Integrating sediment core and satellite sensing approaches to assess recent phytoplankton bloom trajectories in a large reservoir

Abstract

In this work we integrated two approaches that are rarely used together in addressing the responses of biota to multiple environmental stressors sediment cores and satellite images. The study area is a large subtropical reservoir (750 km2) that recurrently suffers severe cyanobacteria blooms, yet the trajectory of blooms since the building of the reservoir (in 1979) until the onset of the ongoing water quality monitoring program (ongoing since the year 2000) was unknown. In 2015 we performed sediment core studies, which were published in 2020, and our findings suggested that phytoplankton blooms initiated in 2003, about 24 years after the reservoir was built. Moreover, in this reservoir, as part of the current monitoring program, specific algorithms for chlorophyll estimation using satellites have been developed and published. Such algorithms allow phytoplankton chlorophyll estimation since 1984, thanks to the continuity of the Landsat mission (about 16 years prior to the monitoring program). Hence, we used images from 1984-2015 to explore if we could complement the information obtained using sediment cores. For this, we processed Landsat 5, 7 and 8 Landsat images (482, 350 and 67, respectively) in different areas of the reservoir, and calculated anomalies in chlorophyll (whenever chlorophyll was > 85 μg L-1). We found that before 2003, in average anomalies were only recorded about one time per year, whereas after 2003 the frequency of anomalies markedly increased (average 8 anomalies per year). Overall, the patterns found using sediment cores were like those encountered using satellite estimation of chlorophyll. Thus, these tools could be complementary used for analyzing recent trends (last 30 years, since satellites are operative in space) of phytoplankton trajectories.