Decadal analysis of chlorophyll fluorescence, algal blooms and driving factors from a fixed-point observing system in the Northern Adriatic Sea

From August 2012 to December 2022, continuous optical measurements of Chlorophyll Fluorescence (ChlF) were conducted at the E1 meteo-oceanographic buoy, a fixed-point observing system located off the Emilia-Romagna coast in the Northern Adriatic Sea (Italy, Mediterranean Sea). Using a WET Labs® ECO Triplet fluorescence sensor, a total of 69,705 hourly ChlF observations revealed significant seasonal and interannual variations in chlorophyll concentrations at the sea surface. ChlF concentrations ranged from below detection limits to a maximum daily average of 41 μg/L, with seasonal peaks during spring (April–June) and winter (January–March). Thanks to an automated detection procedure for algal bloom based on net growth rates and other parameters, it was possible to identify 40 bloom events over the ten years period. Principal Component Analysis (PCA) highlighted the influence of key environmental factors such as temperature, dissolved oxygen, turbidity, salinity, wind speed, as well as processes like nutrient transport and freshwater inputs, on Chl variability. Seasonal patterns emerged as primary drivers: spring blooms were dominated by temperature, salinity, and dissolved oxygen, summer typically featuresstratified conditions with sporadic bloom events, while autumn and winter blooms are more influenced by wind speed and turbidity, which promote nutrient redistribution and mixing. Moreover, the analysis of algal blooms identified two distinct seasonal patterns: Single – Peak blooms, predominant in spring, and Multi – Peak blooms, typical of autumn and winter, each defined by characteristic environmental conditions shaping onset, progression, and duration. These findings highlight the essential role of long-term data series in understanding coastal marine ecosystems and assessing the environmental factors driving algal bloom events.