**2.3 Discussion**

166 Studies on Water Management Issues

**A**

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**1997 1999 2000 2002 2004 1998 2001 2003 2010** 

Fig. 4. Box and whisker plots showing the distribution of chlorophyll *a* concentration (A), total cyanobacteria abundance (B), and microcystin-LR concentration (C) in the Guadiana upper estuary, binned into different periods. Median value is represented by the line within the box, 25th to 75th percentiles are denoted by box edges, 5th to 90th percentiles are depicted by the error bars, outliers are indicated by circles, and extreme values by diamonds. An

extreme chlorophyll *a* value (216.0 µgL-1, year 2001) was omitted for clarity

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In past published reports dealing with the microbial ecology of the Guadiana estuary (Domingues et al., 2005; Domingues & Galvão, 2007; Rocha et al., 2002), the impact of Alqueva dam construction was predicted to increase eutrophication conditions and possibly promote cyanobacterial blooms and associated cyanotoxins. In fact, this has not been observed during the seven-year period after dam completion. Not only cyanobacteria, but overall phytoplankton abundance, biomass and chlorophyll *a* concentrations have decreased markedly and have remained at low levels even in the upper estuary, where peak chlorophyll maxima usually occurred.

Typical estuarine phytoplankton succession observed in the Guadiana estuary from diatoms in early spring, to chlorophytes and finally cyanobacteria in late summer and fall was driven by nutrient regime with high winter loads of nitrogen and phosphorus discharged downriver, and silica depletion after the spring diatom bloom (Rocha et al., 2002). These authors also referred that cyanobacteria dominated the chlorophyll maximum zone in the upper estuary in late summer- early fall, due to warm waters, reduced sinking and grazing, as well as N limitation with low N:P ratio. Nitrogen limitation during summer increased in the period after Alqueva in the upper estuary (Barbosa et al., 2010). Additionally, nutrient enrichment experiments performed during 2008 clearly demonstrated that phytoplankton growth was nitrogen limited (Domingues et al., 2011).

Contrary to more stringent nitrogen limitation, the improved light regime with lower extinction coefficients should have promoted overall phytoplankton growth from 2003

Ecological Tools for the Management of Cyanobacteria

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Blooms in the Guadiana River Watershed, Southwest Iberia 169

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Fig. 7. Log of total cyanobacteria abundance (**A**) and chlorophyll *a* (**B**) versus microcystin concentration in particulate fraction (µg L-1) in upper Guadiana estuary (Alcoutim and

Mértola) from 1997 to 2009. Zero values not log transformed

Fig. 5. Log of total cyanobacteria abundance (cells mL-1) and chlorophyll *a* concentration (µg L-1) in upper Guadiana estuary (pooled data from Alcoutim and Mértola) from 1997 to 2009. Zero abundance values not log transformed

Fig. 6. Microcystin-LR concentration in particulate fraction (µg L-1) over time in upper Guadiana estuary (Alcoutim and Mértola) from 1996 to 2009. Dashed line indicates the 1 µg L-1 limit for drinking water (WHO 1998 guidelines)

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**Chlorophyll** *a* **(µg L-1)**

**MC-LR Alcoutim MC-LR Mértola**

Fig. 5. Log of total cyanobacteria abundance (cells mL-1) and chlorophyll *a* concentration (µg L-1) in upper Guadiana estuary (pooled data from Alcoutim and Mértola) from 1997 to

Fig. 6. Microcystin-LR concentration in particulate fraction (µg L-1) over time in upper Guadiana estuary (Alcoutim and Mértola) from 1996 to 2009. Dashed line indicates the

1 µg L-1 limit for drinking water (WHO 1998 guidelines)

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Fig. 7. Log of total cyanobacteria abundance (**A**) and chlorophyll *a* (**B**) versus microcystin concentration in particulate fraction (µg L-1) in upper Guadiana estuary (Alcoutim and Mértola) from 1997 to 2009. Zero values not log transformed

Ecological Tools for the Management of Cyanobacteria

**3. Guadiana reservoirs management** 

different parts of the watershed.

**3.1 Cyanobacteria management in reservoirs** 

**2.4 Conclusion** 

Blooms in the Guadiana River Watershed, Southwest Iberia 171

natural river flow variations and applying them to flow regulation by dams (Alves & Bernardo, 1998; Alves & Gonçalves, 1994; Chicharo et al., 2006; Chicharo et al., 2009; Wolanski et al., 2008). Flow Incremental Methodology, and other hydrological or ecohydrological approaches, would ensure that natural variations in freshwater flow would be mimicked by dam discharge, albeit dampened. Finally, monitoring of environmental impact usually considers either endangered or economically important vertebrate species existing in the freshwater zone. Yet, marine and freshwater micro- and macroorganisms need also to be considered in terms of whole ecosystem impact. In fact, microorganisms, such as cyanobacteria, appeared to be sensitive indicators of estuarine ecosystem perturbation in this study. Thus, it is proposed that photosynthetic prokaryotes should be

This 13-year study of the Guadiana estuary in Southern Portugal, directed towards assessing the impact of dam construction on cyanobacteria populations in the freshwater zone, revealed that phytoplankton abundance, chlorophyll *a* and diversity decreased markedly from 2003 onwards after Alqueva dam completion. This declining trend in phytoplankton could be explained by both light limitation during dam building coupled with more stringent nitrogen limitation after dam completion. Interestingly, cyanobacteria abundance, diversity and microcystin concentration exhibited an even more pronounced decrease, which could not be attributed to any monitored environmental factors, but instead to perturbations in overall estuarine circulation. The collapse in cyanobacteria populations in the upper estuary warrants a more careful approach towards maintaining ecological river flow in dam discharge. Future research in the Guadiana estuary should address not only the impact of restricted river flow on estuarine circulation, turbidity maximum and associated chlorophyll peaks, as well as provide more adequate approaches towards maintaining an ecological river flow, possibly using cyanobacteria as an indicator of good water quality.

Water management in the Guadiana River watershed is a complex transnational problem and has been object of negotiations between Portugal and Spain for decades now. The last bilateral Agreement assured the integrated management of water and territory, covering quantitative and qualitative features, stipulating minimum flows (under normal rainfall conditions), and foreseeing the permanent exchange of hydrologic and environmental data and information (Mendes, 2010). Environmental laws, in both countries, ensure public access to the monitoring data, allowing for international comparison of water quality in

Normal and drought conditions in the Guadiana catchment have been modelled in multiple hydrologic studies (e.g. Brandão & Rodrigues, 2000), allowing for better management in terms of water availability. Nevertheless, water quality concerns have been mostly ignored in water reservoir management decisions. Impaired sewage treatment and agro-industrial mal-practices have been repeatedly blamed for water quality deterioration in the Guadiana river basin, both in Portugal and Spain. Official reports for the Portuguese part of the

used as indicators of "good" estuarine water quality rather than just "bad".

onwards, after Alqueva dam completion. In view of decreasing trend in chlorophyll and overall phytoplankton abundance from 2003 to 2010, nitrogen availability appeared to play a preponderant role rather than light in this turbid estuary. This shift from light to nutrient limitation was probably the most determinant trend for phytoplankton observed after Alqueva (Barbosa et al., 2010). However, since chroococcoid cyanobacteria have higher affinity for nutrients due to small size, and most filamentous forms as well as some chroococcoid species have nitrogen fixing potential, these photosynthetic prokaryotes should have been less affected by lower nitrogen availability than larger non-nitrogen fixing eukaryotic phytoplankton. Freshwater reservoirs created by dams do not retain only water but also suspended particulate material, including planktonic microorganisms. In consequence, not only are nutritional regimes affected downstream, but also freshwater microbial populations with complex life cycles, such as cyanobacteria. Filamentous cyanobacteria in response to environmental forcing can produce different cell types which are adapted to nitrogen fixation, nutrient storage and reproductive strategies such as winter dormancy and dispersal. Thus, freshwater reservoirs by retaining these morphotypes could seriously affect not only bloom formation but also species composition downstream.

The lack of correlation between chlorophyll and cyanobacteria abundance could be simply explained by predominance of small chroococcoid cells with reduced chlorophyll content. Consequently, poor or absent correlation between chlorophyll and microcystin concentrations should also be expected.

As previously described by Galvão et al. (2008), microcystin concentration were generally not correlated with cyanobacteria abundance or biomass in natural waters (freshwater reservoirs and Guadiana river, South Portugal), since different strains and/or species could produce microcystins at different rates depending on cell cycles and environmental conditions, which has also been documented in laboratory analyses (eg. Kameyama et al., 2004; Rapala et al., 1997; Saker et al., 2005).

Furthermore, in all temperate estuaries, cyanobacteria accumulate and thrive in the chlorophyll *a* peak (Cloern, 1987; Pearl et al., 2006; Pinckney et al., 1998), directly upstream from the turbidity maximum. Restricting river flow can cause perturbations of estuarine circulation, particularly in terms of location and intensity of the turbidity maximum, which in turn will affect the chlorophyll *a* maximum in the upper estuary (Cloern, 1987, 1999). Thus, cyanobacteria decline cannot be simply explained by any one environmental driver, but rather in terms of estuarine circulation. Nutrients tend to be regenerated in the turbidity maximum and phytoplankton bloom directly upstream from this zone, benefiting in this interface between nutrient enriched and clear waters. Unfortunately, how seriously the huge Alqueva reservoir has affected estuarine circulation and the turbidity maximum in the Guadiana estuary has not yet been assessed.

The Alqueva dam not only is the largest dam in the Guadiana watershed but due to its location affects most strongly the estuarine section of the river. In spite of efforts by the Alqueva water management authorities to maintain "ecological" river flow this is not compulsory according to existing Portuguese water resources legislation. Ecological river flow can be broadly defined as the flow necessary to conserve and maintain natural aquatic (freshwater) ecosystems. In Portugal, this is very simply calculated as a value > 2.5 to 5% of the modular water flow to be maintained throughout the year, if conditions permit. Different studies have recently challenged this approach proposing more careful analyses of natural river flow variations and applying them to flow regulation by dams (Alves & Bernardo, 1998; Alves & Gonçalves, 1994; Chicharo et al., 2006; Chicharo et al., 2009; Wolanski et al., 2008). Flow Incremental Methodology, and other hydrological or ecohydrological approaches, would ensure that natural variations in freshwater flow would be mimicked by dam discharge, albeit dampened. Finally, monitoring of environmental impact usually considers either endangered or economically important vertebrate species existing in the freshwater zone. Yet, marine and freshwater micro- and macroorganisms need also to be considered in terms of whole ecosystem impact. In fact, microorganisms, such as cyanobacteria, appeared to be sensitive indicators of estuarine ecosystem perturbation in this study. Thus, it is proposed that photosynthetic prokaryotes should be used as indicators of "good" estuarine water quality rather than just "bad".
