Role of small-sized phytoplankton in triggering an ecosystem disruptive algal bloom in a Mediterranean hypersaline coastal lagoon

Highlights: • Eutrophication has produced a significant alteration of the lagoon phytoplankton. • Synechococcus had a key role in the onset of the ecosystem disruptive algal bloom in the coastal lagoon. • Self-regulation of the Mar Menor in response to environmental changes has been reduced. Monthly samplings carried out in 2016–2019 and satellite color images from 2002 to 2019 have been combined to determine the onset and causative species of the ecosystem disruptive algal bloom (EDAB) that affects the Mar Menor coastal lagoon (Western Mediterranean Sea) since 2015. Substantial changes in satellite spectral reflectance attributable to increasing abundance of Synechococcus were registered in 2014. Furthermore, cell abundances of this species in 2016 were the largest ever obtained in the lagoon (6 10⁶ cells mL⁻¹), with values similar to those reported for other Mediterranean hypertrophic estuaries and coastal lagoons. These results suggest that the early changes leading to the EDAB started in 2014 and that Synechococcus played a relevant role in its development. Moreover, diatom and dinoflagellate abundances changed substantially in 2016–2019, ranging from 10² to more than 10⁴ cells mL⁻¹. Some of these changes were linked to flood, suggesting that EDAB has modified substantially the homeostatic capacity of the lagoon.