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.
||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
|Changes in temperature
||Climate change/global warming
||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
||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.
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
|Changes in nutrient levels
||Waste: sewage discharge
||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
||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.
||Fishing: benthic trawling
||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
||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".