Long-term annual monitoring of rocky intertidal and shallow subtidal shores of an Irish sea lough (1990–2010) documented major regime shifts in the past decade. When population densities of the purple urchin Paracentrotus lividus plummeted in Lough Hyne Marine Reserve in southwestern Ireland, the warm-water fucalean alga Cystoseira foeniculacea and ephemeral algae proliferated shortly after. We discuss the possible influences of release from herbivory and climate change on this algal proliferation, which blanketed the benthos. Smothering of the benthos led to high levels of shallow subtidal anoxia. Furthermore, the invasive fucalean alga Sargassum muticum has made repeated incursions into the reserve. Although being reduced by persistent eradication efforts (2003–2011), Sargassum is spreading within the lough. Limited seawater flushing and propagule dispersal within the lough and eutrophication in coastal waters may have contributed to community-level changes. Whether the regime change is cyclical (contingent on re-establishment of urchins within the lough and continued eradication of S. muticum) or irreversible (the result of critical transitions) remains unclear.
©2011 by Walter de Gruyter Berlin Boston