Effects of rising sea surface temperature and decreasing salinity on kelps and associated macroalgal communities

Abstract

Kelp forests provide a multitude of vital ecosystems services, such as habitat for commercially and recreationally important species, support of complex food webs, and reduction of coastal erosion. The diversity and resilience of kelp forest communities are threatened as the severity of climate change and other anthropogenic stressors continues to mount. Particularly in the North Pacific, sea surface temperature (SST) is warming and glacier melt is discharging into coastal waters, causing decreases in salinity. This study assesses possible impacts of increasing SST and decreasing salinity on kelp forests by examining the response of key kelp species and their associated macroalgal community to these parameters in a North Pacific estuary, using Kachemak Bay as a model system. This two-part study combines both 1) a retrospective analysis of effects of environmental variables on existing kelp populations (Agarum clathratum, Laminaria yezoensis, and Saccharina latissima) and their associated macroalgal communities at three discreet water depths (5, 10 and 15 m), and 2) a factorial laboratory experiment investigating the effects of rising SST and decreasing salinity on kelp spore settlement and initial gametophyte growth in Eualaria fistulosa, Nereocystis luetkeana, and S. latissima. No strong correlations were observed between adult kelp biomass of any individual species with past SST and salinity changes, with the exception of a negative correlation between SST and L. yezoensis biomass at 10 m. In addition, SST and salinity were insignificant factors in shaping the associated macroalgal community biomass. In contrast to the retrospective analyses, the experimental results indicated that the early life-history stages of all kelp study species experienced decreased settlement and growth at elevated temperatures and decreased salinities. Eualaria fistulosa spores and gametophytes were the most negatively impacted, compared to the more widely distributed N. luetkeana and S. latissima. These results suggest that N. luetkeana and S. latissima are more likely to outperform E. fistulosa under projected conditions. By exploring how both early and late life-history stages of several key kelp species are impacted by dual stressors, this research enhances our understanding of how these species and their associated macroalgal communities will respond to projected changes in SST and salinity.