A NEW "SIGHT" ON MICROBIAL PLANKTON ECOLOGY: COASTAL X OCEANIC SYSTEM IN BRAZIL

Abstract

Data from Guanabara Bay, Rio de Janeiro, Brazil (PRONEX), and from a region offshore the Brazilian central coast (REVIZEE) were selected for comparison between the auto­ and heterotrophic components of picoplankton (bacteria, 0.2-2.0 um), nanoplankton (flagellates and diatoms. 2.0-20 um) and microplankton (phyto- and protozooplankton, >20 um). Our goal is to provide a preliminary view of the microbial structure of two contrasting marine environments, as well as methodological recommendations necessary to adjust techniques to particularities of the environments under study. The quantitative (density and carbon biomass) and qualitative (auto- and heterotrophic components) variations indicated a trophic gradient between Guanabara Bay (inner reaches: site Ramos; entrance: site Urca) and REVIZEE (7 stations, from closer to farther from shore). These systems are characterized by the lower salinity and man-induced eutrophication in the coastal area, as opposed to the 1110re saline and oligotrophic waters of the Brazil Current offshore. Total cell density was highest at site Ramos (10¹¹ cel.L^-1), followed by site Urca (10¹⁰ cel.L^-1) and REVIZEE (10⁹ cel.L^-1). Total cell density decreased according to the size class analyzed: picoplankton, dominated by heterotrophs, contributed with more than 99% while nano e microplankton where less representative. Picoplankton biomass at Guanabara Bay (10²-10⁴; ugC.L) was two orders of magnitude higher than at REVIZEE. The nanoplankton did not show a defined pattern. Autotrophs dominated the microphytoplankton in thc bay (>80%, mostly diatoms and euglenophytes) and offshore (>60%, with more heterotrophic dinoflagellates). The greater similarity of the microbial community between site Urca and REVIZEE strengthens the belief in the recovery of the Guanabara Bay ecosystem, especially in areas with tide-induced circulation, despite its advanced state of deterioration due to human impact.