Upwelling and outwelling effects on the benthic regime of the continental shelf off Galicia, NW Spain

The benthic regime off the Galician coast of NW Spain was surveyed to assess biogenic enrichment from coastal upwelling and detritus outwelling from the rías, large coastal embayments: the Rías Altas along the northern and the Rías Bajas along the western coast, which have intense mussel aquaculture. Sediment samples were collected from 1984 to 1986 and used for geological, microbiological, and macrofaunal studies. Sub-bottom acoustic profiler records and grab and core samples identified two main mud deposits on the western shelf that were aligned north to south and parallel with the coastline. The major axis of the mud deposit, which extended south to the Portuguese border, is associated with the three most southern rías (Arosa, Pontevedra and Vigo) along the western shelf. Sediment particle size analysis showed that sediments on the western shelf were heterogeneous, and grain size increased from the inner shelf to the shelf break. On the northern shelf, sediments exhibited a more homogeneous textural distribution. Sediment organic matter followed a similar pattern with that of particle size. The highest organic matter values, 10%, occurred on the western shelf nearest the Rías Bajas, but these values decreased offshore to between 2 and 4%. On the northern shelf organic matter content was generally less than 4% but with patches of higher organic content. \n \nThe composition and structure of macroinfauna on the northern shelf, where seasonal coastal upwelling results in benthic enrichment, showed mainly small, surface feeding, and fast growing polychaetes. In contrast, macroinfauna on the western shelf showed more subsurface, deposit-feeding polychaetes. A main difference between the two shelves is that off the rías Bajas, besides coastal upwelling, outwelling from the highly productive rías Bajas, with their intense mussel aquaculture, also enriches the coastal sediment regime with a steady source of organic matter. Although seasonal and interannual variations occurred in the benthic bacteria, their general density distribution followed the pattern of organic matter content and particle grain size seasonally and interannually. The highest numbers of bacteria occurred in the upwelling region off the northern shelf and nearest the Rías Bajas on the western shelf. \n \nBoth coastal upwelling and organic outwelling from the rías Bajas support benthic production alongthe western Galician shelf. The main commercialdemersal finfishalongthis coast is hake, Merluccius merluccius and blue-whiting, Micomesistius poutassou. The norwegian lobster, Nephrops norvegicus is also an important crop, and is more abundant off the rías Bajas in the finer and organic-richsediments where there are abundant prey resources of benthic infauna.