Influence of Upwelling Events on the Estuaries of the North-Western Coast of the Iberian Peninsula

CSIRO PUBLISHING

Marine and Freshwater Research, 2013, 64, 1123–1134 http://dx.doi.org/10.1071/MF12298

Influence of upwelling events on the estuaries of the north-western coast of the Iberian Peninsula

I. AlvarezA,B, J. M. DiasA,C, M. deCastroB, N. VazA, M. C. SousaA and M. Go´mez-GesteiraB

ACESAM, Departamento de Fı´sica, Universidade de Aveiro, 3810-193 Aveiro, Portugal. BEPhysLab (Environmental Physics Laboratory), Universidade de Vigo, Facultade de Ciencias, Ourense, Spain. CCorresponding author. Email: [email protected]

Abstract. Coastal upwelling is one of the best studied oceanographic phenomena because of its effects on primary production. The western coast of the Iberian Peninsula has high biological diversity, mainly due to this primary production. In this study, the response of salinity and temperature to the occurrence of upwelling was analysed at the Ria de Vigo– and Ria de Aveiro–ocean boundary over the course of a year. Both systems were influenced by similar external forcing, but the response of thermohaline properties differed. Salinity and temperature were dependent on external forcing throughout the water column at Ria de Aveiro, whereas near-bed measurements revealed the presence of upwelled water at Ria de Vigo. Eastern North Atlantic Central Water was observed during spring–summer (summer) at the southern (northern) mouth of Ria de Vigo, but it was not observed at Ria de Aveiro. This difference may be due to the shallowness and narrowness of the Ria de Aveiro mouth, which can limit the entrance of ocean water. The trends found are unlikely to be unique, suggesting that geomorphologic characteristics of system–ocean boundaries determine how physical processes occurring in adjacent coastal areas impact estuarine properties.

Additional keywords: Ekman transport, Ria de Aveiro, Ria de Vigo, salinity, sea surface temperature, temperature, upwelling.

Received 17 October 2012, accepted 11 May 2013, published online 20 September 2013

Introduction northerly winds prevail along the shelf. The existence of these The north-western coast of the Iberian Peninsula (IP) (Fig. 1a)is persistent upwelling-favourable conditions along this coast is the northernmost limit of the Eastern North Atlantic Upwelling possibly the most important physical process determining ocean System, which extends from 108Nto,448N(Wooster et al. conditions at the adjacent shelf. The effect of upwelling can be 1976). Upwelling events along this coast are commonly attrib- observed throughout the year, as it generates a decrease in uted to the action of northerly winds along the shelf that generate nearshore temperature when colder water upwells (Fig. 1a, an Ekman drift directed offshore. Water depletion occurs in the colour map). As a result of spring–summer upwelling events, upper layers and nutrient-rich water from below moves upward cold deep water known as Eastern North Atlantic Central Water and replenishes the space vacated. In general, wind events are (ENACW) (Rı´os et al. 1992; Pe´rez et al. 1995; Fiu´za et al. 1998) usually localised, the coastline is not straight, and upwelling is reaches surface layers. This water mass is rich in nutrients, not at all uniform. Thus, although the onset of coastal upwelling which, together with dissolved CO2 and solar energy, are used is primarily controlled by local wind conditions, geographic by phytoplankton to produce organic compounds through pho- features such as capes and bottom topography may be respon- tosynthesis. This process generates high levels of primary sible for non-uniform upwelling conditions observed along a production, which in turn supports the high biological diversity coast (Lazure and Jegou 1998; Atkinson et al. 2002; Gan and in this region (Tenore et al. 1995; Huthnance et al. 2002; Santos Allen 2002). Several previous studies conducted along the major et al. 2004; Torres and Barton 2006). upwelling regions of the world suggested that capes may pro- The north-western coast of the IP follows a north–south mote locally enhanced upwelling centres (Barton et al. 2001; orientation, and the alongshore topographic features vary greatly. Marchesiello et al. 2003; Torres et al. 2003; Pelegrı´ et al. 2005), Four coastal systems, known as Rias Baixas, are located in the and variations in bottom topography may redistribute upwelled northern area (western Galician coast): from north to south they water unevenly along the coast (Rodrigues and Lorenzzetti are Ria de Muros, Ria de Arousa, Ria de Pontevedra, and Ria de 2001; Glenn et al. 2004; Fre´on et al. 2006). Vigo. South of the Rias Baixas, the coast is an almost continuous Upwelling along the north-western coast of the IP is a line, with the exception of the estuaries of major rivers flowing frequent phenomenon during the spring–summer months when into this area (Minho and Douro) and Ria de Aveiro. The

Journal compilation Ó CSIRO 2013 www.publish.csiro.au/journals/mfr 1124 Marine and Freshwater Research I. Alvarez et al.

(a) 44ЊN

17 (c)

42ЊN 16

40.76ЊN

40ЊN 10ЊW8ЊW

(b)

42.38ЊN 40.62ЊN

42.17ЊN

15 000 8.76ЊW 8.64ЊW 8.95ЊW 8.57ЊW

Fig. 1. (a) Sea surface temperature (SST; 8C) averaged from 1985 to 2007 along the north-west coast of the Iberian Peninsula. Black dots represent the control points where the upwelling index (UI) was calculated using QuikSCAT data. (b) Map of Ria de Vigo and (c) Ria de Aveiro showing the hydrographic sampling stations (black squares) at the mouth of each ria and the meteorological stations (black circles). influence of upwelling events on these estuarine waters has been hydrographic measurements taken during 2003 and 2004 in studied previously, primarily for the Rias Baixas. The great Ria de Vigo (the southernmost ria of the Rias Baixas) and Ria de primary productivity generated by the entrance of ENACW into Aveiro ocean boundaries. The results increase knowledge about these estuaries means that there is extraordinary commercial the processes that determine how upwelling influences interest in the area (Fraga 1981; Blanton et al. 1987; Roso´net al. estuarine waters. Considering that the properties of the ocean 1995; Nogueira et al. 1997; Doval et al. 1998; Pardo et al. 2001; water at the mouth of the rias are mainly governed by upwelling Prego et al. 2001). In fact, this area produces around 250 000 and downwelling events that take place along the coast, the tonnes of mussels per year (,15% of the world production). ria–ocean boundary is a strategic location for analysing the Primary production, biological diversity and estuarine circula- occurrence of such events. Upwelled water may be confined to tion also have been studied in the Minho and Douro estuaries and the bottom layers or it may move onshore in the central water Ria de Aveiro (Vieira and Bordalo 2000; Queiroga 2003; column during the upwelling season (Huyer 1983; Allen et al. Azevedo et al. 2006, 2008; Costa-Dias et al. 2010), but a 1995), thereby generating thermal stratification inside the complete study of the influence of upwelling on these estuaries estuaries. is necessary. Several studies of other coastal systems along different Most previous studies focussed on each estuary indepen- upwelling regions have been conducted to analyse thermohaline dently. Thus, the simultaneous influence of the same upwelling properties at the system–ocean boundary to characterise the events on thermohaline properties of estuaries at different influence of upwelling events; examples include Saldanha Bay locations along this coastline has not been evaluated. The in the Benguela Upwelling System (Monteiro and Largier goal of this study was to fill this knowledge gap using 1999), estuaries along the USA west coast (Hickey and Banas Influence of upwelling events on estuaries Marine and Freshwater Research 1125

Table 1. Characteristics of the Ria de Vigo and the Ria de Aveiro common features. The mouth of Ria de Aveiro and the northern mouth of Ria de Vigo are approximately the same depth Vigo Aveiro (,25 m; see Table 1), and both systems are located along the

2 north-western coast of the IP. In addition, previous studies Surface area (km ) 156 83 (Alvarez et al. 2008a; Go´mez-Gesteira et al. 2011) revealed Length (km) 31 45 Mouth width (km) Southern 5.1 0.350 that the wind regime, air temperature and accumulated precipi- Northern 2.8 tation along the north-west coast of the IP follow approximately Mouth depth (m) Southern 45 25 the same pattern. Thus, the occurrence of upwelling events Northern 25 could have similar effects on the thermohaline properties of both Water content (km3) 3.12 0.15 systems. In this study, the influence of upwelling was compared River (annual mean river Oitabe´n (27.5) Antua (5) between these two systems and the results contribute to the flow m3 s 1) Vouga (50) existing knowledge about the main processes occurring along this coast and to our understanding of the upwelling regime in the region. The goal of this study was to analyse the response of 2003), the Tay Estuary in the UK (McManus 2005), and the thermohaline properties to the occurrence of upwelling events Neuse River Estuary in the USA (Buzzelli et al. 2002; Paerl et al. at the Ria de Vigo– and Ria de Aveiro–ocean boundaries from 2010). These studies reported that primary production in these October 2003 to September 2004. A control station was located estuaries is likely controlled by coastal upwelling and exchange at the mouth of each ria, and hydrographic measurements were of properties across the system–ocean boundary. The analysis of sampled simultaneously every 2 weeks. In addition, the effects thermohaline variables revealed the presence of different water of external forcing (measured as air temperature and river masses, as indicated by abrupt changes in salinity and tempera- discharge) on water temperature and salinity were analysed in ture at the mouths of the systems. Upwelling events can the study area and on the adjacent shelf during this time period. intermittently fill the water column inside the estuaries with these water masses, thereby affecting the macronutrient con- Materials and methods centrations available for production. The inflow of nutrient-rich Thermohaline variables water imposes a control on new production within the estuarine environment (Monteiro and Largier 1999). Moreover, if the Vertical profiles of temperature and salinity were measured at a dissolved oxygen concentration in upwelled water is sufficiently control point at the boundary between the two systems and the low, upwelling may have chronic effects on the fish that are ocean (Fig. 1b, c, black squares). Two control points were used engulfed by these waters (Buzzelli et al. 2002). at Ria de Vigo, one for each mouth. Thermohaline variables and From a morphological point of view, the rias in Rias Baixas water pressure were sampled fortnightly with a time step of 1 s are similar, widening progressively from the innermost part of from October 2003 to September 2004 at both rias using a mini the estuary towards the mouth, and they share some common STD model SD204 (SAIVA/S, Bergen, Norway). Water tem- features (Alvarez et al. 2005). Thus, Ria de Vigo is a good perature was measured using a thermistor with a range from 2 representative for analysing the behaviour of the Rias Baixas. to 408C with an accuracy of 0.018C. Water pressure was This system is connected to the Atlantic Ocean through two measured using a Piezoresistive sensor with an accuracy of entrances, as small islands in the outermost part (Cies Islands) 0.02% of the depth (500 m), and conductivity was measured 1 divide the mouth. It is classified as mesotidal and it behaves as a using an inductive cell in the range of 0 to 70 mS cm with an 1 partially mixed estuary in which partial stratification is main- accuracy of 0.02 mS cm . Salinity was calculated from tained by river discharge in winter and solar heating in summer conductivity, water temperature and water pressure measure- (Prego and Fraga 1992; Nogueira et al. 1997; deCastro et al. ments in the range of 0 to 40 ppt with an accuracy of 0.02 ppt. 2000). The main freshwater input to Ria de Vigo comes from the The surveys at Ria de Vigo were conducted one day after those at Oitaben River (Table 1). Ria de Aveiro. Each survey began ,1 h 40 min after the low tide Ria de Aveiro is a shallow coastal lagoon formed by a predicted for the ria mouth during the flood stream. No specific complex system of shallow channels (four main branches permits were required for the field studies. The survey locations radiating from the sea entrance: Mira, S. Jacinto, I´lhavo and are not privately owned or protected in any way. The field Espinheiro) connected to the Atlantic Ocean through an artifi- studies did not involve endangered or protected species. cial inlet. Thus, it is an interesting system to study in terms of hydrodynamic and hydrographic properties (Lopes et al. 2001, Air temperature and river discharge 2007, 2008; Vaz et al. 2005, 2007, 2012; Dias and Lopes 2006; Air temperature was measured at one meteorological station Vaz and Dias 2008). As a whole, Ria de Aveiro is classified as located near the innermost part of Ria de Vigo (Fornelos de mesotidal and as a vertically homogeneous system, although Montes, 42.328N; 8.48W) and at the meteorological station of some channels vary from well mixed to partially stratified the University of Aveiro (40.638N; 8.658W) (Fig. 1b, c, black depending on the freshwater input (Dias et al. 1999; Vaz et al. circles). Due to the lack of available river flow measurements 2009). Freshwater contributions come essentially from two at Ria de Vigo, daily precipitation data from a meteorological rivers: Antua˜ and Vouga. (Table 1). station (Fornelos de Montes) located near the river mouth were From a morphological point of view, Ria de Vigo and Ria evaluated. Precipitation data used for each survey were the result de Aveiro are different. However, both systems share some of averaging daily measurements from the 4 days before the 1126 Marine and Freshwater Research I. Alvarez et al. date of the hydrographic sampling. The river discharge corre- Sea surface temperature sponding to each survey period at Ria de Aveiro was determined Sea surface temperature (SST) data were obtained from night using measurements of current speed obtained several kilo- time measurements taken with the Advanced Very High Reso- metres upstream from the Vouga River mouth; measurements lution Radiometer on board NOAA series satellites (Kilpatrick were taken 3 h after the low tide predicted for the Ria de Aveiro et al. 2001). Data are available from 1985 and they are distrib- mouth. This procedure guaranteed that the current speed mea- uted in a variety of resolutions and temporal averages. In the surements were taken outside the region of tidal flood influence. present study, a spatial resolution of 4 km and a temporal Current speed data were collected using a current meter model average of 8 days were used. An SST value is computed as the 105 (Valeport, Devon, UK), which has a high-impact styrene average of all cloud-free multichannel measurements available impeller to measure the current speed in the range of 0.1 to for 1 week for each grid point. 5ms1 with an accuracy of 2.5% for readings above 0.5 ms1 and 0.01 ms1 for readings below 0.5 ms1. The current Results direction was measured using a flux gate compass with a range from 0 to 3608 with a resolution of 0.58. River discharge, meteorological conditions and upwelling characterisation Fig. 2a (grey line) shows the Vouga River flow corresponding to Wind each survey from October 2003 to September 2004. The highest Surface wind fields were obtained from the QuikSCAT satellite value (,140 m3 s1) occurred in winter at the end of January, (Perry 2001). The dataset is available from July 1999 and con- and the lowest (,2m3 s1) occurred in summer between July sists of global grid values of meridional and zonal components and September. A large river discharge also took place from , 8 of wind measured twice daily on an 0.25 0.25 grid with November to December, with values ranging from 50 to global coverage. These high-resolution data allow the analysis 110 m3 s1. Precipitation data measured near the innermost part of wind variability near the coast, compensating the lack of wind of Ria de Vigo (black line, Fig. 2a) follow a pattern similar to data at a nearby land station. QuikSCAT data are given in an that of the Vouga River discharge, with the highest value ascending and descending pass. Data corresponding to one pass occurring at the end of January. A high precipitation value also include numerous shadow areas, so the average between both was observed in November. Fig. 2b shows the monthly mean air passes was used to increase the coverage. Wind-speed mea- 1 1 temperature measured from October 2003 to September 2004 at surements range from 3 to 20 ms , with an accuracy of 2 ms both meteorological stations (Fig. 1b, c, black circles). Both 8 and 20 in direction (JPL 2001). The reference height of the wind stations show the same behaviour, with lower values in winter , data is 10 m. Wind data from close to the coast ( 25 km) are not than in summer. However, the air temperature measured at available due to the existence of a small coast mask; neverthe- Ria de Aveiro (grey line) was always higher than that at Ria de less, previous studies have shown that QuikSCAT data are Vigo (black line). comparable to modelled data along the north-west coast of the The UI was calculated at two control points situated in front IP (Gomez-Gesteira et al. 2006; Alvarez et al. 2008b; Sousa of the Ria de Vigo and Ria de Aveiro mouths (Fig. 1a, black et al. 2013). In fact, Penabad et al. (2008) conducted a statistical dots) for each day from October 2003 to September 2004 to comparison between satellite wind measurements and high- analyse upwelling conditions. Data in Fig. 3 represent the resolution numerical models and reported similar results average between both control points in front of each Ria. The between models and satellite data. dates corresponding to each survey are marked by grey dots. UI Ekman transport can be calculated using QuikSCAT wind tended to be positive from April to September, indicating r ¼ 3 speed at the 10 m level (W), sea water density ( 1025 kg m ), upwelling-favourable conditions, whereas the signal alternated ¼ 3 a dimensionless drag coefficient (Cd 1.4 10 ) and air between negative and positive values during autumn and winter. r ¼ 3 density ( a 1.22 kg m ) using the following equation: Fig. 4 shows the oceanic conditions during the study period. The r sea surface temperature anomaly (SSTa) was calculated for ¼ aCd ð 2 þ 2Þ1=2 ð Þ the Ria de Vigo and Ria de Aveiro mouths on the adjacent shelf. Qx Wx Wy Wy 1 r f Both signals follow the same pattern, with the highest positive values occurring in summer due to the influence of solar heating r aCd 2 2 1=2 (Fig. 2b). These results indicate that both locations experienced Qy ¼ ðW þ W Þ Wx ð2Þ r f x y similar water conditions throughout the year. In fact, a significant positive correlation (0.8, significance level 99%) was where f is the Coriolis parameter, defined as twice the vertical obtained between the SSTa values calculated for both places component of the Earth’s angular velocity, O, about the local when the entire available period of SST data (from 1985 to 2007) vertical given by f ¼ 2Osin(y) at latitude y. Finally, the x sub- was analysed. script corresponds to the zonal component and the y subscript to Fig. 5 shows an example of the atmospheric and oceanic the meridional one. Along the western coast of the IP, –Qx can be conditions along the coast corresponding to a cruise conducted considered equal to the rate of water upwelled per kilometre of during upwelling-favourable conditions (18 July 2004). Ekman coast and can be expressed by the upwelling index (UI) in transport and SST data in this figure were averaged for the 7 days m3 s 1 km 1 (Nykjær and Van Camp 1994; Gomez-Gesteira before this cruise. Transport was mainly directed westward all et al. 2006; Santos et al. 2011). Thus, negative (positive) Qx along the coast, indicating strong upwelling-favourable condi- values mean upwelling favourable (unfavourable) conditions. tions (Fig. 5a). As a consequence, a band of cool water was Influence of upwelling events on estuaries Marine and Freshwater Research 1127

150 (a) 2003 2004 24 )

1 Aveiro

Ϫ 20

s Vigo 3 100 16

50 8

4 Precipitation (mm)

River discharge (m River 0 0 Oct 2 Jan 6 Jan Nov 6 Nov Dec 5 Jul 18 Jul 25 Jul Oct 28 Apr 20 Apr 30 Jan 22 Jan 29 Jan 18 Jun 25 Jun Feb 20 Feb 28 Feb Mar 21 Mar 29 Aug 17 Aug 24 Aug Nov 25 Nov Dec 26 Sep 22 May 27 May

25 (b) 2003 2004 ) Њ 20

Aveiro Air temperature ( Air temperature 5 Vigo 0 Oct Nov Dec Jan Feb Mar Apr May Jun Jul Aug Sep

Fig. 2. (a) River flow (m3 s1) and precipitation (mm) corresponding to each survey conducted at Ria de Aveiro (grey line) and Ria de Vigo (black line) from October 2003 to September 2004. The precipitation data are the result of averaging daily measurements from the 4 days before each cruise. (b) Monthly mean air temperature (8C) measured at the meteorological stations (black circles in Fig. 1b, c) of the University of Aveiro (grey line) and the inner part of Ria de Vigo (black line).

3750 Aveiro Vigo

) 2500 1 Ϫ

km 1250 1 Ϫ s

3 0

UI (m Ϫ1250

Ϫ2500 Oct Nov Dec Jan Feb Mar Apr May Jun Jul Aug Sep

Fig. 3. Upwelling Index (UI) calculated from October 2003 to September 2004 at the mouths of Ria de Aveiro (grey line) and Ria de Vigo (black line). Each line corresponds to the average calculated between both points located in front of each ria (Fig. 1a, black points). Dates corresponding to each survey are marked by grey dots.

2.5

1.5

0.5

SSTa Ϫ0.5

Ϫ1.5 Vigo Aveiro Ϫ2.5 Oct Nov Dec Jan Feb Mar Apr May Jun Jul Aug Sep

Fig. 4. Sea surface temperature anomaly (SSTa) calculated at the Ria de Aveiro (grey line) and Ria de Vigo (black line) mouths from October 2003 to September 2004. 1128 Marine and Freshwater Research I. Alvarez et al.

2000 23 (a)(b)

22 Њ 42 N 1500 42ЊN

21 1000 2000 m3 sϪ1 kmϪ1 20

Њ Њ 40 N 500 40 N 19

0 18 12ЊW 10ЊW8ЊW12ЊW10ЊW8ЊW

Fig. 5. (a) Ekman transport (m3 s1 km1) and (b) sea surface temperature (SST; 8C) conditions along the north- western coast of the Iberian Peninsula coast averaged for the 7 days before the selected cruise (18 July 2004). present near shore, which generated a longitudinal SST gradient line) throughout the year, except during the summer months. throughout the region with temperature values of ,18.5–198C Temperature measurements taken near the sea bed and at the sea along the coast (Fig. 5b). surface (Fig. 6d ) showed the same behaviour, with the highest values during summer. Near-bed temperature values at Ria de Thermohaline variables measured in situ Aveiro (dark grey line) also were nearly the same as those near the sea surface (Fig. 6b), indicating similar behaviour for the Fig. 6 shows the annual evolution of salinity and temperature whole water column. Similar temperature values were found at values measured at the control points (Fig. 1b, c, black squares) both of the Ria de Vigo control points, except from July to near the sea surface and near the sea bed. Data from January to September, when lower temperatures were observed at the December clearly show the annual cycle. A running average that southern mouth. took into account the previous and following survey was used to Thermohaline variables also were used to characterise the better observe the annual pattern. In the specific cases of presence of upwelled water at the mouth of both systems. Fig. 7a October 2003 and September 2004, the running average was shows the temperature–salinity diagram corresponding to the calculated assuming signal periodicity. This running average near-bed measurements. The line representing ENACW (Fraga was only used to generate Fig. 6. The original unfiltered 1981) is also shown. Some of the measurements corresponding time series data were considered for all subsequent analyses to the northern and southern mouths of Ria de Vigo appeared to (i.e. presence of upwelled water at the ria mouths and be under the influence of ENACW. In contrast, the water present correlations). at the Ria de Aveiro mouth was not associated with this water Surface salinity values at Ria de Vigo (Fig. 6a) showed mass. Fig. 7b shows an enlargement of the area close to the line nearly the same behaviour at the northern (black line) and of ENACW. Dates corresponding to the data that are most southern mouth (light grey line), with values ranging from 33 similar to the line of ENACW are also included. This water mass in autumn–winter to 35.5 in spring–summer. A different pattern was present in April, June and July (grey squares) at the northern was observed at Ria de Aveiro (dark grey line), with higher mouth of Ria de Vigo. Moreover, the surveys conducted salinity differences between winter and summer. The lowest between March and September (black circles) revealed temper- values were measured in November and January (around 27), ature and salinity values influenced by ENACW at the which shows the effect of river discharge (Fig. 2a). Surface southern mouth. Measurements corresponding to the Ria de salinity values increased during summer (around 35) and Aveiro mouth (grey triangles), however, are far removed reached values similar to those observed at the northern from ENACW. and southern mouth of Ria de Vigo. SST values (Fig. 6b) followed the same pattern at the three control points. The highest values occurred during summer, which shows the influence of Discussion solar heating (Fig. 2b). The northern (black line) and southern Coastal upwelling is one of the most distinctive oceanographic mouth (light grey line) at Ria de Vigo experienced approxi- features attracting the interest of the scientific community mately the same temperature values throughout the year. From throughout the world. In the Northern Hemisphere, when winds October to March it is possible to see lower temperature values blow equatorward along an eastern ocean boundary, surface at Ria de Aveiro (dark grey line) than at Ria de Vigo. In contrast, water moves offshore due to the effect of Ekman transport and SST values were higher from April to September at Ria de is replaced by colder nutrient-rich water from below. Thus, Aveiro compared to the other ria. upwelling regions are areas of high primary production. More Salinity values measured near the sea bed were higher than than 20% of global fish catches occur in areas of upwelling, those near the sea surface at the three control points (Fig. 6c). although these areas occupy less than 1% of the world’s ocean This result indicates that salinity tends to increase from the surface (McGregor et al. 2007). Coastal upwelling events can surface to near the bed throughout the year. Once again, the also affect estuaries and produce thermal stratification inside lowest salinity values were observed at Ria de Aveiro (dark grey these coastal systems. The inflow of cold bottom water is Influence of upwelling events on estuaries Marine and Freshwater Research 1129

Aveiro VigoN VigoS (a) 36

30 Salinity 28 Surface 2004 2003 26 (b) 22 C) Њ 18

14 2004 2003 Surface 10

30 Salinity ( Temperature

28 Bed 2004 2003 26 (d) 22 C) Њ 18

Temperature ( Temperature 2004 2003 Bed 10 Oct 2 Jan 6 Jan Nov 6 Nov Dec 5 Jul 18 Jul 25 Jul Oct 28 Apr 20 Apr 30 Jun 18 Jun 25 Jun Jan 22 Jan 29 Jan Feb 20 Feb 28 Feb Mar 21 Mar 29 Aug 17 Aug 24 Aug Nov 25 Nov Dec 26 Sep 22 May 27 May

Fig. 6. Salinity and temperature measured near the sea surface (a, b) and near the sea bed (c, d) at the Ria de Aveiro mouth (dark grey line) and at the northern (black line) and southern (light grey line) mouth of Ria de Vigo. ecologically critical, as it supplies nutrients to the estuaries and boundary using control points at the mouth of each system. imposes control on new production within the environment Hydrographic measurements were taken simultaneously, fort- (i.e. exports phytoplankton new production to the coast or limits nightly from October 2003 to September 2004. Analysis of the risk of harmful algal blooms) (Monteiro and Largier 1999; external forcing revealed that river flow/precipitation and air Hickey and Banas 2003). temperature showed similar patterns in both systems, with the Upwelling also occurs along the western coast of the IP and highest and lowest values occurring during the same time period is one of the most important physical processes in this region. (Fig. 2). Wind conditions were also analysed using the UI. This This phenomenon determines thermohaline properties of shelf index was calculated as the component of Ekman transport waters and consequently may affect estuarine hydrography. perpendicular to the shoreline, which is the most common It also is responsible for primary production in coastal zones, method for characterising coastal upwelling (Bakun 1990; which generates an extraordinary commercial interest for fish- Nykjær and Van Camp 1994; Gomez-Gesteira et al. 2006; eries (Huthnance et al. 2002; Torres and Barton 2006). Conse- Santos et al. 2011). Note that this UI refers to the possible effect quently, a deep knowledge of the influence of upwelling on of wind on the ocean and does not represent a real oceanic estuarine areas is needed to better determine how to manage variable. Moreover, the UI does not represent the only mecha- many exploited and protected endogenous estuarine species. nism that can result in coastal upwelling. Wind can be strongly In this study, the evolution of water temperature and salinity modulated in the vicinity of capes, which can induce an was analysed at the Ria de Vigo– and Ria de Aveiro–ocean upward pumping of ocean water (Barton et al. 2001; 1130 Marine and Freshwater Research I. Alvarez et al.

(a) 24 Table 2. Correlation coefficient between the river discharge/ Vigo – Northern mouth 22 Precipitation (RD/Pr), air temperature (Tair), Upwelling Index (UI) Vigo – Southern mouth and oceanographic variables (Salinity, S, and Temperature, T) at the 20 Aveiro Ria de Aveiro and Ria de Vigo mouth

18 *RD/Pr and Tair are correlated with the surface salinity and temperature. UI is correlated with near bed salinity and temperature. Only results with 16 a significance level greater than 95% are considered 14

C) RD/Pr Tair UI

Њ 12 10 SVigo(N) 0.30 0.49 – 30 31 32 33 34 35 36 SVigo(S) 0.50 0.40 – 14.0 (b) SAveiro 0.94 0.57 – TVigo(N) – 0.80 0.41

Temperature ( Temperature T – 0.79 0.42 13.5 Vigo(S) TAveiro 0.67 0.89 – 22/09/04 25/07/04 13.0 18/06/04 30/04/04 18/06/04 30/04/04 25/07/04 was found between UI and near-bed temperature at Ria de Vigo. 20/04/04 12.5 27/05/04 No significant correlations were found between UI and near- 18/07/04 18/07/04 29/03/04 bed salinity. Thus, the response of thermohaline variables to external forcing differed for each system. 12.0 35.1 35.2 35.3 35.4 35.5 35.6 35.735.8 35.9 Analysis of the annual evolution of thermohaline properties Salinity revealed that near-surface and near-bed salinity exhibited nearly the same behaviour at the northern and southern mouths of Ria Fig. 7. (a) Temperature–salinity diagram corresponding to near-bed de Vigo, with small differences between winter and summer measurements at the Ria de Aveiro mouth (grey triangles) and the northern measurements (Fig. 6). Important differences were observed in (grey squares) and southern (black circles) mouths of Ria de Vigo for the near-surface and near-bed salinity between winter and summer study period (October 2003 to September 2004) at the sampling stations at Ria de Aveiro, and these difference reflect the effect of river (Fig. 1b, c), black squares). (b) Enlargement of the area around the line of discharge (Fig. 2a). It is important to note that the mean annual Eastern North Atlantic Central Water (ENACW). The line representing river flow at Ria de Vigo is lower than that at Ria de Aveiro ENACW (black line) was obtained from Fraga (1981). (Table 1). In addition, the relationship between river discharge/ water content is considerably higher at Ria de Aveiro (see Table 1), and the effect of the freshwater input inside the system could explain the observed salinity differences in this Marchesiello et al. 2003; Torres et al. 2003; Pelegrı´ et al. 2005). system. On the other hand, SST measurements showed the same Nevertheless, this method has proven to be suitable to charac- pattern at the three control points; the highest values were terise coastal upwelling in the studied area (Nykjær and Van measured from April to September, which illustrates the influ- Camp 1994; Go´mez-Gesteira et al. 2006, 2008, 2011; Alvarez ence of solar heating (Fig. 2b). Near-bed temperature at Ria de et al. 2008a, 2008b; Santos et al. 2011). Analysis of UI (Fig. 3) Aveiro showed nearly the same values as SST, indicating that showed that both systems are influenced by similar wind solar heating affects the whole water column. Similar tempera- conditions, with the prevalence of southward winds (upwelling tures were recorded at both control points at Ria de Vigo, with an favourable) for almost the whole year. important decrease during the summer months regarding mea- The influence of external forcing (i.e. air temperature, river surements at Ria de Aveiro. This difference between the systems discharge and UI) on thermohaline variables was also analysed may be due to the presence of cold oceanic water near the sea by calculating the correlation coefficient (Table 2). Results with bed at the mouth of Ria de Vigo caused by upwelling-favourable a significance level greater than 95% were considered. River conditions along the coast (Fig. 3). Note that the southern mouth discharge/precipitation (RD/Pr) and air temperature (Tair) were of Ria de Vigo is deeper than the northern one, which facilitates correlated with surface salinity and temperature, and UI was the entry of upwelled water (Alvarez et al. 2005). Nevertheless, correlated with near-bed salinity and temperature. A negative the depth of the northern mouth is similar to the depth of the Ria correlation was observed between near-surface salinity and river de Aveiro mouth (Table 1). Thus, if upwelled water is present at discharge at the three control points. The highest correlation the northern entrance of Ria de Vigo, a similar situation is value (about 0.9) was found at Ria de Aveiro. Near-surface expected at the Ria de Aveiro mouth. temperature and river discharge only showed a significant Different types of water masses can be identified by analys- correlation at Ria de Aveiro (about 0.7). Air temperature ing thermohaline variables at the mouth of coastal systems. was positively correlated with SST for both systems Upwelling events can fill the water column inside estuaries with (,0.8–0.9). The correlation observed between surface salinity these water masses, thereby affecting the concentrations of and air temperature was only due to the cross-correlation nutrients available for production. Depending on the water among meteorological factors, as the coldest months are also mass, upwelling can make the systems more productive the wettest ones. Finally, a negative correlation (about 0.4) (Monteiro and Largier 1999; Hickey and Banas 2003) or it can Influence of upwelling events on estuaries Marine and Freshwater Research 1131 make the water more salty and nutrient poor and thus have important influence of air temperature and river discharge a negative impact (Buzzelli et al. 2002). The presence of throughout the whole water column in this system. In fact, the upwelled water at the mouths of Ria de Vigo and Ria de Aveiro strong correlation observed between surface salinity/river was analysed to search for the presence of ENACW. This water discharge and between surface temperature/air temperature mass is present near the western coast of the IP from spring to (Table 2) was also observed near the sea bed, with values of summer (Rıós et al. 1992; Fiu´za et al. 1998) and has salinity 0.43 and 0.88 respectively. Note that these geomorphologic values ranging from 35.67 to 35.83 and temperature values constraints (shallow depth and narrow mouth) could have ranging from 11.8 to 13.58C depending on interannual changes important consequences for nutrient concentration related to in its y–S relationship (Pe´rez et al. 1995; Pollard et al. 1996). salinity and temperature variations. Recent studies reported that The low temperature values measured near the sea bed at both changes in the distribution of thermohaline variables can influ- mouths of Ria de Vigo (Fig. 6) during the summer months could ence the structure and composition of catchment areas (Baptista be explained by the presence of this water mass. In fact, the et al. 2010; Domingues et al. 2011). Some species can disappear, spatial importance and variability of intrusions of ENACW have and a decrease in abundance of species that use estuaries as been extensively studied in the Galician Rias Baixas and its nursery areas can also occur. These circumstances indicate that neighbouring sea area due to the regular occurrence of upwell- further research is necessary to clarify the possible effects of ing events (Fraga 1981; Blanton et al. 1987; Rosoń et al. 1995; geomorphologic factors, such as those observed at mouth of Nogueira et al. 1997; Doval et al. 1998; Pardo et al. 2001; Prego Ria de Aveiro, on primary production and biological diversity. et al. 2001). Previous studies showed that this water mass is rich in nutrients due to decomposition of organic matter sinking from the surface waters, which generates high primary production Conclusions near the coast. Thus, the entrance of ENACW inside the The variability of thermohaline properties was analysed estuaries is a very important topic of study because of its high simultaneously at the Ria de Vigo and Ria de Aveiro mouth from economic importance. October 2003 to September 2004. This study has shown the A temperature–salinity diagram corresponding to near-bed following: measurements was used to characterise the presence of ENACW (1) Analysis of relationships between air temperature, river at the mouths of Ria de Vigo and Ria de Aveiro (Fig. 7). Most of discharge, and thermohaline variables revealed significant the measurements thought to be influenced by ENACW were correlations between surface salinity/temperature and observed at the southern mouth of Ria de Vigo (eight surveys external forcing. A significant correlation was also from March to September). This water mass was only observed found between near-bed thermohaline properties and air at the northern mouth during four surveys (in April, June and temperature/river discharge at Ria de Aveiro, indicating that July). The water present at the Ria de Aveiro mouth was not external forcing can strongly affect near bed properties. associated at all with ENACW, possibly because of the character- (2) Salinity measurements near the sea surface and near the sea istics of the tidal flows at the mouth of this ria. The tidal flows bed at both mouths of Ria de Vigo showed the same block the entrance of ocean water, especially at the beginning of behaviour, with small differences between summer and the flood period when the water entering the ria through the winter. Important salinity differences were observed narrow inlet is strongly influenced by the water that leaves the near the surface and near the bed at Ria de Aveiro between system during the last ebb (Dias et al. 2000; Lopes et al. 2001). summer and winter, revealing a significant effect of Upwelling may exhibit non-uniform conditions along a coast river discharge. due to geographic features, such as capes and bottom topogra- (3) SST measurements exhibited the same pattern at the three phy (Lazure and Jegou 1998; Atkinson et al. 2002; Gan and control points. The highest values were observed during the Allen 2002). Capes may generate upwelling centres (Barton summer months, which illustrates the influence of solar et al. 2001; Marchesiello et al. 2003; Torres et al. 2003; Pelegrı´ heating. Near-bed temperatures were nearly the same as the et al. 2005), and variations in bottom topography may redis- surface ones at Ria de Aveiro, whereas near-bed tempera- tribute upwelled water unevenly along the coast (Rodrigues ture at both control points at Ria de Vigo decreased during and Lorenzzetti 2001; Glenn et al. 2004; Freónet al. 2006). Both the summer months, possibly due to the presence of cold systems studied herein are located outside the region of impact oceanic water. of any major geographic feature, but the mouth of each system (4) ENACW was observed at the southern mouth of Ria de Vigo exhibits different characteristics (Table 1). Thus, the shallow- during spring–summer months and at the northern mouth ness of the Ria de Aveiro mouth (25 m) could possibly limit the only in summer. This water mass was not observed at any penetration of water masses associated with upwelling events. time at Ria de Aveiro. Nevertheless, upwelled water was observed during summer at (5) The shallowness and narrowness of the Ria de Aveiro mouth the northern mouth of Ria de Vigo, which has a depth similar to can limit the entrance of ocean water, even under strong that of Ria de Aveiro (Table 1). The difference between systems upwelling-favourable conditions during summer. Thus, could be due to differences in width of the entrance, as the Ria de the characteristic water masses present at the continental Aveiro mouth is much narrower than the northern mouth of Ria margin were not observed inside this ria. de Vigo. Therefore, the area connecting Ria de Aveiro and the ocean is much smaller than the area connecting the northern The conditions described herein are unlikely to be unique to mouth of Ria de Vigo to the ocean. This situation could affect the Ria de Vigo and Ria de Aveiro. Similar analyses should be thermohaline variables measured at Ria de Aveiro due to the conducted for other tidally dominated systems under the 1132 Marine and Freshwater Research I. Alvarez et al.

influence of upwelling to improve our understanding of how hypoxia and habitat degradation in a shallow estuary. Marine Ecology coastal upwelled water impacts the thermohaline properties Progress Series 230, 103–112. doi:10.3354/MEPS230103 of estuaries. Identification of the dominant mechanism Costa-Dias, S., Freitas, V., Sousa, R., and Antunes, C. (2010). Factors between the local external forcing and the geomorphologic influencing epibenthic assemblages in the Minho Estuary (NW Iberian characteristics of the system–ocean boundary should be the Peninsula). Marine Pollution Bulletin 61, 240–246. doi:10.1016/ main topic of study. J.MARPOLBUL.2010.02.020 deCastro, M., Go´mez-Gesteira, M., Prego, R., Taboada, J. J., Montero, P., Acknowledgements Herbello, P., and Pe´rez-Villar, V. (2000). Wind and tidal influence on water circulation in a Galician Ria (NW Spain). Estuarine, Coastal and This paper was partially supported by the Portuguese Science Foundation Shelf Science 51, 161–176. doi:10.1006/ECSS.2000.0619 through the research projects DyEPlume ((PTDC/MAR/107939/2008), Dias, J. M., and Lopes, J. F. (2006). Implementation and assessment of AdaptaRia (PTDCAAC-CLI/100953/2008), Ecosam (PTDC/AAC-CLI/ hydrodynamic, salt and heat transport models: the case of Ria de Aveiro 104085/2008) and Pac:Man (PTDC/AAC-AMB/113469/2009), co-funded Lagoon (Portugal). Environmental Modelling & Software 21, 1–15. by COMPETE/QREN/UE). The first author was supported through the doi:10.1016/J.ENVSOFT.2004.09.002 Ramon y Cajal Program. The fifth author was supported by the Portuguese Dias, J. M., Lopes, J. F., and Dekeyser, I. (1999). Hydrological characteri- Science Foundation, through a doctoral grant (SFRH/BD/60209/2009). zation of Ria de Aveiro, Portugal, in early summer. Oceanologica Acta 22, 473–485. doi:10.1016/S0399-1784(00)87681-1 References Dias, J. 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