Effects of resources and food web structure on bacterioplankton production in a tropical humic lagoon

The majority of studies of mechanisms regulating bacterioplankton processes havefocused on assessing the isolated effects of nutrients and predation. However, innatural ecosystems, microorganisms may interact strongly with larger organisms inan array of complex, direct and indirect interdependencies. In this work, wereport the results of a field mesocosm experiment in which, over 7 weeks, we eval-uated the individual and interactive effects of resource availability (N and Paddition) and indirect effects originating from the presence of an omnivorous fishspecies (Hyphessobrycon bifasciatus) on bacterioplankton production (BP). Nutrientaddition and fish presence both had individual positive effects on BP, but bottom-up control effects were stronger than indirect top-down control effects. The posi-tive effect of nutrients on BP was mainly direct, through increasing the availabilityof inorganic N and P. There was no significant interaction between fish and nutri-ents. The positive indirect top-down effects of omnivorous fish on BP were prob-ably related to both fish-mediated changes in the zooplankton communitystructure and fish-mediated cross-habitat nutrient regeneration. Our results showthat changes in food web structure, due to the presence or absence of vertebratemacroconsumers, can also affect heterotrophic microbial processes.KEYWORDS: trophic interactions; inorganic nutrients; omnivorous fish;microbial food web; enclosures