Monitoring Techniques

Remote sampling


Towed underwater video

Remotely-operated vehicles (ROVs)

Acoustic surveys

Remote sampling

This heading covers all equipment lowered by cable from the deck of a ship to the sea bed, then hauled back aboard with a sample of the substratum. Variants of this general pattern have been developed for different sediment types and include the Petersen, van Veen and Smith-McIntyre grabs, the Forster anchor-dredge and the Reineck box-corer. Detailed descriptions and illustrations of these and other models can be found in Holme (1971). Sediment samples required for detailed macrofaunal analysis are generally sieved through 1.0 mm or 0.5 mm mesh, and the recovered fauna fixed (4% formalin) and preserved (70% ethanol) for later examination.

All these sampling devices require to be operated from a hard-hulled boat with suitably-sized winch and A-frame. Box-corers are large enough for their use generally to be restricted to ocean-going research vessels. The standard area sampled by the most widely-used grabs is 0.1 m2. In most macrofaunal studies, replicate samples will be taken, totalling 0.5 or 1.0 m2 per station. Anchor-dredges take an unstandardized ‘bite’ out of the substratum and are therefore only semi-quantitative sampling devices. Specimen collection in the field can usually be achieved quickly in good weather conditions, but the sorting and identification of the fauna is very time-consuming and labour-intensive. There has therefore been much debate on the best sampling strategy to employ to maximize the cost-effectiveness of the analysis (Kingston & Riddle, 1989).

Grabs of various kinds have been used as standard sampling devices in countless benthic studies dating back over a century. Their great advantage is that the sampling of a standardized substratum area allows precise quantification of animal densities. Their disadvantage in the present context is that they do not penetrate the sediment deeply enough to reliably sample the deep-burrowing megafauna. Larger, mobile animals (eg. Nephrops) and epifauna occurring at relatively low densities (eg. sea pens in many areas) will also rarely be captured. The recovery of small patches of sediment also gives very little information on sea bed topography or burrow types. The deep-burrowing megafauna can be collected in anchor-dredges, but accurate density measurements are not possible, and the other interpretive problems of grab-sampling also apply.

Advantages and disadvantages of remote sampling


SCUBA diving has been used increasingly since the 1970s for field studies of the ecology of burrowing megafauna, and most of our knowledge of species such as Maxmuelleria lankesteri could not have been gained by any other means. Diving has also been the mainstay of the MNCR biotope surveys around the UK. The overwhelming advantage of the technique is that it allows close-up observations, precise deployment of static camera equipment and application of methods such as resin-casting (Atkinson & Chapman, 1984) that have proved invaluable for burrow identification. Research dives can be carried out from small dories or inflatable boats, or if necessary from the shore, allowing access to shallow or enclosed inlets that larger boats cannot reach.

However, diving does have a number of important drawbacks. Using compressed air as a breathing gas entails strict depth and time limitations. For practical purposes, it is difficult to carry out detailed observations or experiments at depths below 30 m, and most field studies of burrowing megafauna have been conducted in much shallower water. The use of alternative breathing gases promises to extend the depth and time limits for diving studies, but these have not yet come into general use in UK scientific diving. Any form of diving entails exposure to physical hazards (eg. decompression sickness), and as a result the conduct of professional diving operations in the UK is strictly controlled by legislation. Standard training and operational requirements for scientists diving at work are enforced by the Health and Safety Executive.

Divers can examine the sea floor at a finer resolution than any photographic technique, but only relatively small areas can be covered on a single dive. The technique is therefore more suited to repeated monitoring of small fixed sites than to habitat mapping on a scale of kilometres. Observations may be hindered or prevented entirely if the water is highly turbid, which is often the case over the fine-sediment biotopes discussed here. Working effectively over soft muddy sea beds requires a high level of diving expertise, in particular excellent buoyancy control and confidence in low-visibility conditions. These skills are lacking even in some quite experienced divers, but are not difficult to master with sufficient practice.

Advantages and disadvantages of diving

Towed underwater video

Towed video provides a means to visually survey large expanses of sea floor without the depth or time constraints associated with diving. The basic apparatus involved is relatively simple, consisting of a low-light sensitive video camera mounted on a lightweight, runnered metal sledge, towed slowly over the sea bottom by a ship. A number of camera models suitable for this work are now available from commercial manufacturers. The camera is mounted on the sled facing obliquely forwards, usually 70 - 100 cm above the substratum. One or two quartz-iodide lamps are positioned at the front of the sledge, pointing vertically or obliquely downwards to illuminate the sea bed within the camera’s field of view. Red filters can be fitted to the lights to minimize disturbance to light-sensitive benthic animals. The camera is connected to a video recorder on board ship by an umbilical cable loosely attached to the towing warp every few metres along its length.

For optimum picture quality, towing speed has to be carefully controlled and kept at 1 knot or below as far as possible. Positional information during the tow can be recorded using the ship’s navigational system (Decca or GPS). The visual field of the camera can be established prior to the survey by deploying the system with a calibration scale (graduated rule or marked string) fixed to the lower part of the sledge within view of the camera. Analysis of the resulting videotapes usually consists of counting the features of interest (eg. burrow openings, benthic animals) within a strip of known width traversed by the moving camera sledge. The frequency of counts or linear extent of the transect to be analyzed depends on the objectives of the survey and on the time available for the work (videotape analysis can be very time-consuming).

Although the equipment required for towed video surveys is relatively simple, it is expensive and generally confined to large marine laboratories or academic institutions.

Towed video is one of the most valuable tools in the study of megafaunal burrowing communities, and has been used intensively for this purpose in the west of Scotland (Smith, 1988; Atkinson, 1989; Howson & Davies, 1991; Tuck et al., 1997b), the north-eastern Irish Sea (Hughes & Atkinson, 1997), and in the Mediterranean (Marrs et al., 1996). To some extent, the method is not independent of diving studies, since the latter have provided much of the detailed information on burrow types essential for the accurate interpretation of sea floor video recordings. The resolution of camera observations is also lower than that of diving studies, so that smaller burrow openings or fine topographic details may be missed. This deficiency can be partly rectified by mounting a time-lapse still photographic camera on the video sledge,set to take pictures at intervals along the video transect. Still photographs usually give better resolution than videotape, and allow easier quantification of bottom features.

Care must be taken in selection of deployment areas in order to minimize the risk of snagging the camera sledge on wrecks, rock pinnacles or other obstructions. If it is necessary to work near underwater obstructions, a video or still camera mounted on a frame suspended below the ship can be used as an alternative to a towed sledge (C.J. Chapman, personal communication). Grabs or other benthic sampling gear could also be mounted on the frame, allowing samples to be taken. The main drawback to the suspended frame method is that the support vessel has to drift, so that there is little control over transect direction.

Advantages and disadvantages of towed underwater video

Remotely-operated vehicles (ROVs)

ROVs are video camera systems mounted in a compact submersible vehicle whose movements are controlled by a surface operator via an umbilical cable (Auster, 1993). The capacities of ROVs are in some respects intermediate between those of SCUBA diving and towed video. Operations are free from the depth and time constraints imposed on human divers, but have a radius of operation defined by the length of the umbilical cable. Surveying outside this radius is achieved by moving the support vessel. An ROV has the advantage over towed video of being able to hover over a selected point or ‘retrace its steps’, allowing the operator to closely examine a feature of interest. However, quantification of features on the sea bed is more difficult than from a towed video recording, as an ROV does not always remain at a fixed distance from the substratum, and the field of view may therefore change. Because the movements of the ROV are controlled by the surface operator, surveys using this method are by nature more selective than video transects, and so may not give a representative view of the sea floor characters.

Some models of ROV have mechanical ‘arms’ controlled by the surface operator and so have the capacity to take benthic samples.

ROVs are used extensively in the offshore oil and gas industry but have not so far been widely employed in scientific studies in the UK. To date there are no published examples of their application in studies of the biotope complex discussed here.

Advantages and disadvantages of ROVs

Acoustic surveys

Acoustic surveys using the recently-developed RoxAnnTM system are becoming increasingly important in the large-scale mapping of benthic biotopes (Greenstreet et al., 1997). RoxAnnTM is an electronic system connected to the transducer of a conventional echo-sounder in parallel with the existing display. The system functions by processing the first and second echoes returned from the sea bed to derive values for the roughness (ie. topographic irregularity) and hardness (ie. substratum type, rock/sand/mud etc.) of the sea floor. By plotting the roughness and hardness functions against each other and integrating this information with values for water depth, a detailed map of the distribution of substratum types in a survey area can be produced.

The great advantage of RoxAnnTM is that information on substratum types over wide expanses of sea floor (ie. on a scale of tens of kilometres) can be gathered very rapidly, in far less time than it would take to collect and analyse grab samples over such an area (Greenstreet et al., 1997). In addition, the system is sensitive not only to the physical characteristics of the substratum, but also to certain biotic characteristics such as the presence of organisms projecting above the sea bed, or to the presence of large burrows in the sediment. The technique therefore clearly has enormous potential for rapid mapping of marine biotopes.

However, RoxAnnTM data cannot be used in isolation. The substratum types distinguished by the system in its present form must be ‘ground-truthed’, ie. checked by analysis of grab samples, diver survey or photographic observations. In some cases the system distinguishes more sediment ‘types’ than can be recognized by traditional particle size analysis (Greenstreet et al., 1997). Although broad biotope categories can be identified, their precise species composition must still be determined by other means.

Because of its recent origins, RoxAnnTM is only now coming into widespread use as a tool for benthic habitat mapping, and the capabilities and limitations of the system are still in the process of being defined. It has been used in surveys of several candidate SACs, including Strangford Lough (Magorrian et al., 1995), Loch nam Madadh (Entec, 1996), the Sound of Arisaig (Davies et al., 1996) and the Berwickshire/North Northumberland Coast (Foster-Smith et al., 1996). In all of these areas, sedimentary biotopes with sea pens and burrowing megafauna were identified and mapped. This aspect of marine technology is evolving rapidly, and other comparable acoustic systems will doubtless become available in the near future.

Advantages and disadvantages of RoxAnnTM

Summary of the capabilities of the various monitoring techniques.

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