Changes in brown seaweed distributions in Lough Hyne, SW Ireland: a long-term perspective

Abstract

Abstract Distributions of brown seaweeds (Phaeophyceae: Fucales, Laminariales and Tilopteridales) were surveyed in 2011 in Europe’s first marine reserve, Lough Hyne in SW Ireland, and compared with distributions from three historical surveys (1930, 1955 and 1980). The most salient phycological differences were the incursion of the low intertidal and shallow subtidal species Fucus serratus, Himanthalia elongata and Saccharina latissima into the north basin of the marine reserve after the recent mass mortality of the purple urchin (Paracentrotus lividus). Monitoring surveys (1990–2012) at ten sites in the lough indicated that populations of S. latissima peaked in 1994, abruptly crashed in 1996 and then slowly recovered. As well as documenting the expansion of kelp, our annual, whole-lough snorkel surveys (2004–2012) demonstrated the recent proliferation of the introduced Sargassum muticum and native Cystoseira foeniculacea. Although the causal role of top-down factors (fishing ban and/or urchin population crash), bottom-up factors (nutrient enhancement), climatic variables (warming vs. episodic cooling) and pathogens (urchin and algal) is being widely debated, decadal-scale and inter-annual changes are clearly detectable and most consistent with a release from herbivory within the marine protected area.