Effects of seaweed canopies and adult barnacles on barnacle recruitment: The interplay of positive and negative influences Arne J. Beermann a , Julius A. Ellrich a , Markus Molis b , Ricardo A. Scrosati a, ⁎ a Saint Francis Xavier University, Department of Biology, Antigonish, Nova Scotia B2G 2W5, Canada b Alfred Wegener Institute for Polar and Marine Research (AWI), Am Handelshafen 12, 27570 Bremerhaven, Germany abstract article info Article history: Received 15 February 2013 Received in revised form 30 June 2013 Accepted 1 July 2013 Available online xxxx Keywords: Ascophyllum Barnacle Intertidal Seaweed Semibalanus Barnacles are dominant sessile invertebrates on many rocky shores worldwide. Hence, investigating the factors that affect their recruitment is important. Through field experiments done on the Atlantic coast of Canada, we investigated interspecific and intraspecific relationships affecting intertidal barnacle recruitment. Specifically, we evaluated the effects of seaweed canopies (Ascophyllum nodosum) and adult barnacles (Semibalanus balanoides) on the density of barnacle recruits at the end of the recruitment season. The effects of three canopy treatments on barnacle recruitment and understory environmental conditions allowed us to identify positive and negative effects of canopies. At mid-intertidal elevations subjected to a moderate wave action, we found that, during high tides, the flexible algal fronds damage wire sensors established on the substrate (whiplash effect) and limit barnacle recruitment. However, at low tide, algal canopies limit water loss and temperature extremes and enhance barnacle recruitment in understory habitats. The net effect of algal canopies on barnacle recruitment, however, was neutral, as the positive and negative influences balanced out. By manipulating the abundance of adult barnacles under the seaweed canopies, we found that adult barnacles enhance barnacle recruitment, likely due to the known attraction that adults exert on larvae seeking settlement and to the absence of post-settlement events that could otherwise have blurred such effects by the adults. The presence of adult barnacles, however, did not protect developing recruits from canopy whiplash effects. By understanding the con- trasting influences that intertidal algal canopies have on understory abiotic conditions and barnacle recruitment, our ability to predict net canopy effects depending on the relative degree of physiological (e.g., high vs. low intertidal zone) and physical (e.g., sheltered vs. exposed shores) stresses should increase. This study also suggests that recruitment, considered as an important external factor in environmental models of community organization, can also be affected by components of the community itself, potentially triggering local feedbacks. © 2013 Elsevier B.V. All rights reserved. 1. Introduction Barnacles are common organisms in rocky intertidal habitats worldwide. They are often the dominant sessile invertebrates because of their high resistance to emersion stress. As filter-feeders, barnacles convert pelagic biomass into benthic biomass, which eventually sustains upper trophic levels in benthic food webs. Hence, their ecological role is often important (Anderson, 1994; Foster, 1987). To understand how bar- nacle populations persist on the shore, several studies have investigated their recruitment. For intertidal sessile invertebrates with planktonic larvae, recruitment refers to the appearance of new individuals that have developed after larval settlement and have reached an arbitrary size that allows them to be counted (Cole et al., 2011). The transition from settler to recruit takes between a few days and weeks, depending on the species and abiotic conditions (Pineda et al., 2009). For such species, recruitment is an important life-history step that affects popula- tion persistence (Bertness et al., 1992; Broitman et al., 2008; Menge, 2000). Intertidal barnacle recruitment can be affected by non-trophic interspecific interactions. In particular, seaweeds that produce extensive canopies have a variety of positive and negative effects. Tides regularly expose intertidal habitats to aerial conditions, subjecting sessile organ- isms to abiotic stresses such as desiccation and temperature extremes (Eckersley and Scrosati, 2012; Raffaelli and Hawkins, 1999). Flexible macroalgal canopies lay mostly flat during low tides, limiting water loss and temperature variability in understory habitats. Thus, while seaweed canopies facilitate the performance of many understory species (Bertness et al., 1999; Watt and Scrosati, 2013), it has been suggested that canopies may also enhance barnacle recruitment by improving conditions for developing recruits during low tides (Dayton, 1971). During high tides, intertidal habitats are subjected to water move- ment caused by waves (Denny and Wethey, 2001). Thus, at high tide, flexible algal fronds hit and scour the substrate repeatedly (Dayton, 1975). Such a whiplash effect is considered to negatively affect barnacle Journal of Experimental Marine Biology and Ecology 448 (2013) 162–170 ⁎ Corresponding author. Tel.: +1 902 867 5289; fax: +1 902 867 2389. E-mail address: rscrosat@stfx.ca (R.A. Scrosati). 0022-0981/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jembe.2013.07.001 Contents lists available at SciVerse ScienceDirect Journal of Experimental Marine Biology and Ecology journal homepage: www.elsevier.com/locate/jembe