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). Although barnacle recruitment occurs in May and June, we considered April Chl-a
113 because of its possible effects on larval condition ultimately influencing benthic growth (Barnes
114 1956; Emlet and Sadro 2006). Specifically, the nauplius larvae of S. balanoides feed for 5–6
115 weeks in coastal waters before reaching the settling cyprid stage (Bousfield 1954; Drouin et al.
116 2002), and a recent study in our region concluded that most of the larvae that result in recruits
117 are likely in the water in April (Scrosati and Ellrich 2016). The Chl-a values between May and 7
118 the dates when we measured barnacle size were used to represent pelagic food supply for the
119 growing recruits.
120 For each year, we investigated the relationship between phytoplankton abundance and
121 barnacle growth by evaluating Pearson's correlation between the location means of Chl-a and
122 barnacle size. As our hypothesis was directional (a positive association between both variables),
123 we performed one-tailed tests of significance (Quinn and Keough 2002). We also calculated the
124 coefficient of determination for each year to evaluate the amount of variation in barnacle size
125 that was statistically explained by Chl-a. We did the analyses with R version 3.5.1 (R Core
126 Team 2018).
127 Results
128 The observed relationships between Chl-a and barnacle size were always positive. The
129 correlation was significant for 2014, 2016, and 2018 under a significance level of 0.05 and for
130 2015 under a less conservative significance level of 0.10 (Fig. 3). For 2017, the correlation was
131 non-significant, but still associated to a low P value (P = 0.119), suggesting a weak relationship
132 that was hard to detect. As more data (more locations) for 2017 were unavailable, we excluded
133 the southernmost location (L8) from that year's dataset because L8 then exhibited the lowest
134 mean barnacle size (< 0.3 cm) for the entire dataset used for this study. This modification
135 yielded a significant correlation (Fig. 3), indicating that a positive size–Chl-a relationship also
136 existed for 2017, albeit at a slightly more limited geographic extent. Chl-a explained 49 % of the
137 variation in barnacle size in 2014, 32 % in 2015, 62 % in 2016, 47 % in 2017 (excluding L8),
138 and 47 % in 2018. Each year, barnacle size was highest at the same two neighbouring southern
139 locations (L6 and L7). Both such locations also exhibited a higher average Chl-a than the
140 average for the other six locations each year (Fig. 3). 8
141 Discussion
142 Overall, this 5-year study has revealed that intertidal barnacle growth is positively related to
143 phytoplankton abundance along the Nova Scotia coast. This outcome is remarkable because the
144 correlations were based on data for just eight locations (or seven in 2017), a limited number that,
145 by curbing statistical power, often prevents many studies from detecting patterns in ecology.
146 Surveying more wave-exposed locations was not feasible because of safety concerns (rough
147 terrain or access) and the need to sample all locations within a few days of difference every
148 year. Thus, the observed correlations highlight the relevance of Chl-a to statistically explain the
149 alongshore variation in barnacle size. Notably, in four of the five studied years (2014, 2016,
150 2017 –excluding L8–, and 2018), Chl-a explained nearly half, or more, of such variation.
151 Our results are likely explained by the fact that phytoplankton is the main food source for
152 barnacle nauplius larvae and benthic stages (Anderson 1994; Jarrett 2003; Gyory et al. 2013).
153 This consideration bears special relevance in light of the alongshore variation shown by Chl-a,
154 since, for the 5 years of the study, the mean annual coefficient of variation for Chl-a calculated
155 for our locations using our Chl-a data was 61 %. Given the temporal resolution of the Chl-a
156 data, however, it is not possible to ascertain if the purported role of phytoplankton may have
157 differed depending on the developmental stage of barnacles (from pelagic larvae to the
158 successive benthic stages until size was measured). Thus, this study should be best viewed as
159 baseline evidence revealing benthic–pelagic coupling on the NW Atlantic coast using variables
160 not previously examined together for this system (Bryson et al. 2014; Scrosati and Ellrich 2018).
161 Ultimately, this study reveals that a spatial association between phytoplankton abundance and
162 filter-feeder growth can occur on a western ocean boundary coast, adding to the relationships 9
163 previously reported for eastern ocean boundary coasts (e.g., Oregon and New Zealand; Menge et
164 al. 1997, 2003).
165 Given the observed patterns, an emerging question of interest is what caused the Chl-a
166 variation along the Nova Scotia coast. The intermittent upwelling hypothesis (IUH; Menge and
167 Menge 2013) refers to possible mechanisms. Frequent wind-driven upwelling would limit
168 coastal phytoplankton abundance because upwelled nutrients (necessary for phytoplankton
169 development) would be taken offshore before nearshore blooms can occur. Frequent
170 downwelling would also limit coastal phytoplankton abundance by driving nutrient-poor surface
171 waters to the coast. Intermittent upwelling, however, would allow upwelled nutrients to remain
172 near the coast long enough for phytoplankton to bloom, thus favouring the growth of intertidal
173 filter-feeders (Menge and Menge 2013). Wind-driven upwelling has been reported for the
174 Atlantic coast of Nova Scotia (Petrie et al. 1987; Shan et al. 2016), making the IUH worth
175 testing for this coast. In particular, a study in 1984 reported coastal cooling between June and
176 July near L6 and L7, while seawater temperature increased for the same period of time close to
177 our northern locations (Petrie et al. 1987). The localized upwelling on the Atlantic coast of Nova
178 Scotia is probably intermittent, less frequent and intense than on heavy-upwelling coasts like
179 that of California. Therefore, the fact that mean Chl-a for L6 and L7 was higher than for the
180 other locations every year suggests that the IUH might help to understand alongshore variation
181 in phytoplankton abundance and, ultimately, intertidal barnacle growth.
182 We note, however, that alternative analytic approaches have found no support for the
183 mechanisms underlying the IUH, suggesting that surf zone width and tidally generated internal
184 waves can better explain changes in coastal phytoplankton abundance (Salant and Shanks 2018;
185 Shanks and Morgan 2018a). At present, this topic is undergoing an active debate (Menge and 10
186 Menge 2018; Shanks and Morgan 2018b). Whatever the causes of Chl-a variation along the
187 Nova Scotia coast, it is likely that some combination of oceanographic properties is involved.
188 These properties (upwelling, surf zone width, internal waves, etc.) could change differently
189 every year along the coast, thus generating complex scenarios worth investigating from an
190 oceanographic standpoint.
191 Acknowledgements We thank Carmen Denfeld, Willy Petzold, and Maike Willers for
192 field assistance. This project was funded by grants awarded to Ricardo A. Scrosati by the
193 Natural Sciences and Engineering Research Council of Canada (Discovery Grant #311624), the
194 Canada Foundation for Innovation (Leaders Opportunity Grant #202034), and the Canada
195 Research Chairs program (CRC Grant #210283) and by a postdoctoral fellowship (#91617093)
196 awarded to Julius A. Ellrich by the German Academic Exchange Service (DAAD).
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314 Table 1. Basic information on the eight wave-exposed locations examined for this study.
315
Location code Location name Geographic coordinates
L1 Glasgow Head 45.3203, -60.9592
L2 Deming Island 45.2121, -61.1738
L3 Tor Bay Provincial Park 45.1823, -61.3553
L4 Sober Island 44.8223, -62.4573
L5 Duck Reef 44.4913, -63.5270
L6 Western Head 43.9896, -64.6607
L7 West Point 43.6533, -65.1309
L8 Baccaro Point 43.4496, -65.4697
316
317 17
317 Table 2. Dates on which the quadrats were photographed. The number of available quadrats
318 with barnacle size data is provided in parenthesis.
319
Location 2014 2015 2016 2017 2018
L1 17 August (8) 4 September (17) 22 August (8) 16 August (12) 13 August (8)
L2 9 August (8) 28 August (7) 22 August (8) 25 August (8) 13 August (8)
L3 10 August (4) 28 August (11) 25 August (3) 28 August (8) 13 August (7)
L4 13 August (7) 2 September (16) 27 August (4) 19 August (8) 14 August (8)
L5 12 August (7) 1 September (21) 21 August (7) 22 August (8) 11 August (8)
L6 12 August (8) 31 August (20) 20 August (8) 21 August (8) 10 August (8)
L7 11 August (2) 30 August (14) 19 August (7) 18 August (7) 10 August (7)
L8 11 August (6) 29 August (8) 19 August (3) 18 August (5) 10 August (7)
320
321 18
321
322 Fig. 1. Map indicating the eight wave-exposed locations studied along the Atlantic coast of
323 Nova Scotia, Canada.
324 19
324
325 Fig. 2. Example of barnacle size differences between locations: (A) L1 and (B) L6. The frame
326 bordering each photo belongs to the sampling quadrat. One full side of the quadrat (10 cm) is
327 shown at the top of both pictures. The photos were taken by R. A. Scrosati in August 2018.
328 20
328
329 Fig. 3. Relationships between coastal phytoplankton abundance (chlorophyll-a concentration)
330 and intertidal barnacle size in (A) 2014, (B) 2015, (C) 2016, (D) 2017, and (E) 2018. The
331 correlation and functional relationship shown for 2017 were calculated without including L8
332 (see Results for rationale); the data point for L8 is nonetheless shown for visual reference.