Global changes in climatic conditions are expected to disrupt marine ecosystems. Ocean warming is one of many concerns, since more than 90% of the Earth's warming occurs in the oceans. Macrophyte-dominated communities recently have become the focus of climate mitigation due to their high carbon sequestration rate. Therefore, there is an urgent need to understand the effects of environmental variables on the phenological response of photosynthesis in entire macrophyte communities (i.e., community production). We conducted 30 monitoring surveys from May 2015 to February 2017 and collected time-series data of environmental variables in Zostera marina (3 m depth) and Sargassum siliquastrum (1 m depth) communities. The community production and respiration from two different macrophyte communities were calculated from dissolved oxygen time-series. Analysis of the time-series indicated strong diurnal frequencies for dissolved oxygen, light, and net ecosystem production, whereas weekly frequencies dominated for water temperature, chlorophyll a fluorescence, and current speed. Water temperature appeared to drive mean gross ecosystem production over the course of a year and light induced variations in the short-term and were similar in both macrophyte communities.

SUMMARY Approximately one‐third of anthropogenic carbon dioxide is absorbed into the ocean and causes it to become more acidic. The Intergovernmental Panel on Climate Change (IPCC) suggested that the surface ocean pH, by the year 2100, would drop by a further 0.3 and 0.4 pH units under RCP (Representative Concentration Pathway) 6.0 and 8.5 climate scenarios. The macroalgae communities that consisted of Sargassum thunbergii and naturally attached epibionts were exposed to fluctuations of ambient and manipulated pH (0.3–0.4 units below ambient pH). The production and respiration in S. thunbergii communities were calculated from dissolved oxygen time‐series recorded with optical dissolved oxygen sensors. The pH, irradiance, and dissolved oxygen occurred in parallel with diurnal (day/night) patterns. According to net mesocosm production – photosynthetically active radiation (PAR) model, the saturation and compensation PAR, the mean maximum gross mesocosm production (GMP), and daily mesocosm respiration were higher in the CO2 enrichment, than in the ambient condition, while the mean of photosynthetic coefficient was similar. In conclusion, elevated CO2 stimulated oxygen production and consumption of S. thunbergii communities in the mesocosm. Furthermore, the sensitivity of the GMP of the S. thunbergii community to irradiance was reduced, and achieved maximum production rate at higher PAR. These positive responses to CO2 enrichment suggest that S. thunbergii communities may thrive in under high CO2 conditions.