Rocky Shore Transects
Marine Biology Field Work on the Rocky Sea Shore
Students often carry out transects to look objectively at the phenomenon of zonation.
When the tide is out the rocky shore is obviously divided into horizontal bands where different plants and animals live. This subjective observation (what you think you can see) can be studied objectively (not biased by what you think you see) if you carry out a transect of the shore.
Transects
Students usually mark out a line running from the top of the shore down to the edge of the sea. Next they will work out the profile of this part of the shore, often using graph-paper to show how it slopes along the length of the line of the transect. Once this has been done it is possible to decide where to look at the plants and animals in detail.
Vertical height
Complex surveying methods can be used, but a simple technique often works just as well. Two poles are needed for this – one a metre long and the other 50cm. One person takes the longer pole down the shore – along the line that has been marked out – and another person stays at the top with the shorter pole. Using the horizon as the third reference point the person at the top of the shore (with the short pole held vertically and resting on the rock) ‘lines up’ the top of his/her pole with the horizon and the top of the longer pole (also held vertically touching the rock). When the person with the longer pole has moved to the correct position (when the two pole-tops and the horizon line up) you have established where the first station is – i.e. where the first quadrat should be placed. This procedure is carried out along the length of the line, giving the positions of a series of stations at half-metre vertical intervals - the horizontal distance between these stations depends, of course, on the slope of the shore.
Quadrats
Often the best approach is to restrict observations to stations along the line, and to decide in advance on the size of the area to be studied at each station. This is where the quadrat comes in. A good size for quick, but thorough, observations is 50cm x 50cm – the dimensions of the usual wire quadrat frame. These quadrats are placed at intervals along the transect line and all the animals and plants found within the frame are identified and recorded. It is best to work in vertical height above the low-water mark (or below your starting point) because this will allow the results from several transects to be compared most easily.
Keep it Objective
Once the positions of all the stations have been determined it is very important to stick to these places for the detailed study and recording of what is found within the quadrat. It is often tempting to move the quadrat a bit – to a ‘better’ or ‘more typical’ bit of the shore nearby – but if you do this you are no longer making an objective study of the phenomenon of zonation.
Photos of quadrats from a transect of a sheltered shore
Other articles by John Blatchford
