Influence of net-cage fish farming on zooplankton biomass in the Itá reservoir, SC, Brazil

Christina Wyss Castelo Branco,Bruna Roque Loureiro,Evoy Zaniboni Filho

doi:10.1590/s2179-975x2012005000014

Abstract

OBJECTIVES: This study aimed to verify the influence of net-cage fish farming on zooplankton biomass in the Itá reservoir (Uruguay River, Brazil). METHODS: Samples were collected monthly from October/2009 to May/2010 at the surface and at the bottom in two sampling stations, the net-cage area and in a control area using a Van Dorn bottle and a plankton net (68 µm). RESULTS: The Cladocera and Copepoda biomass was estimated by dry weight using a micro-analytical balance, and the Rotifera biomass by Biovolume. Total zooplankton biomass varied between 6.47 and 131.56 mgDW.m-3 Calanoida copepod presented the highest value of biomass (127.56 mgDW.m-3) and rotifers, despite having an important contribution to total density, showed a maximum biomass of 2.01 mgDW.m-3. Zooplankton biomass at the net-cage area surface was higher when compared with the control area during the months of October to January. However, the zooplankton biomass was similar at the bottom of the two areas throughout the studied period. From February until May, zooplankton biomass decreased in both sampling stations, a fact probably associated with the flushing of the reservoir, followed by an increase in water transparency and a decrease in chlorophyll-a concentration in the following months (February to May). CONCLUSIONS: The influence of fish farming on zooplankton biomass was detected at the surface of the net-cage area only from October to January. From February to May this influence was not found, probably by the influence of the flushing of the reservoir.

Highlights

A better understanding of tropical aquatic ecosystems requires more studies, measuring the actual quantity of matter and energy flowing through the ecosystem
The farming activities in the Itá reservoir showed an influence on the zooplankton community on the net-cage area, that showed higher zooplankton density and biomass levels at the surface in comparison at the bottom and when compared to the control area, from October to February
Differences were not significative between most nutrient concentrations when compared both sampling stations, the values of total phosphorus were higher at the bottom of the net-cage area, showing an increase of this nutrient from fish farming decay

Summary

Introduction

A better understanding of tropical aquatic ecosystems requires more studies, measuring the actual quantity of matter and energy flowing through the ecosystem. According to Bozelli and Huszar (2003), biomass estimates are a priority for brazilian environments since it can lead to advances in real assessments of the role of zooplankton on trophic structure. Such information is essential to comparing environments, especially if the objective of the research is the quantification of trophic interactions. Considering that biomass evaluation is a measure of the contribution of a food-chain link to the energy flow and of the available resources in a specific component of the community, the total zooplankton biomass can be useful for estimating the impact of grazers on phytoplankton and food available for fish. Grazing rates on phytoplankton vary with zooplankton taxonomic composition, size structure and biomass of the zooplankton communities (Cyr and Pace 1992; Matsumura-Tundisi, 1999; Espíndola et al, 2000)

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Conclusion