- Email: [email protected]
Hydrographic behavior of the Galician Rias Baixas (NW Spain) under the spring intrusion of the Miño River
Journal of Marine Systems 60 (2006) 144 – 152 www.elsevier.com/locate/jmarsys
Hydrographic behavior of the Galician Rias Baixas (NW Spain) under the spring intrusion of the Miño River I. Alvarez a,⁎, M. deCastro a , M. Gomez-Gesteira a , R. Prego b b
a Grupo de Física de la Atmósfera y del Océano, Universidad de Vigo, Ourense, Spain Grupo de Biogeoquímica Marina, Instituto de Investigaciones Marinas, CSIC, Vigo, Spain
Received 29 April 2005; received in revised form 13 December 2005; accepted 19 December 2005 Available online 13 February 2006
Abstract The hydrographic behavior of the Galician Rias Baixas is studied under the influence of the Miño River outflow. This study is carried out in spring 1998 under a high Miño River discharge and favorable wind patterns to spread the river plume northward of river mouth, toward the Rias Baixas. This freshwater intrusion reverses the normal salinity gradient in the along axis direction in the rias of Vigo and Pontevedra, but not in their neighbor ria of Arousa. © 2005 Elsevier B.V. All rights reserved. Keywords: Hydrography; Freshwater intrusion; SST satellite images; Iberian Peninsula; Rias Baixas; Miño River
1. Introduction The discharge of freshwater from river outflows is particularly important in near coastal regions. The river plume evolution has been largely studied by means of observational and modeling studies. These studies showed that the plume is a moving target, changing direction, thickness, and width with every change in local wind strength or direction (Hickey et al., 1998). In the absence of strong ambient flows, the spreading of plumes is highly dependent on wind stress since their buoyancy usually confines them near the surface layer. The effect of wind direction on a river plume was numerically studied by means of three-dimensional models under upwelling and downwelling favorable
⁎ Corresponding author. E-mail address: [email protected] (I. Alvarez). 0924-7963/$ - see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.jmarsys.2005.12.005
winds (Xing and Davies, 1999; Fong and Geyer, 2001; Garcia-Berdeal et al., 2002). In general terms, at mid latitudes and near the western coasts of Northern Hemisphere, fresh river water usually spread northward and along shore due to the influence of prevailing southwest winds in winter and periods of high river runoff. When river discharge reduces and prevailing winds are from northwest, the northward spreading of plumes may be stopped. In that case, water may be driven offshore or southward. This path change usually occurs in spring. In summer, episodes of inclement weather (when the winds turn northward or have no a clear tendency) and moderate river runoff can result in a plume with a northward orientation. These summer reversals may be relatively common causing a surface salinity decrease northward of river mouth (Fiedler and Laurs, 1990; Lazure and Jegou, 1998). Thus, very fresh water can flood the major coastal estuaries located north of the river estuary for
I. Alvarez et al. / Journal of Marine Systems 60 (2006) 144–152
prolonged periods, reversing the normal estuarine density and salinity gradients. Because the freshwater intrusion can occur in these estuaries, its presence, or absence, can provide an important environmental distinction between estuaries as well as between nearshore coastal regions. The Miño River, situated 30 km south from the Ria of Vigo, is the most important river which flows into the Galician west coast (see Fig. 1). This river has a catchment area of 17,081 km2 and an annual average discharge of 430.8 m3 s− 1. The monthly average discharge oscillates between 100 m3 s− 1 in August and 1000 m3 s− 1 in February, following a pattern similar to the rainfall. Fig. 2 shows the monthly average discharge from 1980 to 1992 (Rio-Barja and RodríguezLestegás, 1996). The western coast is characterized by the presence of four coastal embayment locally named as Rias Baixas (Fig. 1) situated north of Miño River. During the last few years this area has been extensively studied taking into account on the one hand the hydrodynamical and hydrographical behavior of Galician Rias Baixas (Nogueira et al., 1997a,b; ÁlvarezSalgado et al., 2000; Pardo et al., 2001; Prego et al., 2001; deCastro et al., 2004) and on the other hand the
145
circulation at the adjacent shelf (Wooster et al., 1976; Tenore et al., 1984; McClain et al., 1986; Torres et al., 2003; Herrera et al., 2005) (see Alvarez et al., 2005 for a complete description of the different studies carried out in this area). Despite this amount of studies, the Miño River plume influence along the western Galician coast has not been studied in detail. Mouriño and Fraga (1982) analyzed variations in temperature, salinity, nitrate, nitrite and silicate at the southern mouth of the Ria of Vigo from October 1976 to December 1977. They found an important salinity decrease in the estuary mouth from December to March that they attributed to the Miño River freshwater, since the river runoff in the estuary was unable to generate the measured salinity values. The aim of this paper is to describe the northward spread of the Miño River plume following a high river discharge observed in spring 1998. The effect of this plume is analyzed at the Rias Baixas (Ria of Vigo, Ria of Pontevedra and Ria of Arousa) by means of two sampling stations located at the southern mouth and in the inner–middle part of each ria. Under favorable conditions (wind induced northward transport, high Miño River discharge and low river discharge inside the estuaries) the plume is observed to spread about 46 km
Fig. 1. Map of Galician Rias Baixas and sampling stations position (black dots). Triangles mark the meteorological stations at Peinador and Corrubedo.
146
I. Alvarez et al. / Journal of Marine Systems 60 (2006) 144–152
Fig. 2. Monthly average river discharge (m3 s− 1) from 1980 to 1992. To facilitate visual comparison the Oitaben, Lérez and Umia River values were multiplied by 10. The Ulla River values were multiplied by 5.
northward reaching the Ria of Vigo and the Ria of Pontevedra but not the Ria of Arousa. 2. Area under study The Rias Baixas, located in the NW of the Iberian Peninsula (Fig. 1), are incised valleys where the estuarine zone can move according to climatic changes (Evans and Prego, 2003). They are V shaped widening progressively from the innermost part of the estuary toward the mouth with approximately the same transverse geographical orientation in the NE–SW direction. These rias behave as partially mixed estuaries with positive residual circulation where the partial stratification is maintained by the river discharge in winter and by solar heating in summer. In addition, this positive circulation is enhanced by coastal upwelling in summer (Fraga, 1981) which introduces colder nutrientrich deeper water known as Eastern North Atlantic Central Water (ENACW) inside the estuaries (Wooster et al., 1976; Fiuza et al., 1998). In the Rias Baixas, upwelling events last from April to October (McClain et al., 1986; Lopez-Jamar et al., 1992; Tilstone et al., 1994). The tidal regime of the rias is mesotidal, with a mean amplitude between 2 and 4 m, and semidiurnal, the tide being the main forcing of the water circulation, but other factors also contribute as the freshwater discharge (Gomez-Gesteira et al., 2003), the wind regime inside the rias (deCastro et al., 2000), the air temperature (Gomez-Gesteira et al., 2001) and the upwelling events generated by winds on the shelf (Roson et al., 1997; Prego et al., 2001; Alvarez et al., 2003). In this study we consider three rias which are from south to north: the Ria of Vigo, the Ria of Pontevedra
and the Ria of Arousa. They are connected to the open sea by means of two entrances due to the existence of islands in the outermost part. In the innermost parts of these estuaries are located the main freshwater inputs which come from the rivers. The Oitaben River at the Ria of Vigo head, the Lerez River at the Ria of Pontevedra and the Ulla and Umia Rivers at the Ria of Arousa. Fig. 2 shows the monthly average discharges of these rivers from 1980 to 1992. To facilitate visual comparison the Oitaben, Lérez and Umia River values were multiplied by 10. The Ulla River values were multiplied by 5. 3. Materials and methods Thermohaline variables were obtained from two different data sets. In the first one, weekly values of salinity and temperature were measured at two sampling stations located at the southern mouth and in the inner– middle part of the Rias of Vigo, Pontevedra and Arousa during 1998 (Fig. 1). Note that the southern mouth is wider and deeper than the northern one being responsible for most of the water exchange between the estuary and the shelf as shown in Alvarez et al. (2005). In the Ria of Vigo, the southern station (35 m deep) was located at 42°10.7′N, 8°52.5′W and the inner one (18 m deep) at 42°16′N, 8°42′W. In the Ria of Pontevedra, the southern station (35 m deep) was located at 42°19.5′N, 8°53′W and the inner one (17 m deep) at 42°23′N, 8°44′ W. In the Ria of Arousa, the southern station (40 m deep) was located at 42°28.5′N, 8°58′W, and the inner one (33 m deep) at 42°34′N, 8°55′W. The second data set corresponds to the Ria of Pontevedra, which was sampled fortnightly at 19 stations from October 1997 to October 1998. The position and depth of those stations
I. Alvarez et al. / Journal of Marine Systems 60 (2006) 144–152
were described in previous papers by the same authors (deCastro et al., 2000; Gomez-Gesteira et al., 2001, 2003; Prego et al., 2001; Alvarez et al., 2003). In this project, the Lerez River discharge was registered daily at a river gauging station near the river mouth and beyond the limit of tidal influence. In addition, current profiles were measured in the inner–middle part of the estuary by means of an Electromagnetic Current Meter (Valeport Model 808) at six different depths during 5 min at each depth. The same protocol was repeated every 30 min during a tidal cycle. In both data sets salinity and temperature profiles were measured by means of a CTD (Seabird19 and 25). Salinity calibration was previously performed by means of an ‘Autosal’ salinometer. The wind speed, supplied by the ‘Instituto Nacional de Meteorología’, was measured four times at day in two meteorological stations situated at “Peinador” (42°13′N, 8°38′W) and “Corrubedo” (42°35′N, 9°6′ W). Daily Miño River discharge was supplied by the ‘Confederación Hidrográfica del Norte’. 4. Results The salinity pattern in the Rias Baixas is characterized by the presence of saltier water at the outer part of the estuary and fresher water at the middle–inner part in good agreement with the location of rivers at the innermost parts of the estuaries. In particular, in the Ria of Pontevedra, which is a good example of a Ria Baixa, the salinity difference (ΔS = SSouth − SInner) averaged from October 1997 to October 1998 (Fig. 3) shows
Fig. 3. Salinity vertical profile calculated by subtracting internal values from the southern mouth ones in the Ria of Pontevedra. Data were averaged from October 1997 to October 1998.
147
the mouth saltier than the internal part of the estuary with a maximum ΔS = 1.5 psu near surface and close to zero near bed. In contrast with this normal behavior, an abnormal salinity gradient was measured in the along axis direction of the Rias of Vigo and Pontevedra on May 11, 1998 (Fig. 4(a, b)). Southern mouths fresher than the internal part of the estuaries were measured on this date for whole water column. In the Ria of Vigo, a maximum salinity difference (ΔS = SSouth − SInner) was measured near surface (3 m) with a value close to − 3 psu and being at a depth of 15 m close to − 1 psu. A similar behavior was observed in the Ria of Pontevedra with a maximum ΔS = −0.5 psu near surface (3 m) and being close to zero at a depth of 15 m. In this case the abnormal salinity gradient in the along axis direction was not so strong as in the Ria of Vigo. On the contrary, the Ria of Arousa (Fig. 4(c)) shows a completely different salinity gradient with a southern mouth saltier than the inner part of the estuary corresponding to the normal salinity gradient measured in the Ria of Pontevedra (Fig. 3). In this case the maximum ΔS is close to 1 psu near surface (3 m) and close to zero at a depth of 15 m. In order to analyze the origin of the abnormal salinity gradient measured in the Rias of Vigo and Pontevedra in spring, the Lerez (at the Ria of Pontevedra head) and Miño River (30 km south from the Ria of Vigo) discharges were represented in Fig. 5 from January to June 1998. In this figure, an abnormally high Miño River discharge was observed at the beginning of May 1998. The maximum value was observed around May 3, 1998, reaching 1300 m3 s− 1. This value was four times higher than the historical discharge in May as shown in Fig. 2. Actually, it was even higher than the historical discharge in February, the month with the highest runoff. After this date, there was a continuous diminution in river discharge, although the minimum values were always over the historical values corresponding to April–May. The Lerez River discharge followed the rainfall pattern with local maxima in January (80 m3 s− 1) and in April (50 m3 s− 1). After these dates a continuous decrease of the river discharge was observed. The SST satellite image averaged from May 3 to May 9 (Fig. 6(a)) shows cold water at the Miño River mouth. This water, which it is characterized by a temperature between 12 and 13 °C, spreads northward and southward from the river mouth. In contrast, no traces of cold water were observed in the same zone from May 10 to May 16 (Fig. 6(b)), where SST near coast was around 15 °C. Note that the cold water observed from
148
I. Alvarez et al. / Journal of Marine Systems 60 (2006) 144–152
Fig. 4. Salinity vertical profiles at the southern mouth (gray line) and in the inner–middle part (black line) of the estuaries on May 11, 1998: (a) Ria of Vigo, (b) Ria of Pontevedra and (c) Ria of Arousa.
May 3 to May 9 does not correspond to any coastal upwelling event, since the low temperature is localized near the river mouth. In addition, wind conditions measured at the meteorological stations located at Peinador and Corrubedo (see Fig. 1) from May 3 to 15 (Fig. 7) show the existence of surface northward transport along shore from the date of maximum river discharge (May 3) to May 15. This transport tends to displace the Miño River freshwater northward reaching the Rias Baixas. The existence of an important external freshwater supply in an estuary can generate a reverse of the normal salinity and density gradients and consequently a reverse of the normal estuarine circulation. This situation was observed in the inner–middle part of the Ria of Pontevedra on May 13, 1998 (Fig. 8), where a negative estuarine circulation was measured in the along axis direction during a tidal cycle (deCastro et al., 2004).
This negative circulation was characterized by near surface water moving landward and near bed water moving seaward. The mean velocity in the upper layer, 6.1 cm s− 1, was compensated by a negative flow at the lower layer with mean velocity 5.8 cm s− 1. Maximum velocities attained 12 cm s− 1 near surface during flood tide and − 11 cm s− 1 near bed during ebb tide. The zerovelocity line marked with a dark line was observed to sink during flood and to rise during ebb. 5. Discussion It is a well known fact that in winter the river plume usually spreads northward and along shore due to the influence of prevailing southern winds in western coasts of Northern Hemisphere at mid latitudes (Fennel and Mutzke, 1997; Hickey et al., 1998; Garcia-Berdeal et al., 2002; Hickey and Banas, 2003). In spring, even if the
Fig. 5. Daily Miño River (black line) and Lerez River (gray line) discharges (m3 s− 1) from January to June 1998. The maximum Miño River discharge (1300 m3 s− 1) was measured on May 3, 1998.
I. Alvarez et al. / Journal of Marine Systems 60 (2006) 144–152
149
Fig. 6. SST satellite image of the NW Iberian Peninsula. (a) Colder surface water can be observed at the Miño River mouth from May 3 to May 9, 1998 with temperature values ranging from 12 to 13 °C. (b) No traces of colder water were observed at the Miño River mouth from May 10 to May 16, 1998.
river runoff does not suffer a strong decrease and wind keeps a southern component, river outflows still spread northward along the coast (Lazure and Jegou, 1998). A similar behavior was observed for the Miño River which shows an extremely high discharge on May 3 (Fig. 5) and then, under favorable wind induced northward transport (Fig. 7), the river freshwater spreads northward along the west Galician coast (Fig. 6(a)) and
intrudes into the neighbor Rias Baixas. The effect of this freshwater intrusion in these estuaries can be identified by the presence of an abnormal horizontal salinity gradient in the along axis direction. Thus, water at the southern mouths of the Rias of Vigo and Pontevedra is fresher than at the middle–inner stations (Fig. 4(a, b)). In contrast, the neighbor Ria of Arousa shows a normal salinity gradient in the along axis direction (Fig. 4(c)). In
150
I. Alvarez et al. / Journal of Marine Systems 60 (2006) 144–152
Fig. 7. Surface water transport calculated from wind data at the meteorological stations of Peinador and Corrubedo (see Fig. 1) from May 3 to 15, 1998. The light arrows represent the scale. Transport was calculated from wind data following Qx = (ρaCd / ρw f )(Wx2 + Wy2)1/2Wy and Qy = (ρaCd / ρw f ) (Wx2 + Wy2)1/2Wx.
this case, the major freshwater input comes from the river discharge inside the estuary. Both the disappearance of the abnormal salinity distribution in the Ria of Arousa and the decrease of the along-shore salinity (the surface salinity at the southern mouth of the Ria of Vigo is 30.1 psu, being 34.6 psu in Pontevedra and 35.3 psu in Arousa) are in good agreement with the conclusion that the freshwater comes from the Miño River. The increase in salinity from south to north shows that the observed low values are not generated by the rivers placed inside
the rias, since the highest river runoff (Fig. 2) corresponds to the Ria of Arousa (the northernmost). Therefore, the Miño River plume spreads about 46 km northward under favorable conditions (wind induced northward transport, high Miño River discharge and low river discharge inside the estuaries) reaching the Ria of Vigo and the Ria of Pontevedra but not the Ria of Arousa. Following an analysis similar to the one performed by Mouriño and Fraga (1982) to justify the appearance of
Fig. 8. Along estuary negative estuarine circulation (m s− 1) of the Ria of Pontevedra measured during a tidal cycle on May 13, 1998 in a middle–inner station. Maximum velocities about 0.12 m s− 1 were measured near surface during flood tide.
I. Alvarez et al. / Journal of Marine Systems 60 (2006) 144–152
abnormally low salinities in the Ria of Vigo, one can assume that if all freshwater discharges remain in the estuary and there is not an oceanic water input to compensate the loss of salinity, then the salinity diminution can be estimated using dS / dt = − SQ / V, where S is the surface salinity at the southern mouth of the estuary, V the volume of water in a surface layer from 0 to 10 m and Q the freshwater input. Using data from the Ria of Pontevedra and considering the initial situation the one measured during the previous survey, May 4, the estimated salinity diminution would be 0.9 psu, which cannot account from the 1.2 psu decrease measured from May 4 to May 11 in the ria. This is even more patent in the Ria of Vigo, where the observed decrease is about 5 psu and the freshwater input is on the same order of magnitude than the one observed in Pontevedra (see Fig. 2 for river discharge). Freshwater intrusion was not observed in the next survey of the three rias carried out on May 18, 1998. Nevertheless, the persistence of the intrusion can be analyzed in the particular case of the Ria of Pontevedra, were a new survey was carried out on May 13, 1998. On this date, the along axis gradient remained practically unchanged, around − 0.7 psu, although the effect of the Lerez River in the inner station was less clear, in good agreement with the decrease in river runoff described in deCastro et al. (2004). This persistence of the phenomenon for at least 2 days gives rise to a negative estuarine circulation along the estuary (Fig. 8). This circulation pattern was corroborated by a hydrodynamic model in the Ria of Pontevedra (deCastro et al., 2004). As we mentioned above, the main forcing mechanisms giving rise to freshwater intrusion are the existence of high Miño River discharges and favorable winds. In addition, there are other factors that can strongly influence the along axis gradients inside the rias as, for example, the river discharge inside the estuary, which follows a pattern that can be significantly different from the rainfall one since all rivers, except the Lerez, are controlled by dams. Other important factors are the water masses at the adjacent shelf, which follow a marked annual cycle and the vertical mixing generated by wind, which can dilute the plume. Additional surveys should be conducted to identify and describe the evolution of high freshwater discharges from the Miño River and their intrusion into the adjacent estuaries. The appearance of these intrusion patterns can be predicted in terms of the river discharge (controlled by dams) and the prevailing meteorological conditions, which can be obtained from weather forecast agencies. A 3D hydrodynamical model could also contribute to better analyze this kind of phenomenon, although the
151
lack of reliable wind data on the shelf does not allow modeling the present case in an accurate way. Finally, the existence of fresher surface water supplied from the Miño River can result in two opposed biological behaviors. On the one hand, it can generate the inverse estuarine circulation observed in the Ria of Pontevedra, which tends to stop water exchange between the ria and the shelf, increasing the residence time and hence decreasing water quality (deCastro et al., 2004). On the other hand, the high freshwater discharge can induce a phytoplankton bloom at the shelf, which penetrates into the estuary embedded in a fresher water mass, fertilizing the area (deCastro et al., in press). Acknowledgements This work was partially supported by CICYT under projects MAR96-1782 and REN2003-04106-C03 and by ‘Xunta de Galicia’ under project PGIDIT02PXJA38301PR. We acknowledge ‘Confederacion Hidrografica del Norte’ for providing Miño River data. References Álvarez-Salgado, X.A., Gago, J., Míguez, B.M., Gilcoto, M., Pérez, F. F., 2000. Surface waters of the NW Iberian margin: upwelling on the shelf versus outwelling of upwelled waters from the Rías Baixas. Estuarine, Coastal and Shelf Science 51, 821–837. Alvarez, I., deCastro, M., Prego, R., Gomez-Gesteira, M., 2003. Hydrographic characterization of a winter-upwelling event in the Ria of Pontevedra (NW Spain). Estuarine, Coastal and Shelf Science 56, 869–876. Alvarez, I., deCastro, M., Gomez-Gesteira, M., Prego, R., 2005. Interand intra-annual analysis of the salinity and temperature evolution in the Galician Rias Baixas–ocean boundary (NW Spain). Journal of Geophysical Research 110, C04008. doi:10.1029/2004JC002504. deCastro, M., Gomez-Gesteira, M., Prego, R., Taboada, J.J., Montero, P., Herbello, P., Perez-Villar, V., 2000. Wind and tidal influence on water circulation in a Galician Ria (NW Spain). Estuarine, Coastal and Shelf Science 51, 161–176. deCastro, M., Gomez-Gesteira, M., Alvarez, I., Prego, R., 2004. Negative estuarine circulation in the Ria of Pontevedra (NW Spain). Estuarine, Coastal and Shelf Science 60, 301–312. deCastro, M., Alvarez, I., Varela, M., Prego, R., Gomez-Gesteira, M., in press. Effect of an extreme water release from Miño River dams on the neighbor Galician Rias Baixas (NW Iberian Peninsula). Estuarine, Coastal and Shelf Science. Evans, G., Prego, R., 2003. Rias, estuaries and incised valleys: is a ria an estuary? Marine Geology 196, 171–175. Fennel, W., Mutzke, A., 1997. The initial evolution of a buoyant plume. Journal of Marine Systems 12, 53–68. Fiedler, P.C., Laurs, R.M., 1990. Variability of the Columbia River plume observed in visible and infrared satellite imagery. International Journal of Remote Sensing 11, 999–1010. Fiuza, A.F.G., Hamann, M., Ambar, I., Díaz del Río, G., González, N., Cabanas, J., 1998. Water masses and their circulation off western Iberia during May 1993. Deep Sea Research, I 45, 1127–1160.
152
I. Alvarez et al. / Journal of Marine Systems 60 (2006) 144–152
Fong, D.A., Geyer, W.R., 2001. Response of a river plume during an upwelling favorable wind event. Journal of Geophysical Research 106, 1067–1084. Fraga, F., 1981. Upwelling off the Galician Coast, Northwest Spain. In: Richardson, F.A. (Ed.), Coastal Upwelling. American Geophysical Union, Washington, pp. 176–182. Garcia-Berdeal, I., Hickey, B.M., Kawase, M., 2002. Influence of wind stress and ambient flows on a high discharge river plume. Journal of Geophysical Research 107, 3130. Gomez-Gesteira, M., deCastro, M., Prego, R., Perez-Villar, V., 2001. An unusual two layered tidal circulation induced by stratification and wind in the Ria of Pontevedra (NW Spain). Estuarine, Coastal and Shelf Science 52, 555–563. Gomez-Gesteira, M., deCastro, M., Prego, R., 2003. Dependence of the water residence time in Ria of Pontevedra (NW Spain) on the seawater inflow and the river discharge. Estuarine, Coastal and Shelf Science 58, 567–573. Herrera, J.L., Piedracoba, S., Varela, R.A., Roson, G., 2005. Spatial analysis of the wind field don the western coast of Galicia (NW Spain) for in situ measurements. Continental Shelf Research 25, 1728–1748. Hickey, B.M., Banas, N.S., 2003. Oceanography of the U.S. Pacific Northwest Ocean and Estuaries with Application to Coastal Ecology. Estuaries 26 (4B), 1010–1031. Hickey, B.M., Pietrafesa, L.J., Jay, D.A., Boicourt, W.C., 1998. The Columbia River plume study: Subtidal variability in the velocity and salinity fields. Journal of Geophysical Research 103, 10,339–10,368. Lazure, P., Jegou, A.M., 1998. 3D modelling of seasonal evolution of Loire and gironde plumes on Biscay Bay continental shelf. Oceanologica Acta 21 (2), 165–177. Lopez-Jamar, E., Cal, R.M., Gonzalez, G., Hanson, R.B., Rey, J., Santiago, G., Tenore, K.R., 1992. Upwelling and outwelling effects on the benthic regime of the continental shelf off Galicia, NW Spain. Journal of Marine Research 50, 465–488. McClain, C.R., Chao, S., Atkinson, L.P., Blanton, J.O., Castillejo, F., 1986. Wind driven upwelling in the vicinity of Cape Finisterre, Spain. Journal of Geophysial Research 91, 8470–8486. Mouriño, C., Fraga, F., 1982. Hidrografia de la Ria de Vigo. 1976– 1977. Influencia Anormal del rio Miño. Investigaciones Pesqueras 46 (3), 459–468.
Nogueira, E., Pérez, F.F., Rios, A.F., 1997a. Seasonal patterns and long-term trends in an estuarine upwelling ecosystem (ria of Vigo, Spain). Estuarine Coastal and Shelf Science 44, 285–300. Nogueira, E., Pérez, F.F., Ríos, A.F., 1997b. Modeling thermohaline properties in an estuarine upwelling ecosystem (Ría de Vigo: NW Spain) using box-Jenkins transfer function models. Estuarine Coastal and Shelf Science 44, 685–702. Pardo, P.C., Gilcoto, M., Pérez, F.F., 2001. Short-time scale coupling between thermohaline and meteorological forcing in the Ria de Pontevedra. Scientia Marina 65, 229–240. Prego, R., Dale, A., deCastro, M., Gomez-Gesteira, M., Taboada, J.J., Montero, P.P., Perez-Villar, V., 2001. Micro-scale hydrography of the Pontevedra Ría (NW Spain). Journal of Geophysical Research 106 (9), 19845–19857. Rio-Barja, F.X., Rodríguez-Lestegás, F., 1996. Os ríos, in As augas de Galicia. Consello de Cultura, pp. 178–180. Roson, G., Alvarez-Salgado, X.A., Perez, F.F., 1997. A non-stationary box model to determine fluxes in a partially mixed estuary, based on both thermohaline properties: application to the Ria de Arousa (NW Spain). Estuarine, Coastal and Shelf Science 44, 249–262. Tenore, K.R., Cal, R.M., Hanson, R.B., López-Jamar, E., Santiago, G., Tietjen, J.M., 1984. Coastal upwelling off the Rias Bajas, Galicia, Northwest Spain: II. Benthic studies. Rapports et proces verbaux des reunions - Commission internationale pour l'exploration scientifique de la mer Mediterranee 183, 91–100. Tilstone, G.H., Figueras, F.G., Fraga, F., 1994. Upwelling–downwelling sequences in the generation of red tides in a coastal upwelling system. Marine Ecology Progress Series 112, 241–253. Torres, R., Barton, E.D., Miller, P., Fanjul, E., 2003. Spatial patterns of wind and sea surface temperature in the Galician upwelling region. Journal of Geophysical Research 108 (C4), 3130. Wooster, W.S., Bakun, A., McClain, D.R., 1976. The seasonal upwelling cycle along the eastern boundary of the north Atlantic. Journal of Marine Research 34, 131–141. Xing, J., Davies, A.M., 1999. The effect of wind direction and mixing upon the spreading of a buoyant plume in a non-tidal regime. Continental Shelf Research 19, 1437–1483.
Hydrographic behavior of the Galician Rias Baixas (NW Spain) under the spring intrusion of the Miño River I. Alvarez a,⁎, M. deCastro a , M. Gomez-Gesteira a , R. Prego b b
a Grupo de Física de la Atmósfera y del Océano, Universidad de Vigo, Ourense, Spain Grupo de Biogeoquímica Marina, Instituto de Investigaciones Marinas, CSIC, Vigo, Spain
Received 29 April 2005; received in revised form 13 December 2005; accepted 19 December 2005 Available online 13 February 2006
Abstract The hydrographic behavior of the Galician Rias Baixas is studied under the influence of the Miño River outflow. This study is carried out in spring 1998 under a high Miño River discharge and favorable wind patterns to spread the river plume northward of river mouth, toward the Rias Baixas. This freshwater intrusion reverses the normal salinity gradient in the along axis direction in the rias of Vigo and Pontevedra, but not in their neighbor ria of Arousa. © 2005 Elsevier B.V. All rights reserved. Keywords: Hydrography; Freshwater intrusion; SST satellite images; Iberian Peninsula; Rias Baixas; Miño River
1. Introduction The discharge of freshwater from river outflows is particularly important in near coastal regions. The river plume evolution has been largely studied by means of observational and modeling studies. These studies showed that the plume is a moving target, changing direction, thickness, and width with every change in local wind strength or direction (Hickey et al., 1998). In the absence of strong ambient flows, the spreading of plumes is highly dependent on wind stress since their buoyancy usually confines them near the surface layer. The effect of wind direction on a river plume was numerically studied by means of three-dimensional models under upwelling and downwelling favorable
⁎ Corresponding author. E-mail address: [email protected] (I. Alvarez). 0924-7963/$ - see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.jmarsys.2005.12.005
winds (Xing and Davies, 1999; Fong and Geyer, 2001; Garcia-Berdeal et al., 2002). In general terms, at mid latitudes and near the western coasts of Northern Hemisphere, fresh river water usually spread northward and along shore due to the influence of prevailing southwest winds in winter and periods of high river runoff. When river discharge reduces and prevailing winds are from northwest, the northward spreading of plumes may be stopped. In that case, water may be driven offshore or southward. This path change usually occurs in spring. In summer, episodes of inclement weather (when the winds turn northward or have no a clear tendency) and moderate river runoff can result in a plume with a northward orientation. These summer reversals may be relatively common causing a surface salinity decrease northward of river mouth (Fiedler and Laurs, 1990; Lazure and Jegou, 1998). Thus, very fresh water can flood the major coastal estuaries located north of the river estuary for
I. Alvarez et al. / Journal of Marine Systems 60 (2006) 144–152
prolonged periods, reversing the normal estuarine density and salinity gradients. Because the freshwater intrusion can occur in these estuaries, its presence, or absence, can provide an important environmental distinction between estuaries as well as between nearshore coastal regions. The Miño River, situated 30 km south from the Ria of Vigo, is the most important river which flows into the Galician west coast (see Fig. 1). This river has a catchment area of 17,081 km2 and an annual average discharge of 430.8 m3 s− 1. The monthly average discharge oscillates between 100 m3 s− 1 in August and 1000 m3 s− 1 in February, following a pattern similar to the rainfall. Fig. 2 shows the monthly average discharge from 1980 to 1992 (Rio-Barja and RodríguezLestegás, 1996). The western coast is characterized by the presence of four coastal embayment locally named as Rias Baixas (Fig. 1) situated north of Miño River. During the last few years this area has been extensively studied taking into account on the one hand the hydrodynamical and hydrographical behavior of Galician Rias Baixas (Nogueira et al., 1997a,b; ÁlvarezSalgado et al., 2000; Pardo et al., 2001; Prego et al., 2001; deCastro et al., 2004) and on the other hand the
145
circulation at the adjacent shelf (Wooster et al., 1976; Tenore et al., 1984; McClain et al., 1986; Torres et al., 2003; Herrera et al., 2005) (see Alvarez et al., 2005 for a complete description of the different studies carried out in this area). Despite this amount of studies, the Miño River plume influence along the western Galician coast has not been studied in detail. Mouriño and Fraga (1982) analyzed variations in temperature, salinity, nitrate, nitrite and silicate at the southern mouth of the Ria of Vigo from October 1976 to December 1977. They found an important salinity decrease in the estuary mouth from December to March that they attributed to the Miño River freshwater, since the river runoff in the estuary was unable to generate the measured salinity values. The aim of this paper is to describe the northward spread of the Miño River plume following a high river discharge observed in spring 1998. The effect of this plume is analyzed at the Rias Baixas (Ria of Vigo, Ria of Pontevedra and Ria of Arousa) by means of two sampling stations located at the southern mouth and in the inner–middle part of each ria. Under favorable conditions (wind induced northward transport, high Miño River discharge and low river discharge inside the estuaries) the plume is observed to spread about 46 km
Fig. 1. Map of Galician Rias Baixas and sampling stations position (black dots). Triangles mark the meteorological stations at Peinador and Corrubedo.
146
I. Alvarez et al. / Journal of Marine Systems 60 (2006) 144–152
Fig. 2. Monthly average river discharge (m3 s− 1) from 1980 to 1992. To facilitate visual comparison the Oitaben, Lérez and Umia River values were multiplied by 10. The Ulla River values were multiplied by 5.
northward reaching the Ria of Vigo and the Ria of Pontevedra but not the Ria of Arousa. 2. Area under study The Rias Baixas, located in the NW of the Iberian Peninsula (Fig. 1), are incised valleys where the estuarine zone can move according to climatic changes (Evans and Prego, 2003). They are V shaped widening progressively from the innermost part of the estuary toward the mouth with approximately the same transverse geographical orientation in the NE–SW direction. These rias behave as partially mixed estuaries with positive residual circulation where the partial stratification is maintained by the river discharge in winter and by solar heating in summer. In addition, this positive circulation is enhanced by coastal upwelling in summer (Fraga, 1981) which introduces colder nutrientrich deeper water known as Eastern North Atlantic Central Water (ENACW) inside the estuaries (Wooster et al., 1976; Fiuza et al., 1998). In the Rias Baixas, upwelling events last from April to October (McClain et al., 1986; Lopez-Jamar et al., 1992; Tilstone et al., 1994). The tidal regime of the rias is mesotidal, with a mean amplitude between 2 and 4 m, and semidiurnal, the tide being the main forcing of the water circulation, but other factors also contribute as the freshwater discharge (Gomez-Gesteira et al., 2003), the wind regime inside the rias (deCastro et al., 2000), the air temperature (Gomez-Gesteira et al., 2001) and the upwelling events generated by winds on the shelf (Roson et al., 1997; Prego et al., 2001; Alvarez et al., 2003). In this study we consider three rias which are from south to north: the Ria of Vigo, the Ria of Pontevedra
and the Ria of Arousa. They are connected to the open sea by means of two entrances due to the existence of islands in the outermost part. In the innermost parts of these estuaries are located the main freshwater inputs which come from the rivers. The Oitaben River at the Ria of Vigo head, the Lerez River at the Ria of Pontevedra and the Ulla and Umia Rivers at the Ria of Arousa. Fig. 2 shows the monthly average discharges of these rivers from 1980 to 1992. To facilitate visual comparison the Oitaben, Lérez and Umia River values were multiplied by 10. The Ulla River values were multiplied by 5. 3. Materials and methods Thermohaline variables were obtained from two different data sets. In the first one, weekly values of salinity and temperature were measured at two sampling stations located at the southern mouth and in the inner– middle part of the Rias of Vigo, Pontevedra and Arousa during 1998 (Fig. 1). Note that the southern mouth is wider and deeper than the northern one being responsible for most of the water exchange between the estuary and the shelf as shown in Alvarez et al. (2005). In the Ria of Vigo, the southern station (35 m deep) was located at 42°10.7′N, 8°52.5′W and the inner one (18 m deep) at 42°16′N, 8°42′W. In the Ria of Pontevedra, the southern station (35 m deep) was located at 42°19.5′N, 8°53′W and the inner one (17 m deep) at 42°23′N, 8°44′ W. In the Ria of Arousa, the southern station (40 m deep) was located at 42°28.5′N, 8°58′W, and the inner one (33 m deep) at 42°34′N, 8°55′W. The second data set corresponds to the Ria of Pontevedra, which was sampled fortnightly at 19 stations from October 1997 to October 1998. The position and depth of those stations
I. Alvarez et al. / Journal of Marine Systems 60 (2006) 144–152
were described in previous papers by the same authors (deCastro et al., 2000; Gomez-Gesteira et al., 2001, 2003; Prego et al., 2001; Alvarez et al., 2003). In this project, the Lerez River discharge was registered daily at a river gauging station near the river mouth and beyond the limit of tidal influence. In addition, current profiles were measured in the inner–middle part of the estuary by means of an Electromagnetic Current Meter (Valeport Model 808) at six different depths during 5 min at each depth. The same protocol was repeated every 30 min during a tidal cycle. In both data sets salinity and temperature profiles were measured by means of a CTD (Seabird19 and 25). Salinity calibration was previously performed by means of an ‘Autosal’ salinometer. The wind speed, supplied by the ‘Instituto Nacional de Meteorología’, was measured four times at day in two meteorological stations situated at “Peinador” (42°13′N, 8°38′W) and “Corrubedo” (42°35′N, 9°6′ W). Daily Miño River discharge was supplied by the ‘Confederación Hidrográfica del Norte’. 4. Results The salinity pattern in the Rias Baixas is characterized by the presence of saltier water at the outer part of the estuary and fresher water at the middle–inner part in good agreement with the location of rivers at the innermost parts of the estuaries. In particular, in the Ria of Pontevedra, which is a good example of a Ria Baixa, the salinity difference (ΔS = SSouth − SInner) averaged from October 1997 to October 1998 (Fig. 3) shows
Fig. 3. Salinity vertical profile calculated by subtracting internal values from the southern mouth ones in the Ria of Pontevedra. Data were averaged from October 1997 to October 1998.
147
the mouth saltier than the internal part of the estuary with a maximum ΔS = 1.5 psu near surface and close to zero near bed. In contrast with this normal behavior, an abnormal salinity gradient was measured in the along axis direction of the Rias of Vigo and Pontevedra on May 11, 1998 (Fig. 4(a, b)). Southern mouths fresher than the internal part of the estuaries were measured on this date for whole water column. In the Ria of Vigo, a maximum salinity difference (ΔS = SSouth − SInner) was measured near surface (3 m) with a value close to − 3 psu and being at a depth of 15 m close to − 1 psu. A similar behavior was observed in the Ria of Pontevedra with a maximum ΔS = −0.5 psu near surface (3 m) and being close to zero at a depth of 15 m. In this case the abnormal salinity gradient in the along axis direction was not so strong as in the Ria of Vigo. On the contrary, the Ria of Arousa (Fig. 4(c)) shows a completely different salinity gradient with a southern mouth saltier than the inner part of the estuary corresponding to the normal salinity gradient measured in the Ria of Pontevedra (Fig. 3). In this case the maximum ΔS is close to 1 psu near surface (3 m) and close to zero at a depth of 15 m. In order to analyze the origin of the abnormal salinity gradient measured in the Rias of Vigo and Pontevedra in spring, the Lerez (at the Ria of Pontevedra head) and Miño River (30 km south from the Ria of Vigo) discharges were represented in Fig. 5 from January to June 1998. In this figure, an abnormally high Miño River discharge was observed at the beginning of May 1998. The maximum value was observed around May 3, 1998, reaching 1300 m3 s− 1. This value was four times higher than the historical discharge in May as shown in Fig. 2. Actually, it was even higher than the historical discharge in February, the month with the highest runoff. After this date, there was a continuous diminution in river discharge, although the minimum values were always over the historical values corresponding to April–May. The Lerez River discharge followed the rainfall pattern with local maxima in January (80 m3 s− 1) and in April (50 m3 s− 1). After these dates a continuous decrease of the river discharge was observed. The SST satellite image averaged from May 3 to May 9 (Fig. 6(a)) shows cold water at the Miño River mouth. This water, which it is characterized by a temperature between 12 and 13 °C, spreads northward and southward from the river mouth. In contrast, no traces of cold water were observed in the same zone from May 10 to May 16 (Fig. 6(b)), where SST near coast was around 15 °C. Note that the cold water observed from
148
I. Alvarez et al. / Journal of Marine Systems 60 (2006) 144–152
Fig. 4. Salinity vertical profiles at the southern mouth (gray line) and in the inner–middle part (black line) of the estuaries on May 11, 1998: (a) Ria of Vigo, (b) Ria of Pontevedra and (c) Ria of Arousa.
May 3 to May 9 does not correspond to any coastal upwelling event, since the low temperature is localized near the river mouth. In addition, wind conditions measured at the meteorological stations located at Peinador and Corrubedo (see Fig. 1) from May 3 to 15 (Fig. 7) show the existence of surface northward transport along shore from the date of maximum river discharge (May 3) to May 15. This transport tends to displace the Miño River freshwater northward reaching the Rias Baixas. The existence of an important external freshwater supply in an estuary can generate a reverse of the normal salinity and density gradients and consequently a reverse of the normal estuarine circulation. This situation was observed in the inner–middle part of the Ria of Pontevedra on May 13, 1998 (Fig. 8), where a negative estuarine circulation was measured in the along axis direction during a tidal cycle (deCastro et al., 2004).
This negative circulation was characterized by near surface water moving landward and near bed water moving seaward. The mean velocity in the upper layer, 6.1 cm s− 1, was compensated by a negative flow at the lower layer with mean velocity 5.8 cm s− 1. Maximum velocities attained 12 cm s− 1 near surface during flood tide and − 11 cm s− 1 near bed during ebb tide. The zerovelocity line marked with a dark line was observed to sink during flood and to rise during ebb. 5. Discussion It is a well known fact that in winter the river plume usually spreads northward and along shore due to the influence of prevailing southern winds in western coasts of Northern Hemisphere at mid latitudes (Fennel and Mutzke, 1997; Hickey et al., 1998; Garcia-Berdeal et al., 2002; Hickey and Banas, 2003). In spring, even if the
Fig. 5. Daily Miño River (black line) and Lerez River (gray line) discharges (m3 s− 1) from January to June 1998. The maximum Miño River discharge (1300 m3 s− 1) was measured on May 3, 1998.
I. Alvarez et al. / Journal of Marine Systems 60 (2006) 144–152
149
Fig. 6. SST satellite image of the NW Iberian Peninsula. (a) Colder surface water can be observed at the Miño River mouth from May 3 to May 9, 1998 with temperature values ranging from 12 to 13 °C. (b) No traces of colder water were observed at the Miño River mouth from May 10 to May 16, 1998.
river runoff does not suffer a strong decrease and wind keeps a southern component, river outflows still spread northward along the coast (Lazure and Jegou, 1998). A similar behavior was observed for the Miño River which shows an extremely high discharge on May 3 (Fig. 5) and then, under favorable wind induced northward transport (Fig. 7), the river freshwater spreads northward along the west Galician coast (Fig. 6(a)) and
intrudes into the neighbor Rias Baixas. The effect of this freshwater intrusion in these estuaries can be identified by the presence of an abnormal horizontal salinity gradient in the along axis direction. Thus, water at the southern mouths of the Rias of Vigo and Pontevedra is fresher than at the middle–inner stations (Fig. 4(a, b)). In contrast, the neighbor Ria of Arousa shows a normal salinity gradient in the along axis direction (Fig. 4(c)). In
150
I. Alvarez et al. / Journal of Marine Systems 60 (2006) 144–152
Fig. 7. Surface water transport calculated from wind data at the meteorological stations of Peinador and Corrubedo (see Fig. 1) from May 3 to 15, 1998. The light arrows represent the scale. Transport was calculated from wind data following Qx = (ρaCd / ρw f )(Wx2 + Wy2)1/2Wy and Qy = (ρaCd / ρw f ) (Wx2 + Wy2)1/2Wx.
this case, the major freshwater input comes from the river discharge inside the estuary. Both the disappearance of the abnormal salinity distribution in the Ria of Arousa and the decrease of the along-shore salinity (the surface salinity at the southern mouth of the Ria of Vigo is 30.1 psu, being 34.6 psu in Pontevedra and 35.3 psu in Arousa) are in good agreement with the conclusion that the freshwater comes from the Miño River. The increase in salinity from south to north shows that the observed low values are not generated by the rivers placed inside
the rias, since the highest river runoff (Fig. 2) corresponds to the Ria of Arousa (the northernmost). Therefore, the Miño River plume spreads about 46 km northward under favorable conditions (wind induced northward transport, high Miño River discharge and low river discharge inside the estuaries) reaching the Ria of Vigo and the Ria of Pontevedra but not the Ria of Arousa. Following an analysis similar to the one performed by Mouriño and Fraga (1982) to justify the appearance of
Fig. 8. Along estuary negative estuarine circulation (m s− 1) of the Ria of Pontevedra measured during a tidal cycle on May 13, 1998 in a middle–inner station. Maximum velocities about 0.12 m s− 1 were measured near surface during flood tide.
I. Alvarez et al. / Journal of Marine Systems 60 (2006) 144–152
abnormally low salinities in the Ria of Vigo, one can assume that if all freshwater discharges remain in the estuary and there is not an oceanic water input to compensate the loss of salinity, then the salinity diminution can be estimated using dS / dt = − SQ / V, where S is the surface salinity at the southern mouth of the estuary, V the volume of water in a surface layer from 0 to 10 m and Q the freshwater input. Using data from the Ria of Pontevedra and considering the initial situation the one measured during the previous survey, May 4, the estimated salinity diminution would be 0.9 psu, which cannot account from the 1.2 psu decrease measured from May 4 to May 11 in the ria. This is even more patent in the Ria of Vigo, where the observed decrease is about 5 psu and the freshwater input is on the same order of magnitude than the one observed in Pontevedra (see Fig. 2 for river discharge). Freshwater intrusion was not observed in the next survey of the three rias carried out on May 18, 1998. Nevertheless, the persistence of the intrusion can be analyzed in the particular case of the Ria of Pontevedra, were a new survey was carried out on May 13, 1998. On this date, the along axis gradient remained practically unchanged, around − 0.7 psu, although the effect of the Lerez River in the inner station was less clear, in good agreement with the decrease in river runoff described in deCastro et al. (2004). This persistence of the phenomenon for at least 2 days gives rise to a negative estuarine circulation along the estuary (Fig. 8). This circulation pattern was corroborated by a hydrodynamic model in the Ria of Pontevedra (deCastro et al., 2004). As we mentioned above, the main forcing mechanisms giving rise to freshwater intrusion are the existence of high Miño River discharges and favorable winds. In addition, there are other factors that can strongly influence the along axis gradients inside the rias as, for example, the river discharge inside the estuary, which follows a pattern that can be significantly different from the rainfall one since all rivers, except the Lerez, are controlled by dams. Other important factors are the water masses at the adjacent shelf, which follow a marked annual cycle and the vertical mixing generated by wind, which can dilute the plume. Additional surveys should be conducted to identify and describe the evolution of high freshwater discharges from the Miño River and their intrusion into the adjacent estuaries. The appearance of these intrusion patterns can be predicted in terms of the river discharge (controlled by dams) and the prevailing meteorological conditions, which can be obtained from weather forecast agencies. A 3D hydrodynamical model could also contribute to better analyze this kind of phenomenon, although the
151
lack of reliable wind data on the shelf does not allow modeling the present case in an accurate way. Finally, the existence of fresher surface water supplied from the Miño River can result in two opposed biological behaviors. On the one hand, it can generate the inverse estuarine circulation observed in the Ria of Pontevedra, which tends to stop water exchange between the ria and the shelf, increasing the residence time and hence decreasing water quality (deCastro et al., 2004). On the other hand, the high freshwater discharge can induce a phytoplankton bloom at the shelf, which penetrates into the estuary embedded in a fresher water mass, fertilizing the area (deCastro et al., in press). Acknowledgements This work was partially supported by CICYT under projects MAR96-1782 and REN2003-04106-C03 and by ‘Xunta de Galicia’ under project PGIDIT02PXJA38301PR. We acknowledge ‘Confederacion Hidrografica del Norte’ for providing Miño River data. References Álvarez-Salgado, X.A., Gago, J., Míguez, B.M., Gilcoto, M., Pérez, F. F., 2000. Surface waters of the NW Iberian margin: upwelling on the shelf versus outwelling of upwelled waters from the Rías Baixas. Estuarine, Coastal and Shelf Science 51, 821–837. Alvarez, I., deCastro, M., Prego, R., Gomez-Gesteira, M., 2003. Hydrographic characterization of a winter-upwelling event in the Ria of Pontevedra (NW Spain). Estuarine, Coastal and Shelf Science 56, 869–876. Alvarez, I., deCastro, M., Gomez-Gesteira, M., Prego, R., 2005. Interand intra-annual analysis of the salinity and temperature evolution in the Galician Rias Baixas–ocean boundary (NW Spain). Journal of Geophysical Research 110, C04008. doi:10.1029/2004JC002504. deCastro, M., Gomez-Gesteira, M., Prego, R., Taboada, J.J., Montero, P., Herbello, P., Perez-Villar, V., 2000. Wind and tidal influence on water circulation in a Galician Ria (NW Spain). Estuarine, Coastal and Shelf Science 51, 161–176. deCastro, M., Gomez-Gesteira, M., Alvarez, I., Prego, R., 2004. Negative estuarine circulation in the Ria of Pontevedra (NW Spain). Estuarine, Coastal and Shelf Science 60, 301–312. deCastro, M., Alvarez, I., Varela, M., Prego, R., Gomez-Gesteira, M., in press. Effect of an extreme water release from Miño River dams on the neighbor Galician Rias Baixas (NW Iberian Peninsula). Estuarine, Coastal and Shelf Science. Evans, G., Prego, R., 2003. Rias, estuaries and incised valleys: is a ria an estuary? Marine Geology 196, 171–175. Fennel, W., Mutzke, A., 1997. The initial evolution of a buoyant plume. Journal of Marine Systems 12, 53–68. Fiedler, P.C., Laurs, R.M., 1990. Variability of the Columbia River plume observed in visible and infrared satellite imagery. International Journal of Remote Sensing 11, 999–1010. Fiuza, A.F.G., Hamann, M., Ambar, I., Díaz del Río, G., González, N., Cabanas, J., 1998. Water masses and their circulation off western Iberia during May 1993. Deep Sea Research, I 45, 1127–1160.
152
I. Alvarez et al. / Journal of Marine Systems 60 (2006) 144–152
Fong, D.A., Geyer, W.R., 2001. Response of a river plume during an upwelling favorable wind event. Journal of Geophysical Research 106, 1067–1084. Fraga, F., 1981. Upwelling off the Galician Coast, Northwest Spain. In: Richardson, F.A. (Ed.), Coastal Upwelling. American Geophysical Union, Washington, pp. 176–182. Garcia-Berdeal, I., Hickey, B.M., Kawase, M., 2002. Influence of wind stress and ambient flows on a high discharge river plume. Journal of Geophysical Research 107, 3130. Gomez-Gesteira, M., deCastro, M., Prego, R., Perez-Villar, V., 2001. An unusual two layered tidal circulation induced by stratification and wind in the Ria of Pontevedra (NW Spain). Estuarine, Coastal and Shelf Science 52, 555–563. Gomez-Gesteira, M., deCastro, M., Prego, R., 2003. Dependence of the water residence time in Ria of Pontevedra (NW Spain) on the seawater inflow and the river discharge. Estuarine, Coastal and Shelf Science 58, 567–573. Herrera, J.L., Piedracoba, S., Varela, R.A., Roson, G., 2005. Spatial analysis of the wind field don the western coast of Galicia (NW Spain) for in situ measurements. Continental Shelf Research 25, 1728–1748. Hickey, B.M., Banas, N.S., 2003. Oceanography of the U.S. Pacific Northwest Ocean and Estuaries with Application to Coastal Ecology. Estuaries 26 (4B), 1010–1031. Hickey, B.M., Pietrafesa, L.J., Jay, D.A., Boicourt, W.C., 1998. The Columbia River plume study: Subtidal variability in the velocity and salinity fields. Journal of Geophysical Research 103, 10,339–10,368. Lazure, P., Jegou, A.M., 1998. 3D modelling of seasonal evolution of Loire and gironde plumes on Biscay Bay continental shelf. Oceanologica Acta 21 (2), 165–177. Lopez-Jamar, E., Cal, R.M., Gonzalez, G., Hanson, R.B., Rey, J., Santiago, G., Tenore, K.R., 1992. Upwelling and outwelling effects on the benthic regime of the continental shelf off Galicia, NW Spain. Journal of Marine Research 50, 465–488. McClain, C.R., Chao, S., Atkinson, L.P., Blanton, J.O., Castillejo, F., 1986. Wind driven upwelling in the vicinity of Cape Finisterre, Spain. Journal of Geophysial Research 91, 8470–8486. Mouriño, C., Fraga, F., 1982. Hidrografia de la Ria de Vigo. 1976– 1977. Influencia Anormal del rio Miño. Investigaciones Pesqueras 46 (3), 459–468.
Nogueira, E., Pérez, F.F., Rios, A.F., 1997a. Seasonal patterns and long-term trends in an estuarine upwelling ecosystem (ria of Vigo, Spain). Estuarine Coastal and Shelf Science 44, 285–300. Nogueira, E., Pérez, F.F., Ríos, A.F., 1997b. Modeling thermohaline properties in an estuarine upwelling ecosystem (Ría de Vigo: NW Spain) using box-Jenkins transfer function models. Estuarine Coastal and Shelf Science 44, 685–702. Pardo, P.C., Gilcoto, M., Pérez, F.F., 2001. Short-time scale coupling between thermohaline and meteorological forcing in the Ria de Pontevedra. Scientia Marina 65, 229–240. Prego, R., Dale, A., deCastro, M., Gomez-Gesteira, M., Taboada, J.J., Montero, P.P., Perez-Villar, V., 2001. Micro-scale hydrography of the Pontevedra Ría (NW Spain). Journal of Geophysical Research 106 (9), 19845–19857. Rio-Barja, F.X., Rodríguez-Lestegás, F., 1996. Os ríos, in As augas de Galicia. Consello de Cultura, pp. 178–180. Roson, G., Alvarez-Salgado, X.A., Perez, F.F., 1997. A non-stationary box model to determine fluxes in a partially mixed estuary, based on both thermohaline properties: application to the Ria de Arousa (NW Spain). Estuarine, Coastal and Shelf Science 44, 249–262. Tenore, K.R., Cal, R.M., Hanson, R.B., López-Jamar, E., Santiago, G., Tietjen, J.M., 1984. Coastal upwelling off the Rias Bajas, Galicia, Northwest Spain: II. Benthic studies. Rapports et proces verbaux des reunions - Commission internationale pour l'exploration scientifique de la mer Mediterranee 183, 91–100. Tilstone, G.H., Figueras, F.G., Fraga, F., 1994. Upwelling–downwelling sequences in the generation of red tides in a coastal upwelling system. Marine Ecology Progress Series 112, 241–253. Torres, R., Barton, E.D., Miller, P., Fanjul, E., 2003. Spatial patterns of wind and sea surface temperature in the Galician upwelling region. Journal of Geophysical Research 108 (C4), 3130. Wooster, W.S., Bakun, A., McClain, D.R., 1976. The seasonal upwelling cycle along the eastern boundary of the north Atlantic. Journal of Marine Research 34, 131–141. Xing, J., Davies, A.M., 1999. The effect of wind direction and mixing upon the spreading of a buoyant plume in a non-tidal regime. Continental Shelf Research 19, 1437–1483.