish, some while appearing to be trolling for coastal pelagics. Considering the small amount of intact Oculina habitat in the reserve, it wouldnt take much bottom fishing to negate the positive effects of closure.
Our studies in the Oculina reserve are instructive in that they show that highly productive habitat has been and is being destroyed without our knowledge. The net effects of spawning habitat destruction on fishery production are unknown, but are likely to be serious and additive to those effects produced by overfishing.
Enforcement should be a prime consideration in marine reserve establishment, as we have seen in the Oculina reserve. Regulations concerning the reserve must be formulated to make enforcement not only possible but also efficient. As we have seen in the Oculina reserve, allowing one type of fishing (trolling) and disallowing another (bottom fishing) doesnt work because it is virtually impossible for enforcement agents to know how deep one is fishing. Shelf-edge habitats on the west Florida shelf, because of their great distance from land, would require special surveillance probably by air that would include day and night observations. Clearly, without proper enforcement conclusions concerning the effectiveness of reserves as management tools will be seriously hindered as they have been in the Oculina reserve.
If the sediment sample contained gravel, the entire sample was analyzed. If the sample was composed of only sand, silt, and clay, an approximately 50-gram, representative split was analyzed. The sample to be analyzed was placed in a pre-weighed 100-ml beaker, weighed, and dried in a convection oven set at 75°C. When dried, the samples were placed in a desiccator to cool and then weighed. The decrease in weight due to water loss was used to correct for salt. The weight of the sample and beaker less the weight of the beaker and the salt correction gave the sample weight.
The samples were disaggregated and then wet-sieved through a number 230, 62µ (4ø) sieve using distilled water to separate the coarse- and fine-fractions. The fine fraction was sealed in a Mason jar and reserved for analysis by Coulter Counter (Shideler, 1976). The coarse fraction was washed in tap water and reintroduced into the pre-weighed beaker. The coarse fraction was dried in the convection oven at 75°C and weighed. The weight of the coarse (greater than 62µ) fraction is equal to the weight of the sand plus gravel. The weight fines (silt and clay) can also be calculated by subtracting the coarse weight from the sample weight. The coarse fraction was dry-sieved through a number 10, 2.0 mm (-1ø) sieve to separate the sand and gravel. The size distribution within the gravel fraction was determined by sieving.
The sand fraction was dry-sieved at whole phi intervals using a Ro-Tap shaker. The fine fraction was analyzed by Coulter Counter. To mitigate biologic or chemical changes, storage in the Mason jars prior to analysis never exceeded five days. The gravel, sand, and fine fraction data were processed by computer to generate the distributions, statistics, and data base (Poppe and others, 1985). One limitation of using a Coulter Counter to perform fine fraction analyses is that it has only the ability to "see" those particles for which it has been calibrated. Calibration for this study allowed us to determine the distribution down to 0.7µ or about two-thirds of the 11ø fraction. Because clay particles finer than this diameter and all of the colloidal fraction were not determined, a slight decrease in the 11ø (and finer) fraction is present in the size distributions.