Variação espaço-temporal de Trichodesmium thiebautii Gomont ex Gomont (Oscillatoriales - Cyanophyta) em águas tropicais do Nordeste do Brasil

Abstract

The genus Trichodesmium Ehrenberg is very common in the plankton of the tropical and subtropical oceans, and its colonies consist in aggregate filaments which can be seen by naked eye during the flourish period. The genus is composed by 11 species having been the Trichodesmium erythraeum Ehrenberg the only species reported in the continental shelf from Northeast, Brazil sometimes occurring in dense flourishes. This study reports the occurrence of Trichodesmium thiebautii Gomont ex Gomont in tropical waters from the Pernambuco State and it analyzes the spatial and temporal variations in the tricomas density at several points of the platform. Samplings were performed during the rainy period(July, 2005) and dry weather period (November, 2006) at six stations distributed along two profiles perpendiculars to the coast, with distance to the coast of 5, 10 and 20 miles, respectively in the beach of Gaibú (8º18'S and 34º56'W) and in the beach of Serrambi (8º32'S and 35º00'W). The samples for the qualitative and morphological study were collected in superficial drags using 20 μm mesh nets. The samples destined to the quantitative analyses (filament.L-1) were collected with Niskin bottles and the counting accomplished in inverted microscope, using 10 ml sedimentation chambers. Water samples were also collected to identify hydrological variables (temperature, salinity, dissolved oxygen, pH, nitrite,nitrate and phosphate). T. thiebautii was observed during the two annual periods and at the six sampling sites forming colonies in tuft form or rarely bunches, having been observed significant differences in thesize of the cells in relation to the sampling sites and enter the annual periods. However, significant differences were not observed among the tricomas of a same colony. The density of the filaments presented minimum values at sites near the coast (5 miles) and higher values at more distant sites (20 miles) during the dry weather period. Hydrological variables presented short variations noting subtle differences between the surface and bottom, contributing for a wide species distribution along the study area. The registration of T. thiebautii in the continental shelf of Pernambuco enlarges the speciesdistribution for the Brazilian Northeast region.