18 THE ESSEX NATURALIST
and three to the south. The outer row on each side was positioned
about 200m distant from the wall, the central row about 100m
distant, and the inner row was as close to the wall itself as proved
possible. The end stations of each row were in line with the
two ends of the wall, whilst the central stations were in line with
the central "blisters" on the wall (Fig. 2).
Two further groups of three stations each were worked in
1970 on the outfall side (shorewards) of the wall in line with
these central blisters. The two groups were sited approximately
12m and 30m from the wall respectively. In addition, four of the
stations worked by the earlier general survey fall within the area
under consideration here and the results from these stations have
been included in the following account.
Five grab samples were taken from each station, the nature
of the substrate being noted before the sample was hosed over a
3mm mesh sieve. Counts were made of the numbers of each
species collected by each grab at each station. Grab samples
were obtained at high-water-slack-tide (inshore stations) or low-
water-slack-tide (offshore stations), the boat, the M.F.V. "Provi-
dence", being anchored during sampling to prevent drifting
during the lengthy process of obtaining and sorting five satisfac-
tory samples.
Species were identified using the sources listed in the earlier
paper (Barnes and Coughlan, 1971) and in addition Dr Eve C.
Southward kindly identified the phoronid and Dr J. D. George the
terminal fragment of a polychaete as probably belonging to a
terebellid. As in the previous report, a number of species of
large colonial hydroid, including several sertulariids and
plumulariids, were "lumped" together as "colonial hydroid".
THE SUBSTRATE
Four types of substrate were recorded from the area of the
Barrier Wall (Fig. 3), two of them only extremely locally. In the
immediate vicinity of the intake culverts, the substrate consisted
of loose bare stones, presumably as a result of the inflowing
current of water removing all the fine sediment from the area.
In the equivalent position with respect to the outfall culverts (i.e.
the southern side of the wall), the substrate was somewhat vari-
able, but mainly consisted of broken mussel-shell (with a few
larger pieces of Petricola), soft black locally-anoxic mud, and
concrete ballast. Much of the debris was bound by the byssus
attachments of abundant small (i.e. 5-15mm length) mussels. The
ballast, presumably dropped during concrete pouring for the
shafts and tunnels of the cooling water system, extended up to
30m from the wall. Beneath the ballast, debris and mud was
situated a bed of clay, which floored the whole area to the south
of the wall.
Those stations to the north, east and west of the wall (Fig. 3)
possessed a substrate of clean, whole Ostrea and Crepidula shells