Ecological interactions between Nile tilapia (Oreochromis niloticus, L.) and the phytoplanktonic community of the Furnas Reservoir (Brazil)

Abstract

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.

Tilapia rendalli increases phytoplankton biomass of a shallow tropical lake

“…Some studies actually showed a decrease in total phytoplankton biomass (Zhang et al, 2006;Menezes et al, 2010), whereas others report enhanced biomass (Figueredo and Giani, 2005; this study) or no influence (Attayde et al, 2007;Okun et al, 2008;Rondel et al, 2008). Unlike T. rendalli, the exotic Oreochromis niloticus, the Nile tilapia, is a planktivore or herbivore/detritivore in African water bodies (Zengeya et al, 2011).…”

“…However, that was an exceptional situation as both species are omnivores and not planktivores in most water bodies. Furthermore, the predominance of omnivorous fish in tropical and subtropical lakes (Lazzaro, 1997;Jeppesen et al, 2007;González-Bergonzoni et al, 2012) might lead to the direct exploitation of phytoplankton by fish (Starling, 1993a, b;Arcifa 2012), and develops blooms in eutrophic systems with high pH. It has also physiological adaptations to grow in low light intensities (Shafik et al, 1997), showing high absorption capacity of phosphate and ammonium (Padisák, 1997).…”

“…Treatment A (with fish), treatment B (no fish). Giani, 2005;Rondel et al, 2008;Menezes et al, 2010;Salazar-Torres, 2012). A question remains, therefore, on the applicability of biomanipulation techniques developed in higher latitudes to (sub)tropical water bodies.…”

“…Under experimental conditions, it was observed that nutrient cycling (phosphorus) by planktivorous (Vanni and Layne, 1997;Attayde and Hansson, 1999) and omnivorous fish (Starling et al, 2002) could affect phytoplankton community structure. However, the relative importance of direct and indirect mechanisms for phytoplankton dynamics remains unclear (Attayde and Hansson, 2001a, b;Figueredo and Giani, 2005;Attayde et al, 2010).…”

“…Several studies have also investigated the applicability of the top-down control on tropical and subtropical freshwater ecosystems (Arcifa et al, 1986;Northcote et al, 1990;Starling and Rocha, 1990;Starling, 1993b;Figueredo and Giani, 2005;Attayde et al, 2007;Rondel et al, 2008;Menezes et al, 2010), and a recent review compares the differences between (sub)tropical and temperate systems regarding biomanipulation techniques .…”