Habitat-dependent responses of tropical fish assemblages to environmental variables in a marine-estuarine transitional system

Guanabara Bay is located within one of the most populated areas in Brazil, still harbouring a great aquatic biodiversity and supporting important commercial fisheries despite the increased levels of human disturbances. Here, individual and simultaneous effects of water temperature, pH, and salinity on reef-associated and transient fish assemblages in a transitional marine-estuarine zone of Guanabara Bay were addressed. It was predicted that composition and structure of fish assemblages will respond differently to these environmental variables, depending on the species-specific degree of association with rocky substrates. A canonical correspondence analysis applied on all 83 fish species caught revealed that fish assemblages as a whole were mainly driven by salinity and pH, but with no clear trend of species association or seasonal responses. Canonical analysis of principal coordinates revealed a group composed only by reef-associated species, which was separated from a second group of only transient species, but some species of overlapping distribution were also detected (mixed group). Reef-associated species followed mostly a salinity gradient, whereas transient species responded to salinity and temperature, and those of the mixed group were ruled by salinity and pH. Generalized additive models indicated that, except for transient species, fish richness increased with the effects of estuarine waters from inner bay. Our study is the first to highlight the importance of a transitional zone in coastal bays, which can apparently increase the potential of sharing species among marine and estuarine areas, leading thus to positive effects on local biodiversity. We also stress the role of salinity as key structuring factor of the fish assemblages in tropical coastal bays, and its potential to be used, in conjunction with pH and temperature, as a proxy of alternating influence of estuarine and oceanic waters in these systems.