RECURRENT DESTRUCTIVE GRAZING OF SUCCESSIONALLY IMMATURE KELP FORESTS BY GREEN SEA-URCHINS IN VESTFJORDEN, NORTHERN NORWAY

Outbreak populations of the green sea urchin Strongylocentrotus droebachiensis Muller caused widespread decimation of the original Laminaria hyperborea (Gunn.) Fosl. kelp forest in Vestfjorden, Northern Norway, during the early 1980s. In the following decade, some of the resulting urchin-dominated barren grounds reverted to kelp forest, which, after persisting for more than 5 yr, began to be eliminated yet again. Age analysis of annual growth rings in kelp stipes from field sites at Vaeroy Island indicate that the time of initial kelp recovery varied from site to site but took place between 1984 and 1987. The earliest independent observations of localized macrophyte recovery were made in 1984 at the Harbour Pier site, where, by August 1992, the new kelp forest had already been eliminated by recurrent destructive grazing. Sea urchin density inside re-established kelp forests at 3 other sites on Vaeroy Island was 45 to 75 ind. m(-2) in 1992. These urchin populations had significantly aggregated spatial patterns, and recurrent destructive grazing appeared to be imminent. This prediction was verified in 1993 when barren grounds reappeared at all study sites. The threshold conditions for initiation of destructive grazing have been approximated by a curve in the aggregation-density plane. Sea urchins in the Vestjorden area are infected by the recently discovered epizootic endoparasitic nematode Echinomormella matsi. In 1992, the prevalence of E. matsi at Vaeroy Island ranged from 8.8% in the barren ground at the Harbour Pier site, to between 13.6 and 21.6% in the successionally immature kelp forest at the 3 other sites. The observed kelp forest recovery at Vaeroy Island was predicted by the macroparasite hypothesis which states that E. matsi may function as a terminator of sea urchin outbreaks in Northern Norway, However, the succession towards an ecologically mature kelp forest community has been interrupted by the unexpected recurrence of destructive grazing, and the macroparasite hypothesis must therefore be rejected in its present form. Furthermore, these local events may, on the larger time and spatial scales of the current outbreak phenomenon, indicate that the euphotic hard bottom component of the coastal ecosystem in Northern Norway has entered a cyclical domain.