The Emerging Role of Marine Geology in Benthic Ecology
Gordon B. J. Fader, R.A. Pickrill, Brian J. Todd,
Robert C. Courtney and D. Russell Parrott

Introduction
Over the past decade, marine geologists have become increasingly involved in the application of marine geoscience to biological issues of habitat characterization, gear impact assessment and fisheries management. This new application of geoscience information is a direct result of significant advances in the resolution and accuracy of seabed mapping technologies and an improved understanding of complex seabed processes. Early seabed maps were generally inadequate to meet the requirements of the biological community because they could not provide sufficient detail for a direct correlation between sediment type and biological habitat (Kenchington and Halliday, 1994). The goal of these early regional maps was to characterize the seabed sediment distribution over broad areas of continental shelves for geological purposes and was based on sparse samples and acoustic echogram interpretations from widely spaced data sets.(King, 1970; King and Fader, 1986). While not designed for biological purposes, they did assist the fishing community in specific applications, such as the avoidance of regional seabed hazards (boulder zones and rough terrain) and the location of new grounds for potential fisheries such as clam, Icelandic scallop and lobster. Ongoing data collection and supplementation with regional bottom photographic data bases (Lawrence et al., 1989) and data from the use of new technologies, such as sidescan sonar, enhanced these maps. For example, a series of regional maps, know as Canadian Fishermen´s Charts, designed specifically to support the fishing industry, (produced by Nordco Ltd. of Newfoundland) depicted surficial geology from the earlier mapping of the Geological Survey of Canada, bathymetry, and the location of seabed hazards to bottom fishing gear determined by the fishing industry.

Traditional geological seabed mapping has focused on defining surficial sediment units or formations, which are basic map units that can be traced over large areas. These early regional geological maps relied on statistical sample grids spaced at a variety of distances from 100s of metres to kilometres. The advent of sidescan sonar presented the geologist with imagery of the seabed similar to aerial photography of land. Consequently the sample methodology changed to focus on providing ground truth for the sediment acoustic characteristics and features interpreted from this data. That significantly reduced the requirement for a large number of samples and allowed accurate correlation between samples. Until the 1980s, a lack of precise navigation was also a major limiting factor in the production of detailed seabed maps.

Over the past 10 years, major developments in seabed mapping technology have occurred that meet many of the requirements of the marine biological community. These include high-resolution sidescan sonars, multibeam bathymetric mapping systems, precise navigation, precision sampling and photographic systems, and advances in digital data processing and scientific visualization techniques. In this paper we describe recent research at GSC Atlantic utilising these seabed mapping techniques that contributes to habitat mapping, ecological research and fisheries management.

Figure 1: Index map of study area with locations of following illustrations

Geological Habitat
Adequate attention to habitat is considered by some to be the most fundamental of ecological concepts, often missing from ocean fisheries management (Willison and Butler, 1998). To address this issue, closer cooperation between marine biologists and marine geologists is essential. The following is a list of critical geological seabed attributes considered to be of ecological importance based on initial and ongoing discussions through cooperative projects:

- Micro relief - centimetres to decimetres (roughness)

- Macro relief - metres to 100s of metres (topography, morphology, slope)

- Grain size (gravel, sand, silt and clay) - lithology (rock composition)

- Patchiness (local variability, shape, spatial patterns)

- Sediment distribution

- Sediment sorting

- Porosity (pore spaces, packing)

- Grain shape (roundness, sphericity)

- Stratigraphy (layering - centimetres to decimetres)

- Dynamics/processes (relict to modern and combinations)

- Bedforms (all scales, centimetres to 10s of km)

- Sediment transport pathways (net and varying directions)

- Sediment thickness (centimetres to metres)

- Regional setting (e.g. sandbank, moraine, beach ridge, basin)

- Geological history (origin)

- Anthropogenic features (shipwrecks, anchor marks, cables, debris)

Some of these attributes, such as sediment grain size and lithology, are more easily determined than other attributes, provided that valid seabed samples can be collected. This is not a trivial matter in coarse gravely sediment. Other characteristics such as porosity and high-resolution stratigraphy are more difficult to determine, as the process of sampling frequently destroys the fabric of the seabed material, particularly in coarse-grained sediments. For these areas, reliance is placed on remote sensing by acoustic means and insitu geotechnical methods for assessment. The measurement of many of these attributes remains an area of continued research.

Figure 2. 330 kHz sidescan sonogram from Bedford Basin, Halifax Harbour, collected with a neutrally bouyant and decoupled towfish, configured to minimize towfish motion. Image shows flat muddy seabed with linear anchor drag marks. The approximately 20 rectangular features on the seabed are discarded Volvo automobiles whose roofs have been colapsed before dumping. Known targets like these provide calibration for trials to improve sonar resolution. The resolution and image quality is considerably better than data collected with conventional towing arrangements.

Tools and Techniques
The use of sidescan sonar demonstrated to the marine geologist that the morphology and distribution of sediments on the seabed was considerably more complex than previously thought. Additionally, sonograms could be interpreted to assess seabed dynamic conditions using geological knowledge of sediment bedforms, transport pathways and other seabed processes to determine temporal and spatial mobility.

As awareness of the capability of this new technology increased, the biological community recognised an application to address some of its concerns. This led to co-operative programs for assessment of habitats such as lobster off Cape Breton Island; clam, Icelandic scallop and the sea cucumber fishery of the Grand Banks of Newfoundland and the shrimp fishery in Chedabucto Bay. New and unforeseen biological applications of geological mapping technology included the detection of ghost nets on the seabed, the optimum siting of water intakes for lobster pounds, and the assessment of aquaculture sites and their effluent effects on the seabed.

Both the biological and geological communities, desire a capability for higher-and higher- resolution mapping of the seabed. Within the Geological Survey of Canada (Atlantic), one approach has been to maximise the resolution of existing systems that traditionally have been operated in a lower resolution regional mapping mode. For example, 330 kHz sidescan sonars can be operated with ranges of 25 m and slow towing speeds of 2 knots to approach their theoretical resolution limits of approximately 10 cm and 100% seabed insonification. To achieve this resolution routinely, modifications to existing sidescan systems have been made to reduce tow fish motion and increase stability. Depressor weights are positioned on the tow cable, from which a neutrally bouyant fish is towed close to the bottom. This decouples the tow fish from ship motion. In conjunction with improving towing characteristics, positioning has been improved by using short baseline transponders to position the tow fish relative to the ship, while inertial motion sensors provide data on pitch, roll and heave to correct sonograph distortion. The net result of these developments has been a significant improvement in mapping resolution and micro habitat mapping. (Figure 2).

A second approach toward high resolution seabed mapping is the extraction of seabed attributes from digital multibeam bathymetry, sidescan sonar and seismic reflection data. At present much of the backscatter and reflectance data (sediment type) represents only relative changes in seabed character. Through the calibration of existing acoustic surveying systems, additional information on seabed roughness, slope and relief can be extracted. We are in the early stages of this approach and have completed the calibration of seismic and sidescan systems. A series of acoustic experiments were conducted on Browns Bank in the summer of 1998. The results from this effort, when coupled with theoretical modelling, should lead to a better understanding of which of the physical parameters of the seabed are manifested in the acoustic reflection and backscatter data.

Multibeam bathymetric mapping has provided a new opportunity to produce high-resolution maps of seabed morphology over relatively large areas. It was first applied in Canada in the eastern offshore in 1990 (Courtney, 1993; Courtney and Fader, 1994, Courtney et al., 1993), through the use of vessels fitted with an array of boom-mounted transducers, followed by more sophisticated hull-mounted multibeam transducer arrays. The most significant aspects of this new technology are 100% coverage of seabed morphology, an ability for interpretation of regional seabed processes, and the portrayal of subtle aspects of deposition and erosion through techniques of scientific visualization. In addition to the complete coverage of the seabed, producing detailed morphological images (shaded relief bathymetry), these systems also provide backscatter information from the seabed (texture, roughness and lithology). This is a major improvement in seabed geological assessment as it provides a remote mapping tool of seabed type that is georeferenced with morphology, and generated by the same sensor package. These systems, and the products produced, have revolutionized the marine geologists´ view of the seabed and understanding of seabed processes.

Figure 3. High resolution sidescan sonar mosaic of the seabed in Lobster Bay, southwest Nova Scotia. The sidescan data were collected and processed at 50 m range and 330 kHz. This provides very high resolution and can identify individual boulders at the seabed greater than 0.20 m. The image shows hard boulder-covered seabed as an offshore extension of drumlin shoals and islands in Lobster Bay. Deeper areas are sand to muddy sand. These mosaics provide the geological framework as a basis for conducting lobster assessment surveys

PARTNERSHIPS
As a direct result of the applicability of modern geological tools to fisheries and benthic habitat issues, and in response to pressure for sustainable management of fish stocks, GSC Atlantic has partnered with DFO on several new research projects in the last two years. GSCA has provided the geological framework within which these issues can be addressed. The following is a discussion of several of these co-operative projects that illustrate the new and evolving role of geoscience in habitat assessment. These include a zonal lobster research project (ZLORP); an assessment of scallop habitat on Browns Bank and studies on the effects of trawling and clam harvesting on the seabed.

ZLORP
The zonal lobster research program (ZLORP) uses geological mapping to develop an understanding of the spatial dynamics of benthic settlement patterns of lobsters in relation to seabed characteristics in terms of habitat structural elements and biotic assemblages. Fisheries managers require information on the location, nature and areal extent of lobster recruitment habitats within the broad regional lobster production areas. Understanding the ecology of lobsters during the first three years of benthic existence provides essential information for long-range (five to seven year) recruitment forecasting for the Canadian fishery. Additionally, detailed knowledge of habitat location and characteristics is essential for design of a recruitment monitoring program and successful stock management.

High-resolution geological mapping and seabed characterization of nearshore lobster habitat was undertaken in areas of the western Gulf of St. Lawrence and southwestern Nova Scotia. High-resolution sidescan sonar mosaics (Figure 3) have been constructed, geologically interpreted and provided to lobster biologists to form the basis for ground-truthing visual, video and sampling diver transects. Initial results indicate that the geological assessment of seabed sediment distribution, topography and dynamic conditions of sediment transport provides essential information for understanding the spatial and temporal distribution of lobster recruitment habitats.

Scallop Assessment
A new co-operative project between industry, DFO and GSCA has been developed to explore the application of geoscience information to the sea scallop fishery on Browns Bank, western Scotian Shelf. The sea scallop is one of the most important commercial invertebrate species in east ern Canada, second only to lobster in landed value. Sea scallops are generally found in areas of gravely seabed but little is known concerning other preferred bottom characteristics.

Multiple objectives being pursued are:

· To understand the relationship between historical catch effort, returns and substrate

· To understand the relationship between scallop distribution, ecology and seabed habitat,

· To provide information to enhance fishing efficiency such as obstacle and hazard avoidance and optimize fishing practices,

· To provide a knowledge base for the sustainable management of the fishery.

In meeting these objectives, GSC Atlantic has also collected critical information for the primary geoscience program, to understand the glacial and postglacial evolution of the Scotian Shelf which provides the framework for the distribution of surficial sediments and features.

Initiated with the collection, processing and interpretation of multibeam bathymetry, grids of high-resolution seismic reflection and sidescan sonar data have been collected along with large-volume samples, video and photographic information. The integration of these data sets provides considerable insight, enabling the separation of ancient and modern processes acting on Browns Bank, both of which affect the distribution and abundance of scallops. For example, the northern flank of Browns Bank is the Fundian Moraine, a prominent 15-m high, 2-km wide and 80-km long, steep-walled feature, covered with large boulders. The moraine formed during the retreat of glaciers from the western Scotian Shelf approximately 20,000 years ago (Figure 4). Observations from the northern side of the moraine show the presence of dense communities of juvenile scallops associated with gravel in the granule to pebble size range. Other areas to the south of the moraine consist of larger more mature scallops. The distribution of the juvenile scallops in this preferred location suggests that strong currents moving from northwest to southeast across the bank from the Bay of Fundy, are modulated by the prominent moraine, developing suitable seabed habitat characteristics and concentrating a source of abundant food for the juvenile scallops.

Future developments in this project will explore the extraction of additional information on sediment properties from the calibrated multibeam bathymetric backscatter information. This information will be combined with scallop catch data and sediment transport information to better understand the complex relationship between scallop distribution and seabed geology. It is envisioned that a series of geological seabed assessments, provided as layers of information on seabed characteristics and dynamic conditions, will provide an underpinning for both improved fishing practice and fisheries management.

Figure 4. Multibeam bathymetric image of western Browns Bank, western Scotian Shelf. The image shows colour-coded depths and is artificially-shaded from the northwest to enhance the portrayal of morphology. Prominent features include bouldery ridged moraines, fields of sandy bedforms, shelf-edge low stand deltas, and a variety of other subtle terrain types. The bank is largely formed of glacial materials, reworked and modified by a transgressing sea in post glacial time. Both glacial (relict) and modern (present day) seabed features occur adjacent to one another

Effects of Trawling
Of global concern to both the fishing industry and government regulators are the effects of mobile fishing equipment on seabed habitat and benthic ecosystems (productivity and biodiversity). Trawling causes a number of direct and indirect changes in the ecosystem such as changes to fish populations and benthic communities, and the release of organic and inorganic nutrients (De Groot, 1984). Bottom trawling directly alters the structure and morphology of the seabed and the pressure of the gear affects the subsurface sediment fabric including habitat structure. Undisturbed sites commonly contain epifaunal bryozoans, hydrozoans, sponges and polychaete and amphipod tubes which provide protection and large surface areas for fragile taxa such as shrimps, brittle stars and polychaetes (Collie et al., 1997). By contrast, trawled sites often exhibit reductions in epifaunal communities and reduced porosity of coarse-grained sediments.

Previous studies have been largely qualitative and geological characteristics of the seabed have in many cases been omitted or not properly evaluated. A recent project of DFO, NRCan and industry is attempting to address these issues in a three-year study of the effects of bottom trawling on Western Bank and the effects of clam dredging on Banquereau. This is a follow-up to an earlier study on the Grand Banks of Newfoundland over a sandy seabed substrate, which indicated a reduction of 25 % of invertebrates in trawled corridors versus untrawled controlled corridors. (Gordon et al., this publication)

The present area of study is termed the 4TVW Haddock Nursery Area on Western Bank, Scotian Shelf, which has been off limits to mobile groundfish gear since 1987. Controlled trawl surveys have been conducted, together with before and after sidescan sonar, sampling and video observations in both test and control areas. A unique millimetre-resolution acoustic imaging system (DRUMS ^TM) was also deployed to provide very high resolution of the structure beneath the seabed to a depth of 0.5 m. (Schwinghamer et al., 1996). This system provides a calibrated measure of the sediment fabric (habitat structure) as an additional independent acoustic method to quantify disturbance by trawling.

The sidescan sonar data clearly show that the trawl doors make the deepest linear furrows on the flat cobble-pebble, gravel lag surface, but both the cod end and the ground line produce a distinct linearity to the seabed (Figure 5). This is interpreted as a subtle alinement of particles to which the sidescan sonar system is particularly sensitive. Sonar surveys conducted concurrent with the trawling operation correlate the effects of each component of the bottom trawl gear with distinct seabed markings. Benthic samples col lected from the trawled areas are presently under study, but preliminary observations suggest a reduction in biodiversity and a change in sediment porosity. Samples collected from the trawled areas were also reduced in volume. This is interpreted to result from rearrangement of gravel clasts into more closely-packed associations making sampling less effective.

Figure 5. Sidescan sonogram of trawl marks on gravel seabed of Western Bank, Scotian Shelf. The outermost mark produced by the trawl doors is deeper and more continuous than the linear marks made by the ground line and cod end. These marks can persist for long periods of time measured in years, particularly when formed in hard gravely seabeds

Impacts of Clam Harvesting
Two types of clam harvesting are being assessed: in the nearshore and on offshore banks. In New Brunswick and Prince Edward Island, surf clams are harvested from the intertidal zone using small hydraulic rakes. The impact of the rake on the clam population is unknown, but declining returns have forced the closure of the fishery in both provinces. GSC Atlantic has contributed to a controlled experiment to assess the impact of harvesting on juvenile clam recruitment. Hydrau ic rakes fluidise the top 30 cm of sediment. Short core samples driven through the sand before and after raking show that raking increases porosity and decreases bulk density of the clam beds for periods of up to three to four weeks. Recruitment of juvenile clams is higher in these "ploughed" beds than in neighbouring control sites, suggesting that raking has a beneficial effect on surf clams during the early stages of their life cycle. Geotechnical soil properties provide a simple yet effective technique to quantify the physical changes in the clam beds induced by harvesting.

On a larger scale, offshore clam harvesting on Banquereau, eastern Scotian Shelf, uses 4 metre clam dredges equipped with high-pressure water jets to liquefy the sediment. Before, after and control corridor surveys show that the hydraulic dredge is more invasive than bottom trawling, with the water jets producing a trough up to 20 cm in depth. Plumes of sand suspended by the fishing operation fall to the seabed to a distance of 10 to 15 m beside the dredged track and can be clearly seen on the acoustic and photographic data as a fresh deposit of fine-grained sand, largely devoid of browsing organisms (Figure 6). Analysis of the benthic samples is continuing to provide a quantitative assessment of mortality.

The use of sidescan sonar technology provides essential pre-survey confirmation of undredged seabed as well as a map of dredging survey tracks. Despite attempts to dredge parallel, equi-spaced tracks, the sidescan data show that the tracks are often convoluted, criss-crossing and overlapped, as a result of strong currents and inability to sometimes control the track of the dredge on the seabed. The sonar information allows accurate sampling to be conducted post dredging, and to position sampling equipment both in and out of dredged tracks.

Other Habitat Related Projects
In the Bay of Fundy, studies of benthic-pelagic coupling are underway (Wildish et al., 1998 and Wildish and Fader, 1998) involving linear bivalve reefs (horse mussels) that cover large areas of the seabed. These reefs were discovered with sidescan sonar during GSC Atlantic geological mapping projects. They occur as long linear bioherms up to 3 m in height above the surrounding seabed, 20 m in width and km in length. This research has expanded in cooperation with DFO to include assessment of other benthic communities and relationships to sediment provinces within the Bay of Fundy.

The possibility of open ocean scallop aquaculture has been proposed for areas of the Bay of Fundy. Knowledge of seabed sediment type, and especially sediment transport, is essential for such a fishery to be successful.

The provision of geoscience knowledge to the inshore aquaculture industry is also an increasing new opportunity. Like any large seabed facility or engineering installation, assessment of the environmental conditions of proposed aquaculture sites is an essential first step in project location. The new high-resolution mapping tools of the geologist, including multibeam bathymetry, provide a characterization of proposed aquaculture areas to determine transport pathways for contaminants, the presence of seabed and subsurface hazards, and to define conflicting resources such as placer gold and marine aggregate. Subsurface data can be interpreted to extrapolate these characteristics and environments back through time at very high resolution.

Figure 6. Sidescan sonogram of a clam dredge trawled area of southern Banquereau, Scotian Shelf. The untrawled seabed consists of medium sand with a dense benthic community including many siphon holes of propeller and surf clams. It presents a medium intensity backscatter on the sidescan data. The clam dredge marks are 4 m wide, medium intensity backscatter, linear, 20 cm deep depressions. The seabed between dredge marks is of lighter tone indicating low acoustic backscatter. It results from resettling of sand plumes generated by the clam dredging operation. Note the crisscrossing and convoluted pattern of dredge tracks.

Discussion
Globally, and in Canada, the development and application of high-resolution seafloor mapping tools and geotechnical techniques to traditional marine geoscience research has been coincident with declining fish stocks, more efficient fishing technology, exploitation of new commercial fish stocks and a global decline in the sustainability of ocean ecosystems. This has presented fisheries researchers and managers with new challenges to understand seabed ecology for the maintenance of sustainable fisheries. Geoscience tools are recognised as providing valuable information toward resolving some of these questions in benthic ecological research, habitat definition and delineation, fisheries management, and gear impact studies. The long term goal is to systematically understand the coupling between seabed characteristics and associated benthic communities.

Future application of geoscience knowledge to ocean issues will be strengthened, contributing to projects such as the delineation of Marine Protected Areas, the development of strategies for the protection of critical habitat, artificial reef construction for lobster farming, and aquaculture site assessment. Seabed use maps will become common, much as land use maps play a major role in urban and rural development. For the marine geologist, future research will concentrate on the extraction of geological attributes from remotely-acquired acoustic data.

Competing demands for scarce nearshore resources have lead to the development of an integrated coastal zone management policy for Canada. This policy provides a framework within which competing land use, such as aquaculture, fishing, mining, tourism, waste disposal, cable and pipeline corridors and oil field development can be prioritised, controlled and sustained. Under the recently enacted Oceans Act, seabed mapping will provide baseline maps as the essential underpinning to integrated coastal zone management, and links between oceanographers, fisheries managers, industry and marine geologists, will continue to be strengthened.

Acknowledgements
We thank geoscience technicians Austin Boyce and Bob Miller for the collection and processing of sidescan sonar data and mosaic construction. The Canadian Hydrographic Service has provided multibeam bathymetric data over many years of cooperative surveys and projects and we thank them for their support. We have enjoyed many discussions and cooperative projects with marine biologists who have contributed habitat knowledge and enlightened the geological community. This list is long and includes Don Gordon, Peter Lawton, Dave Wildish and their technicians. Our surveys could not be conducted without the support of Canadian Coast Guard vessels. The munuscript was reviewed by Gary Sonnichsen and Don Forbes of the GSC (Atlantic).

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