Bacterial Diversity and Geochemical Profiles in Sediments from Eutrophic Azorean Lakes

Abstract

In the Azores, the advanced trophic state of the lakes requires a fast intervention to achieve the good ecological status prescribed by the Water Framework Directive. Despite the considerable effort made to describe the phytoplankton growing on the water column, the lack of information regarding the microbial processes in sediments is still high. Thus, for the successful implementation of internal management actions, the present work explored the relationships between geochemical profiles and dominant members of the bacterial community in sediments from eutrophic Azorean lakes. Lake Azul geochemical profiles were quite homogeneous for all parameters, while in lake Furnas the total iron profile presented a peak below the aerobic layer. For lake Verde, the concentrations of all studied parameters (20 ± 2% loss-on-ignition; 2.10 ± 0.08 mg g⁻¹ total phosphorus; 1.31 ± 0.50 mg g⁻¹ total nitrogen; 8.06 ± 0.13 mg g⁻¹ total iron) in the uppermost sediment layer were approximately two times higher than the ones in sediments from other lakes, decreasing with sediment depth. The higher amounts of phosphorus and organic matter in lake Verde suggested a higher internal contribution of phosphorus to eutrophication. The dominant members of the sediment bacterial community, investigated by denaturing gradient gel electrophoresis, were mostly affiliated to Proteobacteria phylum (Alpha-, Delta-, and Gamma-subclasses), group Bacteroidetes/Chlorobi and phylum Chloroflexi. The Cyanobacteria phylum was solely detected in sediments from lake Verde and lake Furnas that presented the highest amounts of nitrogen and phosphorus both in the water column and sediments, while the other phyla were detected in sediments from the three studied lakes. In conclusion, management measurers to achieve the good ecological status until 2015 should be distinct for the different lakes taking into account the relative magnitude of the nutrient sources and the bacterial diversity in sediments.

Keywords:

• sediments
• geochemical profiles
• bacterial community composition
• eutrophication

Acknowledgments

The authors are indebted to the Regional Department of Water Resources and Land Planning for the grant (Contrato Excepcionado n° 4/2008/DROTRH), and its staff (Dina Pacheco), to University of Azores (Paulo Antunes), and to INOVA (Manuela Cabral). NIOO-KNAW (Marion Meima) is acknowledged for the collaboration in establishing the molecular methods. The authors also acknowledge the Grant SFRH/BD/25639/2005 from the Foundation for Science and Technology/M.C.T., Portugal, awarded to Gilberto Martins.