Picture of plant transect /main zone transect on Lake Tuusulanjärvi from 1998, transect 12. Drawn by Jari Venetvaara with FreeHand.
Stop points are numbered to indicate the stop point intervals (in metres). When the plant zone widths are measured (stop points), it is easy to see whether and how fast the plant zone broadens (e.g. reeds or yellow water lilies).
Plant transect width in Venetvaara’s method is 5 m. The transect usually starts at the upper part of the shore littoral (upper flood limit) and extends perpendicularly outward from the shore over a distance and depth where aquatic plants occur. Stopping points are situated in places where 1) depth increases by 10 – 20 cm and 2) where the main vegetation zones change. A lake reed belt, for instance, can make up a homogenous transect part of its own between the stop points unless additional points are introduced because of an increase in water depth. Additional points are recommended if the vegetation zone is broad so as to ensure that possible other vegetation changes within the reed belt and changes in belt density can be identified. Abundance is estimated e.g. on a seven-step scale or on a percentage scale 1-100 % in which value 1 = 0,1 and 100 = 10. On the latter scale, both coverage and shoot density, for example, must be taken into consideration.
It is important to ensure that the sampling area on the transect is always the whole area between the stopping points, i.e. plant squares are not made.
The program calculates Venetvaara’s (1995) abundance index number formula (L x n)+(L x n)+...+(L x n)= abundance index number,
in which
L = distance between stop points on the transect,
N = abundance / coverage between the stop points
and
X = multiplication sign
The sum of abundance index numbers is calculated for each species separately using the above formula. To calculate these, a sum is also calculated for the distance between stop points on each transect. This is important with regard to the Wilcoxon’s test that requires a plant-specific abundance index number for each metre of the transect.
The said number increases the sensitivity of the Wilcoxon’s test and of graphic image comparison’s to identify possibly changes in the transect follow-up survey. It also helps better assess the present state of a particular part of the water area.
Transect picture from Tuohtiainen. The picture shows a floating transect measuring rope. One end is attached to the shore and the other 1) to a stick pushed in the lake bottom or 2) to a net post anchored into the bottom.
The transect is examined using a boat, or by wading if the bottom is hard enough. In deep water, where the bottom is not visible, one has to use a long-stemmed iron rake to examine the bottom between the stop points. Even in shallow water raking should be used to collect back-up samples. Plants with floating leaves or aerial shoots can be assessed visually. Make 7 long pulls with the rake over the area between the stop points. The abundance of each species is assessed on the basis of the raking results.