Restoration of a disused dock basin as a habitat for marine benthos and fish in the Mersey Estuary, Liverpool, Merseyside, England
Published Source 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 (added by: Showler D.A. 2007).
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Summary
Changes in the water quality, and in flora and fauna, of a Liverpool dock closed to shipping were monitored subsequent to closure of its gates to shipping in 1977. Following closure, water quality improved and a more diverse marine flora and fauna than in other adjacent unclosed docks and estuary waters outside, developed.


Background
In the UK in recent years, discussions of proposals for dockland development have raised controversy over the question whether, due to decreases in shipping, 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 docks as an aquatic environments for marine benthos and fish following complete closure to shipping.
Action
Study site: Sandon Dock, is located roughly midway along a chain of 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 water, virtually all estuarine in origin. 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 in its wooden gates at the time of closure by removal of six planks, approximately 15 cm in width and 45 cm in length. Benthic organisms are found chiefly on the massive stone and concrete blocks that comprise the dock retaining walls. The gates, ropes, floats, cables and other artificial substrates are also colonized.

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) located 1 km to the north was also 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, a 'Helixor', combined water aerator and circulator. This delivers approximately 5.2 kg oxygen per hour and circulates approximately 68,000 m³ of water per day; achieving complete turn-over of water in the dock in about 6 days.

Consequences
Water salinities fell within the ranges 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 air-lift pump installed in the year after the dock closure effectively prevents stratification during summer and ensured an oxygen-rich water column. The absence of harmful dinoflagellate blooms is attributed to this artificial destratification.

Fauna: Improvements in water quality were accompanied by the development of a more diverse marine flora and fauna than in other unclosed docks. The benthic community is dominated by blue mussel Mytilus edulis which completely dominated (100% cover) most of the Sandon walls down to about 8 m, becoming scarcer below this. The various sea squirts and sea anemones reached maximal abundance at or below the level of the mussel decline. Balanus crenatus (an acorn barnacle) was present at all depths growing only on mussel shells. Occasional starfish (Asteroidea) were seen feeding on mussels. In contrast at a similar open dock 1 km to the north of Sandon, Mytilus was very scarce, the walls being dominated by the sea squirts Ascidiella (60-70% cover) growing on top of B.crenatus (30-40% cover). Sagartia sp. (a sea anemone) was also common at all depths.

Flora: The flora was fairly impoverished but this was considered understandable given the low species diversity of the estuary outside. As Sandon was also almost completely enclosed this must have retarded recruitment. Nevertheless, several species present had not been recorded in the Mersey in recent years including Antithamnion plumula, Giffordia ovata, Punctaria plantaginea and Bryopsis plumosa. Prasiola calophylla (reported relatively rarely from U.K coasts and of uncertain status) was noteworthy. Individual algae were also on the whole, more robust and the vegetation more luxuriant than in the estuary.

At Sandon, introduced sugar kelp Laminaria saccharina grew until water temperatures reached 15ºC, but a few plants survived a 14-week period in summer 1981 when ambient water temperatures were above 15ºC.

Conclusions: Following closure of the dock, water quality improved and a more diverse marine flora and fauna than in other adjacent unclosed docks and estuary waters outside, developed. The authors conclude that a dock of this character can be maintained as a productive marine environment with a variety of educational, amenity and economic uses.


Note: If using or referring to this published study, please read and quote the original paper, this can be viewed at: http://links.jstor.org/sici?sici=0021-8901%28198304%2920%3A1%3C43%3AROADDB%3E2.0.CO%3B2-L