Are the Seasonal Dynamics of Zooplankton Different Between Contrasting Climatic Zones in Brazil? A Comparison Between Tropical and Subtropical Rivers

ABSTRACT The unique environmental characteristics of the tropics and subtropics, including precipitation patterns, photoperiods and temperatures, can influence riverine zooplankton communities in contrasting ways. Therefore, we aimed to investigate the seasonal dynamics of zooplankton in tropical and subtropical rivers and compare the structure of zooplankton in response to distinct climatic conditions. To this end, we collected zooplankton samples from rivers located in two different climatic zones in Brazil (tropical and subtropical) during winter, spring, summer and autumn. Our results showed that testate amoebae and rotifers play a crucial role in zooplankton dynamics in both climatic zones, emerging as the predominant groups regarding density, species richness and diversity. In tropical rivers, seasonal variations in zooplankton are strongly influenced by temperature and flooding events, which affect total suspended solids, turbidity and pH. In subtropical rivers, zooplankton seasonal dynamics are closely linked to fluctuations in temperature and dissolved oxygen. We identified 142 zooplankton taxa, with the tropical zone comprising 125 taxa and the subtropical zone comprising 97 taxa. The results also revealed that tropical rivers had greater species richness and diversity than subtropical rivers. This disparity was primarily attributed to the higher temperatures, greater nutrient concentrations and lower thermal amplitudes in tropical waters compared to those in subtropical waters. Thus, it is evident that different climatic conditions have varying influences on riverine zooplankton dynamics, highlighting the need to consider regional characteristics in the management and conservation of aquatic ecosystems.