Wrack patches and their influence on upper-shore macrofaunal abundance in an Atlantic Canada sandy beach system
Highlights
►We examined wrack features and their influence on supralittoral macrofauna ►Wrack features and associated macrofauna numbers differed among beaches. ►Colonization was higher on rockweed than eelgrass prepared wrack patches. ►These differences were consistent with each plant's nutritional quality. ►They were also consistent with measurements of invertebrate feeding rates.
Introduction
Sandy beaches around the world are characterized by intertidal zones of unconsolidated shifting sands devoid of large primary producers (Ince et al., 2007, Jaramillo et al., 2006). For these habitats, most food availability is allochthonous and limited to the phytoplankton cells transported onshore (McLachlan and Brown, 2006) and the input of nearshore macroalgae and seagrasses (macrophytes; Dugan et al., 2003). The accumulation of patches of stranded macrophytes or “wrack” represents a key food subsidy for resident invertebrate communities, particularly those living at the upper levels of the intertidal (Griffiths et al., 1983, Inglis, 1989, Lastra et al., 2008, McLachlan and Brown, 2006). Wrack is expected to influence these communities, and indeed, several authors have reported increased abundances of macrofauna with wrack cover, volume or standing stock (BehBehani and Croker, 1982, Dugan et al., 2003, Ince et al., 2007, Jaramillo et al., 2006, Rodil et al., 2008, Stenton-Dozey and Griffiths, 1983). Wrack also affects the zonation of macrofauna and some macrofaunal species closely follow the movements of wrack as its position on the beach changes during the semi-lunar cycle (Colombini et al., 2000).
The invertebrates living in the supralittoral zone live buried in the sand beyond the intertidal zone, and therefore are minimally affected by intertidal swash conditions (Jaramillo et al., 2006, Koop and Field, 1980). In temperate regions, the supralittoral fauna in sandy beaches with moderate macrophyte input are often dominated by talitrid amphipods (Colombini et al., 2000, Griffiths and Stenton-Dozey, 1981, Inglis, 1989, Jędrzejczak, 2002). These organisms prefer humid environments and are considered primary colonizers of newly stranded wrack (BehBehani and Croker, 1982, Colombini et al., 2000, Griffiths and Stenton-Dozey, 1981, Inglis, 1989, Marsden, 1991, Stenton-Dozey and Griffiths, 1983), which subsequently attract and sustain secondary (predatory) species (Colombini et al., 2000, Dugan et al., 2003, Griffiths and Stenton-Dozey, 1981, Ince et al., 2007, Jędrzejczak, 2002). As wrack deposits age, they undergo severe dehydration and become covered by windblown sand (Ince et al., 2007, Rodil et al., 2008). Both processes contribute to their decomposition (Ince et al., 2007) and remineralization of seaweeds by bacteria (Kirkman and Kendrick, 1997). Given the importance of wrack as a food source and a refuge from desiccation, it is expected that indirect indicators such as water content and wet mass will show strong relationships with macrofaunal abundances.
Wrack species composition and biomass change spatially and temporally (Dugan et al., 2003, Lastra et al., 2008, Marsden, 1991, Orr et al., 2005, Stenton-Dozey and Griffiths, 1983) in response to local hydrological processes (Ince et al., 2007) or larger-scale processes, such as sea-level change and the dynamics of shoreline erosion (Lastra et al., 2008). Since macrophyte species have different physical and nutritive properties, species of stranded seaweeds are not uniformly used by colonizing invertebrates (Rodil et al., 2008). For example, Lastra et al. (2008) found that amphipods of the genera Megalorchestia and Talitrus rapidly consumed brown algae of the genera Macrocystis and Saccorhiza, respectively, while negligible amounts of the seagrass Phyllospadix were consumed. The causes for these differences are diverse. Seaweeds may vary in physical structure (levels of branching, toughness), nutritional quality and/or quantity, palatability, and decomposition rates while stranded on the beach face (Duarte et al., 2010, Dugan et al., 2003, Rodil et al., 2008, Rossi and Underwood, 2002, Stenton-Dozey and Griffiths, 1983). Despite its importance for upper shore macrofauna, only a few studies so far have attempted to describe the spatial variation of stranded seaweeds, while measuring their individual influence on sandy beach invertebrates.
Several species of seagrasses and rockweeds dominate the wrack in eastern North American shorelines. Despite their abundance, seagrasses are not always the preferred food source due to their structural polysaccharides and low proportion of available nitrogen (Inglis, 1989). Meanwhile, rockweeds are high in nitrogen content and other soluble substances (Inglis, 1989) but contain secondary metabolites that may deter herbivory and decrease herbivore assimilation efficiency and growth (Boettcher and Targett, 1993, Denton and Chapman, 1991, Pennings et al., 2000). Both types of macrophytes are common in the supralittoral wrack on the sandy beaches on the north shore of Prince Edward Island (PEI). Sandy beaches in this region are associated with shorelines backed by sand dunes, till bluffs and sandstone cliffs, among which, sandstone shorelines are known to erode at a much slower rate than dunes and till bluffs (cf. Forbes and Manson, 2002, Hawkins, 2008). These differences suggest that sandy beaches in the area will likely exhibit spatial differences in macrophyte stranding due to those differences in exposure and shoreline sediment mobility. Given that rather little is known about the influence of wrack on the upper shore macrofauna of beaches here and elsewhere, this shoreline system offers a unique opportunity to address this knowledge gap. The goal of this study was to assess the role of macrophyte wrack on the abundance of the supralittoral macrofauna using a combination of exploratory and experimental approaches. Specifically, this study reports (1) a snapshot survey of the standing crop of wrack in seven representative sandy beaches and compares macrofaunal numbers in wrack and bare sediments. (2) a field experiment assessing the rates of colonization on the two most common species of stranded seaweeds, and (3) a comparison of the nutritional quality of those two seaweed species and their corresponding rates of consumption by invertebrates in a laboratory setting.
Section snippets
Study area
Seven sandy beaches located on the north shore of Prince Edward Island were sampled over a period of one week in July 2010 (Fig. 1): Cavendish, Brackley, Ross Lane (all associated to sand dunes), Doyles Cove, Cape Turner (backed by sandstone cliffs), Dalvay west I and II (backed by till bluffs). In addition, a sandy beach at the east end of Dalvay (Dalvay east or #8 in Fig. 1) was used as one of two experimental areas (see Section 2.3 below). The location, mean grain size, slope and
Stranded macrophyte survey
The macrophyte wrack was composed primarily of eelgrass (Z. marina) and a species of rockweed (F. serratus). Other species less frequently collected (< 5%) included Irish moss (Chondrus crispus), sea lettuce (Ulva lactuca) and one or more species of the genus Laminaria. In average, wrack patch density ranged from ~ 8–30 patches per m2 whereas wrack cover varied from ~ 2 to 11% and the water content between ~ 22 and 80% (Fig. 2). In general, average values were higher in Doyles Cove and Cape Turner,
Macrophyte survey and spatial variation
The re-suspension and re-deposition of wrack are typical features of dynamic systems like sandy beaches (Kirkman and Kendrick, 1997, Ochieng and Erftemeijer, 1999). The input of wrack is highly variable and depends on factors such as tides and wave exposure (Ince et al., 2007, Orr et al., 2005), wind (Colombini and Chelazzi, 2003), storms uprooting or breaking algal holdfasts (Griffiths and Stenton-Dozey, 1981, McLachlan and Brown, 2006, Milligan and DeWreede, 2000, Tolley and Christian, 1999),
Acknowledgments
We thank Tim Rawlings (Cape Breton University), Donna Giberson (University of Prince Edward Island), Darren Bardati (Bishop's University), and two anonymous reviewers for their valuable comments on preliminary versions of this manuscript. Our gratitude also goes to Megan Tesch, Veronique Dufour, Lianne Dorion, and Christina Pater, for their assistance in the field, as well as Cristina Duarte (U. Austral de Chile), Bourlaye Fofana, Dave Main, Guru Selvaraj and Kaushik Ghose for their assistance
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