Support for the intermittent upwelling hypothesis using 10 years of barnacle recruitment data from a western ocean boundary in Atlantic Canada

Recruitment is a key demographic step for the persistence of populations, so understanding its drivers has traditionally been a relevant goal of ecology. On marine rocky shores, coastal oceanography is an important driver of the recruitment of intertidal invertebrates that reproduce through pelagic larvae by affecting larval transport and delivery. The intermittent upwelling hypothesis (IUH) posits that coastal pelagic larvae are driven offshore under intense upwelling or to depths under intense downwelling, while weak upwelling allows larvae to stay near the shore, thereby facilitating intertidal recruitment. The IUH thus predicts a unimodal relationship between Bakun's upwelling index (BUI) and intertidal recruitment. The IUH has been supported by studies that plotted simultaneously single average values of recruitment and BUI for various coastal locations that collectively span downwelling to upwelling conditions. Based on its theoretical foundations, the IUH should also hold for a target location analyzed over the years provided enough interannual variation in BUI. On the Atlantic Canadian coast in Nova Scotia, upwelling varies interannually depending on wind patterns. Therefore, for a location that is representative of the abiotic and biotic conditions on this coast (Western Head), we tested the IUH by measuring annual intertidal barnacle recruitment and BUI for the pelagic larval season of barnacles for a period of 10 years (2014-2024). On this coast, BUI averaged for the barnacle larval season varied among years from mild downwelling to clear upwelling. Generalized additive modelling revealed a unimodal relationship between intertidal barnacle recruitment and BUI, thereby supporting the IUH. These results add this western ocean boundary to the known list of coastal systems where upwelling may influence intertidal invertebrate recruitment.