eM 1998/R:4
Mesoscale Physical Phenomena and Biological Production
INTERANNUAL VARIABILITY OF UPWELLING· STRENGTH AND MUSSEL
PRODUCTION ON THE IBERIAN PENINSULA
By
J. O. Blanton .
Skid away Institute of Oceanography
10 Ocean Science Circle
Savannah, GA 31411 USA
and
K. R. Tenore.
Chesapeake Biological Laboratory
University of Maryland Center for Environmental Science
P.O. Box 38
Solomons, MD 20688 USA
1
Abstract
The Atlantic coast along the Iberian Peninsula is one of the world's centers for coastal
upwelling. Downwind of capes like Cabo Finisterre and Cabo da Roca, upwelling is
intensified creating particularly productive zones to the south. The Rias along the
Spanish coast of Galicia are located south of Cabo Finisterre where large quantities of
mussel are cUltivated from rafts. Temperature and nutrients measured in the Rias are
well correlated with theseasonaLwind patterns that generate coastal upwelling. Bottom
temperatures which decrease within 12-24 hours afterlhe onset of upwelling favorable
winds are accompanied by an influx of nutrients into the Rias.
Pressure gradients associated with wind-induced upwelling along the Iberian Peninsula
are located on the northeast flank of high atmospheric pressure centered near the
Azores. There is a positive relationship (I'" = 0.66) between the strength of upwelling
and the percent of solid tissue in the mussels cultivated on ropes suspended from rafts
in the Spanish Rias for each season. A tool that would predict the strength of upwelling
and nutrient supply to the Rias would enable mussel fisherman to optimize the number
of rafts, the number of ropes and the density of mussel seeds on each rope to produce
a more marketable mussel. .
,
Introduction
The western coast of Spain (Fig. 1) is located in one of the main upwelling regimes
(Wooster et aI., 1976; Fraga, 1981; Blanton et a/., 1984; McClain et a/., 1986) making
this region among the more productive for marine shellfish. The Rias situated along this
coast are deep estuaries that have almost unrestricted connections to the ocean
enabling them to receive large doses of nutrients during times of upwelling. This
environment is favorable for the cUltivation' of the edible mussel Myti/us edulis on 20 m
by 20 m rafts, each having an average of 600 hanging ropes on which the mussels
grow (Tenore et a/., 1982; Wiegert and Pensa-Lado, 1982).
The purpose of this paper is to review the relationship between the strength of
upwelling and the quality of the mussels harvested in Ria de Arosa (Blanton et a/.,
1987). Mussel quality was shown to be directly proportional to the strength of upwelling
averaged over each year from 1973 to 1983.
Background
The western coast of Spain is in an eastern boundary current region of the North
Atlantic that is characterized by frequent upwelling events. Upwelling occurs most
2
persistently from April throughSeptember(Blanton .et al., 1984). Daily values of an
index of the strength of upwelling (Bakun, 1973) were used to quantify.the. strength of
upwelling for the period 1973c 1983 where .it was shown thattherE;! was considerable
interannual variability (Blanton et al., 1987).
The upwelling index (UI) is equivalent to the Ekman transportin the surface layers (Ve)
which is related tq the wind stress vector at the sea surface ( 1:) and theCoriolis
parameter (f), or.
Ve
= (
l/j)( ,xk )
where k is a vertical unit vector. Wind stress was calculated from an estimate of the
geostrophic wind adjusted for friction (Blanton et al., 1984) which was derived from the
sea surface pressure field surrounding a point about 150.km west of Cabo Finisterre
(43N, 11W). Positive values of Ve were definedforwestward transport off the coast and
were, therefore, upwelling favorable. The value of Ve averaged from April through
September of a given year is defined as the upwelling index (UI) for that year. UI is
used as a seasonal index of upwelling strength.
Interannua.1 variabilityinUI is undoubtedly related to some, suitably averaged sea level
pressure field. Th.e North Atlantic Oscillation (NAO) Index is the average sea level
pressure differenGe between Iceland and the Azores for the consecutive months of
Decemberthrough March. Large negative differences have been shown to be related to
strong eastward wind stress (Fromentin and Planque,1996). We suspectthat a strong
negative index translates to strong downwelling off the Iberian Peninsula (Fig. 2a)
based on the mean pressure field for January (U.S. Navy, 1955; Blanton et al., 1987).
We are not yet able to say how the summer pressure field (Fig. 2b) which is balanced
by an upwelling-favorable wind regime off Iberia is related to the NAO for the previous
winter, butwe will offer come comments later.
ValuesofVe were averaged for a 17cyear period, 11 of which had data for which the
condition of mussels harvested in Ria de Arosa for that year could be determined. The
condition ofthe meat of the mussel is based on the. meat content (percent solid) of
mussels harvested from Riade Arosa. These data were made.available for several
sitesi~ HiadeArosaby the Spanish Department of Public Health, and year-to-year
changes in the meat content reflect the growing conditions of the mussels (Blanton et
al., 1987).
Review of Past Results·
There is a convincing correlation between the seasonal upwelling regime and nutrients
presentatthe mouth of the Rias (Fig. 3). Bi-weekly averages of Ve show a distinct
upwelling season during 1977 that began in April and lasted until September. Bi-weekly
3
averages in sea level at the mouth of the Rias show negative/positive values during
upwelling/downwelling. Subsurface temperature was relatively cold, and subsurface
nitrate was correspondingly high during upwelling. The subsurface baroclinic pressure
gradient setup by the upwelling of cold water offshoreeasily drives subsurface nitrate
into the Rias.
.
The correlation of mussel condition (MCI)to the upwelling index (UI) for agive~year
(Fig. 4a) shows considerable year-to-year variability. UI decreased from a high orO.51'·
m2 S·1 in 1977 to a minimum of -0.047 m2 S-1 in 1983. This trend suggests that inputs' of
inorganic nitrogen from the ocean decreased. In fact, the lowest UI and MCI of the data
set were for the years 1980-1983. Previous to 1980, UI and MCI were greater for this
11-year time period. The condition of the mussels for 1982 and 1983 were particularly
low (9%). MCI was almost double these values for the 1977 upwelling season.
The sameMCldata are plotted against NAO (Fig; 4b). Note that there isa general
downward trend in mussel condition as NAOincreases to higher values. The
relationship is not as. convincing as the plot against UI (Fig. 4a).
Discussion
Theclimatolog.ical centers of action during January consist of a high pressure cell'
located near the Azores and a deep low pressure cell off the southeast coast of
Greenland (Fig. 2a). The pressure gradient is associated with westerly winds toward the
Iberian Peninsula with a northward component. This induces downwelling favorable
.
wind stress during winter. TheNAO captures the pressure difference bf these two
centersofactibns, sothat high differences (high NAO index) are associated with
stronger than normal eastward streSs (Fromentin and Planque, 1996).
During· July, the pressure difference between these two centers of action' decreases'
(Fig. 2b). The July map indicates that high pressure over the sub-tropical North Atlantic:
drifts westward between January and July by about 20 degrees of longitude. This brings
the Iberian Peninsula into a upwelling favorable wind regime. It is the position of high'
pressure in the sub-tropical Atlantic·and the gradientto low pressure toward Greenland
that sets the stage for persistentupwelling favorable winds off the Iberian Peninsula.
While the dalafor 1973 -1983 are not convincing (Fig. 4b), there is a suggestion that 11
trend to a higher NAO index in winter is associated with weaker upwellingthefollbwing .
summer. We really do not knowhow the-NAO affects the strength of upwelling in the
subsequent summer.
Fromentin and Planque (1996) established that there was a clear upward trend in NAO
during the period 1963 - 1992 associated with increased eastward wind stress toward
Europe. There was considerable interannual variability about this trend: We suspect
that the.diminishedcbrrelation between .NAO and MCI (and NAO versus UI, not shown)
4
.
"
simply reflects that the index used here was not calculated for April through September.
It would be useful to know how the large-scale atmospheric pressure field from April
through September is linked to the NAO from the previous December through March.
Conclusions
The meat content of mussels harvested from Ria de Arosa have a positive correlation
(r2 = 0.66) with thE;! strength of upwelling for the yeaLof harvest. Weak/strong upwelling
results in low/high mussel biomass and poor/good marketability. During the 11 years
examined,lowest upwelling was observed in 1983 when the average UI was negative
(downwelling), and mussel biomass was low (9%). Highest upwelling was observed
during 1977 when mussel biomass wasthe highest observed (16%).
Variations in the strength and position of the Azores High and the Icelandic Low are
likely to have an important bearing on the strength and persistence of upwelling along
the Iberian Peninsula on a seasonaUimescale. We are unable to draw a convincing link
between the NAG calculatedforJhe winter and the strength of upwelling during the 6
months that follow. TheNAO is thought to playan important role in the interannual
variability of some marine resources (Fromentin and Planque, 1996).
The objective still remains to provide long range forecasts of synoptic scale weather
patterns that might.be used as a tool to predict upwelling strength and nutrient supply to
the Spanish Rias for the mussel growing season. The objective would be to optimize
the nutritional value of the mussels. The carrying capacity of the Rias for optimum
mussel production is determined by the number of rafts, the number of hanging ropes
on each raft, and the density of mussels per unit length of rope (Wiegert and PensaLado, 1982). New ropes containing small seed mussels are prepared by fisherman in
early spring before the onset of the upwelling season. If there were advanced
predictions that upwelling would be weaker than normal, the fisherman could deploy
fewer mussels per unit rope or fewer ropes. While the total tons of mussel per raft might
be lower, the resulting higher meat quality would make a more marketable product.
Acknowledgments
We wish to thank Dee Peterson, Anna Boyette and Suzanne Mcintosh at Skidaway
Institute of Oceanography for preparing the text and figures for this paper. We also
thank Alicia Lavin, Instituto Espanol de Oceanografia, Santander, Spain for calculating
the upwelling indices.
This work was sponsored by a cooperative agreement between the Government of
Spain and the United States, the Georgia Sea Grant Program (Grant No. RlFS-1) and
the State of Georgia. We also gratefully acknowledge support from a grant to the senior
5
author from the W.J. Fulbright Commission through the Comissao Cultural LusoAmericana.
References
Bakun, A. 1973. Coastal upwelling indices, west coast of North America, 1946-1971.
NOAA Tech. Rep. NMFS SSRF-671. U.S, Dept. of Commerce, 103 p.
,
Blanton,J.O., Atkinson, L.P., Fernandez de Castillejo, F. and Lavin Montero, A. 1984.
Coastal upwelling off the Rias Bajas, Gallicia, Northwest Spain, I: Hydrography studies.
Rapportset Proc-verbeaux Reun. Cons.lnt., Explor. Mer, 183: 79-90:
Blanton, J.O., Tenore, KR, Castillejo, F., Atkinsoh, L.P., Schwing, F.B. and Lavin, A.
1987. The relationship ofupwelliflg to mussel production in the rias on the western
coastof8pain. J. mar. Res., 45: 497-511.
Fraga,F.1981.Upwelling off the Galician coast, Northwest Spain. In Coastal
upwelling, pp. 176-182. Ed. By F.Richards, American Geophysical Union, Washington,
D.C., USA. .
.. .
..
Fraga, F. and Mourino, C .. 1978. Datos Informativos. Ins!. Pesq., No.6, 78 p.
Frol1lentiri,J.-M. and Planque, B. 1996.Ca/anu5 and environment in the eastern North.
Atlank II. Influcenceof the North Atlantic Oscillationbn C. finmarchicus and C.
helgolandiclls.Mat-ine Ecology Progress Series, 134: 111-118.
McClain,CR, Chao, Shenn~yu; Atkinson, L.P., Blanton, J.O.andCastillejo, F. 1986.
Wind-dreiven upwelling in the vicinity of Cape Finisterre, Spain. J. geophys. Res., 91 :.
8470-8486.
.
. .
Tenore, KR, Boyer, U=.,Cal, RM., Corral, J., Garcia-Fernandez, C., Gonzal~z, N:,
Gonzalez-Gurriaran, E., Hanson, R.B., Iglesias,J., Krom, E., Lopez-Jamar, E., McClain,
J., Pamatmat, M.M., Perez, A., Rhodes, D.C., deSantiago, G., Tietjen, J., Westrich, J.
and Windom, H.L. 1982. Coastal upwelling in the Rias Bajas, NW Spain: Contrasting
the benthic regimes of the Rias de Arosa and de Duras. J. mar. Res., 40: 701-772.
U.S. Navy. 1955. U.S. Navy MarineClimaticAtlas of the World, Vol. 1, North Alla'nllc
Ocean. NAVAIR50~1 C-528. U.S'. Government Printing Office; Washington, D.C. 275, ..
nul1lbefedleaves.
. . ..
6
Wiegert, R.G. and Pensa-Lado. 1982. Optimal exploitation by mussel rafts off the Ria
de Arosa, Spain: predictions of a first-generation model. Proceedings of a Workshop on
Marine Ecosystem Modeling, U.S. Department of Commerce, NOAA, National
Environmental Satellite Data and Information Service, p. 159-171.
seasonal upweliing.cycle.along
Wooster, W.S., Bakun, A. and McLain, D.R. 1976. The
. .
.
the eastern boundary of the North Atlantic. J. mar. Res. 34: 131-141.
7
Figure List
Fig. 1. Location of the Rias Bajas on the western Goast of Spain. The squareshoW$.
location of data presented in Figure 3.
th~
Fig. 2. Climatological sealevel pressure fields (20-year averages) for the North Atlantic,
Oceantaken from U.S. Navy (1955), (a) January;. (b) July.
Fig. 3. Correlation between Ve. sea level. temperature and nitrate at the mouth of Ria
de Vigo. Sea level is referenced to a 4-year average from 1977 - 1980. Temperature
and nitrate data are from Fraga and Mourino (1978).
Fig. 4. The relationship of mussel condition index (MCI) to (a) upwelling index (UI) and
(b) North Atlantic Oscillation (NAO) for the years from 1973 to 1983. NAO data were
downloaded from the web site 'http://ingrid.ldgo.columbia.edu/SOURCES. UI is
calculated as the difference of Ve for a given year from its 20-year average. See
Blanton et al. (1987) for more details.
8
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