Mapping the extent of the biotope

Sampling for species diversity and population abundance

Monitoring rates of growth and production of kelp plants

The UK Marine SACs Project is conducting and publishing the proceedings of a series of workshops devoted to the development of monitoring and management programmes for marine SACs (Worsfold & Dyer, 1997; Hiscock, 1998a). Methods that should be included as part of a monitoring programme would include the following.

At Newcastle University, the BioMar project (Davies et al., 1997) has developed a survey protocol for mapping the sea floor using acoustic techniques, validated by biological sampling, with the data stored and analysed using geographic information systems (GIS). A RoxAnn processor was used for acoustic mapping. Acoustic data have no biological meaning unless they can be related to biological assemblages, determined from direct observations or samples of the seabed at predetermined point locations (see Worsfold et al., 1997 for further discussion).

Based on the video samples, grab samples, diver surveys and previous detailed records of biological surveys in a Scottish sea-loch (Howson et al., 1994), a map of the sea-bed showing the predicted distribution of a total of 23 biotopes was constructed. Acoustic mapping using a RoxAnn system provided data on the physical nature of the sea-bed (depth, smooth/rough, soft/hard), and biological information was then added to the acoustic data. It was not found practicable to relate each biologically based biotope classification to a particular acoustic pattern. Instead, the biotopes determined from a biological approach had to be grouped into 15 much broader categories in which the species component was generally lost and in which there was no possibility of indicating a gradual change from one life form to another.

The recent deployment of the RoxAnn method for a few days in Strangford Lough confirmed its value in mapping the distribution and extent of beds occupied by different benthic animals (M. Service, pers. comm.) and it is possible that the RoxAnn method could be further developed to allow the identification of areas where kelp forests are present. Without subsequent checking by more labour-intensive (and expensive) methods, the method would not differentiate among different kelp species, and neither could it provide useful information on density, growth rates or overall health of the biotope, but it may offer the most cost-effective way of mapping the extent of the kelp forest in an SAC.

It has often been suggested that subtidal kelp beds could be identified from aerial photographs. This method could then be used to make a very broad-scale, rapid assessment of the extent of kelp forests in inshore areas. Attempts to put this idea into practice in Strangford Lough, Northern Ireland (A. Portig, pers. comm.), have largely foundered on the virtual impossibility of getting sunny, calm conditions and a low spring tide to coincide with the availability of an aircraft. The images are also difficult to interpret quantitatively if the orientation of the photograph departs significantly from the vertical. In practice, therefore, and especially given the prevailing weather of the British Isles, it seems that this approach is unlikely to yield detailed, quantitative information, although it could be used to provide a broad over-view of inaccessible areas of coastline.

The methods being developed for monitoring marine SACs (Hiscock, 1998a) which are relevant to kelp biotopes are summarised below but, as with all benthic habitats, the patchy distribution of flora and fauna creates difficulties for objective sampling. The main questions

raised are:

• what is the minimum sample area?
• should samples be distributed randomly or systematically?
• how many samples are needed in order to obtain an adequate representation of the species diversity and biomass of the site?

Hiscock(1998a) provides a discussion of all of these aspects, with particular reference to marine habitats, and also covers the additional question of when is the most appropriate or efficient season to sample a particular type of habitat.

Much of the early work on the extent and biomass of kelp beds in Scotland and Norway was based on samples obtained with a spring grab lowered from a boat (see Chapter III). The major disadvantages of this technique are that the surface area sampled is uncertain, and the grab leaves an unknown (and probably variable) proportion of the kelp population behind. Nevertheless, several of the ecological relationships established by this technique have been confirmed in subsequent work by divers, and it may appear to be a cost-effective way of monitoring kelp plants in relatively dense forests. However, the limitations mentioned above are so serious as to rule it out for detailed, quantitative work on the kelps, and the technique also fails to provide useful data on any of the associated species in the biotope.

Quadrats of known size at an exactly located site are surveyed by divers and the occurrence and abundance of all species on a check list are recorded. This technique has been developed during the workshops preparing methods for the marine SACs project (Worsfold & Dyer, 1997), and is described in detail by Hiscock (1998b). Its efficiency in monitoring sublittoral biotopes has yet to be tested, but it is probably the only technique available for the quantitative recording and monitoring the species associated with kelp biotopes on a broad scale. The development of this technique with specific reference to identifying keystone species in kelp biotopes would make a suitable topic for a CASE/CAST research studentship.

Diver-operated cameras are used to record fixed quadrats at suitable time intervals, and the percentage cover of the most conspicuous species is subsequently determined using a grid of point quadrats over the enlarged or projected photographs. The application of this technique to sublittoral rock biotopes in marine SACs is described in detail by Hiscock and Bullimore (1998). In kelp biotopes, it would only be practicable for monitoring the more obvious flora and sedentary fauna of the substratum below a kelp canopy. The cover of the kelp plants, themselves, could not be determined using this technique because of their size and mobility in water currents.

Measuring the growth rates of individual kelp plants by the "punched-hole technique (i.e. following the movement of punched holes away from the base of the blade) is an elegant method of monitoring the performance of plants in situ, and management agencies would be well advised to make use of it when they are carrying out underwater surveys. A random sample of 20-25 plants in a representative area of kelp forest or parkland should be marked by tagging the lower part of the stipe or the holdfast. A hole (3-5 mm diameter) should then be punched through each blade at a measured distance above the blade-stipe boundary. At intervals of 2-3 months during the spring and summer (depending on the growth rates of the plants) and less frequently at other times, the sites should be re-visited and the distance from the hole to the blade-stipe boundary re-measured. Once the hole is half-way along the blade, a new hole should be punched near the base. Using this technique, the growth rates of plants at different depths, or at different sites, within an SAC can be compared (yielding valuable basic information), and the effects of environmental changes (e.g. eutrophication, pollution, temperature or turbidity changes) on growth rate can be quantitatively established.

The productivity of kelp plants can now be measured in the field using underwater fluorometers ("Diving PAM"; Beer et al., 1998) or submersible recording oxygen electrodes (Birkett et al., in prep.). Such measurements may provide a more rapid indicator of environmental change, but the need for technically elaborate and expensive equipment restricts the number of plants that can be examined. Nevertheless, academic work is continuing with the aim of modelling kelp productivity from continuos measurements of surface irradiance, combined with data on light penetration through the water, and such models may soon require in situ growth data for validation. Once this stage is reached, a broad picture of kelp performance may be obtainable from physical measurements made from the surface, verified by spot checks during diving surveys.

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