The Galician rías are examples of upwelling bays and are therefore among the most productive places on the planet, characterised by high fishery and shellfish production. Although both the bays and the continental-shelf waters are fertilised by the entry of nutrients associated with coastal upwelling, the factors responsible for the extraordinary productivity of the bays were not fully clear.
To shed light on this issue, the IIM is participating in a study led by a team from the Biological Oceanography Group (GOB) of the Marine Research Centre of the University of Vigo, together with scientists from the University of California San Diego and the University of Southampton. The aim of the study was to reveal the keys to phytoplankton growth — the microscopic algae that form the basis of marine food webs — in the Galician rías.
The study was carried out during an oceanographic survey in the Pontevedra and Vigo rías between June and July 2018 as part of the Remedios project. The results show that “the rapid response of phytoplankton growth to wind pulses inside the rías could explain their high productivity”. The conclusions are presented in an article just published in Limnology and Oceanography Letters. The lead author is Esperanza Broullón (the research forms part of her doctoral thesis). The article is also signed by CIM researchers Antonio Fuentes Lema, María Pérez Lorenzo, Emilio Fernández and Beatriz Mouriño, coordinator of the project and supervisor of the doctoral thesis.
Miguel Gil Coto leads the IIM-CSIC participation in the study, which also involved Peter Franks, from the University of California San Diego, and Bieito Fernández Castro, from the University of Southampton.
The role of wind
The study describes how short-term changes in wind forcing play a key role in the high biological productivity of the rías. Local topography channels the winds along the rías, pushing surface water outwards and causing deep, nutrient-rich water to rise rapidly to the surface inside the rías. This circulation responds to wind variability within six hours, three times faster than in shelf waters. In this context, explains Esperanza Broullón, “interestingly, phytoplankton growth in the rías also occurs on similar time scales. In synchrony with the upwelling of nutrient-rich waters, phytoplankton is exposed to sunlight and responds rapidly by absorbing nutrients, resulting in an increase in biomass and production”.
In this way, “the harmony between the time scales of wind-driven upwelling and phytoplankton growth could explain the greater productivity of the Galician rías and of other upwelling bays with similar morphologies and orientations”, says Beatriz Mouriño. The scientific team highlights that this knowledge will benefit local fisheries and improve productivity, and will also be useful for the early detection and monitoring of harmful algal blooms. Esperanza Broullón also explains that “these results reveal that predictions about the evolution of coastal upwelling ecosystems under global-change scenarios should consider not only trends in average wind intensity, but also changes in short-term wind-variability patterns”.
The Remedios project
The work was carried out during three weeks on board the oceanographic vessel Ramón Margalef as part of the multidisciplinary Remedios project, in which scientists from the University of Vigo, the Vigo Oceanographic Centre, the Institute of Marine Research of Vigo, Intecmar, Ifremer, the University of Southampton and the University of California San Diego analysed the factors that determine the richness of the Galician rías, responsible for 95% of Spain’s mussel production and 50% of European production.
The international team, led by CIM researcher Beatriz Mouriño, developed several studies and lines of work to analyse the factors that trigger phytoplankton growth and the importance of mixing among the different layers of the ocean. Within the framework of this project, they also obtained data that made it possible to determine that the bioturbulence generated by fish helps mix and distribute nutrients in coastal waters.