A Comparison of Reproductive Timing of Macrocystis pyrifera and invasive Sargassum horneri on Catalina Island

9 min Regular Session Talk for the Phycological Society of America (PSA) annual conference 2021Observations of terrestrial species shifting their reproductive windows in response to climate change begs the question: Could this occur in aquatic environments too? Giant kelp, Macrocystis pyrifera, is the foundational species along much of California’s coastlines and islands, but in the aftermath of the 2015-16 El Nino on Catalina Island, it appeared to struggle to regain its dominance in the face of an invasive fucoid Sargassum horneri. As a part of a master’s studying the life history interactions between these two species, I performed monthly population and reproductive surveys, July 2018-August 2019, to address when each species was investing most in biomass, reproductive biomass, reproductive output and if there was a correlation between biomass and output in these species. The annual S. horneri dedicated much of its reproductive effort to a few short months in the spring (Feb.- April) and had a strong relationship between individual size and reproductive output. The perennial M. pyrifera peaked in reproductive output in late summer (July-August) and had a weak relationship between an individual’s size and reproduction. The observed timing was surprising as kelp is expected to peak in reproduction in the spring when upwelling is at its strongest. These observations on the kelp forests of Catalina could be an artifact of multiple stressful years of El Nino, heatwaves, and invasive species, but they also ask us to think about the assumptions we have about the classic timing & cycling of algal life histories