Mercury, selenium, and stable carbon and nitrogen isotopes in the striped marlin Kajikia audax and blue marlin Makaira nigricans food web from the Gulf of California.

Mercury (Hg) concentrations based on dietary sources have shown to predict differences in fish; however, they are usually applied at an individual scale and are rarely directed at a known trophic transfer. We combined gut content analysis and stable isotope analysis (δ15N and δ13C) to provide a quantitative estimate of Hg and selenium (Se) biomagnification in the striped marlin (Kajikia audax) and blue marlin (Makaira nigricans) food web from the southwestern Gulf of California. Hg and Se concentrations (mean ± SD; µg g-1, dw) were different among K. audax (Hg = 3.6 ± 2.1, Se = 5.5 ± 5.4) and M. nigricans (Hg = 19.0 ± 29.6, Se = 8.8 ± 10.5). Such variations of element concentrations could be linked to predation with different Hg and Se contents. Diet data presented as prey weight (%W) indicated a higher proportion of large prey fish for the blue marlin than the striped marlin. δ15N and δ13C indicated pelagic food sources with epipelagic preferences for the blue marlin and mesopelagic for the striped marlin. The relationship between Hg concentrations and δ15N was positive along the food web of both marlin species, indicating biomagnification of Hg. However, Se biomagnification was not clearly evidenced, and Se:Hg ratios decreased with δ15N, attributed to increasing Hg concentrations with increased trophic level.

Mercury and selenium in the filter-feeding whale shark (Rhincodon typus) from two areas of the Gulf of California, Mexico.

Mercury and selenium were measured for first time in the endangered species whale shark (Rhyncodon typus) from two areas of the Gulf of California, Bahía Los Angeles (BLA) and Bahía La Paz (LAP) using dermal biopsies of seventy specimens. Additionally, nineteen zooplankton samples from LAP were analysed. Concentrations (ng/g, wet weight (ww)) in biopsies of BLA ranged from 1 to 40 for Hg and 100 to 680 for Se; while in LAP varied from 1 to 9 for Hg and 11 to 850 for Se. A positive correlation was found for Hg in BLA males biopsies with length. Hg and Se concentrations in the zooplankton from LAP were 1.6 ±â€¯1.8 and 770 ±â€¯930 ng/g, respectively. Hg biomagnification factor ranged from 0.8 to 5.3 in sharks. A molar excess of Se over Hg was found in the biopsies and the zooplankton.

The influence of anthropogenic organic matter and nutrient inputs on the food web structure in a coastal lagoon receiving agriculture and shrimp farming effluents.

In this study, we elucidated the impacts of allochthonous organic matter (OM) and nutrients (N and P) inputs coming from agriculture and shrimp aquaculture activities on food web structure in a subtropical coastal lagoon located on the central-east of the Gulf of California. This coastal lagoon is highly influenced by OM and nutrients inputs by a large agriculture district and aquaculture development center in Mexico. We also selected a second coastal lagoon, without direct OM and nutrients inputs, considered as a 'pristine' ecosystem. We evaluated the quantity of OM, N and P in both ecosystems and reconstructed the food webs using isotopic tools of C and N from the base to top. We collected and analyzed autochthonous and allochthonous OM, and organisms including primary producers, and primary to tertiary consumers. Overall, specimens of the same species and/or functional groups showed higher δ15N values in the ecosystem receiving agriculture and shrimp aquaculture effluents than the pristine. Food webs were composed of four and five trophic levels, where fish and birds occupied the top-predator levels. Seasonal increases in OM and N and P quantities in lagoons affected by anthropogenic activities produced high δ15N values in primary producers and consumers.