Effects of fish farming on plankton structure in a Brazilian tropical reservoir

Abstract

This study investigated the effects of fish farming on phytoplankton and zooplankton structure in a Brazilian tropical reservoir. Samplings were undertaken among 15 net cages and upstream and downstream of the cages over a 120-day period. Soon after the tilapia feeding, we observed the highest density of phytoplankton, with dominance of Cyanobacteria which probably increased due to the increase in predation pressure by cladocerans and copepods on the Cryptophyceae. However, after 30 days from that, a reduction in zooplankton density was registered due to the dominance of Cyanobacteria. The absence of a clear spatial difference in the phytoplankton and zooplankton densities was observed. The effects of the net cages on the nutrients and planktonic communities were small, probably due to the low number of net cages and fish employed, the seasons (autumn/ winter) and the hydrodynamics, as lotic traits, the strong influence of the wind, and the large extension of the mixture zone.

Influence of fish farming in net cages on phytoplankton structure: a case study in a subtropical Brazilian reservoir

“…Borges et al (2010), in experiment with tilapia (Oreochromis niloticus L.) in net cages in an arm of Rosana reservoir, observed an increase in the concentration of total nitrogen and ammonium after the installation of the net cages, and also found strong influence of the reservoir hydrodynamics on the cultivation system. Indeed, the hydrodynamic has been pointed out by several authors (Diaz et al, 2001;Silva et al 2005;Araripe et al, 2006;Alves and Baccarin, 2007;Nogueira et al 2010) as driving the changes to the abiotic variables.…”

“…Chlorophyceae and Cyanobacteria are widely distributed and have usually been associated to greater availability of nutrients and light and high temperature conditions (Huszar et al, 2000;Komárek, 2003;Sant'Anna et al, 2006b;Lachi and Sipaúba-Tavares, 2008). In addition, some studies have observed an increase in phytoplankton density and biomass due to the selective consumption of the zooplankton and consequent dominance of Cyanobacteria (Degans and Meester, 2002;Kozak and Goldin, 2004;Borges et al, 2010). Cyanobacteria and Chlorophyceae were particularly represented by Chroococcales and Chlorococcales, respectively, especially colonial nanoplanktonics, that presented high surface/volume ratio, which increases the absorption of nutrients allowing adaptive advantages (Lopes et al, 2005;Dantas et al, 2008;Brasil and Huszar, 2011).…”

“…It's necessary to investigate the impacts of fish waste, considering the intensive farming with a greater number of net cages, especially in high temperatures, when the fish metabolism is increased (Araripe et al, 2006;Borges et al, 2010). In addition, the adjacent areas from the farming should be monitored as well, to verify the real effects on diversity of aquatic biota (Marengoni, 2006;Borges et al, 2010).…”

“…The eutrofization promotes changes in the trophic web and in the balance of aquatic communities, a decrease in phytoplankton diversity and fosters the heavy growth of some species (Crossetti et al, 2008;Borges et al, 2010). As a result of nutrient load, an increase in phytoplankton proliferation, particularly Cyanobacteria, has been recorded and it represents a growing problem in ecosystems such as reservoirs (Borges et al, 2010).…”

“…As a result of nutrient load, an increase in phytoplankton proliferation, particularly Cyanobacteria, has been recorded and it represents a growing problem in ecosystems such as reservoirs (Borges et al, 2010).…”