Transect across the Salt Marsh

It is clear from simple observation that the distribution of the plants on the salt marsh is not random, but rather that different plant species are found in specific locations.

Salt marsh with students and measuring tape along transect

Transect

One way to investigate plant distribution is by of a 'transect'. In this case we will use a line transect to 'sample' the vegetation and obtain a systematic record of the occurrence of individual species and of their contribution to the total ground cover. Some years ago we carefully surveyed the line along which the transect is positioned and installed permanent peg markers. Relative elevations were determined to an accuracy of +/- 5cm.

A quadrat

Quadrat

We set up a series of tapes running from the track towards the bay and then laid down a 250 mm square 'quadrat' made of wire every 2 metres along the tape so that the quadrat is on the East side of the tape and has its rear corner on the metre mark. Observing the plants contained by the quadrat, it is possible to estimate the proportion of the ground which is covered by each species, and to record the result in the table provided using the following convention.

COVER 

SCORE 

COVER 

SCORE 

Absent 

 0

 40% - 59%

 3

0 - 19% 

 1

60% - 79% 

 4

20 %- 39 %

 2

 80% - 100%

 5

 

 

In the example shown, the cover scores were Sarcocornia 3, Sporobolus 2 (Note that it is possible that the total 'cover score' will be greater than 5). Data is accumulated on the score sheet.

Data from transect

When the data from the whole transect has been collated, it can be displayed in graphical form and compared with measurements of some physical parameter - in this case land surface relief and elevation. (This is obviously related to tidal cover.)

Details of the topography are shown overlaying a false colour aerial photo of part of the study area. Mangrove plants appear red. The actual transect coincides with the bottom line of the yellow graph and the position of the marker pegs is shown. The Y axis on the graph shows the 1 metre height difference between Mid tide and Spring tide high. The distribution of salt marsh species along the transect is shown in the three graphs below the aerial photo.

The whole of the salt marsh is covered by water during the spring tides

Spring tide

This view shows the area of the transect being covered by a rising king tide (2 m), about one hour before high tide The water has about 10 cm still to rise and already many of the small plants are completely under water. Look for the bright green ridge of Suaeda australis at the base of the mangroves. This marks the position reached by high tides during the neaps.

What do you think determines the distribution of plants on the salt marsh?