Even protected seaweeds must face a warming ocean: Sea surface temperatures trigger tissue bleaching and breakdown in the unique giant Irish moss (Chondrus crispus).

Current projections of rising sea surface temperatures (SST) pose a threat to marine macroalgae, particularly those living in shallow coastal areas. The giant Irish moss, a unique strain of the common red alga Chondrus crispus, is found solely in a coastal lagoon in Prince Edward Island, Atlantic Canada, and has undergone a two-decade population decline. Despite protection efforts, this alga has not recovered to its pre-decline abundance, which may be due to, among other factors, warming ocean temperatures. This study used laboratory experiments to examine the effects of a range of temperatures, either as steady or discrete temperature spikes, on algal fronds' weight and surface area. The results from both types of experiments were congruent and supported the notion that temperatures in the upper range of those currently occurring or predicted to occur, cause the worst losses in weight and surface area. Forecasted increases in SSTs leave the giant Irish moss vulnerable to further population declines and represent a concern for the long-term sustainability of this unique strain of Irish moss.