Monthly macroalgal surveys reveal a diverse and dynamic community in an urban intertidal zone

Historical baseline data are critical to measuring how communities shift in response to climate change and anthropogenic activity. Macroalgae are marine foundation species that are vulnerable to climate change, but there is a lack of baseline macroalgal biodiversity data for many areas of British Columbia, particularly at a high temporal resolution over years. This presents an obstacle for measuring how communities change in response to shifting average conditions as well as after extreme events, such as the 2021 heat dome. To increase baseline macroalgae biodiversity data in British Columbia, we established a monthly transect-based macroalgal survey in 2021 at an urban intertidal site with a survey protocol that is low cost and easy to replicate. Our datasets and our survey protocol are freely available. Over 35 months we have recorded 59 species of macroalgae, including the canopy forming kelp Nereocystis luetkeana and the non-native fucoid Sargassum muticum. Surveying throughout the year at regular intervals has revealed large-scale seasonal shifts in macroalgal community composition, timing of kelp recruitment, and a decrease in abundance of Fucus distichus in the upper intertidal zone over multiple years. Our publicly accessible data constitute the most comprehensive survey of intertidal macroalgal diversity in Burrard Inlet, illustrating how simple surveying methods can provide high resolution records of macroalgae biodiversity, particularly in easily accessible urban environments.