Resumo

Nitrogen has a great ecological relevance in aquatic systems. Laguna de Rocha is a shallow and highly productive coastal lagoon on the Uruguayan Atlantic South-eastern coast. Heterotrophic bacterial blooms were related to a peak of ammonium (NH₄⁺) concentration but a few days later, blooms disappeared suggesting that predation or viral lysis are involved in the resilience of the system. The aim of this work was to study how trophic cascade (top-down control) affects bacterial ammonium uptake and community composition in Laguna de Rocha. The experimental setup consisted in corers filled with water from two different areas: South (brackish) and North (freshwater). Water pre-filtered by a 35-µm-mesh was the without-mesozooplankton treatment (NZOO) and water without filtering was the whole community treatment (WC). Treatments were spiked with ¹⁵N-NH₄⁺ and incubated during 4 hours. Samples were taken to analyze bacterial community composition (BCC) by CARD-FISH and T-RFLP and NH4+ uptake rates (ρ). In WC, ρ were calculated from 35 µm (ZOO-mesozooplankton), 1.2 µm (PHYTO-phytoplankton) and 0.5 µm (BACT-bacterioplankton) fractions, while in NZOO treatments only from PHYTO and BACT fractions. Isotopic enrichment in ZOO fraction from both zones reflected that predation occurred during the short incubation time. Comparison of ρ from BACT between both treatments showed no significant differences, suggesting that mesozooplankton did not affect the bacterial NH4+ uptake in the studied zones. However, PHYTO fraction from the freshwater zone exhibited lower ρ in the NZOO treatment, indicating that in this zone of the lagoon trophic cascade might affect this community. BCC analyzed by TRFLP showed a shift induced by mesozooplankton presence in the freshwater WC incubations, suggesting that the microbial loop and its connection to higher trophic levels would be more relevant in the North of Laguna de Rocha. Overall, the results from this work propose that top-down control on the microbial ammonium uptake in Laguna de Rocha would depend on the physicochemical environment and hydrology.