Levelling - measurement of tidal elevation

Wave action

Physical structure and Topographical complexity

Biological surveys should be conducted at a range of representative levels or at a single defined shore level on the vertical emersion gradient (measured relative to Chart Datum). Heights of sampling stations should be measured relative to known references such as the level of high or low water (from tide tables and adjusted for atmospheric pressure). The level of a sampling station can be assessed by a number of methods (Hawkins and Jones, 1992). Split prism levels accurate to the nearest centimetre are recommended. Cross staff levels developed by the Field Studies Council are reasonably accurate but only give levels at fixed intervals. An interval of ten percent of the spring tide range is recommended (Baker and Crothers, 1987). When possible, the height of each station should be determined in both directions, away from and back to a reference point.

Measurements of the maximum forces generated by impacting waves have been made on rocky shores by fixing to the rock surface various gauges, meters or force transducers (Field 1968, Jones and Demetropoulos 1968, Harger 1970, Denny 1982, 1983, Palumbi, 1984, Underwood and Jernakoff 1984, Galley 1991). The long-term (>1 year) frequency distribution of wave forces would be the most useful descriptor but such measurement schemes are hardly feasible. The rate of dissolution of balls of plaster of Paris has been used to get an idea of water movement and breakdown of the boundary layer near to organisms (Muus 1968). Recently, various force transducers have become more cheaply available and microcomputers can easily handle the vast amount of data generated (Denny 1982, 1988).

Biological indices (e.g. Ballantine, 1961, see linked figure) can be used to give a quick estimate of exposure. These are based on extensive knowledge of the effects of exposure on the biological community of the shore. Thus changes in the distribution of certain species will give an indication of changes in wave action. Indices may only be locally applicable (Crothers, 1983). Such scales contain an element of circular reasoning: estimates of exposure based on the distribution and abundance of species should not be used to relate community structure to wave action.

Where direct measurements are not possible, map-based methods are recommended. In general terms, wave action increases with fetch, the distance of open sea between the shore and the nearest land. Map based methods (e.g. Wright, 1981, described in Baker and Crothers, 1986) involve a measure of fetch and the angle open to the sea. The exposure index is often modified to take account of such factors as the width of channels, sea depths and the strength and direction of prevailing winds. Some quite complex indices can be found in Thomas (1986). A simple alternative is to measure the angle open to a fetch of 5 or 10km. Shores with greater angles have a higher level of exposure.

One of the best ways to minimise differences due to topographical complexity is to position sampling stations on areas of similar substratum. This will usually be free-draining bedrock unless information on specific microhabitats is required. Whether or not this is possible depends on the design and intended analysis of the sampling scheme. It is perfectly acceptable to place a transect entirely on bedrock. It might be necessary to break up the transect in order to avoid other microhabitats like boulders and rock pools. When the transect reaches one of these features, a new one should be started as close as possible at the same shore level. If this is not done it might not be possible distinguish between the effects of the emersion gradient and microhabitat variation. When conducting stratified random sampling, deliberately placing the quadrat in a chosen location is not permissible. When a quadrat lands on a rock pool, for example, it can be rejected. Alternatively, it can be counted but scored as being in a pool to allow determination of the incidence of the different microhabitat types.

The structure of a site can be described in a number of ways. Most obvious is a descriptive account of the rock type and its main features. A sketch map will help with this. An index of rugosity, the roughness of the rock surface, can be obtained using a length of chain, rope or string. The line is run between two points, along the contours of the rock. It is then pulled taut between the same points. The length along the surface is then divided by the taut length. The resolution of this method depends on the thickness and flexibility of the chain or string used. Similar indices can be obtained using profile gauges. Several methods are compared by McCormick (1994). The methods described by Kostylev et al. (1996) can be used to describe surface complexity in terms of fractal dimension but are probably too complex for most marine conservation oriented studies.

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