| Summary: | Few studies have been made on planktonic food webs of temperate ecosystems, especially those from the Southern Atlantic Ocean, using molecular biomarkers. The fatty acid compositions of suspended particulate matter (SPM), microplankton and mesozooplankton were studied during summer and winter at a sewage-impacted and a control site in the Bahía Blanca Estuary (Argentina). The aim was to identify trophic relationships on a spatial and seasonal scale and to detect allochthonous inputs to the food web. Fatty acid trends were consistent with the seasonal succession of the plankton community structure supporting our underlying hypothesis that regional seasonality is mostly responsible for changes in fatty acid composition. Sewage had no clear impact on the fatty acids and may not be a significant source of SPM in the estuary. However, at the sewage site the composition of the SPM was more related to terrestrially derived compounds, diatoms and bacteria, and mesozooplankton fatty acids suggested grazing on terrestial components and on diatoms over flagellates. Saltmarshes likely have a crucial role as the main contributors to the organic fraction of SPM followed by plankton. The seasonal fatty acid pattern of the mesozooplankton indicated different feeding strategies suggesting an active feeding mode during summer and a more terrestrially associated diet in winter. The fatty acid trophic marker approach provided relevant information to clarify planktonic trophic interactions and to trace the origin of organic matter in this highly dynamic temperate coastal system. Fil: Dutto, María Sofía. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Bahía Blanca. Instituto Argentino de Oceanografía. Universidad Nacional del Sur. Instituto Argentino de Oceanografía; Argentina Fil: Kopprio, Germán Adolfo. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Bahía Blanca. Instituto Argentino de Oceanografía. Universidad Nacional del Sur. Instituto Argentino de Oceanografía; Argentina Fil: Hoffmeyer, Monica Susana. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Bahía Blanca. Instituto Argentino de Oceanografía. Universidad Nacional del Sur. Instituto Argentino de Oceanografía; Argentina Fil: Alonso, Telma Susana. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Bahía Blanca. Instituto de Investigaciones Bioquímicas de Bahía Blanca. Universidad Nacional del Sur. Instituto de Investigaciones Bioquímicas de Bahía Blanca; Argentina Fil: Graeve, Martín. Alfred Wegener Institute Helmholtz Center for Polar and Marine Research; Alemania Fil: Kattner, Gerhard. Alfred Wegener Institute Helmholtz Center for Polar and Marine Research; Alemania
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