Latitudinal variation in seasonal cycle mediates population differences in barnacle reproduction phenology

The timing of life history events around reproduction and early development is critical in population dynamics, and it can determine recruitment success, species dispersal, and population connectivity. In ectotherms, as well as in plants and fungi, phenology is mediated by the nonlinear effects of temperature on physiology and development, meaning that spatiotemporal variation in temperature can exert powerful controls on the timing of local reproduction and recruitment. Here, we examine reproduction phenology (fertilization of embryos, duration of embryonic development during brooding, and larval release) of the intertidal acorn barnacle Semibalanus balanoides in 2002-2004 and in 2019-2024 at up to eight sites along a steep temperature gradient in the northwest Atlantic Ocean. At each site and year, we assessed how phenology varied with intertidal temperature, estimated with a hybrid atmosphere-ocean data assimilation model. Although within-site reproduction was delayed due to interannual and decadal fall warming (3.7 days per 1°C), fertilization at all sites in all years still occurred within a 1-month timeframe. In contrast, latitudinal differences in intertidal temperature resulted in substantially different brooding durations (up to 95 days difference) and, by extension, larval release timing (e.g., Dec 18 vs. Apr 4). Consequently, lower latitude larvae tended to enter the water column at the start of winter, while higher latitude larvae were not released until spring. These different larval release times result in regional differences in temperature-mediated larval development, potentially resulting in lower latitude populations experiencing greater dispersal. Our study is one of the first to evaluate these relationships through both space and time in natural populations, and we show that both spatial gradients and interannual variation in the seasonal temperature cycle can mediate reproductive physiology and dispersal of temperate and polar species.