This study investigated differences in total biomass (ash-free dry weight) of the periphyton and autotrophic biomass (chlorophyll-a content) of benthic diatoms in the absence or presence (No Grazer vs With Grazer) of two invertebrate grazers (Stichopus cf. horrens and Trochus maculatus) under simulated ambient (PRESENT), independent ocean acidification (OA) and warming (OW), and their combination (FUTURE) over an eight-day period. In the absence of a grazer, there were no significant differences in the average of the total and autotrophic biomass among treatments for both experiments. Stichopus significantly reduced the total and autotrophic biomass after 1 day, except under OW. Trochus significantly reduced the total biomass in the OA and OW treatments after 5 days, and the autotrophic biomass in the OA treatment after 1 and 5 days of grazing. In treatments where total and autotrophic biomass were not reduced, nutrients from the fecal matter and metabolic wastes of grazers seemingly stimulated the regeneration of microalgal biomass. The amount of fecal matter produced also affected the rate of microalgal renewal. In addition, due to the unexpected difference in seawater nutrient concentration during the two experiments, comparison of primary production under PRESENT was done to tease out nutrient effects. In PRESENT, autotrophic biomass was higher in Experiment 1 than Experiment 2, which was likely influenced by differences in nutrient concentrations. Results of this study elucidate underlying mechanisms in microalgal interactions with biotic and abiotic factors in tropical systems under changing ocean conditions.