Efeitos top-down e bottom-up sobre a comunidade fitoplanctônica: uma análise experimental

Abstract

The phytoplankton community can be regulated by the bottom-up controls (nutrients) and top-down (predators). The availability of nutrients has been considered as the most significant factor in this regulation, however, studies show that phytoplankton can also be strongly regulated by zooplankton. Experiments were conducted to study the effects of varying the concentration of nutrients (N: P ratio) and herbivory exercised by zooplankton on the structure and biomass of phytoplankton, with emphasis on species-specific response of phytoplankton to the effects of these factors. Sub-samples of 700 ml of water in a eutrophic environment (Apipucos reservoir, Brazil) were placed in 1 liter Erlenmeyer flasks and maintained in the laboratory for 7 days. Three experiments were conducted separately and maintained for each of these treatments included the addition of nutrients to maintain DIN different reasons: E (N: P ratio 16, 60 and 5), presence and absence of zooplankton. The nitrogen source used was a phosphorus NaNO3 and the KH2PO4. Zooplankton added was obtained in the reservoir water. Phytoplankton community in terms of total biomass was not affected when kept under different nitrogen and phosphorus reasons while herbivory caused significant reduction. The community composition has changed, with the response to the effects of these factors are species-specific. The diatom Cyclotella meneghiniana Kützing was dominant in the treatments and the main species preyed upon by zooplankton. In the absence of zooplankton, this species showed exponential growth in all the experiments, mainly in the N: P ratio 60. The ratio N: P optimal for the growth of green algae ranged from 5: 1 to 60: 1, with species with more a great reason to grow. The green algae Desmodesmus protuberans (Fritsch and Rich) E. Hegewald, D. quadricauda (Turpin) Hegewald and Pediastrum duplex Meyen did not have growth negatively affected by zooplankton, while Cylindrospermopsis raciborskii (Woloszynska) Seenayya & Subba Raju, growth was stimulated by the presence zooplankton. These results demonstrate the importance of both nutrients as of herbivory in regulating the composition and biomass of phytoplankton and reinforce the need for evaluation of phytoplankton response to specific level for a better understanding of the importance and magnitude of these factors.