Climate change represents a threat to coastal marine ecosystems through variable effects on community structure and function due to increasing mean sea-surface temperatures (SST), marine heatwaves, variation in salinity, and ocean acidification. Among the most at risk species are California kelps, which have already experienced significant die-offs over the past several years as a result of elevated SST and urchin grazing. However, the effects of these stresses on the very sensitive microscopic kelp life stage (gametophyte) are much less understood. Gametophytes are generally less resilient to changes in abiotic conditions, so global environmental change could result in drastic changes in kelp forest community structure and composition via impacts on this life stage. My dissertation research used manipulative laboratory experiments to investigate the interacting role of abiotic stressors on kelp reproduction and community compositions, specifically, the growth and survival of early kelp life stages. My first chapter focused on the effects of climate-driven temperature increases and ocean acidification on bull kelp (Nereocystis luetkeana) gametophytes from Point Arena, CA (Korabik et al. 2023). From 2014 to 2016, the largest marine heatwave in history appeared off the coast of California resulting in large kelp die off events. In this chapter, I asked how increased temperature and lowered pH impact the survival of bull kelp gametophytes and the production of juvenile bull kelp sporophytes. My results showed that increased temperature resulted in a significant decrease in the survival of gametophytes and a lower number of juveniles produced, whereas lowered pH only had a significant effect on the production of juveniles, slowing their rate of development. These results indicate that the predicted increase of marine heatwaves could have devastating effects on the persistence of bull kelp forest ecosystems. My second chapter considered the interacting effects of climate driven changes in temperature and salinity and interactions with the invasive seaweed (Sargassum muticum) on the growth and survival of giant kelp (Macrocystis pyrifera) gametophytes from Tomales Bay, CA. In my experiments, I tested: 1) how different salinities and temperatures impact giant kelp early life stages from different sources within Tomales Bay, 2) how the presence of invasive Sargassum propagules affect giant kelp gametophyte development, and 3) how the combined effects of salinity, temperature, and Sargassum presence affect giant kelp early life stages. My results indicate that 1) the presence of Sargassum had little effect on the survival of giant kelp gametophytes, 2) Sargassum accelerated development of giant kelp juvenile sporophytes, and 3) high temperatures resulted in the greatest reduction of giant kelp gametophyte survival. These results imply that giant kelp reproduction and presence within estuaries is more influenced by temperature than salinity and microscopic-stage competition with invasive species. My third chapter examined the effects of increased temperature and lowered salinities on invasive Wakame (Undaria pinnatifida) gametophytes in the San Francisco Bay. Previous studies have shown that low salinity can limit the distribution of Undaria, but there is no information about these effects on gametophyte stages. Using a full factorial design, I exposed Undaria gametophytes to five salinity conditions ranging from low to ambient salinity and two temperatures representing pre-2013 temperature maxima in San Francisco and maximum increased temperatures experienced under the 2014-2016 marine heatwave. I found that Undaria microstages were unable to survive below 20 psu and generally survived better under warmer temperatures of 18°C. Climate change in California is predicted to result in higher temperatures and reduced annual rainfall in drought years, which may facilitate future northward expansion of Wakame populations. With this research, I can better predict the impacts of climate change on kelp ecosystems to help coastal managers prioritize future protection efforts. Early life stages are often the most vulnerable to stress, and in this era of rapid climate change, understanding early life stage responses to stress will allow scientists and managers to better work towards the protection of our planet.