Environmental Management (2017) 59:635–651 DOI 10.1007/s00267-016-0815-y Controlling Eutrophication in A Mediterranean Shallow Reservoir by Phosphorus Loading Reduction: The Need for an Integrated Management Approach Mikel Zaragüeta 1 ● Pablo Acebes 2 Received: 2 May 2016 / Accepted: 27 December 2016 / Published online: 13 January 2017 © Springer Science+Business Media New York 2016 Abstract Increased nutrient enrichment in Mediterranean standing waters has enhanced the risk of being affected by cyanobacterial blooms. Because phosphorus abatement is shaped as a crucial strategy for controlling eutrophication, this study introduces a structural thinking, experiential learning laboratory with animation dynamic model elabo- rated for Cazalegas Reservoir (Spain) to assess the feasi- bility of implementing a set of internal and external control measures and hydromorphological adjustments to meet the goal of oligotrophication. This shallow reservoir is another case where recurrent eutrophication has led to reach annual mean total phosphorus concentrations (0.16 ± 0.08 mg total phosphorus/L) over the threshold of current water policies, triggering cyanobacterial growth up to undesirable levels in summer time (approximately 50,000 cells/mL). Modeling results showed that (i) after upgrading water treatment in the main tributary, (ii) applying a lanthanum-modified bento- nite into the water column and sediment, and (iii) increasing reservoir water level, in-lake P concentrations and cyano- bacterial abundance decreased in an 88% (below 0.01 mg total phosphorus/L) and 84% (below 6000 cells/mL), respectively in the most critical periods. However, the constraints of the proposed management strategies are associated with their costs of implementation and the time span for a stable trophic recovery of the reservoir. In that end, integrated management approaches are aimed to be adopted by water managers to reach adequate ecological status of freshwater bodies. Keywords Cyanobacterial bloom ● External/Internal P- loading ● Mitigation ● Nutrient management ● Restoration ● STELLA model Introduction Most lentic freshwater bodies are threatened by cultural eutrophication caused by an increase of nutrient-rich inputs from agricultural, industrial and untreated household waste discharges (Smith et al. 1999; Smith and Schindler 2009). As a consequence, the increasing cyanobacterial blooms result in the impairment of lakes and reservoirs functioning and the ecosystem services they provide (Ibelings et al. 2016). Under such circumstances water authorities must design suitable strategies to prevent, control and mitigate eutrophication (Stroom and Kardinaal 2016). Considering that nitrogen (N) and phosphorus (P) are the main nutrients limiting phytoplankton productivity, an ongo- ing debate has been posed among the scientific community to determine whether actions should be addressed to reduce N or P sources to reverse this trend towards a re-oligotrophication (Conley et al. 2009; Paerl et al. 2011). However, clear evi- dences have pointed P abatement as the most feasible approach to attenuate cyanobacterial dominance in freshwaters (Correll 1998; Jeppesen et al. 2005; Schindler et al. 2008; Smith and Schindler 2009; Carvalho et al. 2013). Multiple restoration action plans including hydrological management (Day et al. 2005), reduction of external P loads (Jeppesen et al. 1999) and complementary ecological * Mikel Zaragüeta mzaragueta@cimera.es 1 Cimera Estudios Aplicados S.L. Science to Business, C/. Santiago Grisolía, 2., Madrid, Tres Cantos 28760, Spain 2 Terrestrial Ecology Group-TEG. Departamento de Ecología, Facultad de Ciencias, Universidad Autónoma de Madrid, C/. Darwin, 2, Madrid 28049, Spain