Stable Carbon Isotope Ratios of the Planktonic Food Web in the Northern Gulf of Mexico
Abstract:Analyses of stable carbon isotope ratios were conducted on components of the planktonic food web in the northern Gulf of Mexico to evaluate the importance of terrestrial organic matter as a source of carbon to this food web. These analyses were made on samples collected in areas of high (Southwest Pass, Louisiana) and low (Cape San Bias, Florida) riverine input. Dissolved organic carbon at 7 and 26 km from Southwest Pass and the 0.45–20 μm particulate organic carbon size fraction at 7, 26, and 43 km from the Pass were the only components that displayed isotope ratios approaching terrestrial carbon values, means of –24.0‰ and –24.60/0‰ respectively. Phytoplankton had a mean δ 13C of –22.7‰ for both northern Gulf areas, whereas three copepod genera and total zooplankton had mean values of –20.5‰ and –21.9‰, respectively. Four species of larval fish had similar δ 13C values in both areas (–21.1‰), suggesting their tissue carbon was derived ultimately from phytoplankton. Gut analyses indicated an intermediate zooplankton link for three species. Gulf menhaden appeared to derive their carbon directly from phytoplankton as well as through the phytoplankton-zooplankton pathway. Isotopic fractionation values between trophic levels did not exceed ±1.8‰, a range similar to that reported for other ecological systems.
Document Type: Research Article
Publication date: April 1, 1983
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