Human activities potentially affecting brittlestar beds

Organic pollution and eutrophication

Other pollutants

Coastal alteration

Introduced species

There are very few recorded examples of brittlestar beds being directly affected by human activities. Most of this chapter therefore consists of a summary of the factors that could be expected to have an influence on these biotopes. There are no detailed case studies but the few relevant observations are mentioned where appropriate.

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 consequently little likelihood of damage to brittlestar beds by fishing activities. Aronson & Harms (1985) speculated that human overexploitation of fish resources could favour the spread of brittlestar aggregations by reducing predation pressure on the animals. The seas around the British Isles, in which brittlestar beds are common, have certainly been the sites of intensive fishing activity for many decades. There is no evidence of any causal connection between these two aspects, but it is not impossible that human activities may be changing the ecology of the British seas in ways that favour particular benthic communities. Lindley et al. (1995) described changes in the zooplankton of the North Sea over the past few decades, specifically a marked increase in the proportion of echinoderm larvae (brittlestars and sea urchins) relative to copepods. The increased dominance of echinoderms was apparent from the early 1980s onwards, beginning over the Dogger Bank and extending northwards to the level of southern Scotland. The sediment-dwelling brittlestar Amphiura filiformis accounted for most of the larval echinoderm increase over the Dogger Bank. Suggested human-induced causes of this phenomenon were increased eutrophication of the North Sea, and a reduction in predation pressure caused by overfishing. Demonstration of a cause-and-effect relationship in cases such as this is difficult, but both processes might conceivably have similar effects on the abundance of epifaunal brittlestars such as Ophiothrix and Ophiocomina.

The input of dissolved nutrients or particulate organic matter (eg. from sewage or aquaculture waste) to coastal areas may in some cases favour the proliferation of brittlestar beds by increasing the supply of suspended food (Hily, 1991). Raymont (1950) recorded an increase in Ophiocomina nigra populations following the addition of fertilizers to the waters of an enclosed basin of Loch Sween, Argyll. However, high levels of organic enrichment would be expected to have deleterious effects on brittlestars and other suspension feeders by excessive sedimentation and hypoxia. Organic pollution may well have contributed to the environmental oxygen depletion causing mass mortality of brittlestars in the Gulf of Trieste (Stachowitsch, 1984). The imprecision of these statements regarding the levels of organic input having beneficial or harmful consequences for brittlestar beds reflects the lack of any quantitative study of the question. Logically, the tolerance of beds to organic pollution should depend on the local geography and hydrodynamic regime, with highest tolerance in open-coast, well-flushed areas.

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 (sea lochs/loughs) (Black, 1996). The effects of this on brittlestar beds 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 sandy 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 the past ten years, the extent and density of the brittlestar beds appear not to have changed (B. Ball, personal communication), although an increase in siltation has taken place.

It is logical to suppose that brittlestar beds would be adversely affected by major pollution incidents such as oil spills, or by continuous exposure to toxic metals, pesticides, or the anti-parasite chemicals used in cage aquaculture. 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.

Aronson (1989) refers to the demise of Warner’s (1971) Ophiothrix bed in Torbay, and tentatively attributes this to increased sedimentation caused by the localized dumping of construction materials. There appear to be no published details of this, but human alteration of the coastal environment clearly has the potential to affect brittlestar beds and other benthic communities, particularly if this involves changes to important parameters such as current regime and sedimentation rate. The potential effects of dredging and other forms of coastal engineering on benthic biotopes is an area requiring more detailed study.

There is currently increasing concern about the effects on marine ecosystems arising from the introduction of non-native species, this process often occurring accidentally as a result of human activities (eg. transport in ships’ ballast water) (Carlton, 1996). To date, a number of non-native species have become established in British waters, some very locally, others distributed more widely (Eno et al., 1997). No biological invasions of relevance to brittlestar beds have been detected, but the possibility that such an event might occur in the future cannot be discounted. As an example of what can occur, the mass mortality of the sea urchin Diadema antillarum throughout the Caribbean during the period 1983-84 may have been caused by an exotic pathogen introduced in ballast water by a ship transiting the Panama Canal (Lessios, 1988).

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