Long-term trends in algal-dominated rocky subtidal communities on the Swedish west coast - a transitional system?

Abstract

Rocky subtidal communities on the Swedish west coast were monitored over approximately 16 years (1970–1985) by means of stereophotographic recordings, at intervals, of fixed sites.

Dominating elements of community structure were studied at the 5- and 10 m depth levels of an outer ar-chipelago locality. Excluding seasonal variations, the basic community structure, largely dominated by the perennial phaeophycean Halidrys siliquosa (L.), remained relatively stable over the first 7–8 years of study, but with an increasing trend in cover of filamentous epiphytic red algae. Later, the structure of the studied communities changed radically. The changes initially involved intense settling of the blue mussel Mytilus edu-lis L. and reduced cover of red algae. The mussels settled largely on Halidrys plants which eventually broke down almost completely. Later, there was intensive predation on Mytilus by the starfish Asterias rubens Linné. After the elimination of Mytilus, the studied test areas were dominated by filamentous ephemeral red algae for the rest of the study period, but with some regeneration of Halidrys at the 5 m level.

The observed changes were thought to be indicative of a major disturbance in the coastal ecosystems off the Swedish west coast during the studied period. This notion is discussed in relation to studies of other components of the ecosystem during the same time period and in relation to possible causes. Among the latter may be mentioned a suspected eutrophication process, long-term temperature anomalies and resulting effects on biological interrelationships such as competition, predation and chemical defense.

The observed trends included a shift in dominance from slow-growing perennial algae to fast-growing ephemeral algae. If this shift was a general phenomenon, for which there is some circumstantial evidence, it could have important repercussions on the cycling of nutrients and organic matter in the coastal ecosystem. Possible effects are discussed on hypothetical terms in relation to observed anomalies in phytoplankton blooms and benthic oxygenation.