In the present study, we report the concentrations of 21 trace elements (V, Cr, Mn, Co, Cu, Zn, As, Se, Rb, Sr, Mo, Ag, Cd, Sn, Sb, Cs, Ba, Hg, Tl, Pb, and Bi), as well as the results of the analysis of stable carbon and nitrogen isotopes, of the various biota that make up the food web in the main stream of the Mekong Delta near Can Tho, South Vietnam. A significant trophic level-dependent increase was found in concentrations of Se, Rb, and Hg with increasing delta(15)N, indicating that an overall biomagnification of these elements occurred. However, the increase of Hg concentration per trophic level was lower than in previous studies. In contrast, the concentration of Mn showed an opposite trend in the food web of the Mekong Delta. In addition to these overall trends, the present study revealed that the biomagnification profiles of trace metals differ between crustaceans and fishes; concentrations of Mn, Cu, Zn, As, Sr, Mo, Ag, Cd, Sb, Cs, Ba, Tl, and Pb were significantly higher in crustaceans, whereas fishes showed higher concentrations of Cr, Rb, and Hg (trophic level determined by delta(15)N). The differences in the biomagnification profile between the major taxa might be attributed to differences in metal accumulation and in detoxification abilities such as possessing a metal-binding protein, e.g., metallothionein (MT).

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Eastern boundary current (EBC) upwelling zones are among the most productive of marine ecosystems globally and have been generalized in terms of their food web structure. Little empirically based evidence exists to suggest that there is any one form of trophic control of EBC systems and, because of logistical constraints, knowledge of food web structure is limited in these large marine ecosystems. To determine principal trophic links within the pelagic food web, we combined stable isotope and diet analysis of 21 dominant species of nekton found within the northern California Current, a highly productive EBC upwelling ecosystem. Samples of nekton and zooplankton were collected in June and August 2000 and 2002 from the shelf-slope waters of northern California to Washington. Results showed most nekton consumed mixed diets of zooplankton and larval-juvenile or adult fishes, indicating the importance of omnivory in this system. Euphausiids were overwhelmingly the most prevalent zooplankton taxa in the diets of nekton across multiple trophic levels, accounting for > 50% of the index of relative importance in 10 of 21 nekton species analyzed. Nitrogen stable isotopes (δ 15 N) reflected trophic patterns observed in the diets, with most nekton species (~62%) falling between trophic levels (TL) of full zooplanktivores (TL = 3) and full piscivores (TL = 4). The prevalence of omnivory observed here is probably due to high primary production that supports an abundance of large zooplankton prey, such as euphausiids, which may be a more available prey resource for a broad range of predators. A modification of this form of bottom-up control, termed bottom-up omnivory, is presented.

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In this study, two sediment cores (*70 cm) were collected from separate mangrove forests straddling the Ba Lat Estuary, Red River of northern Vietnam, to examine the origins of sedimentary organic carbon (SOC) and reconstruct the paleoenvironment. In addition, mangrove leaves and particulate organic matter were collected and measured for d 13 C to trace the origins of SOC. The cores were analyzed by high-resolution sections for d 13 C, TOC, C/N ratios, sediment grain size, water content, and porosity, with values of d 13 C, TOC, and C/N ratios ranging from -28.19 to -22.5%, 2.14-30.94 mg/g, and 10.29-18.32, respectively. The d 13 C and TOC relationship indicated that there were some small residual effects of diagenetic processes on TOC and d 13 C values in mangrove sediments. However, the shifts of d 13 C and C/N ratios from the bottom to the surface sediment of the cores explained the change in organic matter sources, with values of C/N [ 12 and d 13 C \ -25%, and C/N \ 12 and d 13 C [ -25% indicated terrestrial (e.g., mangrove litter) and marine phytoplankton sources, respectively. The covarying d 13 C, C/N ratios, and sediment grain sizes during the past 100 years in sediment cores showed that the paleoenvironment may be reconstructed into three environments (subtidal, tidal flat, and intertidal mangrove). General trends in d 13 C and C/N followed a gradual increase in the C/N ratio and a concomitant decrease in d 13 C from the subtidal, through to tidal flat, and to the intertidal mangrove. d 13 C and C/N ratios are therefore effective in measuring the continuum of environmental change in mangrove ecosystem.