THE BOTTOM FAUNA OF THE BLACKWATER ESTUARY 25
(twenty-seven being taken by seven grabs). Larger numbers
of Buccinum were recorded from this area than from any other.
The seed mussels and Mytilus-debris are probably derived from
animals growing within the culverts. Additionally, at the Brad-
well Generating Station the washings from the cooling water
intake screens are returned continuously to the outfall culvert
and will inevitably include potential food fragments. This
obviously supports considerable numbers of organisms; some of
the finer fragments may even support the large population of
Phoronis dwelling nearby. Audouinia occurred in the areas
where the gritty mud partially replaced the black anoxic mud,
and the ballast provided suitable attachment sites for Tubularia
and anemones, which to a considerable extent replaced the
various sessile and sedentary species found on the "shell on
mud". It is of interest to note that Ostrea edulis was recorded
from two stations in this area; also present in the immediate
vicinity of the outfalls were Pilumnus and Cereus (see 'Discus-
sion').
Comparison of Table 1, in which the presence of the species
is listed on the basis of position relative to the Barrier Wall,
and Table 2, in which the data is analysed in relation to sub-
strate, discloses the extent to which the fauna is distributed on
a substrate specific basis (see Fig. 4). Particularly relevant here
are stations 'A' (one of the western controls) and '3' (to the north
—intake side—of the wall). These have the "mud on clay"
substrate and possess the reduced fauna characteristic of the
substrate and not that typical of the geographical area in which
they are situated.
DISCUSSION
The following discussion considers the effect of the Barrier
Wall, with its intakes and outfalls, on the macroscopic benthic
fauna of the Blackwater Estuary in the light of other studies
of areas subjected to heated discharges and examines the nature
of the species which appear to be restricted to the Barrier Wall
area.
The effects of thermal discharges on aquatic organisms have
been shown to vary with the nature of the fauna and the size
and flushing time of the receiving body of water. Brackish-water
faunas are, in general, more eurythermal than those from marine
FOOTNOTES TO TABLE TWO
"shell on mud": stns X, Y, Z, B, C, 1, 4, 5, 6, 7, 8, 9
"mud on clay": stns A, i, ii, iii, iv, v, vi, vii, ix, 3
intake: stn 2
outfall: stn viii
x = present at between 1 & 24% of stations
xx = present at between 25 & 49% of stations
xxx = present at between 50 & 74% of stations
xxxx = present at between 75 & 100% of stations
+ = present at the single intake or outfall station