A METHOD OF QUALITATIVE BIOLOGICAL RIVER SURVEY 255
Stone Transect
This method of sampling is selected primarily for the estima-
tion of those organisms to be found living in the shelter of the
stones. As indicated (Fig. 5a) stones can provide shelter from
the current for quite a large number of organisms. The stone
transect is bisected and five stones from each quadrat selected at
random. Each stone is lifted by the downstream end with a
triangular net mesh (20 strands per square inch) placed near the
base of the stone (Fig. 5b). The contents of the net for each
quadrat are then examined separately and the quantity of
organisms noted. The riffle current is sufficient to dislodge most
of the sheltering organisms into the net.
Gravel Square
The riffle bed selected for this sampling method should consist
of small stones, average size about one inch in diameter with a
maximum diameter of two inches. A foot square of river bed
is marked out with a wire frame of these dimensions and with
the triangular net of 20 meshes per square inch held immediately
down-stream, the first l"-2" of gravel bed is removed into a bucket
using a small shovel. The larger stones are examined first and
their surfaces brushed into a tray with the contents of the collect-
ing net. This method of sampling is used to estimate those
organisms if any, which are associated with the smaller stones
of the riffle.
Sparganium Sweep
The net used for this sweep should be circular, with a mesh
size of 20 strands per square inch and should be slowly moved
upstream through the Sparganium bed. The contents of the net
are then examined and the predominating genera assessed. The
reason for this examination is to see if any members of this
community have become dislodged from the riffle community and
should be regarded as "invasion species".
DISCUSSION OF THE RESULTS
The Hirudinea (Leeches) were one of the commonest groups in
the riffle habitat. The most abundant of this group was Erpobdella
octoculata. The next most abundant was Glossophonia complanata
of the same family. Helobdella stagnalis was present, but rare, as
was also Pisciola geometra. As would be expected most leeches
were found on the larger stones of the riffle bed as sampled in the
Stone Transect and Quadrats and not in the Gravel Square and
Sparganium Sweeps. The very large numbers of Erpobdella found
on the riffle bed showed that the station was an ideal habitat.
Of the Oligochaeta (Worms), Tubifex spp. were most abundant
although they most certainly originated from the mud of the silted
reach upstream of the riffle. Asellus aquaticus, the isopod crusta-
cean, was rare possibly because of competition with gammarids for