Temporal and spatial variability in the Guadalquivirestuary: a challenge for real-time telemetry

Gabriel Navarro & Francisco Javier Gutiérrez &

Manuel Díez-Minguito & Miguel Angel Losada &

Javier Ruiz

Received: 27 September 2010 /Accepted: 9 January 2011# Springer-Verlag 2011

Abstract Meteorological, hydrological, and hydrodynamicdata for 3 years (2008–2010) have been used to documentand explain the temporal and spatial variability of thephysical–biogeochemical interactions in the GuadalquivirRiver Estuary. A real-time, remote monitoring network hasbeen deployed along the course of the river between itsmouth and Seville to study a broad range of temporal scales(semidiurnal, diurnal, fortnightly, and seasonal). Thisnetwork consists of eight hydrological monitoring stationscapable of measuring temperature, conductivity, dissolvedoxygen, turbidity, and chlorophyll fluorescence at fourdepths. In addition, six stations have been deployed tostudy hydrodynamics, obtaining 20-cell water columncurrent profiles, and there is a meteorological station atthe river mouth providing data for understanding atmosphericinteractions. Completing this data-gathering network, thereare several moorings (tide gauges, current/wave sensors, and athermistor chain) deployed in the estuary and river mouth.Various sources of physical forcing, such as wind, tide-associated currents, and river discharge, are responsible for theparticular temporal and spatial patterns of turbidity andsalinity found in the estuary. These variables force thedistribution of biogeochemical variables, such as dissolved

oxygen and chlorophyll fluorescence. In particular, episodesof elevated turbidity (when suspended particle matter concen-tration >3,000 mg/l) have been detected by the network,together with episodes of declining values of salinity anddissolved oxygen. All these patterns are related to riverdischarge and tidal dynamics (spring/neap and high/low tide).

Keywords Guadalquivir estuary . Real-time remotemonitoring . Sensor technology . Telemetry .Water quality

1 Introduction

The Guadalquivir River estuary is located on the SW coastof the Iberian Peninsula (Fig. 1). The source of the river isin the Cazorla mountains at about 1,400 m above sea level,and it flows into the Gulf of Cadiz; its total length is680 km and it drains a basin of 63,822 km2 in area(Granado-Lorencio 1991). The present Guadalquivir estuaryis enclosed by the spits of Doñana and La Algaida, in theinland of which there is a large area of freshwater marshlandof 140,000 ha. (Rodriguez-Ramirez and Yáñez-Camacho2008). The estuary extends as far as the Alcalá del Río dam,110 km upstream from the river mouth at Sanlucar deBarrameda (Fig. 1). This estuary presents several notablecharacteristics, including the largely protected estuarymarshes forming part of the Doñana Natural and NationalParks, which are a UNESCO-MAB Biosphere Reserve. Theriver between Sanlucar de Barrameda and Seville is unique,being the only navigable river in Spain; its navigationchannel carries a relatively heavy traffic and has a minimumdepth of 6.5 m, requiring annual dredging. Moreover, theestuary has undergone rapid agricultural, fisheries, andanthropogenic development, particularly in recent decades(Ruiz et al. 2009). The catchment area of Guadalquivir has

Responsible Editor: Aida Alvera-Azcárate

G. Navarro (*) : F. J. Gutiérrez : J. RuizDepartment of Ecology and Coastal Management,Instituto de Ciencias Marinas de Andalucía ICMAN-CSIC,Puerto Real, Spaine-mail: gabriel.navarro@icman.csic.es

M. Díez-Minguito :M. A. LosadaGrupo de Dinámica de Flujos Ambientales,Universidad de Granada,Centro Andaluz de Medio Ambiente,Granada, Spain

Ocean DynamicsDOI 10.1007/s10236-011-0379-6

recently increased the use of watering for olive trees,probably resulting in acute erosion and sediment loading tothe river. On the other hand, the original salt marshesadjacent to the estuary have been transformed to grow ricethat contributes significantly to nutrient loadings, and theamount of nutrients reaching the coastal fringe is very large(Navarrro et al. 2006; Prieto et al. 2009).

In spite of its importance, until very recently, only a fewstudies have been carried out in the estuary; these havefocused mainly on fisheries, crustacean decapods (Baldóand Drake 2002; Fernández-Delgado et al. 2007; González-Ortegón et al. 2010), and chemical contamination followingthe Aznalcollar mining waste spill (Grimalt et al. 1999;Blasco et al. 1999). Three years ago, to obtain a betterunderstanding of the physical and biological processesgoverning the estuary ecosystem, an interdisciplinaryresearch program was established, with the following maingoal: “to develop an integral method to diagnose and

forecast the consequences of human actions on theGuadalquivir estuary.” To achieve this goal, a comprehensivemonitoring program has been established in the estuary andriver mouth. This program has comprised three main parts: thereal-time remote monitoring (RTRM) network, the deploymentof several moorings in the estuary and river mouth, andmonthly cruises for water sampling. The stations of the networkand moorings are strategically positioned at locations that areimportant for hydrological dynamics. The locations span thewhole estuary but with increased spatial resolution and shorterdistance between stations close to the mouth, where environ-mental gradients are more acute and dynamic (Fig. 1).

This monitoring program was set up in the summer of2007 in the estuary and near the continental shelf and isproviding detailed data for research, policy-makers, gov-ernment agencies, and education/outreach applications oncoastal and transition waters management. In this paper, wereport on the technical layout of the various stations and on

Fig. 1 a Inset, the area of interest located in the SW IberianPeninsula; b inset and main illustration, locations of the variousstations of the RTRM network, moorings, and water sampling pointsof the Agencia Andaluza del Agua in the Guadalquivir estuary and

along the river. Other insets are photographs/drawings of variousbuoys: c the Don Isaias water quality station, no. 52; d the waterdynamics station, no. 47; e the Salmedina buoy; f water quality stationno. 9; and g water dynamics station no. 7

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the experience gained of their performance during the last3 years of operation. The paper is focused mainly on theRTRM network that has been operated in the study area bythe Instituto de Ciencias Marinas de Andalucía (ICMAN).This network provides data with a high degree of temporalresolution within a Eulerian framework, and has theadvantage that constant surveillance can be carried out torapidly detect any changes and trends in critical indicators.Remotely acquired data from continuous in situ monitoringprovides important early warning information to decision-makers so that they can respond appropriately. RTRMtechnologies have emerged as an economically viablemeans of monitoring key hydrological parameters (Glasgowet al. 2004). The calibration/validation of the instruments isdescribed in the “Results and discussion” section togetherwith the approaches adopted to overcome the limitations thatwere encountered during the operational use of these stations.Also, in the “Results and discussion” section, we present apreliminary analysis of the data sets obtained with thenetwork with the aim of understanding the temporal andspatial variability of the physical and biological processes thatdominate in the estuary. Finally, in the “Conclusions” section,some conclusions about the RTRM network are presented.

2 Materials and methods

2.1 Location

The Guadalquivir estuary (SW Spain: 36º 45′ - 37º 15′ N, 6º00′ - 6º 22′ W) is a well-mixed system with a longitudinalsalinity gradient (Vanney 1970). The tidal influenceextends up to the Alcalá del Río dam (Álvarez et al.2001) and the maximum tidal range for the Guadalquivirmouth is 3.86 m (Rodriguez-Ramirez and Yáñez-Camacho2008). Between the river mouth and the Port of Seville,several navigation buoys (Fig. 1) are deployed by the PortAuthority of Seville, and these have been transformed intoenvironmental laboratories.

2.2 Monitoring program

The three parts that comprised the monitoring programwere set up on different dates: monthly oceanographiccruises started in June 2007 and the installation of themoorings and RTRM stations started in March 2008 and atthe beginning of 2008, respectively. The real-time telemetrysystem was aimed at providing online continuous meteoro-logical, hydrographic, and water quality information. Thewater samples were used both to supplement the telemeteredwater quality information and to provide a cross-check onsome of the other important parameters. To complete themonitoring program, daily data on discharges from the Alcala

del Rio dam were obtained from the regional water manage-ment agency (Agencia Andaluza del Agua, Junta de Andalucía,http://www.juntadeandalucia.es/agenciadelagua/saih/). Thisagency also carried out several cruises in the estuary tomeasure hydrological parameters (conductivity, dissolvedoxygen, and turbidity), and its water quality stations aresituated along the estuary (Fig. 1). Moreover, there are somemeteorological stations located in Sanlucar de Barrameda andChipiona at the estuary mouth that we have used to validatedata obtained by our meteorological station. In addition, RGBMODIS images have been downloaded from the NASAserver (http://rapidfire.sci.gsfc.nasa.gov/). These images areprocessed and projected using MATLAB(c) software. Satelliteimagery can resolve patterns of turbidity on a large spatialscale but is restricted to surface data.

Water samples were collected each month at the fieldstations from June 2007. Parameters measured in theroutine water sampling included: nutrient concentration(nitrite, nitrate, phosphate, and silicate) following JGOFSstandards (UNESCO 1994), chlorophyll concentrationusing fluorometric methods following JGOFS protocols(UNESCO 1994), dissolved organic carbon and totalnitrogen (TOC-VCPH, Shimadzu, Japan), dissolved oxygen(Winkler method), total alkalinity (Pérez and Fraga 1987),suspended particle matter (SPM) using the gravimetricmethod, phytoplankton, and zooplankton. Vertical profilesof temperature, salinity, dissolved oxygen, turbidity, andchlorophyll fluorescence were measured with a CTD-Seabird 19 probe with external sensor. Water samples weretaken at the same depths as the pumping levels of theRTRM station to validate quality data from the RTRM.

Several moorings were deployed in the estuary to studythe tidal regime and hydrodynamics. Seven tide gauges(Aqualogger 520PT and NKE-SP2T logger) were deployedon the bank of the river between Sanlucar de Barramedaand Seville (Fig. 1). Currents at 1-m intervals and surfacewaves were measured with an upward-looking Nortekmodel AWAC-AST acoustic Doppler profiler, which wasput into operation in May 2008 at a depth of 16 m in theriver mouth (Fig. 1). A thermistor chain was moored on thecontinental shelf nearby to study the seasonal stratification/mixing and to establish the boundary conditions for models.More details on the sensor equipment deployed in themoorings are given in Table 1.

The RTRM network comprises three types of stations:for water dynamics, six stations; for water quality, eightstations; and one meteorological station. These stations areinstalled on the navigation buoys positioned between theriver mouth and Seville harbor for water dynamics andwater quality and on the Salmedina buoy for meteorologicalmonitoring (Fig. 1). The stations were designed for year-round operation, with planned maintenance quarterly. Allstations were strategically positioned. The technologies

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employed offer several advantages over historical monitoringtechniques: they streamline the data collection process, theypotentially minimize human errors and time delays, theyreduce the overall cost of data collection, and they signifi-cantly increase the quantity and potentially increase thequality of data obtained on temporal and spatial scales(Glasgow et al. 2004). Details of the technology applied inthe construction of the prototype stations for water dynamicsand water quality can be found in Gutierrez et al. (2009).

2.2.1 Meteorological station

The meteorological station was installed on the Salmedinabuoy (Fig. 1e) located over the continental shelf offChipiona (Fig. 1, map) in May 2008. The system comprisesan array of sensors for meteorological measurements (airtemperature, relative humidity, incident solar radiation,barometric pressure, and wind speed and direction).Meteorological data is collected as follows. Barometricpressure is obtained with a Young 61202 L barometer. Airtemperature and relative humidity data are acquired via aGeonica STH-5031 instrument (Geonica(c), Spain). Windspeed and direction are obtained using a marine windsensor (R.M. YoungWindMonitor). Incident solar radiation iscollected by a pyranometer (model Licor Li200 LiCorBiosciences, Lincoln, NE, USA). Different measuring inter-vals can be selected but this station was designed to sampleevery second, and the average, maximum, and minimumvalues for a 10-min interval are sent to the laboratory bytelemetry. A Geonica Hydrodata 2008CP datalogger serves asthe central processing unit of the system and the energy supplyis from a gel battery charged by a bank of three solar panelsconnected in parallel. Bi-directional communication betweenthe meteorological station and the laboratory enables theinstruments and sensors installed to be controlled and servicedremotely.

2.2.2 Water dynamics stations

Six stations have been deployed along the estuary (Fig. 1,map) since February 2008. Figure 1d, g also showsphotographs of the transformed navigation buoy at a currenttelemetry station. These stations are equipped with a NortekAS Aquadopp acoustic Doppler current profiler (ADP)operating at a frequency of 1,000 kHz. The deployment

configuration has been set to a cell size of 1 m, with anintegration period of 2 min and four profiles per hour. TheADP has been programmed independently from thetelemetry module as the two modules work asynchronously.The ADP also measures temperature, head pressure, pitchand roll. These parameters are used in real time to assessthe quality of data. The ADP is controlled by a GeonicaHydrodata 2008CP datalogger, and the data are sent to thelaboratory every 15 min by GPRS system. The energysupply to the ADP and datalogger is from a gel batterycharged by a bank of three 45-W solar panels connected inparallel. The compass was calibrated following the manu-facturer’s specifications. Information on the amount ofsuspended particles can be deduced from the intensity ofthe backscatter signal measured by the ADP (Hill et al.2003).

2.2.3 Water quality stations

Eight stations have been deployed along the estuary (Fig. 1,map) since February 2008. Figure 1c, f also showsphotographs of the transformed navigation buoys operatingas water quality laboratories. This type of station comprisesfour modules: the power module (a bank of three 120-Wsolar panels connected in parallel, charging a gel battery),the hydraulic module (a SHURflo suction pump, flowmeter, batch filter, and silicon piping), the measurementmodule, and the control module (a Geonica Hydrodata2008CP datalogger). The functioning of this station isdescribed in more detail in Gutierrez et al. (2009). Routinemaintenance of the water quality stations was carried outmonthly and involved replacing the pumping module andcleaning the CTD and external sensors. When necessary,the anti-fouling cylinders (SBE A24173) inside the CTDwere replaced.

The measurement module is comprised of a SeabirdElectronics SBE16plus conductivity and temperature recorderwith external sensors for dissolved oxygen (SBE43), achlorophyll fluorometer (Turner Designs, model Cyclops-7),and a turbidimeter (Turner Designs, model Cyclops-7). Thesalinity (S) and density (ρ) are calculated from temperature,conductivity, and hydrostatic pressure (p) with the equationsof state of seawater (UNESCO 1985). Measuring intervalscan be selected, but the standard measuring rate is two cyclesper hour.

Table 1 Sensor equipment installed in moorings

Tide gauges Aqualogger 520 PT, between March and June 2008. sampling rate, 10 min

NKE SP2T logger between June 2008 and March 2009. Sampling rate, 10 min

Thermistor chain RBR-Thermometrics, between November 2007 and August 2008. Sampling rate, 1 min

NKE-S2T. From June 2009. Sampling rate, 10 min

Currents/waves AWAC-AST Nortek 1,000 kHz. Currents: sampling rate, 10 min; cell size, 1 m. Waves: sampling rate, 1 h

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During the first 9 months, the water quality stations tookmeasurements at four depths (1-, 2-, 3-, and 4-m depth fromthe sea surface). After November 2008, measurements wereonly taken at a depth of 1 m because the estuary was wellmixed and the maintenance cost could thus be reduced.Continuous monitoring generates more than 7,000 waterquality records per day, accessible in near real-time.

Logged data were uploaded to a data acquisitioncomputer located in the laboratory via GPRS communicationsand posted in graphical form on the website (http://www.guadalquivir.csic.es); this webpage is currently password-protected and therefore not accessible to the general public.Data acquisition programs were modified for the specificinstrument packages.

3 Results and discussion

3.1 Calibration/validation program

The Guadalquivir estuary is characterized by high currentvelocities, high loadings of suspended matter (González-Ortegón et al. 2010), and high biological productivity (Ruizand Navarro 2008). All of these characteristics affect thestability of the instruments, which have been deployed forextensive periods of time. Therefore, a calibration/validationprogram is required to monitor and maintain the quality of thedata collected by the RTRM network during its operation.

3.1.1 Calibration program

ADP instruments (an AWAC-AST located at the rivermouth and the several Aquapro’s deployed at waterdynamics stations) and meteorological sensors were cali-brated by their manufacturers. For the water dynamicsstations, every compass was calibrated at the buoys tocompensate for the spurious or residual magnetic effect.The tide gauges (Aqualogger 520 PT and NKE SP2T) andthe thermistor sensor of the thermistor chain were alsocalibrated by their manufacturers.

Sensors for temperature (SBE16plus), conductivity(SBE16plus), and dissolved oxygen (SBE43) installed atthe water quality stations were calibrated by the manufacturer,Seabird Electronics. Turbidity and chlorophyll fluorescencesensors (Turner Design's Cyclops) were calibrated in ourlaboratory as detailed in the following subsections.

High turbidity levels are very common in the Guadalquivirestuary (González-Ortegón et al. 2010). When turbidity levelsare above 1,000 Formazin Nephelometric Unit (FNU), theresponse of the turbidity sensor becomes nonlinear and adetailed calibration of every sensor becomes necessary. Forthis purpose, a protocol was designed to characterize theresponse of the turbidity sensors up to about 8,000 FNU,

using formazin dilutions from 400 to 4,000 FNU (Anderson,2005) and milk dilutions up to 8,000 FNU. Figure 2arepresents the response of each turbidity meter versus thefraction of milk dilution. Differences between sensors makeit necessary to calibrate each sensor individually beforebeing used. The relationship between the sensor response(volts) and the turbidity dilution (FNU) for sensor number634 is represented in Fig. 2b, c and it has been fitted by afourth-degree polynomial. We carried out this analysis foreach of the nine turbidity sensors to establish the polynomialrelationship between volts and turbidity standard units(FNU). The results of calibration experiments are presentedin Table 2.

The dynamics of suspended particulate matter areimportant for the functioning of the ecosystems of theGuadalquivir estuary and coastal zone. These dynamicsgovern processes such as sediment transport and primaryproduction, the latter being limited by the availability oflight in the water column. To measure the SPM with theRTRM network, we have established a relationship betweenSPM (measured by gravimetric method) and turbidity (FNUunits). This relationship for all water quality stations isdepicted in Fig. 3.

With respect to chlorophyll fluorescence and due to thehigh turbidity levels found in the Guadalquivir estuary,turbidity has two primary effects on chlorophyll fluorescencereadings: first, it may increase blank readings due to increasedlight scatter and, second, it may reduce the fluorescencereading due to light absorption. To calibrate the chlorophyllfluorometer in turbid water, we used the method proposed bythe manufacturer (Turner Designs, Application Note S-0035),whereby we have simultaneously measured turbidity andfluorescence in estuary waters. We have demonstrated thatfluorometers run correctly when the turbidity is lower than1,500 FNU.

3.1.2 Validation program

To validate the quality of the data collected through theRTRM network, we have obtained external data asmeasured by other institutions (see “Materials and methods”section) and in our own monthly cruises (ICMAN) forcomparison purposes. For example, Fig. 4 shows the timeseries of air temperature measured by our own meteorolog-ical station and by another station belonging to IFAPA, partof the Junta de Andalucía (http://www.juntadeandalucia.es/agriculturaypesca/ifapa/ria/). The correlation is very good forthis and the rest of the parameters (data not shown).

Currents measured by the water dynamics stations werevalidated with a different ADP (ADP-1,000 kHz SonTek)which integrates a GPS and bottom tracking. During a tidalcycle, we measured water velocity profiles near the buoyusing the same equipment configuration (cell size, integration

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time, etc). The results show that the water velocity profilesrecorded by the RTRM stations are measured correctly (datanot shown).

Figure 5 shows an example of the validation of theparameters measured by several water quality stations. Forthis validation, we have used data from cruises carried outby the Agencia Andaluza del Agua (GQ) and our ownmonthly cruises (ICMAN). A comparison between theRTRM network and in situ measurement shows a goodvalidation for all parameters. This figure shows clearly the

advantage of the RTRM over cruises; information capturedonly by cruises is not enough to resolve all frequenciesinvolved in the estuarine dynamics.

3.2 General field operation of the RTRM

The RTRM network has been operating successfully,providing good quality data since its installation (Figs. 4and 5). One criterion for time series quality is theavailability of data. Figure 6 presents the availability of

Fig. 2 a Comparison betweensensor response in volts andfraction of milk dilution for eachturbidimeter (see Table 2). bRelationship between turbiditysensor response (in volts) forsensor 634 and fraction of milkdilution. The horizontal greenline corresponds to 1.6094 V,which is 90% of the maximumsaturation signal (∼1.7882 V, at∼0.2 % dilution). The verticalgreen line corresponds to thefraction of dilution for thatvoltage (0.1325%). The blackline represents the sensorresponse in volts (∼1.1512 V)for a calibrated formazin dilu-tion of 4000 FNU. Assuming alinear relationship between frac-tion of milk dilution and turbid-ity level of this dilution, it ispossible to apply a linear corre-spondence between turbidity (inFNU) and fraction of milk dilu-tion in the range between 0 and0.1325%. c Turbidity in FNU(calculated from fraction of milkdilution) versus maximumdynamic range of sensor signal.The fourth-degree polynomialfitting has been calculatedfrom this dataset

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data for each station (water quality, a to h; water dynamics,i; and meteorological data, j). In June of 2008, all theinstruments of all the water quality stations, except number52, were transported to the laboratory for general mainte-nance. This work caused the 1-week gap observed in thetime series at that time. To prevent the time series frombeing interrupted for such a long period by the generalmaintenance required in the following summers, the instru-ments removed from the stations and brought to the lab formaintenance were substituted by similar ones in the field,thus avoiding such big data gaps. However, some other datagaps occurred between maintenance dates: the main reasonfor data gaps at the water quality stations was suction lossat the pump module (filter saturation, premature wear ofpump membrane). Another reason was that the stainlesssteel cases of the Cyclops fluorometers systematicallybecame corroded after 20 months of operation and theyhad to be returned to the factory. A third problem was that

the data reception pin at the male end connector of theAquadopp cables started to fail: data continued to bereceived by the datalogger, but the operator was preventedfrom re-programming the equipment in the field andretrieving the full data files.

Figure 6 shows that data availability can be consideredexcellent since more than 97% of the total expected datawas acquired. In addition, the data quality is sufficient interms of accuracy, as it is evident from Figs. 4 and 5.System reliability is guaranteed by the fact that measure-ment and analysis were still operational under severeconditions, including storms, high winds, spring tides, andhuge river discharges. Results obtained during some ofthese extreme events will be analyzed, and they will beused to validate hydrodynamic and ecological models.

3.3 Multi-year time series data

This paper presents high-resolution, long-term hydrologicaland hydrodynamics measurements obtained at the Guadalqui-vir estuary. The data reveal the wide fluctuations of waterquality parameters in highly dynamic tidal regimes. Inaddition, special events, such as a high river discharge, arecaptured (April–May 2008, March–April 2009, and December2009; Fig. 5). These data are valuable for estimating theimpact of such events on the biogeochemical dynamics in theGuadalquivir estuary. For instance, they can be used tovalidate model calculations. These topics will be discussed inmore detail in a separate paper.

Most of the time, the tidal regime controls the morpho-hydrodynamics of the estuary (Fig. 7). Only high riverdischarges (Q>1,000 m3/s) during the wet season arecapable of reversing this situation, influencing free surfaceelevation, tidal currents (see Fig. 8), and sediment transport.The RTRM network also diagnoses the transition between atidally and a discharge-dominated regime in the estuary (Díez-Minguito et al. 2009). These results are particularly relevantfor the estuary managers and public decision-makers.

Fig. 3 Relationship between suspended particle matter (SPM, in mg/L)and turbidity levels (in FNU) in the Guadalquivir estuary (SPM=1.6015* turbidityCTD , R2=0.93, n=25 samples from five stations)

Table 2 Fourth-degree polynomial calibration coefficients for turbid-ity sensor. Turbidity (FNU)=p1 * V3+p2 * V2+p3 * V+p4, where V isthe turbidity sensor response in volts (Gain 1x). R2 is regression

coefficient, Voffset is minimum sensor response in millivolts, Vmax is90% of the sensor saturation signal, and Tmax represents the turbidityin FNU corresponding to Vmax

Sensor p1 p2 p3 p4 R2 Voffset (mV) Vmax (mV) Tmax (FNU)

T632 118.83 −54.73 1,107.82 −83.97 0.999 76 3.5008 8,600

T633 593.16 −335.53 1,840.72 −124.99 0.999 68.7 2.1183 8,200

T634 1,243.68 −420.99 2,460.51 −125.24 0.999 51.3 1.6094 8,200

T635 229.49 −160.58 1,285.21 −96.15 0.999 75.4 2.9043 8,200

T636 477.79 −262.97 1,702.19 −118.47 0.999 70.3 2.253 8,100

T637 495.97 −163.21 1,600.33 −107.39 0.999 67.5 2.1783 8,100

T638 416.76 −81.42 1,547.69 −99.30 0.999 64.3 2.312 8,500

T639 1,716.89 −676.78 2,569.83 −141.33 0.999 55.7 1.4972 8,200

T397 188.68 −209.86 1,460.40 −113.23 0.999 78.4 3.18 8,800

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Fig. 5 Real-time remote monitoring data (n>40,000 observations at30-min intervals) for temperature (a), conductivity (b), dissolvedoxygen (c), turbidity (d), and chlorophyll fluorescence (e) measuredby several water quality stations from March 2008 to August 2010.

Green and red dots correspond to data from our own monthly cruises(ICMAN) and from the water sampling station of the AgenciaAndaluza del Agua (GQ), respectively

Fig. 4 Time series of air temperature. Blue dots correspond to datafrom our own meteorological station located on the Salmedina buoy(10-min intervals). Red and green dots correspond to daily mean data

from the meteorological stations belonging to the Junta de Andalucía,located at Chipiona and Sanlucar de Barrameda, respectively

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Over the 3-year period, the biogeochemical parametersclearly show several sources of variability. Figure 9 showsthe time series of buoy 34 during the 2 years that theRTRM system was in operation. From the data, theimportance of the tidal dynamics and periodic inputs of

fresh water from the Alcala del Río dam can be clearlyseen. We can observe the seasonal cycle of temperaturewith maxima in summer and minima in winter. Temperaturemonitoring is basic to understanding the current on thecontinental shelf (García-Lafuente et al. 2006; García-

Fig. 6 Bar chart of daily data availability (more than 97%). Panels a–h represent each water quality station. Color indicates each parameter:Blue is temperature, red is conductivity, green is dissolved oxygen,yellow is turbidity, and cyan is chlorophyll fluorescence. Panel i isdata availability for water dynamics stations. Color indicates each

station: blue, red, green, yellow, cyan, and magenta are buoys 7, 11,16, 20, 30, and 47, respectively. Panel j is data availability for themeteorological station. Color indicates each parameter: blue, red,green, yellow, and cyan are wind, temperature, radiation, barometricpressure, and relative humidity, respectively

Fig. 7 Time series of the predicted tidal amplitude (a), daily waterriver discharge from the Alcala del Río dam (b), barometric pressurefrom the Salmedina buoy (c), air temperature from the Salmedina

buoy (d), wind velocity (e) and direction (f) from the Salmedina buoy,and g water current velocity from water dynamics station no. 47(negative, downstream current in the estuary) at 1-m depth

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Lafuente and Ruiz 2007). Salinity shows higher short-termfluctuations than the water temperature, and salinity is muchless dominated by seasonal factors, although there are highcorrelations with heavy discharges into the river from theAlcala del Río dam. The maxima of river discharges (April2008, February 2009, January and March 2010) coincidewith increases in turbidity and decreases in salinity,especially during the winter of 2009–2010 (Fig. 10). The

salinity pattern in the estuary is very important since it playsa dominant role in determining the composition and spatialdistribution of its aquatic communities (Drake et al. 2002;Cuesta et al. 2006; González-Ortegón et al. 2006). Therefore,time series measurements offer valuable possibilities foridentifying trends in environmental parameters, e.g., seawa-ter temperature and salinity, which have been presentedabove.

Fig. 8 Time series of the predicted tidal amplitude (a), daily river discharge from the Alcalá del Río dam (b), and water current velocity fromwater dynamics station no. 47 (negative, downstream current in the estuary) at 1-m depth (c)

Fig. 9 Time series of the predicted tidal amplitude (a); daily waterriver discharge from the Alcala del Río dam (b). Hydrologicalparameters measured by water quality station no. 34 at 1-m depth

for temperature (c), salinity (d), dissolved oxygen (e), chlorophyllfluorescence (f) and turbidity (g)

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Discharges into the river, together with tidal dynamics,also affect the levels of dissolved oxygen and turbidity.During the 2 years of study, there were several dry and wetseasons, with minima and maxima, respectively, in theturbidity pattern. The importance of the turbidity pattern isthat it affects many diverse variables, from primaryproduction (Gonzalez-Ortegón et al. 2010) to tourismactivity. During the dry season, normally the summermonths, the turbidity pattern seems to follow the springtides, as has been reported for other areas like the Tagus(Valente and Da Silva 2009) and the German Wadden sea(Bartholomä et al. 2009). In fact, the RTRM results enableall the mechanisms involved in the salinity and turbiditypatterns to be differentiated. Figure 11 shows the timeseries of salinity and turbidity data from buoy 34 during thedry season in summer 2008. The salinity distribution isgoverned by tidal dynamics (the main harmonic is M2),whereas in the turbidity distribution harmonic componentslike M4, which are related to hydrodynamics, appear.Moreover, the turbidity pattern shows a fortnightly regimeassociated with spring tides, also related to estuaryhydrodynamics. The dynamics of chlorophyll fluorescenceis highly complex and does not appear to follow any clearseasonal pattern.

4 Conclusions

After more than 2 years of operation, the RTRM systemdescribed provides an effective means of monitoring

estuarine Guadalquivir water, with high-temporal resolution.This platform is very important because estuarine ecosystemsare characterized by high inherent spatio-temporal variability(Day et al. 1989). Reference has been made both to themechanical modification of navigation buoys to convertthem into valuable environmental laboratories and to thesensors and loggers shown to be adequate for reliablymeasuring the key meteorological, hydrodynamic, andhydrological parameters. These parameters are used byspecialists to make decisions on maintenance and theoperation of each station for monitoring water dynamicsand water quality. The stations can be mounted without anyalteration of the buoy, and this can be performed in situ in afew hours (two or three stations can be mounted per day).The modularity of the stations allows them to be adaptedeasily and deployed in other estuaries, ports, dikes, piers,etc., with minimal maintenance.

The high data availability and the good quality of thedata obtained make RTRM a powerful tool for monitoringand analyzing the diverse processes that govern this kind ofenvironment. Moreover, in this case, RTRM has beencomplemented with moorings and sampling cruises thatprovide additional information for understanding theestuary ecosystem. The limited information that can becaptured by cruises alone is not enough for a fullunderstanding of the estuary ecosystem. Another advantageof the RTRM network is that it has been shown to becapable of continuing to operate reliably during exceptionallyhydrographic conditions, like a huge river discharge, highcurrent velocities, and high water turbidity. Figure 12 shows

Fig. 10 Time series of the predicted tidal amplitude (a), daily water river discharge from the Alcala del Río dam (b), and salinity patterns fromwater quality stations (c–i)

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some RGB MODIS images of the river plume before,during, and after a large river discharge. This plume has anenormous amplitude and accounts for the large quantity ofsuspended matter that the estuary contains. The RTRMnetwork has proved to be an excellent platform for obtaininglong-term hydrologic data irrespective of weather conditions.

RTRM network enables a variety of types of study andapplication to be carried out, as demonstrated in theprevious section. The sampling rates were 10, 15, and30 min for meteorological, water dynamics, and waterquality stations, respectively. These rates allow processessuch as overtides (harmonics like M6 and M4) and inter-

annual variability to be resolved. Numerical models havebeen applied to this large volume of high-frequencyinformation to make forecasts of how ecosystems in theestuary are likely to develop. In addition, real-time remotemonitoring could be used to generate early warnings fornavigation, algal bloom, turbidity increases, etc. The datacan also be sent automatically to other cooperatingagencies.

This type of system would aid in the earlier forwardplanning of measures to mitigate undesirable events orchanges. In the near future and given the modularity of thenetwork, we hope to install a new type of biosensor

Fig. 12 Time series of the daily water river discharge from Alcala del Río dam (a) and turbidity patterns from water quality station number 34(b). Shaded areas indicate the data corresponding to the dates of the RGB MODIS images presented in the bottom panel

Fig 11 Tidal amplitude (a, e), time series of salinity (b, f), turbidity (c, g) and power spectrum for salinity (d) and turbidity (h). Significantconstituents are included

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(Altamirano et al. 2004) to detect and give immediatewarnings of phytoplankton blooms, a crude oil sensor towarn about oil spills, and a nutrient analyser to study theeutrophication of the estuary. Despite the present economiccontext, the long-term continuity of the system is underconsideration by regional administration given its diagnosingpower, autonomous simplicity, and low-cost robustness.

Acknowledgements This project was supported by the AutoridadPortuaria de Sevilla (APS) and by the Consejería de Innovación,Ciencia y Empresa (Junta de Andalucía). MODIS images have beenprocessed in the context of P09-RNM-4583 Project. Thanks are due toall the APS technicians and divers of AGUAYO S.L. who participated.The authors also thank, in particular, David Roque, Raúl García, JoaquínPampin, and Antonio Moreno (ICMAN-CSIC) for their assistance.

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