Biological modification of coastal pH depends on community composition and time.

Biological processes play important roles in determining how global changes manifests at local scales. Primary producers can absorb increased CO via daytime photosynthesis, modifying pH in aquatic ecosystems. Yet producers and consumers also increase CO via respiration. It is unclear whether biological modification of pH differs across the year, and, if so, what biotic and abiotic drivers underlie temporal differences. We addressed these questions using intensive study of tide pool ecosystems in Alaska, USA, including quarterly surveys of 34 pools over one year and monthly surveys of 5 pools from spring to fall in a second year. We measured physical conditions, community composition, and changes in pH and dissolved oxygen during day and night. We detected strong temporal patterns in pH dynamics. Our measurements indicate that pH modification varies spatially (between tide pools) and temporally (across months). This variation in pH dynamics mirrored changes in dissolved oxygen and was associated with community composition, including both relative abundance and diversity of benthic producers and consumers, whose role differed across the year, particularly at night. These results highlight the importance of time of year when considering the ways that community composition influences pH conditions in aquatic ecosystems.