Barnacle recruitment on ice-scoured shores in eastern Canada

Abstract

Previous observations in the St Lawrence Estuary (eastern Canada) suggested that larvae of intertidal barnacles (Semibalanus balanoides) would settle almost exclusively inside crevices on shores that are scoured by sea ice every winter. It was suggested that the strong ice scour in winter on that coast (which removes organisms outside of crevices) would select for such a larval behaviour. We tested the generality of this pattern by sampling other ice-scoured shores within the Gulf of St Lawrence system. In particular, we surveyed a shore in Nova Scotia where exposed habitats (subjected to strong ice scour in winter) are interspersed with sheltered habitats (which suffer milder ice scour). Such a topographical complexity might allow for the coastal larval pool to contain a proportion of larvae that have no particular settlement preference for crevices, as selective pressures for such a behaviour would be minimal in ice-sheltered habitats. Consistently with this notion, barnacle recruits (which appear after the winter ice melts) occurred abundantly both inside and outside of crevices across the shore in the spring seasons of 2005 and 2006. Average recruit density on rocky surfaces ranged between 337 and 588 recruits dm−2, depending on the habitat. It is therefore possible that barnacle recruitment patterns on ice-scoured shores may be affected indirectly by the structural complexity of the coast.

Intertidal barnacles as indicators of the intensity of scour by sea ice

“…First, the supply of new organisms does not explain differences in adult barnacle density between exposed and sheltered habitats. On our studied shore, new individuals of Semibalanus balanoides are recruited only after ice melt every year, and measurements done in 2 consecutive years indicated that recruit density is similar in sheltered and exposed habitats (MacPherson et al 2008). Interspecific interactions also do not seem to affect barnacle abundance in the high intertidal zone.…”

“…It is also worth noting that restricting density measurements to the high intertidal zone, as opposed to lower elevations at which barnacles also occur (MacPherson & Scrosati 2008), confers a practical advantage. High elevations are exposed to the air for longer periods of time than lower elevations because of tide dynamics (Raffaelli & Hawkins 1996), which should allow surveyors to take more field measurements per tidal cycle.…”

“…However, those organisms are virtually restricted to mid-and low intertidal elevations on the Gulf of St. Lawrence coast of Nova Scotia (Scrosati & Heaven 2007). Also, on our studied shore, the rocky substrate becomes widely covered by barnacles every autumn, as the numerous recruits from the preceding spring grow to adult size by autumn (MacPherson & Scrosati 2008). The entire coast is later covered by ice for the winter, and, thus, ice scour intensity emerges as the main factor likely explaining the differences in adult barnacle density between exposed and sheltered habitats in the high intertidal zone in spring.…”

“…In rocky intertidal habitats that suffer intense ice scour in winter, adult barnacles occur almost exclusively in cracks and crevices because such places constitute refuges against the physical abrasion caused by the ice (Bergeron & Bourget 1986). A study done at 1 location in northern Nova Scotia suggests that the density of adult barnacles in spring might be inversely related to the intensity of winter ice scour, as many individuals survive outside of crevices in habitats where ice scour is moderate (MacPherson et al 2008). However, the lack of location replication in that study precludes the establishment of any generalizations.…”

“…wave action, wind) seem to have determined the differences in adult barnacle density between exposed and sheltered sites between ice melt and the time of our measurements. Our studied coast became free of ice only at the end of April (Canadian Ice Service 2008), and we never observed any noticeable change in adult barnacle density at any site during our month-long study shortly after ice melt.It is also worth noting that restricting density measurements to the high intertidal zone, as opposed to lower elevations at which barnacles also occur (MacPherson & Scrosati 2008), confers a practical advantage. High elevations are exposed to the air for longer periods of time than lower elevations because of tide dynamics (Raffaelli & Hawkins 1996), which should allow surveyors to take more field measurements per tidal cycle.…”