Tramas tróficas en sistemas frontales del Mar Argentino: estructura, dinámica y complejidad analizada mediante isótopos estables
Two of the most important frontal systems of the Argentine Sea (SW Atlantic Ocean) were compared to obtain evidences on the regulatory mechanisms of food web length, incorporating both spatial (areas within fronts) and temporal (presence or absence of frontal structure) variability. The stable isotopes of Carbon and Nitrogen were used to evaluate the origin and composition of the organic matter used by primary consumers, the structure of food webs, the trophic levels of several taxa and the benthic-pelagic coupling within each system. In the Río de la Plata estuarine front, different sources of organic matter would support the food webs occurring in the three areas of the system. The particulate organic material (POM) would be the main resource supporting the food web in the mixohaline area, although with different composition between surface and bottom layers. The similarity of the ~d<sup>13</sup>C signature between the fluvial detritus and the surface POM revealed a strong river-estuary coupling. On the contrary, the salt marsh area did not contribute significantly to POM composition. Consumers of the three areas used both in situ generated and alocthonous resources. A clear division between those species that used planktonic (estuarine or river species) and benthic resources (sea-originated) was observed. Invertebrates such as Acartia tonsa and Neomysis americana were the base of the planktonic food web and canalized most part of the energy present in the system (detritus and POM). The salt marsh contribution to the estuarine system was restricted to crabs from the tidal plain. The trophic level of fish community mostly corresponded to secondary consumers. In the tidal front, there were differences in the POM composition between homogeneous and stratified sectors. Notwithstanding that, the trophic structure was similar between sectors, with variations in the composition of primary consumers. The different signatures of lower consumers were integrated by higher order consumers (fishes). Two trophic pathways (herbivorous and microbial) may dominate in different areas and periods. The fish community was characterized by a higher trophic level (mostly third consumers). The productivity, environmental compartmentation and species diversity affected the trophic structure differently in each front. The structure of the estuarine trophic food web was based on a few dominant species in the lower levels that canalized most part of the energy, a high trophic dependence of these species to fish community and, as a corollary, a classic and short trophic food web stable through time. In the tidal front, the trophic food web presented spatio-temporal fluctuations in the POM composition, a strong benthic-pelagic coupling with a great proportion of fish using benthic preys and a larger food web than in the estuary. This study indicates that high resource availability alone is not an indicator of a longer food web.