Coupling high-resolution measurements to a three-dimensional lake model to assess the spatial and temporal dynamics of the cyanobacterium Planktothrix rubescens in a medium-sized lake
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In a medium-sized pre-alpine lake (North Italy) the cyanobacterium Planktothrix rubescens has strongly dominated the phytoplankton assemblage since 2000, similar to many pre-alpine lakes, despite improvements in water quality. The objective of this study was to determine the factors governing the spatial distribution of P. rubescens, including the major hydrodynamic processes and the influence of long-term reduction in nutrient concentrations during a period of climate warming. We used an intensive field campaign conducted from February 2010 to January 2011, to evaluate distributions of phytoplankton phyla, as well as P. rubescens, using spectrally resolved fluorescence measurements. These data provided highly spatially and temporally resolved phytoplankton population data suitable to calibrate and validate a coupled three-dimensional hydrodynamic (ELCOM) and ecological model (CAEDYM) of the lake ecosystem. The simulations revealed the fundamental role of physiological features of P. rubescens that led to observed vertical patterns of distribution, notably a deep chlorophyll maximum, and a strong influence of lake hydrodynamic processes, particularly during high-discharge inflows in summer stratification. The simulations are used to examine growth-limiting factors that help to explain the increased prevalence of P. rubescens during re-oligotrophication.
KeywordsMetalimnion Hydrodynamics Deep chlorophyll maximum Phytoplankton ELCOM–CAEDYM
The authors wish to thank Maurizio Maierna and Fabio Buzzi (Regional Agency for Environmental Protection, Department of Lecco—ARPAL) for the precious collaboration in phytoplankton cells identification and counting, Giuseppe Morabito (Institute of Ecosystem Study-CNR) for the support of a second Fluroprobe and Marco Seminara (University La Sapienza of Rome) for the zooplankton survey. A special thank is due to Franceso Spada, Alessandro Perotto and EPSON Meteo Centre (http://www.meteo.it/) for the computational support to modelling, to Mr. Giudici for his fundamental contribute during field activities, to Vladimir Krivtsov for kindly revealing the key-role of Diatoms in any ecosystem and to Laura Marziali for a unique and precious support.
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