There have been no long-term studies of British sea pen populations, so the degree of temporal stability they exhibit is unknown. In California, Davis & Van Blaricom (1978) monitored densities of the sea pansy Renilla kollikeri on a subtidal sand plain from March 1974 to September 1975 and compared these with figures recorded at the same site in the period 1957-63. The density of Renilla remained fairly constant at about 1.68 individuals m^-2 throughout 1957-63. Mean density in 1974-75 was much lower (0.06 individuals m^-2). A major recruitment was observed in August 1974, the first seen at the study site since 1971. In the northern part of the study area, Renilla density decreased rapidly after the August larval settlement, eventually stabilizing at approximately pre-recruitment levels. Starfish predation was suggested as a cause of the post-recruitment population decrease. In the southern study area, the Renilla population stabilized at a higher density of 4 m^-2, possibly because starfish foraging was inhibited here by wave surge.

Information on changes in megafaunal burrower populations is also very fragmentary. Off the Northumberland coast, the density of Calocaris macandreae was found to be very stable, remaining at about 18 individuals m^-2 over a period of ten years (Buchanan, 1974). In contrast, in the German Bight (southern North Sea) populations of Echiurus echiurus fluctuate widely over a much shorter time-scale (Rachor & Bartel, 1981). From 1973 to spring 1976, the echiuran was apparently absent from the study area in the eastern German Bight. A mass recruitment of juvenile worms was recorded in spring 1976, with extremely high densities (averaging 250 individuals m^-2) over an area of about 40 km². By mid-late summer 1977, this area had shrunk to about 12.5 km² and population density had dropped to 140 m^-2. In summer 1978 Echiurus was found only in about 2.5 km² at a density of 22 individuals m^-2. This population had disappeared entirely by autumn 1978. From these observations and earlier records, Rachor & Bartel (1981) noted that successful recruitment of Echiurus seemed to follow very cold or stormy winters, perhaps as consequence of increased oxygenation of the bottom sediments.

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