Summer 2021

Document Type

Master's Thesis (Open Access)

Degree Name

Master of Science (M.S.)


Moss Landing Marine Laboratories


Climate change is driving an increase in the number of El Niño and marine heatwave events, especially in the last decade. These events when coupled with localized stressors can lead to dramatic changes to the local ecology. Kelp forests, found in temporal ecosystems across the world, are among the most vulnerable to these extreme environmental shifts. Off the southern west coast of North America, kelp forests are experiencing warmer waters and additional stressors from the introductions of new species brought in by boats through busy ports. This study examined how the 2014 heatwave and the 2015/16 El Niño coupled with the reproductive strategies of native giant kelp, Macrocystis pyrifera, and invasive fucoid, Sargassum horneri, may have contributed to patterns of deforestation of kelp forests on Catalina Island. This study sought to understand the sudden disappearance of kelp beds at the end of 2016 by conducting monthly field surveys to observe current density and demography, temporal fluctuations of vegetative vs. reproductive biomass investment, and the temporal variation in gamete release for both species. Surveys were conducted on the west-end of Catalina Island from July 2018 to August 2019 observing M. pyrifera and S. horneri at four sites that were kelp forests prior to the 2014 heatwave. This fieldwork was complimented by experimental lab cultures growing M. pyrifera with and without S. horneri settled concurrently at current, medium, and high temperature predictions. The lab experiments focused on understanding competition between these two species at their microscopic life stages and what role temperature plays in gamete development success. The results of the field surveys observed some deforested areas regaining canopy forming adult kelp, but not all. Sites with kelp had fluctuations in density and fecundity. Peaks in kelp biomass occurred in summer and coincided with peaks in zoospore production, however the size of the individual did not correlate strongly to its zoospore production. Sargassum horneri biomass and reproduction peaked between February and April when the majority biomass of this annual was dedicated to reproduction. Macrocystis pyrifera produced more zoospores at its peak reproductive period in summer than S. horneri and had smaller pulses of zoospore release throughout the year. But S. horneri released all its zygotes earlier and because of its animalistic life history can grow a faster rate. In the controlled lab experiment the development of these gametes was observed to be most influenced by temperature, with some limited influence from S. horneri density. Kelp gametophytes developed into fewer sporophytes at the highest temperature treatment and sargassum zygotes developed more quickly in this same treatment. These results support the narrative that there is nothing inherently unusual or toxic about S. horneri. Sargassum horneri has likely been successful on Catalina Island because of the cumulative effects of available space and above average temperatures brought on by a sequence of a marine heatwave, fall typhoons, and the hottest El Niño recorded in recent history. Concurrently, these conditions also made it difficult for M. pyrifera to grow and reproduce successfully. Further research focused on how high ocean temperatures impact M. pyrifera resilience and how kelp forest ecological processes can aid in better understanding the trajectory of current and future introduced species, such as invasive S. horneri. Understanding biotic interactions between species and abiotic oceanographic conditions can lead to better management strategies that support healthy ecosystems in the changing oceans of the future.