Exploiting high frequency monitoring and satellite imagery for assessing chlorophyll-a dynamics in a shallow eutrophic lake

Freshwater ecosystems are challenged by cultural eutrophication across the globe, and it is apriority for water managers to implement water quality monitoring at different spatio-temporalscales to control and mitigate the eutrophication process. Phytoplankton abundance is a keyindicator of the trophic and water quality status of lakes. Phytoplankton dynamics are characterizedby high spatio-temporal variation, driven by physical, chemical and biological factors, thatchallenge the capacity of routine monitoring with conventional sampling techniques (i.e., boatbased sampling) to characterise these complex relationships. In this study, high frequency in situmeasurements and multispectral satellite data were used in a synergistic way to explore temporal(diurnal and seasonal) dynamics and spatial distribution of Chlorophyll-a (Chl-a) concentration, aproxy of phytoplankton abundance, together with physico-chemical water parameters in a shallowfluvial-lake system (Mantua Lakes). A good agreement was found between Chl-a retrieved by remote sensing data and Chl-a fluorescence data recorded by multi-parameters probes (R2 = 0.94).The Chl-a maps allowed a seasonal classification of the Mantua Lakes system as eutrophic orhypertrophic. Along the Mantua lakes system an increasing gradient in Chl-a concentration wasrecorded following the transition from a fluvial to lacustrine system. There was significantseasonal heterogeneity among the sub-basins, probably due to different hydrodynamics, influencedalso by macrophyte stands. High-frequency data revealed the importance of rainfall events in thetiming and growth dynamics of phytoplankton, particularly for spring and late summer blooms.Combining temporal and spatial data at high resolution improves the understanding of complexfluvial-lake systems. This technique can allow managers to target blooms in near-real time as theymove through a system and guide them to localized hot spots enabling timely management actionin ecosystems of high conservation and recreational value.