THE BOTTOM FAUNA OF THE BLACKWATER ESTUARY 19
Fig. 3. The substrates of the study area
on an aerobic mud base, with a considerable quantity of similar
shell-remains mixed with the mud. The stations due north of
the wall possessed, in general, more and cleaner shells than those
to the north-east and north-west, which were characterised by a
gritty shell and mud mix with little surface whole-shell.
The remaining stations, to the south of the wall, had a sub-
strate composed of a layer of soft aerobic mud (of up to 10cm
thickness) overlying hard black mud (of up to 20cm thickness)
and stiff clay (which was reached from 0.5 to 25cm below the
surface). The western area was often floored by almost bare grey
clay, but towards the east the layers of mud and silt increased
in thickness. Surface shell was absent.
THE NATURE OF THE FAUNA
Since samples were sorted by hosing over a 3mm mesh sieve,
small or fragile species were not retained and all such species
were discounted with respect to the present surveys. In particular
in this context, amphipods were seldom retained and were
ignored. In total, fifty-one species (including the "colonial
hydroid" category as a single species) were recorded and a
further two "incidental" species (the small or fragile species re-
ferred to above) were identified. These species are listed in
Table 1. They include ten species not recorded by the general
survey of 1970, of which three are not listed by Davis (1967) and
are therefore new records for the Blackwater (these are
asterisked in that table, and see also Appendix 1). In addition,