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

Abstract

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 baseline macroalgal biodiversity data are lacking 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an urban intertidal site with a low-cost and easily replicable survey protocol, in addition to publishing a previously unpublished 1983-1984 historical dataset of the same area. Our datasets and our survey protocol are freely available. Over 35 months we have recorded 61 taxa of macroalgae, including the canopy forming kelp Nereocystis luetkeana and the introduced fucoid Sargassum muticum. Surveying throughout the year at regular intervals has revealed large-scale seasonal shifts in macroalgal community composition, the 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.