The western English Channel: changes in predation intensity

The Gulf of Trieste: oxygen depletion

The Oosterschelde Estuary: temperature

Relevance of these examples to beds in other areas

Fluctuations in the extent of Ophiothrix fragilis beds in the western Channel were reviewed by Holme (1984), using records made by various workers over a period of almost a century. Dense beds were first recorded in the Eddystone area by Allen (1899), but records from the 1920s and 30s suggest that Ophiothrix had become much less common by that time. In the early 1950s, beds were once more recorded in the western Channel using sea-floor photography (Vevers, 1952), and these persisted until the late 1960s. From about 1970 onwards, there was a marked decline in Ophiothrix populations in the Plymouth area, and the formerly dense aggregations disappeared. This situation prevailed at the time of Holme’s review (1984). The 1970s decrease in Ophiothrix populations was apparently confined to the English side of the western Channel, and no comparable decline occurred off the French coast, or in the eastern Channel.

Holme considered that the most likely factor underlying these cyclical changes was the intensity of predation by the large starfish Luidia ciliaris. Records of this species in the Plymouth area show a roughly inverse relationship to the abundance of Ophiothrix fragilis (Aronson, 1992). Luidia ciliaris, and the related L. sarsi, showed a marked increase in abundance near Plymouth during the early 1970s, the period during which the Ophiothrix beds were declining. Records from earlier decades are more fragmentary, but there are suggestions that the starfish were rare or absent at the times when the brittlestar beds were flourishing. The increase in Luidia numbers from 1970 onwards was apparently confined to the English side of the western Channel, so that Ophiothrix populations outside this area were not affected.

Luidia ciliaris is known to prey upon Ophiothrix fragilis and other echinoderms (Brun, 1972). It is uncertain whether the observed rate of feeding of Luidia upon Ophiothrix (2.4 individuals day^-1 in Brun’s laboratory study) is high enough to directly account for the disappearance of the beds, but it is likely that brittlestar mortality rates would be increased by the disruptive effect of the starfish in addition to direct predation. By breaking up the dense aggregations of Ophiothrix, Luidia would render the beds less stable and more likely to be dislodged by strong currents.

The increase in Luidia populations during the 1970s may have been driven by the ‘Russell Cycle’, a multidecadal oceanographic cycle affecting the western Channel (Russell, 1935; Southward, 1980), and expressed by changes in sea surface temperature, circulation patterns, nutrient levels and plankton communities. A change in sea conditions and plankton composition occurred off Plymouth in 1968 (Southward, 1980), probably due to the inflow of water from the Celtic Sea, and this may have brought an influx of Luidia larvae and created the conditions necessary for their successful development.

Ball et al. (1995) found that the stomachs of Luidia ciliaris specimens from Kinsale Harbour, Ireland, contained up to 13 Ophiothrix, but considered that the starfish had little effect on the density of the brittlestar aggregations owing to the continuous presence of juveniles able to colonize newly cleared surfaces. It is possible therefore that the cyclical changes in the western Channel might have a more complex causal mechanism, involving a failure of Ophiothrix recruitment as well as increased predation by Luidia.

Stachowitsch (1984) observed a mass mortality of benthic organisms in the Gulf of Trieste, northern Adriatic Sea, apparently caused by the onset of severe hypoxia (oxygen depletion) in the near-bottom water. A wide variety of organisms were affected, including burrowing invertebrates, sponges, and the brittlestar Ophiothrix quinquemaculata, a dominant component of the local epifaunal community (Fedra et al., 1976). The area affected by hypoxia covered several hundred km². The mass mortality proceeded very rapidly. On September 10, 1983, no abnormal signs were visible, but by September 12 conditions had deteriorated severely. All brittlestars and sponges were dead within 2 - 3 days of the onset of hypoxia.

This event appears to have been caused by a combination of unfavourable weather and tidal conditions, coinciding with a period of maximal organic input from coastal pollution and sedimenting phytoplankton. The 1983 summer was unusually hot, and a strong thermocline had developed only 2 - 3 m above the sea bottom in the Gulf of Trieste. Water exchange in the gulf is poor, and the area tends to accumulate sediment and suspended organic material. Very high productivity in the water column, combined with sewage input throughout the summer tourist season, probably led to the consumption of most of the dissolved oxygen by microbial activity. Mortality occurred when the oxygen-deficient water mass extended to the sea floor.

Leewis et al. (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 substratum 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.

The cyclical changes in the western English Channel show that dense populations of brittlestars can persist for several decades, but may also decline sharply in the space of a few years and remain at low levels for a decade or more if conditions remain unfavourable. These changes appear to be natural in origin, and associated with large-scale oceanographic cycles (mediated through changes in predator populations). It is possible that analogous processes operate in other areas, but have not been detected due to the absence of comparable long-term records.

The examples from the Gulf of Trieste and the Oosterschelde Estuary are probably of more limited general relevance. Hypoxia of the severity described in the Adriatic is extremely unlikely to occur in the current-swept environments typical of most brittlestar beds around the UK and Ireland, although the necessary combination of environmental conditions (poor water circulation, high organic input) can be found in sea lochs or other semi-enclosed localities. The majority of brittlestar beds are found in much deeper waters than those of the Oosterschelde, and are therefore better insulated from exposure to extremes of temperature.

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