Effects of Increasing Temperature and Ocean Acidification on the Microstages of two Populations of Saccharina latissima in the Northwest Atlantic

Saccharina latissima (Linnaeus) C.E.Lane, C.Mayes, L.D. Druehl and G.W.Saunders, is the most widely distributed species of kelp in the western North Atlantic, occurring from the Arctic to Long Island Sound. The effects of global climate change on these ecologically and economically important cold temperate species at the southern range of their distribution are unknown. This study investigated the impact of the combined stressors of increased temperature (16, 19, 22, 25 & 28°C) and reduced pH (7.9, 7.8, 7.7, & 7.6) on the gametophyte and juvenile sporophyte stages of sugar kelp populations from Maine and Long Island Sound. Spore germination and growth, male and female ratio, fecundity, reproductive success of female gametophytes, and growth of juvenile sporophytes were investigated on crossed gradient temperature tables with CO2-adjusted pH levels. The upper critical thermal limit for gametophytes in all trials for both populations was 22°C, with full mortality of gametophytes occurring at all temperatures tested above this limit (i.e. 25° and 28°C). Gametophyte survival, growth, and male and female ratios were similar in all trials for both populations at 16° and 19°C, but gametogenesis was suppressed at temperatures above ca. 17°C. There were no consistent effects of pH in any trials, though the lower pH values (7.6-7.7) did result in slightly larger gametophytes (primary cell diameter & gametophyte length) than the highest value (7.9) at 16° and 19°C in some of the trials. These results support the hypothesis that the predicted increase in seawater temperatures will shift the distributional boundary of these cold temperate seaweeds northward, resulting in the loss of populations at the southernmost boundary.