Potential of the bivalve Perna perna as bioindicator of eutrophication in tropical coastal ecosystems: carbon and nitrogen stable isotopes, fatty acids and shell morphometry

Stable isotopes, fatty acids (FAs), and morphometric data are used to describe how eutrophication influences the mussel Perna perna along the coast of Rio de Janeiro (Brazil). Three sites were selected according to their nutrients and chlorophyll a concentration: MAM oligotrophic, ITA mesotrophic to eutrophic and BV, eutrophic. Particulate organic matter (POM) was a mixture of few amounts of cyanobacteria and a predominant source composed in recycled terrigenous OM and bacteria in MAM, of phytoplankton in ITA and BV with a higher contribution of bacterioplankton and a lower food quality in BV. POM was more abundant at ITA and BV but with a lower content of essential poli-unsaturated fatty acids compared to MAM. In the digestive gland, isotopic enrichment factors and FAs profiles reflected intense feeding selectivity. Mussels in the eutrophic sites were able to maintain higher FA concentrations in their muscles and selectively retain essential PUFAs (20:5ω3, 22:6ω3), causing a stronger isotopic fractionation, consistent with long-term integration of assimilated resources and physiological adjustments. In contrast, mussels at the oligotrophic site were apparently starved with a reduced metabolism to maintain essential FAs threshold in their tissues. Shell morphometric traits also differed among sites, with greater morphological heterogeneity observed under eutrophic conditions, particularly in larger individuals (≥7.5 cm). Overall, the combined biochemical and morphometric responses of P. perna reflect trophic differences among sites, suggesting that this species can provide useful information on eutrophication gradients in tropical coastal ecosystems.