Bacterioplankton Activity in a Meso-eutrophic Subtropical Coastal Lagoon.

The aim of this study was to investigate whether the bacterioplankton activity in the meso-eutrophic Conceição Lagoon would increase significantly under allochthonous inputs of inorganic nutrients and organic carbon. Abundance and biomass of bacterioplankton were evaluated under three treatments: light (14 h light/10 h dark), complete darkness (dark-control), and nutrient (C + N + P-dark, 100 : 10 : 1) enrichments during 72 h. Nutrient enrichments promoted a significant increase in abundance (maximum of 19.0 ×10 cells·L in the first 32 hours) and biomass of the heterotrophic bacterioplankton, which induced the formation of large clusters. Bacterial biomass remained constant in the non-enriched incubations (dark-control and light). Bacterial growth rates were significantly higher after nutrient additions (1.35 d), followed by control (0.79 d), and light (0.63 d) treatments, which were statistically equal ( > 0.05). Bacterial production rates were also significantly higher under nutrient additions (1.28 d), compared to the control and light (0.50 d and 0.44 d, respectively), demonstrating that bacterial growth and production in this meso-eutrophic lagoon are under an immediate "bottom-up" regulation, followed by a potential top-down effect. These facts reinforce the urgency on improving the local wastewater management plan in order to prevent further expansion of anoxic waters.