THE BOTTOM FAUNA OF THE BLACKWATER ESTUARY 27
environments and hence they are to some extent preadapted to
withstand increased environmental temperatures (c.f. Mar-
kowski, 1959, 1960, 1962; Naylor, 1959, 1965a). The hydrographic
and geographic features of the discharge area influence the
amount by which the temperature is raised by the discharge.
In small enclosed bodies of water the temperature rise will be
higher than in large bodies or those with fast flushing rates, e.g.
Naylor (1965a) recorded mean temperatures some 10°C higher
than the mean ambient temperature in Queen's Dock, Swansea,
whilst the temperature increase produced by discharging into
large bodies can be very small and localised (Gilet, 1960). The
fauna of the Blackwater is predominantly marine in character
and hence can be expected to be sensitive to temperature
changes, but the volume of water is large and tidal and hence,
a priori, any temperature increase will be small.
Naylor (1965b) classified the observed effects of heated
effluents on marine and estuarine environments in the latitudes
of Britain into: (a) the elimination of native boreal or cool-
temperate species; (b) the introduction of exotic warm-temperate
or subtropical species; (c) effects upon temperature-controlled
or temperature-sensitive biological processes, e.g. spawning
times, growth, etc.; and (d) direct effects on ecologically important
physical variables, e.g. oxygen concentration. Temperature can
also have synergistic effects on the toxicity of other pollutants
present in the water (Milne, 1940; Hynes, 1959). If one examines
the species to be found near the outfall culverts of Bradwell
Power Station, it can be seen that exotic species (such as those
listed by Naylor, 1965b, pp 87-89) do not contribute to the fauna.
One species is, however, of a south-western distribution and
otherwise unknown from the North Sea area (and a second
approaches this state), and Phoronis, whilst not exotic is at least
unusual: these species are considered in more detail below.
As noted above, there does not appear to be any marked
impoverishment of the Blackwater fauna in the vicinity of the
outfall region, other than that which can be attributed to lack
of niches for epifaunal organisms. Further study of similar
"mud on clay" substrates from elsewhere in the Blackwater is
required, however, before the degree of impoverishment, or
otherwise, can be fully assessed. A number of stations with a
soft mud substrate were sampled by the general survey of 1970
(Barnes & Coughlan, 1971, and unpublished). These possessed a
sparse fauna which partially confirmed Davis's (1967) statement
that the fauna "is composed mostly of polychaetes, Nereis sp.,
Nephthys sp., Pectinaria koreni and Clymeme (sic) sp., and
molluscs, Abra alba and Macoma balthica". It should be noted,
however, that the present authors failed to record Pectinaria
and Clymene, although it is likely that Clymene would not have
been retained by the combination of 3mm mesh sieve and a hose.
The distribution of Macoma is predominantly littoral and there-