188
THE ESSEX NATURALIST
Pymmes Brook rarely exceeds 20 million gallons per day and (b)
the sewage effluent entering Pymmes Brook is only treated to the
Royal Commission standard of 30 p.p.m. and 20 p.p.m. for sus-
pended solids and biological oxygen demand, respectively, there
is insufficient dilution in the Lee and a serious deterioration in
water quality occurs immediately downstream. Table 2 shews
the chemical quality of the River Lee at about 75 metres upstream
of the confluence with Pymmes Brook and these figures, par-
ticularly those for dissolved oxygen, should be compared with the
Hackney results. Since the poor quality of the water produces a
barrier for many species the effect on the fauna and flora is con-
siderable but it may have aided the establishment of the viable
guppy population since other fish, save Three-spined Stickleback,
Gasterosteus aculeatus, can only rarely penetrate into the Lee as
far downstream as Hackney.
Throughout most of its length in the. lower reaches the River
Lee Navigation is about 19 metres wide with vertical sides and
a maximum depth of about 2.5 metres. At all times of the year
the river bed at Hackney contains extensive deposits of decompos-
ing organic mud and this is anaerobic below 2 cm depth. The bed
provides optimum conditions for vast numbers of oligochaet worms,
which may occur at densities greater than 20,000 individuals