Fisheries and oceanography off Galicia, NW Spain: Mesoscale spatial and temporal changes in physical processes and resultant patterns of biological productivity

Abstract

Abstract. The Galician shelf off NW Spain (43N ø 9W ø) exhibits mesoscale spatial and temporal changes in biological productivity associated with upwelling. Spatial heterogeneity results from local geomorphic and land-sea interactions superimposed on the large scale atmospheric processes that produce upwelling. Wind-induced upwelling events, commonly of short (i.e., week) duration, are more common in the summer than in the winter. A series of cruises, including some time series sampling, and satellite imagery analysis showed that surface upwelling was more common and persistent on the northern coast compared with the western coast off the coastal embayments, the Rias Bajas. Nearshore off the rias, coastal runoff, which is greater in the rainy winter/spring versus the dry summer, affected upwelling. In early summer, upwelling less often reaches the surface because of increased water column stratification associated with lower surface salinities and thus upwelling is not detected by satellite imagery. Conversely, in late summer, upwelling more often reaches the surface because coastal runoff is reduced during the dry summer months and the water column tends to be less stratified. Plankton biomass and rate processes along the Galician shelf reflected both ambient hydrographic conditions as well as prior history of upwelling or downwelling. Phytoplankton and bacterioplankton were in greatest abundance during upwelling conditions (June through August); in contrast, both zooplankton and fish larvae exhibited highest abundances in March, when there were upwelling conditions prior to our cruise. Spatial differences in the duration and frequency of upwelling events, in combination with advection of water masses, are critical to the patterns of water column productivity and sardine fisheries production off the Galician coast. More persistent upwelling at this NW corner of the Iberian peninsula supports large sardine fisheries because zooplankton and larval fish populations have time to respond to the higher primary production. Farther down the western Galician coast, the episodic upwelling and resultant intermittent primary production does not support a stable food supply needed to support fisheries. Times series sampling revealed mean response times of bacteria, phytoplankton, and zooplankton to be on the order of a day, days, and weeks, respectively. Sardines showed no spawning response in the relatively short time series sampling. The observed distributional patterns of fish eggs and larvae showed some offshore transport of fish larvae that were spawned inshore during upwelling periods and aggregation of larvae in a convergence zone northwest of Cabo Villano.

Mesoscale distribution patterns of diatoms in surface sediments as tracers of coastal upwelling of the Galician Shelf (NW Iberian Peninsula)

“…In the western Galician shelf maximum values for this species are located in middle shelf positions, far from the coastline, in an area where upwelling influence decreases (Prego and Bao, 1997). Its preferential distribution in the western shelf is in accordance with the more episodic upwelling conditions in this area (Tenore et al, 1995). Moreover, it has been suggested that small Thalassiosira species become more important in the sediments in areas where the nutrient supply is more consistent (Abrantes, 1988).…”

“…The general hydrographical effects of upwelling off the Galician coast have been studied during the last two decades (Fraga, 1981;Fraga et al, 1982;Blanton et al, 1984;McClain et al, 1986;Tenore et al, 1995) but a detailed mesoscale description of upwelling patterns in the Galician coast has still not been considered until recently (Prego and Bao, 1997). Using the hydrodynamical and biological characteristics of upwelling, the Galician shelf waters can hypothetically be divided into two contrasting sectors south (western shelf) and north (northern shelf) of Cape Finisterre (Estrada, 1984;Tenore et al, 1995;Prego and Bao, 1997). A third hydrodynamically delimited area, the Rias Baixas, could be defined according to its characteristic positive residual circulation (Prego, 1990).…”

Temporal dynamics of gastropod fauna on subtidal sandy sediments of the Ensenada de Baiona (NW Iberian Peninsula)

“…In the Galician rias, this situation might be related to the high primary production. In fact, upwelling events are known to occur in the rias usually twice a year, namely in spring and autumn, due to the particular hydroclimatic conditions and geographical features (Tenore et al 1995;Figueiras et al 2002). This grants a priori an important food supply until autumn which might favour to recruits from summer-autumn.…”

Aspects of dissolved inorganic carbon dynamics in the upwelling system off the Galician coast

“…Also, off capes, upwelling can occur with a wider range of wind directions than along a relatively linear coastline (Huthnance et al, 2002). Also, it should be noted that surface seawater temperatures lower in the CFA than in the RBA is a feature that can be noticed in several published surface water temperature distributions based on field data (see Fraga, 1981;Castro et al, 1994;Tenore et al, 1995) or based on the output from ''realistic'' physical models (see Stevens et al, 2000).…”