A novel earth observation based ecological indicator for cyanobacterial blooms

Abstract Cyanobacteria form spectacular mass occurrences almost annually in the Baltic Sea. These harmful algal bloomsare the most visible consequences of marine eutrophication, driven by a surplus of nutrients from anthropogenic sources and internal processes of the ecosystem. We present a novel Cyanobacterial BloomIndicator (CyaBI) targeted for the ecosystem assessment of eutrophication in marine areas. The method measures the current cyanobacterial bloomsituation (an average condition of recent 5 years) and compares this to the estimated target level for ‘ good environmental status’ (GES). The current status is derived with an index combining indicative bloomevent variables. As such we used seasonal information from the duration, volume and severity of algal bloomsderived from earth observation (EO) data. The target level for GES was set by using a remote sensing based data set named Fraction with Cyanobacterial Accumulations (FCA; Kahru & Elmgren, 2014) covering years 1979–2014. Here a shift-detection algorithm for time series was applied to detect time-periods in the FCA data where the level of bloomsremained low several consecutive years. The average conditions from these time periods were transformed into respective CyaBI target values to represent target level for GES. The indicator is shown to pass the three critical factors set for marine indicator development, namely it measures the current status accurately, the target setting can be scientifically proven and it can be connected to the ecosystem managementgoal. An advantage of the CyaBI method is that it’s not restricted to the data used in the development work, but can be complemented, or fully applied, by using different types of data sources providing information on cyanobacterial accumulations.