Study of two major events of thin layers of phytoplankton in the Galician rias using a 3D ocean model

V Encuentro de Oceanografía Física 2018 (EOF), Vigo (Spain), 20-22th June 2018

The Galician Rias, located in NW Iberia, are highly productive coastal ecosystems due to the influence of upwelling events occurring seasonally during spring-summer. Thus, a series of phytoplankton blooms occur in the Rias along the year. However, the mechanism of formation and the persistence of some of these blooms are poorly understood. Thin layers of phytoplankton (TLP) are a particular case of phytoplankton blooms that occur when large numbers of photosynthetic cells are located within a narrow depth interval. They have been recurrently observed in the Galician Rias, sometimes related to the occurrence of harmful algal bloom species. The analysis of the dataset collected weekly by the INTECMAR (Instituto Tecnolóxico para o control do medio mariño de Galicia) showed that two major events of TLP were observed, in May 2012 and June 2013, extending over several stations, throughout the Rias of Vigo, Pontevedra and Arousa. In this study we used a 3D ocean circulation model of the southern Galician Rias coupled to a biogeochemical model in order to reproduce the oceanographic conditions during these events. Our mail goal was to investigate the possible drivers responsible for the formation of the observed TLPs. The model was able to reproduce the formation of the blooms, their approximate horizontal extension and their time evolution, although the modeled chlorophyll fields were smoother, with lower concentrations and more vertically spread maximums, than observed. In both events, the bloom formation was associated with strong surface haline stratification within the Rias. The source of the freshwater driving this stratification during the 2013 event was associated with the northward advection of the river Miño plume, whereas in 2012 it was associated with local river discharges. These results demonstrate the utility of modeling approaches for the study of the dynamics of TLPs in this ecosystem

This is a contribution to the REMEDIOS project (CTM 2016-75451-C2-1-R) funded by Ministerio de Economía y Competitividad

Peer reviewed