Bioindicators or sediment relationships: Evaluating ecological responses from sandy beach nematodes

Sandy beaches from tropical countries are currently facing many challenges due to rapid urban development and its impacts on coastal areas. Despite their aesthetic, recreational and ecological importance, these environments are neglected when it comes to assessment and conservation measures, which can be done in several ways, including biomonitoring. Nematodes are amongst the most suitable organisms to be used as ecological indicators in sandy beaches due to their high abundance on marine sediments and its rapid responses to different sorts of impacts. We analyzed the nematode assemblages and the environmental variables from seven sandy beaches under different types of pressure from Rio de Janeiro state, in Brazil, and applied the biological indexes that use nematodes as bioindicators in order to evaluate differences on the ecological status of these beaches. Sampling took place in the summer of 2015, during the spring low tide. Physicochemical characteristics, microbiology and granulometry were measured alongside human density on each of the beaches and linked to nematode distribution found on the intertidal zone. Botafogo beach, already known for being highly affected by sewage disposal, showed high values for organic matter content and the least diverse assemblage, with high dominance from genus Chromadorita. Despite the higher human density, most urban and touristic beaches (Barra da Tijuca and Copacabana) did not show distinct values for the diversity measures when compared to the most preserved beaches (Restinga da Marambaia and Fora - Urea). While the applied biological indexes did not show clear answers concerning the ecological health of the studied sites; the sediment sorting coefficient was the variable that best explained the nematode distribution and some genera are associated to certain spectrums with this variable. For instance, opportunist genera such as Daptonema and Theristus were found on broader spectrums of sorting coefficient while predators with bigger body diameters showed a more restricted distribution. As the mean grain size is still considered the main variable on nematode distribution for sandy beaches, this study reveals that the sorting coefficient also gives important answers for the nematode distribution.