Effects of a La Niña event on hydrological patterns and copepod community structure in a shallow tropical estuary (Taperaçu, Northern Brazil)

• We examine effects of a La Niña event on the structure of a copepod community.
• Salinity, dissolved nutrients and phytoplankton biomass varied with rainfall levels.
• La Niña influenced the recruitment of nauplii and the density of the dominant species.
• Anomalous rainfall-induced forces resulted in a reduction in the copepod diversity.

The influences of the 2011 La Niña event on the hydrological patterns and copepod community structure were investigated in a shallow tropical estuary, the Taperaçu, in northern Brazil. Specifically, this study aimed to explore the response of the most dominant copepod species and ecological indices (diversity, evenness and richness) to temporal changes in rainfall regime and water parameters in a tropical meso-macro tidal setting. Zooplankton samples were collected from three sampling sites using a conical plankton net (120 μm mesh), with both water and zooplankton samples analyzed by standard methods. In 2011, the physical, chemical and biological parameters of the water were exacerbated by increased rainfall levels resulting from the La Niña event. This resulted in a reduction in the salinity and an increase in dissolved nutrient concentrations and phytoplankton biomass in the study area. These conditions had a direct effect on the monthly dynamics of copepods as a whole, and in particular of five species, O. oswaldocruzi (169,090 ± 254,609 ind. m− 3; p < 0.0001), P. acutus (301,133 ± 518,065 ind. m− 3; p < 0.05), P. marshi (329,391 ± 563,009 ind. m− 3; p > 0.05), O. hebes (40,888 ± 64,893 ind. m− 3; p < 0.05) and A. tonsa (10,680 ± 13,877 ind. m− 3; p > 0.05), all of which were represented by higher densities in February. An extremely high recruitment rate of copepod nauplii was also observed during this month (3,088,309 ± 5,206,645 ind. m− 3; p < 0.05), with this peak in density thus overlapping that of the adult forms. The anomalous period of rainfall was also reflected in reduced species richness and diversity, which not only affected the structure of the copepod community, but may also have provoked shifts in trophic dynamics at higher levels, such as zooplanktivorous fishes.