The Guajará Bay, a component of the Marajó Bay (located at the Amazon River
mouth), is an important coastal ecosystem of Pará State (Brazil). It is formed by
the confluence of Pará, Guamá and Acará rivers, and has slowly received human
domestic, industrial and agro-industry effluents. That system, dominated by
mesotides is dynamic and unique, due to the large volume of freshwater brought
by the river. Microalgae include aquatic unicellular organisms, autotrophs that
occur in the plankton or adhered to various substrates, which may be indicators of
hydrological conditions and possible environmental modifications. This paper
presents a characterization Guajará Bay, based on phytoplankton composition,
corresponding to an annual cycle of sampling. To achieve this, we made a
reasearch on composition, seasonal and spatial distribution, as well as their
ecological tolerances to salinity. Samplings were taken at six fixed stations along
the estuary from March to December 2005 (Project "Diagnosis of environmental
quality in Guajará Bay, Belém-PA /CT-HIDRO/CNPq). The biological material
collected with van Dorn bottle, was fixed with formaldehyde and analyzed by the
Uthermöhl technique to determine floristic composition, richness and density.
Climatic parameters (rainfall) and hydrological (turbidity, temperature, conductivity
and pH) were also analyzed. It was observed that the temperature does not
constitute a limiting factor for phytoplankton due to evident regional thermal
stability. The low turbidity of the water may be cited as a reason for the low
density of dinoflagellates. The pH proved to be acid to slightly acid, the lower
values occurred in the innermost stations and in months of highest rainfall. The
conductivity had a similar behavior. Both were determinants of phytoplankton
species. 83 taxa were identified under Bacillariophyta, Chlorophyta, Cyanophyta
and Dinophyta divisions. Diatoms were the dominant group. Aulacoseira
granulata, Rhizosolenia setigera, Achtinoptychus splendens, Coscinodiscus
oculus-iridis, Polymyxus coronalis e Triceratium favus were the most abundant
and constant species. Both groups and species were good indicators of
environmental conditions and changes occurring on the site. The occurrences of
algae along the flows of fresh water as Maguari River and the association of
cyanobacteria with the seasons enriched by nutrients or organic matter can be
highlighted. The density was highest in periods of low rainfall (September to
June). The number of species varied with the density. The planktonic species
showed subtle dominance in relation to benthic, the last are favored by periods of
turbulent flow of the bay. There was no clear spatial gradient in the species
distribution in relation to salinity. We identified oligo-mesohalobian; mesohalobian;
meso-euhalobians and euryhaline individuals. Thus, from the standpoint of
phycology, this environment has estuarine characteristics and can be classified
as mesohalobian. The precipitation and circulation patterns, and especially the
entrance of the saline tide, which causes changes in physical and chemical
properties of these waters and in the concentration of suspended particles,
conferring specific characteristics to the system. The phytoplankton of Guajará
Bay proved to be a good bioindicator of water quality and its trophic state, and the
estuarine system.
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