314
THE ESSEX NATURALIST
glossum woodwardii, Polynema gmelinii, Corallina officinalis,
Polysiphonia nigra, P. insidiosa, Cladophora glaucesens, Erythro-
trichea carnea, Ulothrix flacca, Enteromorpha torta.
5. The Sea Walls
Salt marshes have been reclaimed for agricultural land for at
least three centuries and now much of the shore line of the Black-
water Estuary is made up of sea walls. The steep face of the wall
extends some way below highwater mark and is colonised by a
rich growth of algae. In fact almost all of the Phaeophyta in the
area are restricted to this type of habitat. As on a rocky shore,
zonation, related to the duration of exposure at low tide, is
apparent.
High-water mark:
Fucus spiralis and Fucus ceranoides
Ascophyllum nodosum
Fucus vesiculosus
Chondrus crispus
Low-water mark:
The Chondrus crispus zone is actually on the mud at the foot
of the wall and the growth in this area is as sparse as in other
muddy areas.
Also found on the sea walls were: Enteromorpha intestinalis,
E. clathrata, Leptonematella fasiculata, Pilayella littoralis,
Ectocarpus arctus, Erythrotrichea carnea.
The sea walls were examined at Bradwell and Osea Island.
6. Surfaces provided by the Barrier Wall and Ships Anchored in the
Estuary
The hulls of ships anchored in the estuary provide large, con-
stantly submerged surfaces for colonization by algae. At the
waterline the surface is exposed by wave motion, thus there is some
slight differentiation of the algae in the top 11/2 to 2 in. which are
mainly Enteromorpha intestinalis. Below the waterline there is a
transition to a zone of Porphyra with Ulva, Pilayella and Entero-
morpha. On one of the ships which had been in the area for a
long time there was an additional band below these two consisting
of Laminaria saccharina. The Barrier Wall of Bradwell Power
Station shows similar colonization but as it is fixed, there is a
considerable surface which is exposed by the tides.
The species recorded were: Enteromorpha intestinalis, Ulva
lactuca, Porphyra umbilicalis, Pilayella littoralis, Ectocarpus
arctus and Laminaria saccharina.
7. Wrecks
There are several submerged wrecks of boats and old wharves
in the area which are covered by a considerable growth of Fucus