3.2 The Ebro River Basin

The Ebro river basin (Chapters 4, 5, and 6, Section 3) is the biggest catchment of the Iberian Peninsula flowing west to east, coursing into the Mediterranean (Figure 6). It is more than 85,000 km² wide and exposed to quite variable climates due to its size, wherein the west rainfall is heavier and flows are higher (Barceló et al. 2011). Orography does also vary. The southern riverbank is relatively low in altitude in relation to the northern. The latter is mostly located on the Basque-Cantabrian mountains and the Pyrenees. The eastern river side passes the lower Catalan coastal mountain ridge. 

Figure 6 Subdivisions of the Ebro river Basin into Ecoregions. Points = exemplary diatom sampling in 2013.  

The Ebro river basin is conformed by 702 rivers and tributaries that conform this basin with a combined length of approximately 12,00 km [www.chebro.es]. Natural rivers predominate, with 70 very modified rivers and two “artificial” ones. There are 102 lakes in the basin. In the delta region there are 16 transitional water bodies and 3 coastal lakes. The average flow in the whole basin is of 14,623 hm³/year, albeit with substantial interannual variations due to climate.

The Main River Axis is characterized by a high anthropological input due to the location of big cities on its catchment. This implies high industrialization, a potential heavy metal input and priority substance pollution. The population density varies highly throughout the catchment. Big cities alternate with agricultural land or even the Monegros desert (Romaní et al. 2011). Population surpassed three million in 2013.

The whole basin has 125 principal reservoirs (Prats et al.), of capacities over 1 hm³. These water course changes are not regulated by the WFD. The course of the main axis is also affected by agricultural irrigation canals. These consume an average of 5085 hm³/year of water. A total 430 hm³/year is transferred or used for irrigation, drinking water, energy gain, or industry.

In 2005 a report on preliminary evaluation of climate change was made for this basin. An increase of temperature and a reduction of rainfall were foreseen, that would be added to the already erratic effect of the Mediterranean climate (García Vera et al. 2005). A significant reduction of water flow down to 5-50% on the volume measured in the previously studied interval from 1970-2000 were also predicted. This will reduce water quality even further (Aguilera et al. 2015; Bovolo et al. 2011). Thus, water quality needs to be improved to counteract this effect. Rivers of the southern Ebro riverbank are expected to be most strongly affected. The biological study of this basin has lagged in relation to the formal studies. 

Big studies were made into flora and fauna. Studies involved creating check-lists and species distribution, based mostly on diatoms and macroinvertebrates, and to a lesser degree fish and macrophytes (Sabater et al.; Oscoz et al.). Nonetheless, further, more physiological understanding of the organisms could improve biomonitoring.

The rivers of the Ebro basin are in two main biogeographical regions, the Euro-Siberian region, located at the higher mountains, and the Mediterranean region, mostly at the river axis. In 2007, the Water Framework Directive established river ecotypes only based on biogeochemical and extrinsic traits (Annex Table 1), later translated for use in Spanish rivers (Ministerio de Agricultura). A previous subdivision of the Ebro Basin had been attempted by Munné and Prat in 1999. They used physical and chemical parameters, cross-referencing them with macroinvertebrate community structures (Munné and Prat).

A new subdivision, including the degree of alteration of the riverbed, was established in 2016 (BOE 2016). In this thesis we have not used this new subdivision, since it follows the previous one, only including a mention when altered.

Bioindicator distribution of the IPS index was calculated for each of the river ecotypes separately. A calculated Ecological Quality Ratio (EQR) was obtained from the respective reference sites to account for the variability between them (Table 3).

Table 3 Comparative ecological state ranks for each ecotype’s IPS values established in 2010 (Ministerio de Medio Ambiente y Medio Rural y Marino. 2008, ARM/2656/2008). 109 = mineralized rivers from low Mediterranean mountains, 111 = Mediterranean siliceous mountain rivers, 112 = Mediterranean calcareous mountain rivers, 115 = Continental and Mediterranean slightly mineralized axes, 116 = Continental and Mediterranean mineralized axes, 117 = Main axes in Mediterranean environment, 126 = Rivers of wet calcareous mountains, 127 = High mountain river.

Ecological Scale	109	111	112	115	116	117	126	127
High	>16.8	>16.2	>16	>15.1	>14.2	>11.7	>16.3	>17.4
Good	12.6 – 16.8	12.2 – 16.2	11.9 – 16	11.3 – 15.1	10.6 – 14.2	8.8 – 11.7	12.2 – 16.3	13.1 – 17.3
Moderate	8.4 – 12.5	8.1 – 12.1	8 – 11.8	7.6 – 11.2	7.1 – 10.5	5.9 – 8.7	8.1 – 12.1	8.8 – 13
Poor	4.2 – 8.3	4.1 – 8	3.9 – 7.9	3.8 – 7.5	3.5 – 7	3 – 5.8	4.1 – 8	4.38 – 8.7
Bad	<4.2	<4.1	<3.9	<3.8	<3.5	<3	<4.1	<4.3

 

 

 

 

 

 

3.2.1.1 Diatoms at the Ebro Basin

The diatom river quality control network of this basin was established in 2002, trying to comply with the Water Framework Directive (Gomà et al.). Over time, from control network instauration to 2013, a total of 459 sites have been sampled at least once. A thorough observation of the diatom flora and subdivision of the Ebro river basin (2005 – 2006) appeared in the thesis of Ortiz-Lerín (2012).

An inspection of the total flora and fauna of the Ebro river basin was included into a compendium (Sabater et al.), describing patterns of diatom distribution. Thus, low diversity communities at headwaters were defined by Achnanthidium minutissimum (Kützing) Czarnecki and Achnanthidium pyrenaicum (Hustedt) Kobayasi. The upper Segre, due to its silicate content, had a quite unique community, with Achnanthidium subatomus (Hustedt) Lange-Bertalot, Diatoma mesodon (Ehrenberg) Kützing, Encyonema silesiacum (Bleisch) D. G. Mann, Hannaea arcus (Ehrenberg) R. M. Patrick, Fragilaria capucina Desmàzieres, Gomphonema pumilum (Grunow) E. Reichardt & Lange-Bertalot, Meridion circulare (Greville) C. Agardh and Nitzschia pura Hustedt. 

Slower waters at the middle part of the river, were characterized by slower water flow and high mineral contents, as shown by the preponderance of Amphora pediculus (Kützing) Grunow ex A. Schmidt and Cocconeis placentula Ehrenberg. Finally, with more pollution, communities were described by Navicula cryptotenella Lange-Bertalot, Sellaphora atomoides (Grunow) Wetzel & Van de Vijver, Craticula subminuscula (Manguin) Wetzel & Ector, and Nitzschia inconspicua Grunow. In oligotrophic sites with phosphate access, appearance and even mass-formation of Didymosphenia geminata (Lyngbye) Mart. Schmidt was observed.

3.2.1.2 Description of exemplary sites

Here we will describe a typical site for each of the river ecotypes established for the Ebro river basin (Ministerio de Medio Ambiente y Medio Rural y Marino 2008; European Commission 2000, Annex II). We have selected sites from each river ecotype that was monitored in the morphology section of chapter 6. Since no site of ecotype 109- mineralized rivers from low Mediterranean mountains was selected, we have taken a reference site. The physicochemical description of the sites is listed in AnnexTable 2.

Ecotype 109 – Mineralized rivers from low Mediterranean mountains (Reference site 1141)

Alcanadre is a tributary to the Aragon river. It can suffer point pollution due to spills and debris and is modified upstream by two diversion dams for irrigation (Confederación Hidrográfica del Ebro and Gobierno de Aragón). Despite of these caveats, both diatom and macroinvertebrate biomonitoring display high water quality (Confederación Hidrográfica del Ebro and Gobierno de Aragón). The river at this site has wide catchments without slopes, moderate salinity (⌀ 371, 297 – 493 µS/cm) and temperature (⌀ 12.8, 0.8 – 24.5 ºC).

Ecotype 111 – Mediterranean siliceous mountain rivers (Referebce site 1178)

Najerilla is a stream of 72.4 km in length with a 1.107 km² wide catchment. Its total elevation gradient is 1,595 m. 8 annual samples were taken in the site of Villavelayo. Its population of over 50 citizens is surrounded by natural and forest land uses. Its water quality has always been good-very good. Nonetheless, urban spills are found in summers (Confederación Hidrográfica del Ebro et al. 2007). The main defining attribute is the soil composition. All sites of this ecotype are on the Iberian ridge.

The water composition was characterized by relatively low conductivity (⌀ 506.8 µS/cm, 272.3 – 658 µS/cm), nitrate and phosphate concentration levels were erratic, mostly low, with a peak in the years 2008 – 2009. The water temperature is moderate (⌀ 11.3 ºC, 2.4 – 16.8 ºC). The most crucial difference is the concentration of Silica (⌀ 6.787 mg/l, 5.9 – 8.53 mg/l) and a tendency to alkalinity (⌀ pH=8.4, 7.05 – 8.7).

Ecotype 112 – Mediterranean calcareous mountain rivers (site 0038)

The Najerilla river ends in the Guatizalema river, a tributary to the Ebro. The lower catchment is warm and wide, and its main land use are viticulture and irrigated land. Its sediment is mostly calcareous and porous. The catchment is 1,107 km² wide and has an average flow of 13.7 m³/s. It has two main alterations given by hydroelectric plants that vary its hydrology (Confederación Hidrográfica del Ebro et al. 2007). 

Its waters are very carbonated, with sulphate peaks due to gypsum in the substrate (⌀ 81.3 mg/l, 22.2-129 mg/l). Main conductivity ranges mainly between 200 and 400 µS/cm and can go up to 700 µS/cm, with an increase between 2007 and 2013. Nitrate concentration can reach up to 18 mg/l. Industrial point pollution has been recorded, and thus the protocol for dangerous substances has been activated (European Parliament and Council of the European Union, “Decision N 2455/2001/EC of the European Parliament and of the Council of 20 November 2001 Establishing the List of Priority Substances in the Field of Water Policy and Amending Directive 2000/60/EC”; European Parliament and Council of the European Union, “Directive 2006/11/EC of THE EUROPEAN PARLIAMENT AND THE COUNCIL OF THE EUROPEAN UNION of 15 February 2006 on Pollution Caused by Certain Dangerous Substances Discharged into the Aquatic Environment of the Community”).

Ecotype 115 – Continental and Mediterranean slightly mineralized river axes (site 0101)

Aragón is a main tributary of the left catchment side of the Ebro. The transversal profile is open and less sloped. It has a catchment size of 2,171 km² upstream, and an optimum ecological flow of 4,520 m³/s. Its quality is good, even though the conductivity tends to increase, due to flow reduction (271 – 361 µS/cm). 28 % of the hydrologic regime are modified due to hydroelectric plants. Nutrient values are below 2 mg/l, no phosphate concentrations were registered, chemical oxygen demand and dissolved oxygen were maintained through time (Confederación Hidrográfica del Ebro et al. 2008).

Ecotype 116 – Continental and Mediterranean mineralized axes (site 0009)

Jalón is a main tributary to the Ebro river on the left side. The slope is reduced and characterized by gypsum sediments. It has an actual flow of 386 hm³/year, when it should be 396 hm³/year to maintain ecological function. It can have point pollution due to small townships (under 2,000 inhabitants), diffuse pollution because of dryland agriculture, water extraction and morphological alterations. (Confederación Hidrográfica del Ebro, Gobierno de Aragón, Junta de Castilla y León, et al.). Conductivity was relatively high (927 – 1869 µS/cm), as are nutrient concentrations (nitrate <2 mg/l, phosphate 0.09 mg/l, nitrites and ammonium were also found), and temperature (4.3 – 19.9 ºC) (Confederación Hidrográfica del Ebro 2015).

Ecotype 117- Main axes in Mediterranean environments (site 0512)

The lower Ebro has a catchment of 3,800 km² with an average flow of 215 hm³/year. The site located in Xerta is characterized by its high regulation and depletion. This is due to its use as main water supply for the municipality of Tarragona, bringing water to at least 453,000 inhabitants (Prats et al.). Its riverbed consists of vegetation of holm oak groves (Quercus ilex L.) and scrubland, with some vineyards. This site is located at the main river axis after polluting urbanizations. Thus, water quality is not more than mediocre – good.

This river type has just theoretical reference sites, necessary to establish reference diatom communities for the European Ecological Quality Ratio (EQR). Due to being on the main axes of such a populated river, it is highly altered. It is chemically influenced by industry, by a high input of mercury. It may be affected by several industrial spills both in Flix – Ascó , located upstream of the site. A flow volume of 5.4 hm³/s is diverted to a concession to produce 18,000 kW of electricity (Prats et al.). It has relatively high conductivity (360 – 1,709 µS/cm), relatively high phosphate concentration (0.05 – 0.51 mg/l) and nitrate levels (1 – 27.83 mg/l) and high temperature (17.6, 7.9 – 26.7 ºC).

Ecotype 126 – Rivers of wet calcareous mountains (site 0022)

Valira is a 44 km long Andorran tributary to the Segre sub-basin of the Ebro. It is characterized by granodiorites and metamorphic sediment formations in a sloped terrain with meadows and conifers as main coverage, followed by low crops. Although this site is found at a high altitude, the physico-chemical composition is defined as bad. A part of the Flow is used for a hydroelectric power. It is located after a wastewater treatment plant (WWTP) and thus, the concentrations of phosphates (0.34 mg/l) are high and dissolved oxygen (6.5 mg/l) is relatively low. Nitrates were relatively high (1.3 – 12.7 mg/l), ammonium was present (0.1 – 2.7 mg/l) and chemical oxygen demand (1.1 – 2 mg/l) and nitrite (0.01 – 0.36 mg/l)are also present (Confederación Hidrográfica del Ebro et al. 2008).

Ecotype 127- High mountain rivers (Reference site 1448)

The Veral River has its headwaters in the Aragonese Pyrenees and has a 47 km long calcareous catchment. The average river flow is of 1.9 m3/s with a quite undulating pattern (Balcells). The land uses are mainly meadows and natural beech (Fagus) woods. The woods reduce probability of point pollution and reduce runoff. Water quality is thus good to very good. Nutrient concentrations were merely traces, see AnnexTable 2 (Confederación del Ebro 2015).

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