Trophic niche shifts driven by phytoplankton in sandy beach ecosystems Leandro Bergamino a, * , Ana Martínez b , Eunah Han c , Diego Lercari d, a , Omar Defeo d, a a Centro Universitario Regional Este (CURE), Universidad de la República, Rutas 9 y 15 s/n, Rocha, Uruguay b Unidad de Gesti on Pesquera Atl antica, Direcci on Nacional de Recursos Acu aticos (DINARA), Av del Puerto s/n La Paloma, Rocha, Uruguay c School of Environmental Science & Engineering, Gwangju Institute of Science and Technology, Gwangju, 500-712, Republic of Korea d UNDECIMAR, Facultad de Ciencias, Igu a 4225, Montevideo, 11400, Uruguay article info Article history: Received 16 January 2016 Received in revised form 4 May 2016 Accepted 26 June 2016 Available online 27 June 2016 Keywords: Stable isotopes Food webs Diatoms Asterionellopsis guyunusae sandy beaches Uruguay abstract Stable isotopes (d 13 C and d 15 N) together with chlorophyll a and densities of surf diatoms were used to analyze changes in trophic niches of species in two sandy beaches of Uruguay with contrasting mor- phodynamics (i.e. dissipative vs. reflective). Consumers and food sources were collected over four sea- sons, including sediment organic matter (SOM), suspended particulate organic matter (POM) and the surf zone diatom Asterionellopsis guyunusae. Circular statistics and a Bayesian isotope mixing model were used to quantify food web differences between beaches. Consumers changed their trophic niche between beaches in the same direction of the food web space towards higher reliance on surf diatoms in the dissipative beach. Mixing models indicated that A. guyunusae was the primary nutrition source for suspension feeders in the dissipative beach, explaining their change in dietary niche compared to the reflective beach where the proportional contribution of surf diatoms was low. The high C/N ratios in A. guyunusae indicated its high nutritional value and N content, and may help to explain the high assimilation by suspension feeders at the dissipative beach. Furthermore, density of A. guyunusae was higher in the dissipative than in the reflective beach, and cell density was positively correlated with chlorophyll a only in the dissipative beach. Therefore, surf diatoms are important drivers in the dynamics of sandy beach food webs, determining the trophic niche space and productivity. Our study provides valuable insights on shifting foraging behavior by beach fauna in response to changes in resource availability. © 2016 Elsevier Ltd. All rights reserved. 1. Introduction Exposed sandy beaches constitute physically stressful environ- ments with strong spatial and temporal dynamics (McLachlan and Brown, 2006). Interactions between sand, waves, and tides produce a wide range of beach morphodynamic types ranging from narrow and steep, with coarse sediments and no surf zone (reflective) to wide and flat, with fine sediments and a large surf zone (dissipa- tive) (Short, 1999; Finkl, 2004). Abiotic factors, including grain size, beach slope and swash processes, are usually considered the main drivers regulating the abundance and distribution of the resident fauna, whereas biological factors appear to have less influence (Defeo and McLachlan, 2005; McLachlan and Brown, 2006). However, understanding the role that biotic interactions can play in structuring sandy beach ecosystems is increasingly important, elemental and challenging (Defeo et al., 1997; Dugan et al., 2004; Ortega Cisneros et al., 2011; Rodil et al., 2012). In this context, the examination of the variability in space and time of food web structure and trophic interactions can improve our understanding of functional response to environmental variation and disturbance (Tewfik et al., 2016). Sandy beach food webs are fuelled by marine derived organic matter, including surf zone phytoplankton, carrion, seagrasses and macroalgae (wrack), supporting a diverse community (McLachlan and Brown, 2006; Schlacher et al., 2008). However, food web structure markedly differs between beaches with contrasting morphodynamics (Bergamino et al., 2011). Particularly, reflective beaches are mainly subsidized, processing organic material inputs from the sea and land (McLachlan and Brown, 2006). By contrast, high diatom accumulations in the surf zone is a characteristic * Corresponding author. E-mail address: lbergamino@gmail.com (L. Bergamino). Contents lists available at ScienceDirect Estuarine, Coastal and Shelf Science journal homepage: www.elsevier.com/locate/ecss http://dx.doi.org/10.1016/j.ecss.2016.06.023 0272-7714/© 2016 Elsevier Ltd. All rights reserved. Estuarine, Coastal and Shelf Science 180 (2016) 33e40