28
THE ESSEX NATURALIST
fore outside the scope of the present surveys. Notwithstanding
these differences in the published faunas, the fauna of the, out-
fall side of the Barrier Wall appears to correspond fairly well
with those recorded from the comparatively few other investi-
gated Sublittoral mud deposits. The presence of Phoronis in
large numbers in 1971 in the area of the wall is anomalous as
indicated earlier. It was not recorded from there in 1970 and
similar habitats have not been reinvestigated since that date: it
is therefore quite possible that it occurs elsewhere in the estuary.
This survey was not designed to shed light on Naylor's cate-
gories (c) and (d) and for these the reader must be referred to
the literature, e.g. Barnett (1972), Naylor (1965b), etc.
Hence, at least with respect to the benthic macrofauna and
excluding from the present discussion Cereus, Pilumnus and
Phoronis, the outfall discharge only modifies the composition
and distribution of the local fauna to the extent of providing a
substantial but localised supply of small Mytilus, organic debris,
etc. for carnivorous and scavenging species which would probably
not otherwise be present in such numbers in the limited area
concerned. In fact the siting of such discharges adjacent to
deep water ensures that the discharged water, being of higher
temperature and, if of the same salinity, therefore less dense
than the receiving water, floats as a surface layer having little
effect on the benthos.
Six species, Cereus pedunculatus, Kefersteinia cirrata,
Scolelepis sp., Cirratulus cirratus, Pilumnus hirtellus and
Phoronis ?mulleri, are to date only known in the Blackwater
Estuary from the area adjacent to the Barrier Wall. The question
of whether they are introductions dependent upon the conditions
provided by the wall and its culverts therefore arises. Kefer-
steinia was only recorded by accident, i.e. it is a species which
would not normally be obtained using the collecting methods
adopted by these surveys. Hence knowledge of its Blackwater
distribution must await surveys of the smaller macrofauna. It is,
however, known from the North Sea (Fauvel, 1923). Cirratulus
and Scolelepis are widely distributed polychaetes of muddy sub-
strates : both are known from the North Sea and the presence of
neither would appear to correlate with any attribute of the dis-
charge area other than the substrate.
Pilumnus has also been recorded from the southern fringes
of the North Sea, i.e. Norfolk, Suffolk, Kent and the Waddensea
(Christiansen, 1969), although it is relatively infrequent on the
British North Sea coastline (see Bell, 1853 and subsequent
authors). Indeed, this Blackwater record is the first from Essex.
Pilumnus may be associated with the abundant Mytilus debris in
the outfall region of the Barrier Wall, as it is known to feed on
this lamellibranch (Lobb, unpublished), and although once again
it would appear to be the nature of the substrate which is mainly
responsible for the presence of Pilumnus near the wall, never-