Coastal upwelling, primary production and mussel growth in the Rias Baixas of Galicia

The Rias Baixas are four flooded tectonic valleys located on the northwest Iberian Peninsula that support a mussel production of about 250x10(6) kg y(-1) from 3337 mussel rafts. Mussel production in this region is the highest in Europe and one of the most intensive in the world, giving employment to 9000 people directly and 20 000 indirectly. The causes of this high mussel production are discussed through the analysis of published and some unpublished information. The interaction between coastal upwelling and the circulation patterns in the Rias, which channel the 3-dimensional variability of the open ocean into a 2-dimensional system, promotes a massive response in the productivity of phytoplankton populations inside the Rias, even during weak upwelling events along the coast. Coastal upwelling in the area typically occurs between March-April and September-October. The mean value of gross primary production during the whole upwelling season is similar to1.4 g C m(-2) d(-1), although high sporadic values of similar to4 g C m(-2) d(-1) may occur during upwelling relaxation events, when phytoplankton export to the coastal shelf is restricted. Mussel growth occurs mainly during the upwelling season. It is estimated that mussel harvest extracts similar to10% of the primary production. The phytoplankton response to upwelling provides food of high quality (fsimilar to0.5) that determines high absorption efficiency (similar to0.6), whereas the characteristics of the Rias maintains the seston concentration at levels (0.5-1.3 mg TPM l(-1) and < 5 mg Chl a m(-3)) below the threshold of pseudo-faeces production. The physiological behaviour of mussels indicates that the high yield of mussel culture in the Rias of Galicia is a consequence of the particular characteristics of the seston.