Composição e biomassa microfitoplanctônica associadas a variáveis físico e químicas em dois transectos da zona estuarina do Rio Amazonas (Amapá, Amazônia, Brasil)

Abstract

The estuary of the Amazon River is the largest river system recognized worldwide with a drainage area of 7,050,000 km2. Due to its great ecological importance and its extensive area of influence, most of the work performed in this area focuses on quantitative and functional aspects of biogeochemical production and cycling, not including surveys of various taxonomic and ecological niches that inhabit this ecosystem. This research aimed at studying the composition and biomass of phytoplankton associated with environmental variables in an stretch (North and South Channel) of the Amazon River estuarine ecosystem in Amapá / Brazil. Therefore, monthly and quarterly samplings were conducted in six points that intersect the channels in the estuary from March/2011 to July/2012. Samples of phytoplankton were collected with a 60 μm plankton net and were fixed with transeau solution for further analysis with light microscopy. Other parameters were measured in the field, like temperature, electrical conductivity, pH and dissolved oxygen. Samples were also collected for determination of chlorophyll-a concentration by spectrophotometry. Finally, the data were statistically studied using descriptive, cluster and multivariate linear regression analysis. A total of 155 taxa were identified and distributed in the following classes: Zygnematophyceae, Cyanophyceae, Chlorophyceae, Bacillariophyceae, Coscinodiscophyceae, Ulvophyceae, and Mediophyceae Euglenophyceae. The class Zygnematophyceae was the richest with 86 taxa (55.48%), followed by Cyanophyceae (15.48%). In contrast, classes Mediophyceae (1.29%) and Euglenophycea (1.29%) were less representative with only 2.58% of the total recorded taxa. With regard to frequency of occurrence, 40% of the taxa were classified as sporadic (S), 36.13% as uncommon (UN), 15.48% as frequent (F) and 8.39% as very common (VC). The values of chlorophyll-a showed averages that varied in the sampling sites from 4.36 ± 5.41 μg / L to 10.59 ± 6.39 μg / L (p <0.05), presenting a significant variation (p = 0.003) in time with the highest values related to the rainy season. The linear regression analysis showed that no environmental variable significantly influenced (p> 0.05) in phytoplankton richness and biomass, due to the spatial uniformity observed in all parameters measured (p <0.05). This trend differs from studies performed in estuary regions, as well as the phytoplankton composition recorded, showing that the Amazon River estuarine system has unique characteristics, yet little known and studied. Therefore, this work is one of the pioneers in what concerns to the phycological knowledge of this ecosystem, as well as a significant contribution to the characterization of Amazon and local phycoflora. Of the total taxa identified by this survey, 73 are new records for the state of Amapá