Nekton migration and feeding location in a coastal area – A stable isotope approach

Abstract

Stable isotope analysis was used to investigate nekton movements and feeding location in a coastal area adjacent to a major European river, the Tagus, Portugal. Particulate organic matter isotopic signatures presented a gradient from the river towards the sea. Phytoplankton, zooplankton, polychaetes and the crab, Polybius henslowii, provided evidence of the incorporation of terrestrial organic matter into the lower levels of the food web, reflecting local isotopic signatures. Two fish species reflected the coastal isotopic gradient in δ 13C, Diplodus vulgaris and Arnoglossus imperialis and the latter also presented isotopic differerences among the sites for δ 15N. Alloteuthis subulata, Trisopterus luscus and Callionymus lyra were isotopicaly distinct among sites for δ 15N. An increase of δ 15N with length was detected for T. luscus and C. lyra, possibly showing ontogenic trophic level changes. Since A. subulata did not present differences in length and still showed isotopic distinction for δ 15N, among areas, it was concluded that local biogeochemical factors may also have an influence. Diplodus bellottii and Dicologlossa cuneata did not reflect any isotopic signature reflecting their wide migration and feeding across the coastal area. Central isotopic ranges, defined as the site mean values for δ 13C and δ 15N ± 1‰ were determined for each species and site and those deviating from these were considered transient individuals. Central isotopic ranges accounted for 87% of A. imperialis, 80% of A. subulata, 77% of T. luscus, 67% of C. lyra and 50% of D. vulgaris. The number of individuals within each central isotopic range was surprisingly high for an open coastal area and comparable to those of more structured environments.