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Barnacle recruitment is often studied in rocky intertidal habitats due to the relevant role that barnacles can play in intertidal communities. In 2014, barnacle (Semibalanus balanoides) recruitment was measured at high elevations in wave-exposed intertidal habitats on the NW Atlantic coast in Nova Scotia, Canada. Values were considerably lower than previously reported for middle elevations in wave-exposed intertidal habitats on the NE Atlantic and NE Pacific coasts. To determine if such differences in recruitment may have resulted from elevation influences, I did a field experiment in 2019 in wave-exposed intertidal habitats in Nova Scotia to test the hypothesis that recruitment is higher at middle than at high elevations, based on known environmental differences between both elevation zones. Based on data from three locations spanning 158 km of the Nova Scotia coast, barnacle recruitment was, on average, nearly 200 % higher (and recruits were larger) at middle than at high elevations. However, even with this increase, barnacle recruitment on this NW Atlantic coast is still lower than for comparable habitats on the NE Atlantic and NE Pacific coasts, and also lower than previously reported for wave-exposed locations farther south on the NW Atlantic coast, in Maine, USA. Therefore, barnacle recruitment in wave-exposed intertidal environments in Nova Scotia appears to be only moderate relative to other shores. This difference in the supply of barnacle recruits might influence the intensity of interspecific interactions involving barnacles.

In rocky intertidal environments, the vertical gradient of abiotic stress generates, directly or indirectly, significant spatial variation in community structure. Along shorelines within biogeographic regions, abiotic changes also generate horizontal biological variation, which when measured at large sampling intervals may surpass vertical biological variation. Little is known, however, on how vertical variation compares with horizontal variation measured at multiple spatial scales in habitats with similar environmental conditions. Here, we compare spatial variability in rocky-intertidal communities between vertical stress gradients and three horizontal spatial scales (sampling interval) across habitats experiencing the same wave exposure on the Northwest Atlantic (NWA) and Southeast Pacific (SEP) coasts. For both regions, the vertical variation in species richness and composition (Raup-Crick and Bray-Curtis indices) was higher than the variation measured at all horizontal scales, from a few cm to hundreds of km. The patterns of variation in community structure matched those of abundance for the dominant sessile organisms, the foundation species Ascophyllum nodosum (seaweed) in NWA and Perumytilus purpuratus (mussel) in SEP. This interhemispheric comparison reveals the tight link between environmental and biological variation, indicating that studies comparing spatial scales of biological variation must consider the underlying environmental variation in addition to simply scale alone.

Recruitment is a key demographic step for population persistence. This paper focuses on understanding barnacle (Semibalanus balanoides) recruitment. In rocky intertidal habitats from the Gulf of St. Lawrence coast of Nova Scotia (Canada), winter ice scour is common. At the onset of intertidal barnacle recruitment in the spring, mostly only adult barnacles and bare substrate are visible at high elevations on the shore. We conducted a multiannual study to investigate if small-scale barnacle recruitment could be predicted from the density of pre-existing adult barnacles. In a year that exhibited a wide adult density range (ca. 0–130 individuals dm-2), the relationship between adult density and recruit density (referred to the available area for recruitment, which excluded adult barnacles) was unimodal. In years that exhibited a lower adult density range (ca. 0–40/50 individuals dm-2), the relationship between adult and recruit density was positive but simply resembling the lower half of the unimodal relationship. Overall, adult barnacle density was able to explain 26–40 % of the observed variation in recruit density. The unimodal adult–recruit relationship is consistent with previously documented intraspecific interactions. Between low and intermediate adult densities, the positive nature of the relationship relates to the ability of adult barnacles to attract settlement-seeking larvae, likely enhancing local population persistence where it is most needed. Between intermediate and high adult densities, where population persistence may be less compromised and the abundant adults may be detrimental to recruit growth, the negative nature of the relationship suggests that larvae are stimulated to settle elsewhere. This unimodal pattern may be particularly common on shores with moderate rates of larval supply to the shore, as high larval supply rates may swamp the coast with settlers, decoupling recruit density from local adult abundance.

On the Gulf of St. Lawrence coast of Nova Scotia (Canada), recruitment of the barnacle Semibalanus balanoides occurs in May and June. Every year in June between 2005 and 2016, we recorded recruit density for this barnacle at the same wave-exposed rocky intertidal location on this coast. During these 12 years, mean recruit density was lowest in 2015 (198 recruits dm-2) and highest in 2007 (969 recruits dm-2). The highest recruit density observed in a single quadrat was 1457 recruits dm-2 (in 2011) and the lowest density was 34 recruits dm-2 (in 2015). Most barnacle recruits appear during May, which suggests that most pelagic larvae, which develop over five-to-six weeks and originate the recruits, are in the water column in April. A model selection approach identified sea surface temperature (SST) in April and the abundance of phytoplankton (food for barnacle larvae, measured as chlorophyll-a concentration –Chl-a–) in April as good explanatory variables. Together, April SST and Chl-a explained 51 % of the observed interannual variation in recruit density with an overall positive influence. April SST was positively related to March–April air temperature. April Chl-a was negatively related to the April ratio between the number of days with onshore winds (which blow from phytoplankton-limited offshore waters) and the number of days with alongshore winds (coastal phytoplankton is higher on coastal waters). Therefore, these observations suggest that climatic processes affecting April SST and Chl-a indirectly influence intertidal barnacle recruitment by influencing larval performance.