|Patterns of variations in C and N stable isotope ratios in size-fractionated zooplankton in the Gulf of Lion, NW Mediterranean Sea |
(Article) Publié: Journal Of Plankton Research, vol. 36 p.1204-1215 (2014)
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Stable isotope ratios for six size fractions of zooplankton (80 to >2000 μm) were analyzed in the Gulf of Lion in spring 2010 and winter 2011. Environmental and plankton community variables were also recorded. The originality of this study is the use of a Lagrangian transport modeling system to determine the origin of the water masses and the assessment of the proportion of detritus in the plankton samples. The highest δ15N values were observed in the 1000–2000 μm fraction in January and in the 500–1000 μm fraction in May. The largest size class (>2000 μm), dominated by salps, had lower δ15N values owing to the low isotopic signatures of these organisms. The history of the water masses resulted in two main patterns with different isotopic signatures: the water masses which resided on the shelf and the waters masses carried onto the shelf from off-shelf region by the Northern Current. The δ13C values varied strongly between January and May, mainly owing to changes in hydrographic conditions. The δ15N values, plankton size structure and zooplankton feeding activity varied depending on the season, revealing differences in the seasonal trophic structure of the plankton communities. The trophic structure was characterized in January by a high chlorophyl a (Chl-a) concentration, a population dominated by small organisms and herbivores, and in May by patchy Chl-a distribution, higher particulate organic matter concentration, a population dominated by large size organisms and an increase in the number of omnivores.