Spatiotemporal Variation of Bacterial Assemblages in a Shallow Subtropical Coastal Lagoon in Southern Brazil

A study on the bacterioplankton of Conceição Lagoon (27°34′ S–48°27′ W), Southern Brazil, was carried out in July 2005 (austral winter) and January 2006 (austral summer) to characterize the bacterial spatiotemporal distribution and to determine the heterotrophic and photoautotrophic bacterial dominance in hypoxic/oxic stratified waters. Bacterial abundance increased significantly (p?5 (winter) to 3.21?×?10⁶ cells mL^?1 (summer), heterotrophic coccus/rod-shaped (HCR) cells from 7.00?×?10⁴ to 3.60?×?10⁶ cells mL^?1, and heterotrophic filamentous (HF) bacteria from 2.90?×?10³ to 2.74?×?10⁵ cells mL^?1. Bacterial biovolumes also increased in summer with mean biovolumes of CCY ranging from 0.38 to 1.37 μm³, HCR cells from 0.31 to 1.12 μm³, and HF from 3.32 to 11.34 μm³. Principal component analysis showed that salinity, temperature, and light were the abiotic factors that better explained the temporal variability of bacterial assemblages. Bacterial heterotrophy dominated in the lagoon, excepted by the southern and part of central sector in January 2006, when autotrophic-dominated microbial community occurred. Spatially, bacterial assemblages were influenced by nutrient gradient, oxygen, and salinity with a positive relationship between biovolumes and nutrients and a negative relationship between abundance of coccus cyanobacteria and nutrients. area revealed a singular temporal pattern with hypoxic bottom waters in winter and oxygen-rich waters appearing in summer related with the availability of light and predominant microbes. Thus, oxygen consumption/production is likely to be regulated by the amount of light reaching the bottom, stimulating the production of oxygen by oxygenic phototrophs.