ABSTRACT
Biomanipulations are almost exclusively performed on temperate North American and European lakes to reduce excessive phytoplankton abundance. They use piscivore stocking and/or zooplanktivore removal to enhance zooplankton grazing on algae. Their success depend on intense trophic cascade interactions along a linear food chain from piscivores to visual zooplanktivores, herbivorous zooplankton and phytoplankton. Piscivore cascading effects down to phytoplankton requires a substantial grazing pressure by large D aphnia, the key invertebrate herbivores, when relaxed from zooplanktivory. In contrast, tropical lakes have a weak, zooplanktonphytoplankton link due to the lack of D aphnia and prevalence of large algae, mostly inedible by zooplankton, particularly cyanobacteria that may form efflorescences. Their more complex pelagic food webs make
the fish-algae relationships less predictable. Dominant planktivores are filter-feeding omnivores that consume large algae in addition to zooplankton and detritus. Most omnivores have benthic habits (e.g., Nile tilapia and common carp), whereas only few are pelagic feeders (e.g., silver carp). Not limited by resources, filter-feeding planktivores often reach high biomasses and enhance internal nutrient cycling, thus promoting algal growth. Because of these peculiarities, biomanipulation in the Tropics should target the control of cyanobacteria by enhancing filter feeding by pelagic planktivores and/ or reducing nutrient loading by benthic omnivores. We present results of laboratory trials (10-50-L aquaria), situ experiments (2.5-6.5-m3 mesocosms, 1,000-m2 littoral lilmnocorrals), an overall acoustic fish stock assessment, and a massive accidental fish kill (300 tons) to demonstrate the applicability of fish manipulation within CAESB Restoration Program of Lago Paranoa, the reservoir of Brasilia. We use them to test a biomanipulation approach based on the reduction of the benthic tilapia overpopulatioon ( Oniloticus plus Tilapia rendalli) and stocking of pelagic silver carp ( molitrix) to reduce phosphorus loading and thus control the occurrence of cyanobacteria blooms (Microcystis aeruginosa, Botryococcus spp.,
Cylindrospermopsisraciborskii). We also explore the benefits of stocking herbivorous grass carp (Ctenopharyngodon idella) to control excessive macrophytes growth, as well as implementing environment-friendly silver carp cage culture and promoting sport fishery of native piscivores (tucunare Cichla ocellaris, dourado maxillosus, surubim, Pseudoplatystoma coruscans) to improve water quality. We discuss the potential and effectiveness of food-web manipulations for the restauration of shallow eutrophic tropical lakes, emphasizing their peculiarities, e.g. the importance of omnivory and internal nutrient cycling favoring nitrogen limitation.
