Effects of environmental variables on mesozooplankton dynamics in an Amazonian estuary.

In the present study, it was analyzed the structure of mesozooplankton community, and their spatio-temporal variations along the Caeté River estuary (northeast of Pará, Brazil). Samples were collected during spring tides conducted in nychthemeral cycles, in three fixed sectors located along the studied estuary (inner-S1, middle-S2 and outer-S3), using conical plankton nets of 200 μm mesh size. Samplings were performed in September 2014 (dry season) and March 2015 (rainy season), totaling 54 samples. A total of 25 mesozooplankton species were recorded, being the copepods the most diversified group (21 species), mostly dominated by estuarine and coastal species. The most abundant mesozooplankton species presented significantly higher densities (p <0.0001) between the middle (Pseudodiaptomus richardi , 583.8±956.5 ind m⁻³, Paracalanus quasimodo, 108.9±181.4 ind m⁻³ and Pseudodiaptomus acutus, 23.4±66.6 ind m⁻³) and outer (Acartia tonsa, 476.9±1,298.8 ind m⁻³ and Acartia lilljeborgii (407.9±1,038.1 ind m⁻³ and Parasagitta tenuis , 59.9±117.1 ind m⁻³) sectors. The non-metric multidimensional ordination analysis highlighted that the average density of the main species varied mainly at a spatial scale, with density increasing toward the mouth of the estuary (except for P. richardi). Spearman's correlation (rs) demonstrated that the salinity, turbidity, dissolved nutrients and chlorophyll a concentrations were the prevailing environmental variables associated with significant spatial changes of mesozooplankton distribution in this area. Overall, the results showed that spatial variation in the water physicochemical characteristics of the Caeté Estuary have significant effects on the structure and succession of the mesozooplankton, which may potentially affect the functioning and biodiversity of this coastal ecosystem. [ABSTRACT FROM AUTHOR]