Coupling between heterotrophic nanoflagellates and bacteria in fresh waters: does latitude make a difference?

SEGOVIA, BIANCA; DOMINGUES, CAROLINA; MEIRA, BIANCA; LANSAC-TOHA; FERMANI, P.; UNREIN, F.; VELHO, L.; SARMENTO, H.

Congreso; Aquatic Sciences Meeting (ASLO). Aquatic Sciences: Global and Regional Perspectives-North Meets South; 2015

Environmental conditions and food web structure of freshwater ecosystems present some consistent differences along latitudinal gradients. The literature proposes that heterotrophic bacteria (HB) abundance and growth efficiency is lower in tropical freshwaters, comparing to temperate systems, indicating that protist grazing pressure on bacteria in the tropics could be stronger. In order to resolve this question, we gathered a large dataset (n=1148, 381 tropical and 767 temperate) of heterotrophic nanoflagellates (HNF) and HB abundance from both latitudes in different types of ecosystems (shallow and deep lakes), and inferred the HNF-HB coupling. Our hypotheses were that in tropical systems i) HB abundance would be lower, ii) HNF abundance would be higher, and iii) HNF-HB coupling would be stronger. Considering the whole dataset, both HB and HNF abundances were lower in tropical systems. Separating the ecosystems by depth, HB was significantly lower in both shallow and deep tropical lakes, while HNF was lower in shallow, but higher in deep tropical lakes. Global regression of HNF vs HB abundance showed a steeper slope for tropical data, suggesting a higher top-down regulation in tropical habitats.