The ecology of the benthos in Liverpool Docks.
A broadscale survey of the benthos was carried out in the docks
around Liverpool and the results indicated that the walls of the
majority of the South Docks were dominated by Mytilus edulis.
This species tended to be less abundant in docks close to the
intake from the Mersey while Molgula manmattensis tended to be
more abundant at these sites. These differences may be due to a
combination of either increased suspended solids or decreased
phytoplankton. The walls of Albert, Queens and Princes Docks were
surveyed over a three year period. The results indicated that the
abundance of Mytilus was relatively constant between years. Other
more ephemeral species, such as Ciona, showed considerable
variation both within and between years.
Closer examination of the Mytilus population structure has
indicated that it was dominated by one or two cohorts which had
settled in 1988 - 1989. Much less recruitment has occurred
subsequently. Reasons for this lack of recruitment are examined,
however, the most likely explanations are increased predation
from Carcinus and/or intraspecific interactions from the adult
bed, either filtering out Mytilus larvae or reducing food supply
to new settlers.
Monitoring of the zooplankton indicated considerable temporal and
spatial variation, despite the fact that the docks are
effectively a closed ecosystem. The observed variations are
attributed to either adult or larval behaviour or changes in
primary production affecting secondary production.
Tiles have been used to follow the pattern of annual succession
and the effect of timing of available space on this pattern of
succession. Results were integrated with changes observed in the
wall benthos and variations in larval supply. Considerable
differences were observed in the community that developed on
suspended tiles, tiles fixed to the dock wall and cleared areas
of the dock wall. One of the primary factors affecting this was
thought to be reduced food supply on the wall due to the dense
filter feeding assemblage there. No evidence was found of any
strong interspecific interactions in the successional sequence.
Rather, the community composition was typical for the time of
year. The community development is described with regard to the
life-history strategies of the species in the fouling assemblage.
Tiles were also used to look at the annual pattern of algal
settlement. This indicated that diatoms were the principal
settlers early in the year, brown ephemerals such as Giffordia
and Punctaria during spring and green ephemerals such as
Enteromorpha and Cladophora over the summer period. Amphipods
were the dominant grazers of this assemblage. Tiles left in place
for two to three months initially developed dense algal growth
but this was subsequently replaced by a cover of Botryllus. This
change was thought to be accelerated by the grazing amphipods.
No perennial algae were recorded in the docks; reasons for this
Finally, an assessment is made of the overall stability of the
benthic ecosystem found in the docks around Liverpool and a
number of possible management options, which could be used to
improve the stability, are suggested.