Food web structure of sandy beaches: Temporal and spatial variation using stable isotope analysis Leandro Bergamino, Diego Lercari * , Omar Defeo UNDECIMAR, Facultad de Ciencias, Iguá 4225, Montevideo 11400, Uruguay article info Article history: Received 28 April 2010 Accepted 4 December 2010 Available online 15 December 2010 Keywords: food webs benthic invertebrates sandy beaches stable isotopes Uruguay abstract The food web structure of two sandy beach ecosystems with contrasting morphodynamics (dissipative vs. reflective) was examined using stable carbon (d 13 C) and nitrogen (d 15 N) isotope analysis. Organic matter sources (POM: particulate organic matter; SOM: sediment organic matter) and consumers (zooplankton, benthic invertebrates and fishes) were sampled seasonally in both sandy beaches. Food webs significantly differed between beaches: even though both webs were mainly supported by POM, depleted d 13 C and d 15 N values for food sources and consumers were found in the dissipative system (following the reverse pattern in d 13 C values for consumers) for all the four seasons. Primary consumers (zooplankton and benthic invertebrates) use different organic matter sources on each beach and these differences are propagated up in the food web. The higher productivity found in the dissipative beach provided a significant amount of food for primary consumers, notably suspension feeders. Thus, the dissipative beach supported a more complex food web with more trophic links and a higher number of prey and top predators than the reflective beach. Morphodynamic factors could explain the contrasting differences in food web structure. The high degree of retention (nutrients and phytoplankton) recorded for the surf zone of the dissipative beach would result in the renewed accumulation of POM that sustains a more diverse and richer fauna than the reflective beach. Further studies directed to assess connections between the macroscopic food web, the surf-zone microbial loop and the interstitial compartment will provide a deeper understanding on the functioning of sandy beach ecosystems. Ó 2010 Elsevier Ltd. All rights reserved. 1. Introduction Sandy beach ecosystems constitute an important transition between continental and marine zones where sand grain size, beach slope and tidal and swash processes mainly regulate abun- dance and distribution of the resident fauna (Defeo and McLachlan, 2005). Interactions between these environmental variables and processes produce a wide range of beach morphodynamic types ranging from narrow and steep (reflective) to wide and flat (dissi- pative), as sand becomes finer and waves and tides larger (Short, 1999; Finkl, 2004). Reflective beaches are coarse-grained and have narrow surf zones, whereas dissipative beaches have finer sediments and extensive surf zones. Species richness, total abun- dance and biomass of the resident biota increase from reflective to dissipative beaches (McLachlan, 1990; Defeo and McLachlan, 2005; McLachlan and Dorvlo, 2005). Food webs in beach ecosystems are mainly based on marine sources, such as phytoplankton, wrack (stranded algae and sea- grasses) and carrion (McLachlan and Brown, 2006). These systems may support up to three food webs: (1) a discrete food web constituted by interstitial organisms (bacteria, protozoans and meiofauna); (2) a microbial loop in the surf zone (phytoplankton, bacteria and protozoans); and (3) a macroscopic food web struc- tured by macrofauna, zooplankton, fishes and birds (Heymans and McLachlan, 1996; McLachlan and Brown, 2006). Interstitial and macroscopic food webs are unconnected and have no evident trophic links between them, and the relative importance of these food webs differs among beach types (McLachlan and Brown, 2006). Despite the above mentioned concepts, little is known about the role of trophic interactions in the structure and dynamics of macrofauna communities in exposed sandy beaches (McLachlan, 1983; Heymans and McLachlan, 1996). Stable isotope ratios of carbon (d 13 C) and nitrogen (d 15 N) have been widely used for assessing trophic relationships in coastal ecosystems (Michener and Schell, 1994; Kwak and Zedler, 1997). On average, d 13 C values of consumers are expected to increase by w1& compared to those of their food and therefore can provide * Corresponding author. E-mail address: lercari@fcien.edu.uy (D. Lercari). Contents lists available at ScienceDirect Estuarine, Coastal and Shelf Science journal homepage: www.elsevier.com/locate/ecss 0272-7714/$ e see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.ecss.2010.12.007 Estuarine, Coastal and Shelf Science 91 (2011) 536e543