ABSTRACT Intensive growth of aquatic macrophytes interferes with water quality and ecosystem dynamics worldwide. Although mechanically removing macrophytes is the most commonly used method for their eradication, it can also cause undesirable disturbances in aquatic reservoir communities. We performed laboratory incubations of phytoplankton sampled before and after macrophytes were mechanically removed from the Piraquara II reservoir, South Brazil. We analyzed changes in growth and composition of the main phytoplankton groups with respect to nutrient shifting. Prior to removing the macrophytes, the phytoplankton community was dominated by low cell abundances of diatoms and flagellates. In contrast, growth rates of cyanobacteria (mainly Cylindrospermopsis raciborskii, Pseudanabaena sp., and Geitlerinema sp.) and of colonial chlorophytes were favored after macrophyte removal, while the abundances of diatoms and flagellates decreased. Our results suggest that removing macrophytes causes dramatic changes in phytoplankton composition and biomass and selects for toxigenic species of cyanobacteria. These changes were probably associated with the disturbance caused by removing the macrophytes, which immediately created new environmental conditions prone to species competition. These findings indicate that the use of mechanical techniques to manage macrophytes should be carefully considered, along with monitoring of harmful species and changes of limnological parameters.

RESUMO A eutrofização de corpos d’água tem causado desiquilíbrios ambientais, alterando dinâmicas populacionais, como o crescimento excessivo de macrófitas aquáticas. Havendo necessidade de controle dessas comunidades, os métodos mecânicos são os mais empregados. O presente trabalho teve o objetivo de avaliar o rendimento operacional conjunto e individual, de um barco removedor e de uma escavadeira hidráulica de braço longo, na retirada de plantas aquáticas flutuantes (e seus conteúdos de Nt e Pt) do Reservatório Piraquara II (Piraquara – PR). Os tempos gastos em 100 ciclos operacionais, no trabalho conjunto dos equipamentos (carregamento, transporte e descarregamento de caminhões), entre outras atividades complementares, foram medidos por cronometragem. A escavadeira hidráulica de braço longo apresentou rendimentos satisfatórios na retirada de macrófitas aquáticas do corpo hídrico, quando comparados com outros trabalhos reportados na literatura. No entanto, o apoio do referido barco aumentou consideravelmente o rendimento (ordem de 115%). O rendimento médio foi de 35,57 ± 0,27 m3/h ou 284,67 m3/dia. Aplicando-se relações de área, massa e volume para o caso avaliado, isto equivale a 3273,7 m2/dia (considerando 11,5 m2/m3) ou a 142,28 t úmidas/dia (considerando a densidade úmida do material = 0,5 ± 0,08; n = 30). Foi constatado que, para situações similares, ambos os equipamentos são recomendados para o controle de infestações de plantas aquáticas em corpos hídricos.

ABSTRACT Eutrophication of water bodies has caused environmental imbalance, changing population dynamics, as the excessive growth of aquatic macrophytes. The mechanical methods are the most used to control these communities. The aim of this study was to evaluate the operational yield of a harvesting boat and a long-arm hydraulic excavator (in group and individually) in the removal of floating aquatic weeds (and their Nt and Pt contents) in Piraquara II Reservoir, in Southern Brazil. The duration of each of the 100 operational cycles (including loading, transport and unloading of trucks), besides other activities were measured through timing. The hydraulic excavator presented right yields in the removal of weeds from the water body as compared to other studies reported in the literature. However, the adding of the harvesting boat led to an increase of productivity (about 115%). The average yield was 35.57 ± 0.27 m3/h or 284.67 m3/day. Applying area, mass and volume relations for the studied site, this is equivalent to 3273.7 m2/day (considering 11.5 m2/m3) or 142.28 humid tons/day (humid density of 0.5 ± 0.08; n = 30). Therefore, for similar situations, both equipment (mainly in a joint operation) are recommended to aquatic weed infestation control.