312
THE ESSEX NATURALIST
ditches cut at the seaward edge. In these shallow runnels which
are only flooded by the spring tides, the species found to be
common were restricted to Ulva lactuca, Enteromorpha intes-
tinalis, smaller amounts of Pilayella littoralis, and some stunted
specimens of Fucus vesiculosus. The larger runnels may be up to
3 ft. or 4 ft. deep and are flooded by all tides. The species found
were Ulva lactuca, Enteromorpha intestinalis, Pilayella littoralis,
Rhizocolonium implexum, one species of Vaucheria and on two
occasions Polysiphonia violacea (Bradwell Creek and Ray Creek).
Fucus vesiculosus may be found growing where the substratum is
firm.
In certain parts of the marshes, which are only flooded at high
tide, mud banks are thickly colonised by spongy, blue-green
masses of Cyanophyceae and Xanthophyceae. The principal
species found here is Vaucheria (thuretii?) with which are mixed
several species of blue-green algae including Microcoleus chthono-
plastes, Hydrocoleum lyngbyaccum, Oscillatoria margaritifera,
Oscillatoria nigroviridis, Oscillatoria amphibia, Spirulina subsalsa,
Symploca hydenoides, and Anabaena torrulosa. Only the surface
filaments of these growths are living, bound by some 3 in. of dead
filaments to the mud beneath, which is black and anaerobic.
In one area, in the saltings at the east end of Osea Island,
Ascophyllum nodosum var. scorpiodes is found growing among the
marsh grass.
Saltings were examined at the entrance to Bradwell Creek, the
head of Ray Creek, the north side of Mersea Island, the East
Mersea Stone, Fingringhoe Wick and the east end of Osea Island.
2. Mud Flats
Tidal flats composed mainly of soft mud extend in some cases
for over a mile from high-water mark. The water which passes
over these areas as the tide rises becomes very turbid and it is
reasonable to suppose that, when covered with water, very little
light penetrates to the substrate where the algae are sparsely dis-
tributed and half buried in the mud. In many areas the mud is
very unstable to a depth of 2 or 3 ft., so no fixed algae can grow.
Although algae are not abundant there is a large variety of
isolated species attached to occasional rocks, stones and shells.
The most abundant species was a filamentous diatom, Navicula
grevillei, with the following species as common occurrences: Ulva
lactuca, Enteromorpha intestinalis, E. clathrata, E. torta, E.
prolifera, Cladophora hutchinsiae, Bryopsis plumosa, Ectocarpus
aratus, Pilayella littoralis, Dictyota dichotoma, Porphyra
umbilicalis, Chondria dasyphylla, Polysiphonia elongata, P.
nigrescens, P. simulans, Griffithsia flosculosa, Antithamnion
plumula, Ceramium deslongchampsii, C. rubrum, Chondrus crispus,
Cystoclonium purpureum, Gracilaria confervoides and Plocamium
vulgare.
Also found living on the mud flats but in far less abundance
were : Spirulina subsalsa, Protococcus marinus, Cladophora albida,