THE ALGAL FLORA OF THE RIVER LEE 175
The section of the river treated here in detail is that between the
confluence with the Stort at Rye Meads and the Waltham Cross—
Waltham Abbey road. There are several channels in places, of which
the largest is the River Lee Navigation Canal, but the diatom flora of
each of these seems to be similar. In addition, collections were also
made at Hertford, Bow Lock (just above tidal water) and Buntingford
on the upper part of the Rib, a tributary of the Lee.
A detailed investigation of the plankton of the Rye Meads—Waltham
Abbey stretch of the river is now being carried out (Swale, in prepara-
tion) and some of the results have been incorporated into the table.
The plankton diatoms seem to consist of two groups. The first of these
comprises typical benthic forms swept up from the bed of the river,
including Nitzschia, Navicula and Cocconeis. These forms are particu-
larly evident in the plankton after storms. The second group includes
the true plankton forms and is most highly developed during the spring
and early summer, being almost non-existent during the winter months.
The dominant diatom of this group is Stephanodiscus hantzschii, which
has exceeded 40,000 cells per cubic centimetre in the water of the
Lee.
Among the benthic diatoms a large proportion are sessile, epiphytic on
the dead or living parts of other algae and higher plants, or growing
attached to concrete or wood. A few, including Amphora ovalis var.
Pediculus and Synedra parasitica, are epiphytic on the larger benthic
diatoms, especially Nitzschia Sigmoidea. In the part of the Lee studied
no difference was noted between the epiphytic communities and those
on stones, in contrast with the findings of Round (1957). This may be
due to stones in the muddy river bed being too scarce for a proper
community to develop. The stony river bed at Buntingford supported
a flora more comparable with the epilithic ones of Round.
Other benthic diatoms lie or move freely and form a living film
over the bottom mud. Weirs provide abundant material of these free-
living species, and below a small weir near King's Weir the large
Nitzschia Sigmoidea forms thick dark brown patches on wooden boards
and on the mud of the stream bed.
These associations are apparently constant over the middle reaches
of the river, though above Bow Lock, where the river joins the Thames,
most of the larger forms are absent and brackish water species are
present, such as Raphoneis amphiceros.
Listed in the table are most species of diatom which occur in the
River Lee. The first column shows the relative quantity of each species
in the plankton collected from the stretch of the Lee studied in detail.
The second deals with the quantity in the benthos from this stretch
and the third gives the distribution. The fourth and fifth columns give
the species from Buntingford and Bow respectively, since the benthic
communities here are rather different from those of the middle reaches
of the river.
Identification of species was carried out using Hustedt (1930).