SEAWEEDS OF THE BLACKWATER ESTUARY 311
no measurements have been taken. This suspended material,
constantly settling and re-settling, covers the bottom of the river
and everything lying on it with a fine loose sediment.
In such conditions of unstable bottom, and sometimes rapidly
changing salinities and temperatures, growth conditions for algae
are not ideal, and seaweeds are sparse in the estuary compared
with the amounts found on a rocky coast. Growth tends to be
poor, and is rarely luxuriant; individuals, half-buried in the mud,
are often widely separated from one another. The mobility of
the substrates and the gentle slope of the shore seem to provide
an almost homogeneous environment throughout which algae
(with certain exceptions) are distributed at random and with little
relationship to the presence of particular substrates, distance from
the mouth of the estuary, or intertidal level.
Ninety-two species have been recorded from the Blackwater;
of these only Enteromorpha intestinalis, Ascophyllum nodosum,
Dictyota dichotoma, Fucus vesiculosus, Fucus spiralis, Chondrus
crispus, Griffithsia flosculosa, and Vaucheria thuretii are ever
abundant; the others are rarely conspicuous. On some occasions,
loose-lying specimens were found alive and in growing condition.
Although it is probable that many of these were torn loose as a
result of storms, some of them may have grown unattached. No
species were found which grew only unattached.
HABITATS
The main habitats which were examined have been put into
categories which are dealt with separately.
1. Saltmarshes and Pools
Saltmarshes are the natural margin of the Blackwater Estuary
but due to submergence and erosion and the constuction of sea
walls, their area is now greatly reduced. The flora is mainly
composed of halophytic (salt-tolerant) angiosperms, but marine
algae are found between their roots and in the shallow pools.
There are shallow pools in the marshes (often old oyster pits)
with water 1-2 ft. deep and with bottoms of soft mud. At neap
tides sea water does not flood over the marshes, so these pools
are exposed to the sun, which in summer may cause rapid increases
in temperature and salinity. These pools are refilled with water
at the next spring tide.
The algae most abundant in the pools were: Ulva lactuca,
Enteromorpha intestinalis, E. prolifera, E. ramulosa, Cladophora
fracta, and Rhizoclonium implexum.
There were also large numbers of diatoms on the mud at the
bottom of each pool.
The soil in the saltmarshes is very impervious to water, so
water left by the receding tide runs off over the surface, forming
erosion ditches or runnels. The very shallow landward ends of
these runnels have a different character from the deeper and wider