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 change |
Waste: spoil dumping |
Intermediate |
Heavy sedimentation will inhibit bed occurrence by clogging
the brittlestar feeding organs. Aronson (1989) refers to the demise of Warner’s
(1971) Ophiothrix bed in Torbay, and tentatively attributes this to increased
sedimentation caused by the localised dumping of construction materials. |
| Changes in temperature |
Climate change/global warming |
High |
Leewis, Waarenburg & van der Tol (1994) described
fluctuations in the abundance of Ophiothrix fragilis in the Dutch Oosterschelde
Estuary over the period 1979-90. These changes appeared to be driven by winter
temperatures. Following the mild winters of 1979-80 and 1987-88, populations of
brittlestars increased enormously, the animals occupying 60-90% of the available hard
substrata in layers up to 5 cm deep. Populations were greatly reduced (to less than 10%
spatial coverage) following cold winters in 1978-79, 1984-85 and 1985-86. The populations
undergoing these changes were living in very shallow water (5-7 m depth) and were
therefore vulnerable to spells of unusually cold weather. |
| Heavy metal contamination |
Waste: industrial effluent discharge |
Not sensitive |
Gounin, Davoult & Richard (1995) studied the transfer of
heavy metals (iron, manganese, lead, copper and cadmium) through Ophiothrix beds.
They concluded that heavy metals ingested or absorbed by the animals transited rapidly
through the body and were expelled in the faeces. The brittlestars did not appear to
accumulate metals in the tissues and so would not act to decontaminate the near-bottom
water mass. |
| Hydrocarbon contamination |
Uses: boats/shipping (oil spills) |
Low |
The water-accumulated fraction of diesel oil has been found
to be acutely toxic to Ophiothrix fragilis and Ophiocomina nigra (Newton
1995). So far, however, there are no field observations of epifaunal brittlestar beds
being damaged by any of these forms of pollution. It is logical to suppose that
brittlestar beds would be adversely affected by major pollution incidents such as oil
spills. |
| Changes in nutrient levels |
Aquaculture: fin-fish |
Low |
The expansion of cage aquaculture of Atlantic salmon along
the fiordic coastlines of western Scotland and Ireland over the past few decades has led
to increased local inputs of organic material into many semi-enclosed water bodies (Black
1996). The effects of this on brittlestars have not been studied in detail, but some
relevant observations have been made in Killary Harbour, western Ireland (Keegan &
Mercer 1986). A dense aggregation of Ophiothrix and Ophiocomina was recorded
in 1974 from a site at the mouth of the harbour, mainly on rocky outcrops but extending
out onto adjacent sand silt areas. A salmon farm was established at the site in the late
1980s, within 100 m of the main beds. Despite the presence of this farm for ten years, the
extent and density of the brittlestar beds appeared not to have changed (B. Ball pers.
com.), although an increase in siltation had taken place. |
| Changes in oxygenation |
Aquaculture: fin-fish |
Intermediate |
High levels of organic enrichment such as that expected from
aquaculture waste would have deleterious effects on brittlestars and other suspension
feeders by excessive sedimentation and hypoxia. |
| Displacement |
Fishing: benthic trawling |
Low |
Brittlestars themselves are of no economic value, and their
aggregations are not significant habitats for any commercially important fish or
shellfish. Fishermen tend to avoid areas with dense brittlestar populations because the
animals foul their nets (Aronson 1989). There is little likelihood of damage to
brittlestar beds by fishing activities. In fact Aronson & Harms (1985) speculated that
human overexploitation of fish resources could favour the spread of brittlestar
aggregations by reducing predation pressure on the animals. |
