Ceratium furcoides (Dinophyceae) and Microcystis spp. (Cyanophyceae) segregation patterns are ruled by nutrient availability and meteorological shifts in a subtropical region

Ceratium furcoides is a dinoflagellate that exhibits invasive behavior in aquatic ecosystems of South America and its presence alters local phytoplankton communities. It may coexist with Microcystis, together constituting a functional assemblage in temperate regions. However, their interaction in the subtropics is still unclear. In this study, 151 southern Brazilian municipalities were monitored between 2012 and 2022, aiming to understand whether both species co-occur and given their co-occurrence, how it progresses, as well as to investigate which variables drive the observed patterns. The results indicated that both present similar spatial preferences, occurring and blooming in the same hydrographic regions. Distribution models showed that they responded similarly to the same predictors, the most important being temperature (seasonality, annual range, and mean diurnal range), in addition to flow accumulation. Despite this, co-occurrence analysis showed high segregation at all scales, driven by shifts in precipitation and temperature provided by El Niño and La Niña, associated with nutritional increase. In the subtropics, C. furcoides was dominant, in terms of frequency and abundance, with organic matter being key for its development and dominance; in contrast, Microcystis was sensitive to disturbances, with sporadic blooms supported by nitrites and nitrates.