C-13 enrichment and oceanic food web structure in the northwestern Gulf of Mexico.
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The use of 8C measurements to indicate trophic levels of offshore animals was tested by analyzing four components of food webs in the northwestern Gulf of Mexico. A progression of increasing 13C contents (less negative 8C values) occurred from particulate organic Carbon (x=-21.7%) to whole benthic crustaceans (x = -16.9%) so that the degree of C enrichment functioned as a crude indicator of trophic level. Six transects made off Texas and Louisiana from near shore to approximately 160 m during 1979 and 1980 showed that considerable seasonal and spatial variation occurred in this average pattern of 13C enrichment, and that this variation was not linked to inputs of terrestrial carbon. Benthic crustaceans collected in the same trawl had very similar 8C values (within 1.3%), regardless of species. Shipboard experiments showed that assimilation could account for the observed 0.9 %C enrichment in these animals vs. their diets. Previous findings of a .35% increase in C of offshore plankton per C increase in temperature (Fontugne and DuPlessy 1981) were not confirmed in this study. While considerable isotopic variation occurs offshore and complicates simple assessment of trophic level from C enrichment data, reduced isotopic variation appears useful for identifying consumers at higher trophic levels.