This is a non-peer-reviewed article published at MarXiv (doi: 10.31230/osf.io/5e9cs) 1 Positive relationship between coastal phytoplankton abundance and 2 intertidal barnacle growth along the Nova Scotia coast 3 Ricardo A. Scrosati 1 and Julius A. Ellrich 4 St. Francis Xavier University, Department of Biology, Antigonish, Nova Scotia B2G 2W5, 5 Canada 6 1 Corresponding author. Phone: +1-902-867-5289. Email: [email protected] 7 2 7 Abstract 8 Benthic–pelagic coupling refers to the ecological relationships between benthic and pelagic 9 environments. Studying such links is particularly useful to understand biological variation in 10 intertidal organisms along marine coasts. Filter-feeding invertebrates are important on marine 11 rocky shores, so they have often been used to investigate benthic–pelagic coupling. Most 12 studies, however, were done on eastern ocean boundary coasts highly influenced by upwelling. 13 To evaluate the extent of benthic–pelagic coupling on a western ocean boundary coast, we 14 conducted a 5-year study spanning 415 km of the Atlantic coast of Nova Scotia (Canada). 15 Between 2014 and 2018, we annually measured intertidal barnacle growth in experimental 16 clearings created on the rocky substrate at eight wave-exposed locations. We then examined the 17 relationships with chlorophyll-a concentration (Chl-a), a commonly used proxy for the 18 abundance of phytoplankton (food for barnacle nauplius larvae and benthic stages). For every 19 year and location, we used satellite data to calculate Chl-a averages for a period ranging from 20 the early spring (when likely most larvae were in the water) to the summer (when barnacle size 21 was measured after weeks of growth following spring benthic recruitment). The relationships 22 were always positive, Chl-a explaining nearly half, or more, of the variation in barnacle size in 23 four of the five studied years. These are remarkable results because they were based on a 24 relatively limited number of locations (which often curtails statistical power) and point to the 25 relevance of pelagic food supply to explain variation in barnacle growth along this western 26 ocean boundary coast. 27 3 27 Introduction 28 Benthic–pelagic coupling refers to the ecological relationships that exist between benthic 29 and pelagic environments (Griffiths et al. 2017). Recognition of such links has particularly 30 facilitated progress in the field of intertidal ecology. For example, understanding how pelagic 31 food supply and oceanographic properties vary along coastlines often helps to predict, directly 32 or indirectly, the alongshore distribution of intertidal species (Navarrete et al. 2005; Blanchette 33 et al. 2008; Menge and Menge 2013). Such studies, however, have overwhelmingly been 34 conducted on coasts strongly influenced by upwelling (Menge and Menge 2013), as such shores 35 are normally productive and host important fisheries (FAO 2018). Those systems are mainly 36 located on eastern ocean boundaries in both hemispheres (Kämpf and Chapman 2016). 37 For western ocean boundaries, knowledge on benthic–pelagic coupling is more limited, 38 although it is progressively accumulating. An important question is to what extent alongshore 39 variation in intertidal processes can be inferred from nearshore pelagic variables on western 40 ocean boundary coasts. On the SW Atlantic coast, for example, the recruitment of intertidal 41 filter-feeders (barnacles and mussels) was recently found related to the abundance of 42 phytoplankton (their main food source) and oceanographic features such as wave exposure and 43 seawater temperature (Arribas et al. 2014; Mazzuco et al. 2015). On the NW Atlantic coast, 44 surveys in the Gulf of Maine suggested that intertidal filter-feeder recruitment might be 45 influenced by currents affecting larval supply and that intertidal algal growth might be enhanced 46 by nutrients brought by localized upwelling (Bryson et al. 2014). Larger-scale NW Atlantic 47 surveys including sites on Canadian and American shores have found links between coastal 48 phytoplankton abundance and intertidal barnacle recruitment (Cole et al. 2011) and between 49 thermal stress during low tides and intertidal mussel abundance (Tam and Scrosati 2011). 4 50 The Atlantic Canadian coast in Nova Scotia is well suited to study benthic–pelagic 51 coupling, as it runs for some hundreds of km facing the open ocean. A study done in 2014 52 revealed that the recruitment of intertidal barnacles and mussels in wave-exposed locations was 53 positively related to pelagic food supply and, to a lesser degree, seawater temperature along this 54 coast. Recruitment of these filter-feeders was in turn related to their abundance later in the year 55 and, ultimately, to the abundance of their main predators (dogwhelks), suggesting bottom-up 56 community regulation (Scrosati and Ellrich 2018). While filter-feeder recruitment may predict 57 predator abundance and even facilitation on other organisms (Menge 1976), filter-feeder growth 58 is another important aspect of bottom-up regulation, as larger sizes represent more food for 59 higher trophic levels (Dunkin and Hughes 1984; Carroll and Wethey 1990). Therefore, in this 60 paper, we focus on barnacle growth. Using field data for 5 consecutive years (2014–2018), we 61 test the hypothesis that phytoplankton abundance is positively related to intertidal barnacle 62 growth along this western ocean boundary coast. 63 Materials and methods 64 To test our hypothesis, we collected data from 2014 to 2018 at eight intertidal locations 65 spanning 415 km of the Atlantic coast of Nova Scotia (Fig. 1). For ease of interpretation, these 66 locations are referred to as L1 to L8, from north to south (their names and coordinates are given 67 in Table 1). They all have stable bedrock as substrate and are wave-exposed, as they face the 68 open ocean directly. Daily maximum water velocity (an indication of wave exposure) measured 69 in exposed intertidal habitats along this coast ranges between 6-12 m s-1 (Hunt and Scheibling 70 2001; Scrosati and Heaven 2007; Ellrich and Scrosati 2017). Using wave-exposed intertidal 71 habitats to study benthic–pelagic coupling is particularly fitting because such places face the 72 open ocean, which facilitates the identification of pelagic influences. 5 73 We measured the growth of Semibalanus balanoides, as this is the only intertidal barnacle 74 species on this coast. For each location, we considered the intertidal range to be the vertical 75 distance between chart datum (0 m in elevation, or lowest normal tide in Canada) and the 76 highest elevation where sessile perennial organisms (coincidentally, S. balanoides) occurred on 77 the substrate outside of crevices (Scrosati and Heaven 2007). We then divided the intertidal 78 range by three and measured barnacle growth just above the bottom boundary of the upper third 79 of the intertidal range. As tidal amplitude increases approximately by 33 % from L1 to L8 80 (Tide-Forecast 2018), this method allowed us to measure barnacle growth along the coast at 81 comparable elevations in terms of exposure to aerial conditions during low tides. 82 In Atlantic Canada, Semibalanus balanoides mates in autumn, broods in winter, and 83 releases pelagic larvae in spring (Bousfield 1954; Crisp 1968; Bouchard and Aiken 2012). 84 Larvae settle in intertidal habitats and metamorphose into benthic recruits during May and June, 85 which is thus considered to be the recruitment season (Ellrich et al. 2015). To measure barnacle 86 growth unaffected by other sessile species (Beermann et al. 2013), we made clearings (100-cm2 87 quadrats) of the substrate in late April of each year. Clearings were achieved by removing all 88 pre-existing organisms from the substrate using a chisel and a metallic mesh scourer. We 89 measured the size of the barnacles recruited therein as they looked in summer after growth 90 (Table 2, Fig. 2). We determined barnacle size as the basal shell diameter measured along a 91 straight line passing through the middle of the rostrum and the carina (Chan et al. 2006) using 92 photographs of the quadrats analyzed in a computer. To avoid influences of intraspecific 93 crowding (Bertness 1989) on the size data, we measured barnacles that were not in contact with 94 any neighbouring barnacles. Such organisms were common because, in summer, barnacles 95 constituted almost the only macroscopic species in the quadrats and their density was not 6 96 particularly high (Fig. 2). We measured size for a maximum of 10 random barnacles per 97 quadrat. For data analyses, we first calculated mean barnacle size for each quadrat and, then, 98 averaged the corresponding quadrat means to generate a value of mean barnacle size per 99 location and year. The number of quadrats with barnacle size data for each location and year is 100 given in Table 2. As recruits appear in Nova Scotia during a short period (May–June, although 101 mainly in May; Scrosati and Ellrich 2016), we hereby use the summer size values to compare 102 barnacle growth among locations. 103 To describe phytoplankton abundance, we used satellite data (MODIS-Aqua) on the 104 concentration of chlorophyll-a in seawater (Chl-a, hereafter) for the 4-km-x-4-km cells that 105 include our eight locations (NASA 2018). Satellite Chl-a data are often used in intertidal 106 ecology (Navarrete et al. 2005; Burrows et al. 2010; Arribas et al. 2014; Mazzuco et al. 2015; 107 Lara et al. 2016) and are especially useful when studying hundreds of km of coastline for which 108 in-situ data are lacking (Legaard and Thomas 2006). For this study, satellite data should be 109 appropriate because neighbouring locations are considerably more distant from one another than 110 the data cell size (Fig. 1). For each location and year, we calculated the mean of all Chl-a values 111 available from the beginning of April to the date when we measured barnacle size in summer 112 (Table 2).