**2.4 Conclusion**

170 Studies on Water Management Issues

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.

should also be expected.

2004; Rapala et al., 1997; Saker et al., 2005).

Guadiana estuary has not yet been assessed.

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

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.,

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

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 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.
