Environmental drivers of algal bloom timing and magnitude estimated from Sentinel-2 imagery

Freshwater ecosystems can experience shifts in aquatic primary production that are driven by disturbances in temperature, river discharge, and nutrient cycle dynamics – with interacting and cumulative effects that are poorly understood, but have strong implications for ecosystem health. These shifts can reduce biodiversity, alter food web structures, degrade water quality, and negatively impact aquatic communities. Changes in the timing of phytoplankton growth cycles are often associated with different algae groups dominating the assemblage. While cyanobacterial blooms are of primary concern due to their toxicity and tight association with high summer temperatures and nutrient loads, other harmful and potentially toxic algae may proliferate as well. In the summer of 2022, the Oder River experienced a harmful algal bloom caused by the brackish-water haptophyte Prymnesium parvum. This unprecedented event culminated in an environmental disaster in one of Europe's last rivers with a free-flowing lower course and several regions of extraordinary ecological importance. Here, we integrate long-term records of hydrometeorological variables with the estimations of chlorophyll content derived from the Copernicus satellite Sentinel-2 along the full extent of the Oder River and its most relevant tributaries to (1) characterize the timing (i.e., phenology of each growth cycle) and magnitude (i.e., peak chlorophyll-a activity) of the phytoplankton dynamics over the last 10 years; (2) evaluate the role of temperature and discharge anomalies, and of increased saline inputs in driving these blooms; and (3) compare these assemblages with those observed in other European and global river systems. Our findings reveal distinct patterns of spring and summer blooms alternating between years in magnitude as well as in onset timing. When incorporating cumulative anomalies of temperature and discharge the sensitivity of the phytoplankton community dynamics to the interplay of environmental drivers becomes clearer. We discuss the implications of these patterns in a context of rapid global change.