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Processing of Allochthonous Macrophyte Subsidies by Sandy Beach Consumers: Estimates of Feeding Rates and Impacts on Food Resources

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Processing of Allochthonous Macrophyte Subsidies by Sandy Beach Consumers: Estimates of Feeding Rates and Impacts on Food Resources Mar Biol DOI 10.1007/s00227-008-0913-3 RESEARCH ARTICLE Processing of allochthonous macrophyte subsidies by sandy beach consumers: estimates of feeding rates and impacts on food resources Mariano Lastra · Henry M. Page · Jenifer E. Dugan · David M. Hubbard · Ivan F. Rodil Received: 29 January 2007 / Accepted: 8 January 2008 © Springer-Verlag 2008 Abstract Allochthonous subsidies of organic material impact on drift macrophyte processing and fate and that the can profoundly inXuence population and community struc- quantity and composition of drift macrophytes could, in ture; however, the role of consumers in the processing of turn, limit populations of beach consumers. these inputs is less understood but may be closely linked to community and ecosystem function. Inputs of drift macro- phytes subsidize sandy beach communities and food webs Introduction in many regions. We estimated feeding rates of dominant sandy beach consumers, the talitrid amphipods (Megalor- Allochthonous inputs of organic matter can strongly inXu- chestia corniculata, in southern California, USA, and Tali- ence population and community structure in many eco- trus saltator, in southern Galicia, Spain), and their impacts systems (e.g., Polis and Hurd 1996; Cross et al. 2006). on drift macrophyte subsidies in Weld and laboratory exper- Such eVects are expected to be greatest where a highly iments. Feeding rate varied with macrophyte type and, for productive system interfaces with and exports materials to T. saltator, air temperature. Size-speciWc feeding rates of a relatively less productive system (Barrett et al. 2005). talitrid amphipods were greatest on brown macroalgae Ecosystems that are subsidized by allochthonous inputs (Macrocystis, Egregia, Saccorhiza and Fucus). Rates for often support a high abundance and diversity of primary large individuals of both species ranged from »40 mg wet and secondary consumers (Polis and Hurd 1996; Busta- wt individual¡112 h¡1 on brown macroalgae to negligible mante et al. 1995; Anderson and Polis 1999; Dugan et al. feeding by M. corniculata on a vascular plant (surfgrass). 2003), however, the role of subsidized consumers in the Amphipod growth rates were also greatest on Macrocystis processing of these inputs is less understood. and lowest on surfgrass, Phyllospadix. For a Californian Sandy beach ecosystems form a dynamic interface beach with substantial inputs of macrophyte wrack (>70 kg between marine and terrestrial environments. These sys- wet wt m¡1 month¡1 in summer), we estimated that the tems are generally characterized by low in situ primary population of talitrid amphipods could process an average productivity (McLachlan and Brown 2006) and beach food of 55% of the palatable Macrocystis input. Our results indi- webs are subsidized by allochthonous inputs from the cate that talitrid amphipod populations can have a signiWcant coastal ocean (GriYths et al. 1983; Dugan et al. 2003; reviewed by Colombini and Chelazzi 2003). Beach con- sumers are supported primarily by these allochthonous Communicated by U. Sommer. inputs, which include macrophyte wrack, consisting of drift macroalgal and vascular plant material, as well as phyto- M. Lastra (&) · I. F. Rodil Y Departamento de Ecología y Biología Animal, plankton, carrion, and other organic material (Gri ths et al. Universidad de Vigo, 36310 Vigo, Spain 1983; McLachlan and Brown 2006). In regions where the e-mail: [email protected] production of reef macroalgae, such as kelps, is high (e.g., Kirkman 1984; Dayton 1985; Hobday 2000), inputs of drift H. M. Page · J. E. Dugan · D. M. Hubbard Marine Science Insitute, University of California, macrophytes to beaches can be extensive. For example, Santa Barbara, CA 93106, USA annual inputs of kelp wrack exceeding 470 and up to 123 Mar Biol 2,100 kg wet weight m¡1 of shoreline were estimated for rate of macrophyte wrack by talitrid amphipods, including sandy beaches of southern California and the west coast of macrophyte species, amphipod size and abundance, and air South Africa, respectively (Hayes 1974; Stenton-Dozey temperature, and to explore the eVect of macrophyte spe- and GriYths 1983). The species composition and biomass cies on the growth performance of these amphipods. Spe- of macrophyte inputs to beaches vary both spatially and ciWcally, we (1) estimated the size-speciWc feeding rates of temporally (e.g., Stenton-Dozey and GriYths 1983; Mars- talitrid amphipods on common drift macrophytes in the den 1991; Dugan et al. 2003; Orr et al. 2005). Following Weld and laboratory in two widely separated geographic deposition on the beach as wrack, drift macrophytes are regions, (2) explored amphipod food preferences in a Weld subjected to a variety of processes including consumption experiment, (3) evaluated the eVect of macrophyte species by beach herbivores, in situ microbial degradation, desicca- on amphipod growth, and (4) estimated the impact of tali- tion, and export by tides and currents (GriYths et al. 1983; trid amphipods on drift macrophyte processing and fate. GriYths and Stenton-Dozey 1981; Inglis 1989; Jddrzejczak 2002; Orr et al. 2005). Biological processing of macrophyte inputs through con- Materials and methods sumption by herbivores is a potentially important pathway by which this imported detrital material is cycled through Study sites the beach ecosystem (Hayes 1974; GriYths et al. 1983; Chown 1996). Talitrid amphipods are often the most abun- Field sampling and experiments were carried out on sandy dant herbivores on exposed sandy beaches, comprising up beaches in southern California, USA, and the southeast to 90% of the abundance (Stenton-Dozey and GriYths 1983) coast of Galicia, Spain (Fig. 1). Talitrid amphipods are and achieving high densities (>10,000 individuals m¡1) important primary consumers of drift macrophyte wrack in on some beaches (e.g., Jaramillo and McLachlan 1993; both regions. Isla Vista beach, located along protected outer Dugan et al. 2003; Lastra et al. 2006). These abundant coast in Santa Barbara County, southern California herbivores are supported by macrophyte wrack and are (34°24Ј33ЉN, 119°52Ј9ЉW) is a narrow bluV-backed, reXec- important trophic intermediates between macrophytes and tive to low intermediate state beach (Dean’s Parameter, higher level insect and avian consumers (GriYths et al. 1983; = 0.7–3.8, Short and Wright 1983) »3 km in length that Dugan et al. 2003). experiences mixed semi-diurnal tides. The intertidal width Although feeding by herbivores may remove appreciable of the beach (bluV base to low swash level) ranges from amounts of recently deposited macrophytes from a beach, »30 to 70 m. Isla Vista beach receives inputs of the kelps, few data are available that permit general estimates of the Macrocystis pyrifera (giant kelp, hereafter Macrocystis) importance of this pathway to the fate of macrophyte wrack and Egregia menziesii (feather boa kelp, hereafter Egregia) and reported Wgures vary widely. For example, estimated and the vascular plant, Phyllospadix torreyi (surfgrass, consumption of brown macroalgae inputs by beach herbi- hereafter Phyllospadix), as well as a variety of other brown, vores has been reported as negligible (<1%, Inglis 1989) or very low (4–9%, Hayes 1974; Koop and Lucas 1983) to substantial (>70%, GriYths et al. 1983). The rates of consumption of drift macrophytes by diVerent herbivore species may depend on the macrophyte species and associated physicochemical and morphological characteris- tics (WakeWeld and Murray 1998; Van Alstyne et al. 2001; Pennings et al. 2000). Thus, biomass, composition and palat- ability of macrophytes arriving on the beach may play an important role in determining the fate of these inputs (Van Alstyne et al. 1999; 2001; Orr et al. 2005). Likewise, the Isla Vista abundance, composition and structure of the consumer com- 43° 00’ X 34°30’ Beach Santa munity inhabiting a beach could in uence the processing and Barbara availability of macrophyte wrack. In turn, spatial and tempo- Vigo ral variability in wrack composition and biomass may aVect 33°45’ 42° 00’ composition, abundance and demographic rates of amphi- O Muiño pods and other consumers within the beach community 120° 00’ 118° 30’ Beach (Stenton-Dozey and GriYths 1983; Dugan et al. 2003). 9° 00’ The overall goal of this study was to investigate factors Fig. 1 Location of the study beaches: Isla Vista beach in southern that potentially inXuence the processing and consumption California, USA and O Muiño beach in southwest Galicia, Spain 123 Mar Biol red and green algal species from rocky reefs and kelp for- species were calculated by averaging data from the four ests located just oVshore. This beach supports an abundant, transects and expressed in terms of grams wet weight of species rich (39 species) community of intertidal macro- each species per meter of beach shoreline. invertebrates (Hubbard and Dugan 2003). O Muiño beach, located in southern Galicia (northwest Input of macrophyte wrack coast of Spain) (41°52Ј21ЉN, 8°52Ј9ЉW) is a protected, dune-backed reXective beach ( =0.2) »700 m in length. To estimate the potential food resources available and The intertidal width of the beach from the edge of the dunes allow calculations of macrophyte processing rates by tali- to the low swash is »100 m. O Muiño beach receives trid amphipod populations, we measured the input of drift inputs of the brown macroalgae, Saccorhiza polyschides macrophytes for 30 days on Isla Vista Beach in July– (furbelow, hereafter Saccorhiza) and Fucus spiralis (spiral August 2002. This period coincides with a time of year that wrack, hereafter Fucus) and the green alga, Ulva lactuca lacks storm activity and has consistently small waves and (sea lettuce, hereafter Ulva), as well as several species of minimal beach erosion. Four 25-m wide plots were selected red algae from nearby rocky intertidal and subtidal habitats. from a 300 m segment of beach using a random number Large beds of macroalgae comparable to those oVshore of table. These plots were initially cleared of surface and bur- Isla Vista beach above were absent. The number of macro- ied wrack by hand on 9 July.
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