SEAWEEDS OF THE BLACKWATER ESTUARY 313
C. flexuosa, C. glomerata, Rhizoclonium implexum, Bryopsis
hypnoides, Giffordia granulosa, Fucus vesiculosus, Ascophyllum
nodosum, Dumontia incrasata, Hypoglossum woodwardii,
Polyneura gmelinii, Polysiphonia nigra, and Ceramium
diaphanum.
Areas of mud flats were examined at Sales Point, Mersea Flats,
Bradwell Creek, Southey Creek, Stansgate, Decoy Point and East
Mersea.
3. Stony Beaches
In the littoral zones patches of sand and gravel are distributed
haphazardly at all levels. Stones, which are not too mobile, or
are held fast by clumps of mussels, are generally encrusted with
barnacles. In places where the stones are mobile they are
encrusted with Protococcus and Lithothamnion. Stones which
are only moved with difficulty provide a substrate for Corallina
officinalis also. In some places where there is a continuous flow of
water of low salinity through the stones (e.g. near the Weymark's
Sluice at Bradwell) the seaweeds Dumontia incrassata and Clado-
phora hutchinsiae are commonly found, also Antithamnion
cruciatum..
Other species recorded on the stony beaches were: Porphyra
umbilicalis, and Porphyrodiscus simulans as abundant, Ulva
lactuca, Enteromorpha intestinalis, Dictyota dichotoma, and
Chondrus crispus as common, and also Antithamnion plumula and
Hildenbrandia prototypus.
Stony beaches were examined at Osea Island, Stone, Colne
Point and Cocum Hills.
4. Sand and Shell Areas
On Colne Point, Cocum Hills and parts of the Mersea Flats
some areas of sand and shell are exposed by extremely low tides.
Here the coarse sand and shell overlies mud. Pools in this area
support a luxuriant and conspicuous growth of algae.
Recorded as abundant were: Cladophora hutchinsiae, Entero-
morpha clathrata, Dictyota dichotoma, Porphyra umbilicalis,
Antithamnion plumula, Griffithsia flosculosa, Polysiphonia
nigrescens, P. elongata, Ceramium rubrum, Ceramium deslong-
champsii, and Plocamium vulgare.
Less common species were: Navicula grevillei, Ulva lactuca,
Enteromorpha ramulosa, E. intestinalis, Bryopsis plumosa,
Rhizoclonium implexum, Chaetomorpha aerea, C. melagonium,
Chorda filum, Gracilaria confervoides, Laurencia obtusa, Anti-
thamnion cruciatum, Chondria dasyphylla, C. tenuissima,
Chondrus crispus, Porphyrodiscus simulans, Fucus vesiculosus,
Leptonematella fasiculata.
Also recorded were: Protococcus marinus, Cladostephus
spongiosus, Chaetopteris plumosa, Ectocarpus erectus, Laminaria
saccharina, Pilayella littoralis, Calliblepharis lanceolata, Hypo-