COASTAL UPWELLING IN THE RIAS BAJAS, NW SPAIN: CONTRASTING THE BENTHIC REGIMES OF THE RIAS DE AROSA AND DE MUROS

Abstract

Nutrient-rich North Atlantic water upwells off the NW coast of Spain and intermittently
\nintrudes into the rias, coastal embayments, by displacement during periods of offshore winds.
\nHigh primary production associated with the upwelling supports an intensive raft culture of the
\nedible mussel, M y tilus edulis. This culture is most intensive (ca. 2000 rafts) in the Ria de Arosa,
\nand results in one of the highest protein yields per unit area on earth. Mussel feces, in tum,
\nserve as food for other species of epifauna which live on, or between the densely-packed mussels. These epifauna, in tum, serve as food for demersal fish and crabs. The epifauna mineralize
\na great deal of the mussel feces but still there is high deposition of organic-rich deposits to the
\nseafloor, where they are utilized by a low diversity benthic assemblage dominated by the polychaete Spiochaetopterus costarum and comesomatid nematodes.
\nSedimentary structures, profiles of pore-water constituents, and C/ N and 210Pb vertical profiles show that sediment deposits under the rafts are mixed to a depth of about 20 cm. Bioturbation is a relatively unimportant advective process in Ria de Arosa sediments, and mixing is
\napparently accomplished by bottom current erosion. Mixing may take place with a frequency
\nof less than one month, releasing pore water ammonium and phosphate into the water column
\nand replenishing pore water sulphate.
\nThe Ria de Muros, an embayment north of the Ria de Arosa, has few (< 100) mussel rafts.
\nThe benthos in muddy sediments of the Muros consists of a high diversity, equilibrium assemblage dominated by a subsurface deposit-feeding polychaete (Maldane glebifex), an ophiuroid
\n(Amphiura chiajei), a holothurian (Leptosynapta bergensis) and desmodorid nematodes. Bioturbation is an important mixing process in Muros sediments. This biogenic process is reflected
\nin high rates of pore-water exchange and mixed vertical profiles of Pb