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&e Relationship of Upwelling to Mussel
Production in the Rias on the Western Coast of
Spain
Jack O. Blanton
Kenneth R. Tenore
F. Castillejo
Larry P. Atkinson
Old Dominion University4);316:77,<-,<
Franklin B. Schwing
See next page for additional authors
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16;0-91):76;0->-:;-96+7):;7.$8)16 Journal of Marine Research, 45,71
Journal of Marine Research, 45, 497-511, 1987
The relationship of upwelling to mussel production in the rias
on the western coast of Spain
by
J.
O. Blanton,
I
K. R. Tenore/ F. Castillejo/ L. P. Atkinson,
1
F. B. Scbwing
l
and A. Lavin
4
ABSTRACT
We have calculated an upwelling index for each month over a 17-year period (1969-1985) for
a point offthe western coast of Spain. We interpret April through September values of the index
to indicate the flux of nitrate-rich water into the Spanish Rias. The index representing the
6-month upwelling series has been correlated with an index representing the conditions of
mussels grown during that season on rafts in Ria de Arosa. Two seasons represent extreme
upwelling conditions over the 17-year sampling period: 1977 when the upwelling index was the
highest, and 1983 when it was the lowest. A comparison of the condition of mussels during these
years showed that meat content was double in 1977.
We suggest, by this study, that long range forecasts of synoptic scale weather patterns could
be used to predict the potential nutritional value of mussels harvested in the rias of Spain.
1. Introduction
The Rias Bajas on the northwest coast of Spain (Fig. 1) have one of the highest
known protein yields of the edible mussel Mytilus edu/is. This production is carried out
on 20 m x 20 m rafts with an average of 600 hanging ropes on which the mussels grow.
There are over 2,000 rafts in the Ria de Arosa, the largest of the Rias Bajas. Ria de
Arosa annually produces over 100,000 metric tons total wet weight over a total area of
230 km
2
(Tenore et al., 1982). The high production is supported, in great part, from
nutrient inputs from the upwelling of nutrient-rich North Atlantic central water along
the coast that subsequently is advected into the rias. For further information on the raft
culture see Tenore et al., 1982.
Upwelling occurs between the months of April and October (Blanton et al., 1984)
when the average winds along the coast exert a southward surface stress causing an
Ekman transport offshore. The displaced surface water is replaced by the colder,
nutrient-rich deeper water. During the "upwelling" season, the winds actually cycle
I. Skidaway Institute of Oceanography, P. O. Box 13687, Savannah, Georgia, 31416, U.S.A.
2. Chesapeake Biological Laboratory, University of Maryland, P. O. Box 38, Solomons, Maryland,
20688-0038, U.S.A.
3. Instituto Espanol de Oceanografia, Laboratorio de Malaga, Paseo de la Farola 27, Malaga 16, Spain.
4. Instituto Espanol de Oceanografia, Centro Costero de Santander, Santander, Spain.
497
498
Journal of Marine Research
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upwelling indices were calculated.
between episodes favoring upwelling and downwelling. These cycles are governed by
the synoptic weather patterns which change every 5 to 10 days. Thus upwelling occurs
on two basic time scales.
This paper discusses the link between seasonally varying coastal upwelling, nutrient
supply to the rias, and the atmospheric system that governs them, An upwelling index,
which is related to wind stress, illustrates variations, over time, in the upwelling process
(Bakun, 1973). The upwelling index indicates periods during which upwelling theoreti-
cally occurs and thus should be related to the influx of nutrient-rich water to the rias.
Finally, we examine interannual variations in the strength of upwelling along the coast
to determine its relationship to good and bad years for mussel production. The strength
and length of upwelling seasons depends, to a large extent, on the position and strength
1987]
Blanton et al.: Upwelling
&
mussel production in Spain
499
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200
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Figure 2. Vertical section off Arosa, 13-14 Oct 1977. (a)
111;
(b) nitrate.
of the Azores High pressure cell. This will govern the overall wind field along the
western coast of Spain. We will use a mussel condition parameter based on percent
solids (meat content) to provide an index of the overall health of the mussel population
in a given year. If the position and strength of the Azores High can be successfully
predicted, we hypothesize that upwelling along the coast, influx of nutrients into the
rias, and hence, the condition of mussels in the rias can also be predicted.
2.
The upwelling regime
The western coast of Spain (Galicia) is in an eastern boundary current region of the
Atlantic and is influenced by upwelling favorable winds. A vertical section across the
continental shelf (Fig. I) shows that isopycnals several hundred kilometers off the
coast and at depths less than 800 m are uplifted approximately 50 m as the edge of the
shelf is approached (Fig. 2a). The uplifted water is cold and nutrient-rich (Fig. 2b)
thus introducing nutrients to the continental shelf environment when upwelling occurs.
A small amount of upwelling can lower temperature by I-2°C and significantly
increase the concentration of nitrate (Fig. 3) in the surface waters. High nitrate values
are clearly associated with cold water.
Upwelling occurs frequently off Galicia from April through September (Blanton et
al.,
1984) due to winds blowing southward along the coast. The upwelling season in a
given year can be depicted by graphing daily values of an index representing the
onshore or offshore transport of surface water. The index (Bakun, 1973) is equivalent
to the Ekman transport derived from surface winds. As pointed out by Wooster et
at.
(1976), "To the extent that this transport is replaced by water from deeper layers,