Invasive species can alter patterns of abundance and diversity in species that are ecosystem engineers, which in turn may impact the ecosystem services these species provide. This study addresses how the recent invasion of the brown algae, Sargassum horneri, alters native algal abundances and rates of primary production on Catalina Island, California, USA. Specifically, I identified what drove diurnal patterns of Gross Primary Production (GPP) and Net Primary Production (NPP) in benthic algal communities on a subtidal rocky reef on Catalina Island that had been invaded by S. horneri. Additionally, I conducted periodic S. horneri removals to examine how S. horneri removal may influence native algal abundances, GPP and NPP. My study revealed that variation in morning GPP among months is driven primarily by changes in S. horneri biomass. In contrast, variation in afternoon GPP among months is primarily driven by differences in ocean temperature. Daily variations in GPP is strongly influenced by changes in temperature, algal metabolism and light conditions, ultimately resulting in net respiration by late afternoon. Moreover, S. horneri removal led to 9x more adult giant kelp stipes, Macrocystis pyrifera and 3x more juvenile kelps ( Ecklonia arborea and M. pyrifera) in S. horneri removal plots than in control plots. However, these changes in algal abundances upon S. horneri removal did not scale up to drive significant changes in GPP or NPP among treatments over the course of this study, suggesting that S. horneri removals may need to be conducted for more than a year to see significant changes in primary production. This study provides evidence that the invasion of S. horneri negatively alters patterns of native algae abundance and primary production on subtidal rocky reefs at Catalina Island.