1. Exotic invasive species modify natural food webs in a way frequently hard to predict. In several aquatic environments in Brazil the introduction of Oreochromis niloticus (tilapia) was followed by changes in water quality. Yet, because of its rapid and easy growth, this fish has been used in many aquaculture programmes around the country. 2. To measure the effects of tilapia on the phytoplankton community and on water conditions of a large tropical reservoir in south-eastern Brazil (Furnas Reservoir), we performed two in situ experiments using three controls (no fish) and three tilapia enclosures (high fish density). Abiotic and biotic parameters were measured at 4 day intervals for 28 days. 3. Fish presence increased nitrogen (N) and phosphorus (P) availability (ammonium 260 and 70% mean increase -first and second experiment; and total phosphorus 540 and 270% mean increase) via excretion. Nutrient recycling by fish can thus be significant in the nutrient dynamics of the reservoir. The higher chlorophyll a concentration in the experimental fish tanks (86 and 34 lg L )1 , first and second experiment, respectively) was the result of a positive bottom-up effect on the phytoplankton community (approximately 2 lg L )1 in the reservoir and control tank). 4. Because tilapia feed selectively on large algae (mainly cyanobacteria and diatoms), several small-sized or mucilaginous colonial chlorophyceans proliferated at the end of the experiments. Thus, the trophic cascade revealed strong influences on algal composition as well as on biomass. 5. Tilapia can contribute to the eutrophication of a waterbody by both top-down and bottom-up forces. In particular, by supplying considerable amount of nutrients it promotes the increase of fast growing algae. Tilapia must be used cautiously in aquaculture to avoid unexpected environmental degradation.

No abstract

a b s t r a c tThis study presents data on seasonal changes of the phytoplanktonic community of a small tropical lake, Lagoa Santa, in the Brazilian cerrado. Temporal variation in the community structure was small and we observed a permanent dominance of the cyanobacteria Cylindrospermopsis raciborskii. This dominance could be associated to the stability of several environmental conditions. Especially water temperature, always within the optimal growth range for C. raciborskii, and the constant mixed water column may have been important factors driving to the long term dominance of this species in the lake. Remarkable seasonal variation in rainfall, as well as the occasional and non-seasonal variation in nutrient concentrations, were important to explain changes in environmental and biological variables, but were not related to the dominance of C. raciborskii. Pearson's correlation and PCA could just in part explain the stable dominance of this cyanobacterium. Meteorological and chemical factors seem to have no clear control on the variability of the phytoplankton dynamics in Lagoa Santa. The weak relationship between C. raciborskii abundance and environmental variables points to additional intrinsic factors associated to this species that may be important in structuring the phytoplankton assemblage.

Cylindrospermopsis raciborskii (Wołosz.) Seenayya et Subba Raju is a planktonic filamentous cyanobacterium whose sudden worldwide proliferation and ability to produce toxins are a reason for concern. In this paper, we suggest that its ecological dominance might be explained by antagonistic interaction with other phytoplankton species due to production of allelopathic metabolites. To test this hypothesis, experiments were run with exudates of natural phytoplankton and C. raciborskii strains isolated from Lagoa Santa, a small natural lake in southeastern Brazil, where this species has become dominant in recent years. The exudates were added to different algal species obtained from the same environment and maintained in culture. After 24 h incubation, PAM fluorometry was used to compare control and treatment photosynthetic responses (relative electron transport rate) to the dissolved extracellular products. Results indicate that most of the target species were sensitive to C. raciborskii exudates, which showed strong inhibitory effects on their photosynthetic activities. These results provide evidence that allelopathy may offer a competitive benefit to C. raciborskii and contribute to its stable dominance in Lagoa Santa. A potential allelopathic advantage could also help to explain the geographic expansion of this species at midlatitudes.

No abstract