Sensitivity to human activities

Activities listed are those which influence, or are likely to influence this habitat and which are assessed in the UK marine SAC project review. The sensitivity rank may require amendment in the light of new information becoming available.

Sensitivity to: Human activity Rank Comments
Substratum loss Extraction: maerl High Dead maerl extraction is liable to lead to muddy plumes and excessive sediment load in the water column. Heavy siltation also clogs up the maerl matrix, which serves as a habitat for infauna, and once clogged, the passage of oxygenated seawater between the maerl fragments is restricted and the number of infaunal species is reduced.
Changes in temperature Climate change/global warming Intermediate Even in the relatively short term, global warming of the anticipated 1-3oC within the next century could have an effect on the composition of maerl beds in the UK, in that the cold-intolerant species Lithothamnion corallioides might be able to extend its distribution northwards.
Changes in turbidity Extraction: navigational/ maintenance dredging Intermediate Dredging results in the suspension of the fine silt and clay fractions of the sediment, which is deposited by inshore currents. This will increase turbidity and decrease the amount of penetrating light as well as smothering other algae.

If the underlying substratum is altered, it is unlikely that maerl will be able to re-establish itself at the site, given the probable method of reproduction of the species involved.

Changes in nutrient levels Waste: sewage discharge Intermediate The increase in levels of macronutrients (particularly nitrogen and phosphorus) in European coastal waters results in the excessive growth of ephemeral macroalgal species which may smother the maerl. Increased turbidity in coastal waters may also occur as a result of prolific phytoplankton growth, thus reducing available penetrating light.
Changes in oxygenation Aquaculture: fin-fish Intermediate Positioning cages over a maerl biotope would lead to fish faeces and partly-consumed food pellets contaminating the bed and may result in anaerobiosis due to the oxygen demand of the decomposing material. Detrital rain from the cages would act in a similar way to terrigenous silt, reducing light penetration through the water column and smothering the maerl surface so that the stabilising epiphytic algae could no longer establish themselves.
Abrasion Fishing: benthic trawling Intermediate The removal of living maerl thalli from the biotope surface, the loss of the stabilising algae and the disruption of the structure the community structure occur. These major changes have been reported in Rade de Brest, France where the maerl beds support populations of the scallop Chlamys varia, which are locally abundant and are intensively fished during the winter months. The dredging activity has been reported as resulting in severe disruption to the maerl bed and associated flora and fauna (Hily & Le Foll 1990).
Removal of target species Extraction: maerl High Maerl is extracted in large amounts for use in animal food additives, water filtration systems, but mostly to replace lime as an agricultural soil conditioner. Live maerl extraction is very problematic as the growth rates for replacement are so slow. Hall-Spencer (1995) expressed the view that "commercial dredging of maerl deposits is particularly destructive since this removes the productive surface layer and dumps sediment on any plants which escape dredging, inhibiting habitat recovery".

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