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The study of the feeding habits variability (spatial and temporal scales) allows us to evaluate the trophic interactions between species, thus, the short and long-term effects of the removal of different species by the presence of different phenomena. In this study, we carried out stable isotope (δ13C and δ15N) and stomach content analyses to infer the trophic linkages between two snapper species (Lutjanus synagris and Lutjanus griseus) captured in the littoral of Campeche, Mexico. In total, 781 specimens were collected (528 L. synagris and 253 L. griseus) and based on relative importance index [%PSIRI] both snapper species consumed based on the prey-specific index of relative importance (PSIRI), Penaeidae family (PSIRI = 23.41%) was the most important species in the diet of Lutjanus synagris, while Callinectes sapidus (PSIRI = 21.45%) was the primary prey of Lutjanus griseus. The isotopic analyses indicated that both snapper species feed in the coastal-marine (δ15N: 10.6 to 12.1‰ and δ13C: -15.7 to -12.7‰); however, according to PERMANOVA, low diet similarity was found between snapper species (R = 0.07, P < 0.01), also, significant differences was detected in the δ15N and δ13C values between L. synagris, and L. griseus. Therefore, despite both snapper species presents similar trophic positions (L. synagris: 4.2 ± 0.2; L. griseus: 3.9 ± 0.1) and feeding behavior (according to SIBER = opportunistic predators), a low trophic overlap was observed, probably associated with the differential use of habitat in the coastal zone; where L. synagris is ecologically fed in areas of seagrass, while L. griseus is probably intermittent between pasture and mangrove areas, therefore, the role of each species is crucial in the dynamics of coastal-marine ecosystems as predators and potential structuring of the populations of their prey.

The abundance of major components (Fe, Ca, K, and organic carbon) and trace elements was analyzed in surface sediments and core samples from the Colorado River delta (CRD) and the Upper Gulf of California (UGC) using instrumental neutron activation analysis. The spatial distribution patterns of the elements studied are consistent with the model of sedimentary dynamics proposed for this area [Mar. Geol. 158 (1999) 125]: intense tidal resuspension of sediments in the delta with subsequent transport of suspended particulate matter in a southerly direction, followed by sedimentation of fine-grained material in a depocenter near the southwestern margin of the UGC. Concentrations of most of the elements are higher in the surface sediments of this depocenter. The gradual mixing of terrigenous and marine biogenic materials, normally expected for the estuarine sediments, was not detected in the CRD–UGC system because of homogenization of the sediments by tides and wind. Vertical profiles of element contents in samples of the sediment core collected in the depocenter area revealed (i) almost no anthropogenic contamination of the area by environmentally important trace elements such as Cr, Co, Sb, and As; (ii) a twofold decrease of Fe, Sc, Cr, and Co in upper core sediments; and (iii) the enrichment of the sediments at 60–62 cm depth in the core, in calcium carbonate, Ca, Sr, and the Eun/Smn shale-normalized ratio along with a depletion in this layer of Fe, Sc, Cr, Co, light rare-earth elements (REEs), and some other elements of terrigenous origin, presumably caused by the dilution of fluvial terrigenous material by biogenic carbonates, which were probably introduced at this level in the sediments by the action of a strong episodic winter storm, followed by the advective transport of shell fragments from the coastal clam banks or as a result of strong planktonic bloom. D 2002 Elsevier Science B.V. All rights reserved.