BOTTOM FAUNA OF BLACKWATER ESTUARY: 1970 271
Fig. 3. Productivity of the Blackwater in terms of numbers of macro-
scopic species per station.
attempt to be as accurate as possible within the limitations of
the data.
Davis maintained records of the following thirteen species:—
Porcellana longicornis, Eupagurus bernhardus, Carcinus maenas,
Hyas araneus, Macropodia rostrata, Crepidula fornicata, Buc-
cinum undatum, Urosalpinx cinerea, Ostrea edulis, Solaster
papposus, Asterias rubens, Ophiothrix fragilis and Ophiura sp.
All but five of these appeared to have been affected by the
severe winter as evidenced by decreased frequency.
Porcellana appears to have disappeared from the Blackwater
in a crash between the time of Davis's December 1961 sampling
and that of the following month. The only record of this species
in the Blackwater since December 1961 is that "from the Colne
Bar area at the mouth of the Blackwater in May 1962" (Davis,
1967), i.e. it had disappeared before the advent of the severe
winter, although the 1961-62 winter was also particularly cold.
The species does not appear to have recolonised this region yet,
as the 1970 survey failed to detect it. Ophiura (Fig. 4a), of which
the early data consist of O. albida and O. texturata records in
unknown proportion and the 1970 data of only O. albida, main-
tained an almost constant frequency from 1960 to 1965, the
severe winter apparently having no effect. Buccinum and Hyas
frequencies within the Blackwater increased between 1962 and
1963, slightly in the former (Fig. 4b) and markedly in the latter
(Fig. 4c). No effect of the severe winter was apparent on the
frequency of Crepidula (Davis, unpublished).