Résumé
The finding of invariant structures in species assemblages is of primary importance for ecology because it would suggest that, despite species turnover and environmental gradients, some self-organizing principles may shape these assemblages. Tropical estuarine and coastal ecosystems are ideal for investigating patterns in trophic structures because they contain many species and are characterized by a high variability for both biotic and abiotic variables. We used the data from a 150 km long transect in the Terminos Lagoon region (Campeche State, Mexico) where 37 stations were sampled monthly during 1 yr for both abiotic parameters and nektonic assemblages. We then quantified 3 complementary components of trophic diversity (trophic richness, trophic evenness and trophic divergence) and then challenged the idea that some stable structures may emerge. We found that abiotic parameters, space and time have weak explanatory power on trophic diversity indices. We also observed a high species turnover both at local and regional scales, but it was unrelated to the small variations of trophic diversity indices. This stability of trophic structure is partly due to the predominance of the trophic class 3.25 to 3.5, which accounted invariably for between 50 and 60% of the total nekton biomass across space and time. These findings suggest that the species turnover observed in our system is not random but, rather, allows maintenance of the same abundance distribution along the trophic axis. The mechanisms underlying these emergent properties of trophic structures deserve to be investigated through the use of trophodynamic models.