The La Mancha Húmeda Biosphere Reserve is the largest wetland district in the Iberian Peninsula, containing many temporary saline lakes undergoing different anthropogenic pressures. Eleven of these lakes were selected to assess phosphorus (P) burial, fractionation and mobilization in their sediments. Wastewater inputs, wherever they occurred, favored accumulation of all P sedimentary fractions, which took place mainly via precipitation with divalent cations (i.e. calcium or magnesium) and as buried organic matter, both representing relatively occluded P forms, although they can also be involved in P release. P immobilization would be partially restrained in the volcanic lakes of the region, making them more prone to eutrophication. Our seasonal survey indicated that the natural drying and re-flooding of lakes replenishes the availability of labile-P in water, and often contributed to the lakes' productivity. However, artificial flooding alters redox conditions and thereby enhances a reductive dissolution of P. This further suggests that the internal P supply might not diminish in the polluted lakes after external input shortcomings, thus counteracting any restoration measures. On the other hand, condensed P, which occurs when the excess P is taken up by microorganisms and stored as polyphosphate, was also important in sediments. Aside from the wastewater purification performed in treatment plants, we propose additional countermeasures such as extending the use of tributary slow flow channels and/or artificial wetlands. These can act as "tertiary treatments" that would promote biogeochemical transformations involving P binding before treated wastewater is poured into the lakes.
Keywords: Internal phosphorus load; La Mancha Húmeda Biosphere Reserve; Saline shallow lakes; Sediment–water phosphorus exchange; Water management.
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