Individual study: Restoration of a disused dock basin as a habitat for marine benthos and fish in the Mersey Estuary, Liverpool, Merseyside, England
Russell G., Hawkins S.J., Evans L.C., Jones H.D. & Holmes G.D. (1983) Restoration of a disused dock basin as a habitat for marine benthos and fish. Journal of Applied Ecology, 20, 43-58
In the UK in recent years, proposals for dockland development have raised controversy over whether now more-or-less redundant dock basins should be filled in or retained as water-filled structures. The following study assessed the value of one such dock as an aquatic refuge for marine benthos and fish following closure to shipping.
Study site: Sandon Dock, lies about midway along a chain of concreate ann stone docks that line the west-facing bank of the River Mersey. Sandon is roughly trapezium-shaped, with a maximum depth of 10-11 m and contains about 425,000 m³ of sea water. The dock was closed to shipping and its gates closed in March 1977. The dock bottom consists of fine estuarine sediments. Water exchange with other docks takes place via gaps made by removal of six planks in its wooden gates at the time of closure by , approximately 15 cm in width and 45 cm in length.
Monitoring: Through 1980-1981 water quality, surface temperature, water transparency (using a Secchi disc) and monitoring of the flora and fauna was undertaken. A similar nearby dock (Brocklebank Dock) open to boats 1 km to the north, was monitored for comparison. During summer, the water begins to stratify into a warm, oxygen-saturated surface layer and a cool, subsaturated, bottom layer. Destratification was brought about by introduction in June 1978, of a 'Helixor', combined water aerator and circulator, delivering approximately 5.2 kg oxygen per hour and circulating approximately 68,000 m³ of water per day.
Water salinity varied from 25-31‰, and temperatures 0.3-19 ºC. Secchi disc extinction depths averaged 5-6 m in the dock whilst in the adjacent Mersey estuary it was only 10 cm. The pump installed in the year after closure effectively prevented stratification during summer and ensured an oxygen-rich water column. The absence of harmful dinoflagellate blooms is attributed to this artificial destratification.
Fauna: As water quality improved a more diverse fauna developed than in open docks. The benthic community beacame dominated by blue mussel Mytilus edulis which dominated (100% cover) most of the Sandon walls down to about 8 m, becoming scarcer below this. Various sea squirts and sea anemones reached maximum abundance at or below the level of the mussel decline. Balanus crenatus (an acorn barnacle) was present at all depths growing on mussel shells. Occasional starfish (Asteroidea) were seen feeding on mussels. In contrast at the similar open dock, Mytilus was very scarce, the walls being dominated by sea squirts Ascidiella (60-70% cover) growing on top of B.crenatus (30-40% cover); Sagartia sp. (a sea anemone) was common at all depths.
Flora: The flora was fairly impoverished but this was attributed in part at least, due to low species diversity of the estuary outside. As Sandon was also almost completely enclosed this was assumed to have retarded recruitment. Nevertheless, several species not recorded in the estuary in recent years were present: Antithamnion plumula, Giffordia ovata, Punctaria plantaginea and Bryopsis plumosa. Prasiola calophylla (reported relatively rarely from UK coasts) was noteworthy. Individual algae were also on the whole, more robust and the vegetation more luxuriant than in the estuary.
Conclusions: Following closure of the dock, water quality improved and use of a pump prevented stratification during summer. A more diverse marine flora and fauna than in adjacent unclosed docks developed.