Abstract Studies considering the functional traits of organisms, populations, and communities functional indices increase the understanding of many factors on ecosystem functioning. Here, we analyze the predation effect (by fish) on zooplankton functional diversity and the effects of biomass and density of periphytic algae on zooplankton feeding type trait and body size. We expect that intense predation by fish on zooplankton leads to higher values of zooplankton functional diversity and that food resource will be positively related to the abundance of zooplankton trait and body size. For that, microcosms were established (T1- fish-absence, and T2- fish-presence, both with periphytic algae as food). We observed that fish presence decreased zooplankton functional diversity through modifications in the availability of nutrients and algae, through the middle-out effect. We also observed that body size had a negative relationship with the food resource, reaffirming that high food availability in subtropical lakes is linked to small-bodied zooplankton. The raptorial copepods covariate positively with the periphytic algae, which was an alternative food resource and, in this case, the main form of carbon input into the system. In this study, omnivorous fish reduced zooplankton functional traits, which can alter the energy stock and energy flow in aquatic ecosystems.

O objetivo deste estudo foi investigar o impacto da piscicultura em tanques-rede sobre a estrutura da comunidade zooplanctônica em um reservatório tropical. Acredita-se que a abundância do zooplâncton é maior perto dos tanques-rede e aumenta ao longo do tempo devido ao aumento da disponibilidade de alimento. As coletas foram realizadas próximo aos tanques-rede, bem como a montante e a jusante dos mesmos, antes e após a instalação dos tanques. A abundância do zooplâncton aumentou 15 dias depois da instalação dos tanques, seguida por uma redução e posterior aumento no fim do experimento. A abundância dos rotíferos apresentou diferenças significativas entre os pontos (p<0,05) e períodos de amostragem (p<0,001). Diferenças significativas também foram observadas para a abundância do zooplâncton total e de cladóceros (p<0,001). A variação espacial e temporal das variáveis físicas e químicas apresentou uma relação indireta com a estrutura e dinâmica da comunidade zooplanctônica, pois essa variação indicou a produção primária no ambiente. A hipótese foi rejeitada, pois o zooplâncton foi mais abundante no ponto de controle. Apenas os rotíferos apresentaram maior abundância perto dos tanques, devido à influência da disponibilidade de alimento. No entanto, a dinâmica da comunidade durante o experimento também foi relacionada com a disponibilidade de alimento. Os resultados sugerem o impacto da piscicultura em tanques-rede na comunidade zooplanctônica.

We investigated the impact of fish cage culture on the zooplankton community structure in a tropical reservoir. We hypothesized that community abundance is greater near cages and increases over time due to the increase in food availability. Samplings were performed near, upstream and downstream from net cages, and before and after net cage installation. The abundance of zooplankton increased 15 days after the experiment was set up, followed by a reduction and finally increased. Rotifer abundance showed significant differences among sites (p<0.05) and sampling periods (p<0.001). Significant differences were also observed in total zooplankton and cladoceran abundance (p<0.001). The spatial and temporal variation of the physical and chemical variables were indirectly correlated with the structure and dynamic of the zooplankton community, as they indicated the primary production in the environment. Our hypothesis was rejected, since the zooplankton was abundant at the reference site. Only rotifers showed higher abundance near cages, due to the influence of food availability. Community dynamics during the experiment was also correlated to food availability. Our results suggest an impact of fish farming on the zooplankton community.

A composição e ocorrência dos rotíferos foi investigada during dois períodos hidrológicos distintos em dois ambientes (rio e lagoa). As coletas foram realizadas diariamente (14 dias) à superfície na região pelágica de cada ambiente. A similaridade faunística entre os ambientes foi avaliada através do índice de Jaccard. Foram identificados 143 táxons. O maior número de táxons foi registrado no rio. A similaridade faunística entre os ambientes foi maior no período de águas altas. Os táxons planctônicos predominaram, sendo que a diferença para os não planctônicos não foi expressiva. No rio, destacaram-se esses últimos táxons, enquanto que na lagoa prevaleceram tanto os táxons planctônicos como os não planctônicos. Verificou-se que ocorreu um maior número de táxons de menor tamanho (> 200µm a 600µm) nos dois ambientes. Esse fato está relacionado com a pequena profundidade dos ambientes e a velocidade de corrente no rio, o que permite o deslocamento dos táxons bentônicos e perifíticos do fundo e da região litorânea para a coluna de água.

Investigations were carried out for the composition and occurrence of rotifer taxa of different forms and sizes during two distinct hydrological periods, in two environments (river and lake). Two sampling stations were established in open water of each environments. Surface samples were taken daily for 14 days. The faunistic similarity between the environments was established using Jaccard's index. One hundred and fourty three taxa were identified. The largest number of taxa was recorded in the river. Faunistic similarity between these environments was greater during high water. The planktonic taxa predominated and the difference for the non-planktonic ones was not marked. Non-planktonic taxa predominated in the river, while both types were numerous in the lake. We observed a larger number of smaller-sized taxa in both environments (> 200µm to 600µm in length). This was related to the shallowest depths and to current velocity, which moved benthic and periphytic taxa from the bottom and littoral zone into the water column.