SEAWEEDS OF THE BLACKWATER ESTUARY 309
The Seaweeds of the Blackwater
Estuary
with a complete list of Marine Algae recorded
from Essex
By G. Marlene Milligan
This survey was made as part of the ecological investigations conducted in
the Blackwater Estuary by the Central Electricity Research Laboratory.
It is published here by permission of the Director.
The Blackwater Estuary, a part of the Thames Estuary
complex, is an elongated tidal inlet, with a tidal height range of
approximately 20 feet. The river has gently sloping sides, large
areas of which are exposed at low water.
These inter-tidal shores, although for the most part covered
with soft mud, consist in some places of stones, sand and/or shell.
The mud is anaerobic below 3 inches. The shores are backed
either by storm beaches of sand and shell, salt marshes or sea
walls. There is a main water channel running from Maldon to
the sea, and this gradually widens and deepens towards the sea-
ward end. The depth opposite West Mersea Church is approxi-
mately 7 fathoms; near Osea Island, 2 fathoms. The bottom
constituents of this channel vary much as those of the shore.
The salinity of the water in the estuary is dependent on weather
conditions, and may vary considerably in certain areas. Although
salinity is generally uniform throughout the river at high tide,
varying between 30 and 34.5 P.P.T. (Spencer, 1963), it may
reach as low as 25-26 P.P.T. in some areas at low water and after
heavy rainfall, under the influence of fresh water which flows
continuously through Maldon towards the sea, into the mouth of
the estuary from the River Colne, and off the land.
The temperature varies between about 0°C. in winter and a
maximum of about 20°C. in summer (Orton and Lewis, 1930;
Spencer, 1963). Again this factor is generally uniform through-
out the estuary at any given time, but with long periods of
sunshine or cold weather on shallow areas or mud flats at low tide,
the temperature of the water on incoming tides may be
temporarily changed in certain localities relative to ambient
(Spencer, 1963).
Dependent on wind, weather, tide and other factors, the water
in the river carries in suspension changing amounts of material,
much of which comes from the mud bottom. The amount of this
suspended matter may vary by as much as 70 to 700 P.P.T. (J. F.
Spencer, private communication). This silt, etc., in the water
probably prevents a large portion of the light utilizable in
photosynthesis, from reaching the bottom, but as far as is known