Estrutura da comunidade fitoplanctônica como alternativa para avaliação da sustentabilidade biológica de áreas com potencialidade aqüícola estabelecida pelo zoneamento costeiro do Estado do Maranhão

Abstract

The estuarine areas in São José de Ribamar, Cedral and Cajapió counties, Maranhão State (Brazil), are between 2° e 3° S and 44° e 45° W. Six superficial collections at high tide in morning time were carried out during the period from May/2005 to April/2006 for each sample station to know the phytoplanktonic biomass, ecology and flora composition, seasonal variations and the hydrological and climatic variables. These stations were defined according to the Costal Zoning of Maranhão State, which considered them as medium and high potentiality for aquatical organisms cultures which are economically important. Two seasonal periods were considered: the wet weather (from January to June) and the dry one (from July to December). Climatological data were taken at meteorological stations at São Luís, Alcântara and Pinheiro counties. In situ data about local depth, temperature, water transparency were registered and, concomitantly, water samples were collected for biological and hydrological variables analysis. Microphytoplankton samples were obtained through superficial horizontal drags, with a dragnet of 1 m length, 30 cm ring and 45 mm mesh size, during 5 minutes. The phytoplanktonic biomass (total and sizefractionated) was measured through chlorophyll a concentrations. A number of 124 taxa had been identified, mainly diatoms, composing 87,1 % of the total taxa. In relation to occurrence frequency, the most important were the diatoms Cyclotella stylorum, Ditylum brightwellii, Thalassionema frauenfeldii and Thalassiosira subtilis, with 94 % of occurrence in the collections and Actinoptychus annulatus, Coscinodiscus centralis, Odontella aurita, O. mobiliensis and Triceratium favus, frequents in 89 % of the samples. The dinoflagellates constituted the second flora group, followed by Chlorophyceae, Cyanophyceae and Euglenophyceae. Shannon’s diversity index (H’) characterizes the environments with medium and high specific diversity degrees. The water transparency varied from 12 cm to 130 cm; the water temperature (from 27,8 to 38,5 °C); salinity (from 10,7 to 33,0) varied from mesohalyne to euhalyne environment; saturation rate of dissolved oxygen (from 61,44 to 160,55 %); pH keep alkaline, varying from 7,40 to 8,68; nitrate (from 1,39 to 24,43 mM), phosphate (from 0,15 to 1,72 mM) and silicate (from 3,25 to 65,03 mM); algal biomass varied from 2,40 to 16,56 mgClorofila.m-3, showing a light seasonality with shorter concentrations during the wet weather and generally with high indices, characterizing the area as an eutrophic environment. In that areas there are serious biological and chemical limitations for aquaculture. In São José de Ribamar and Cajapió, the low phytoplanktonic biomass can be named as inappropriate in some periods of the year, while in São José de Ribamar and Cedral some species with toxic potentiality are presents, as two genera of diatoms (Nitzschia and Pseudo-nitzschia), dinoflagelates of the genera Protoperidinium and cyanobateria of the genera Oscillatoria, Such species can be developed by the consequent environment enrichment with nitrate and phosphate deriving from aquacultural activities.