276
THE ESSEX NATURALIST
frequent in 1970 than in 1965, and that most were more frequent
in 1970 than at any time between 1960 and 1965.
Fig. 6 shows the approximate abundances of four species
(Crepidula, Ostrea, Solaster and Ophiothrix) within the Black-
water. As has been stated earlier, these are based on less than
one fortnight's data, etc., and therefore they can only be regarded
as guidelines for future study.
Acknowledgements
The authors are indebted to Mr P. R. French and sons of West
Mersea for the use of their boats and for much assistance in
handling the apparatus, to Mr J. M. Fleming for his assistance in
the field, and to Mr A. Vowles and Mr K. Emerson for the use of
the facilities of the District Survey Laboratory, C.E.G.B., Brad-
well. This paper is published with the permission of the Director
of the Central Electricity Research Laboratories, Leatherhead.
References
Allen, J. A. (1967). The fauna of the Clyde Sea area. Crustacea:
Euphausiacea and Decapoda, with art illustrated key to the British
species. S.M.B.A., Millport.
Crisp, D. J. (Edit.) (1964). The effects of the severe winter of 1962-63 on
marine life in Britain. J. anim. Ecol., 33: 165-210.
Davis. D. S. (1967). The marine fauna of the Blackwater Estuary and
adjacent waters, Essex. Essex Nat., 32 : 2-61.
Eales, N. B. (1967). The littoral fauna of the British Isles. A handbook
for collectors. 4th Ed. C.U.P., Cambridge.
Millar, R. H. (1970). British Ascidians. (Synopses of the British fauna,
new series, No. 1). Academic Press, London and New York.
Milligan, G. M. (1966). Seaweeds of the Blackwater Estuary. Essex
Nat., 31 : 309-327.
Orton, J. H. & Lewis, H. M. (1931). The effects of the severe winter of
1928-29 on the oyster drills, with records of five years tempera-
ture observations on the oyster beds of the Blackwater Estuary.
J. mar. biol. Ass. U.K., 17: 301-313.
Tebble, N. (1966). British Bivalve Seashells. B.M.(N.H.), London.