Microbial Food-Web Drivers in Tropical Reservoirs

Area of Science:

• Aquatic Ecology
• Limnology
• Biogeochemistry

Background:

• Planktonic processes drive element cycling in aquatic systems, with food web structure dictating carbon transfer efficiency.
• Tropical aquatic ecosystems, particularly reservoirs, remain understudied regarding their complete planktonic food web structure and carbon dynamics.

Purpose of the Study:

• To investigate top-down (metazooplankton), bottom-up (resource availability), and environmental controls on microbial food web (MFW) carbon stocks in tropical reservoirs.
• To utilize structural equation models (SEM) to analyze these complex interactions within diverse trophic states.

Main Methods:

• Field study across four Brazilian reservoirs (Balbina, Tucuruí, Três Marias, Funil) representing oligotrophic to eutrophic conditions.
• Application of SEM to assess relationships between environmental variables, metazooplankton structure, and MFW components (phototrophic picoplankton, heterotrophic nanoflagellates, heterotrophic bacteria).

Main Results:

• The MFW contributes substantially (up to 50%) to total planktonic carbon, particularly in less eutrophic reservoirs.
• SEM revealed negative interactions between soluble reactive phosphorus and phototrophic picoplankton (PPP), and dissolved inorganic nitrogen and heterotrophic nanoflagellates (HNF).
• Copepods positively influenced ciliates and cladocerans influenced heterotrophic bacteria (HB) and PPP, suggesting complex trophic linkages and potential strengthening of HB-HNF coupling.

Conclusions:

• Evaluating the entire planktonic size spectrum, including microbial components, is crucial for understanding carbon dynamics in tropical aquatic ecosystems.
• Observed low degree of uncoupling (D) and HNF/HB ratios suggest efficient microbial loop functioning in these tropical systems compared to temperate ones.