Trophic status and metabolic rates of threatened shallow saline lakes in Central Spain: providing diagnostic elements for improving management strategies

Shallow saline lakes in the La Mancha Húmeda Biosphere Reserve in Central Spain show diverse degrees of cultural and natural eutrophication, prompting urgent conservation measures. This study focuses on 17 representative lakes from the site to assess seasonal nutrient dynamics and their connection to plankton metabolism (photosynthesis and respiration) during two successive hydrological periods. Effect of environmental factors was evaluated on a combination of several response variables, demonstrating that source of the nutrient inputs (ranging from natural to anthropic) had the highest influence on the nutrients stoichiometry and metabolic rates. Regarding the source of eutrophication, the model demonstrated that effects of urban wastewaters exceed that of agricultural runoff, and moreover lead to more prolonged hydroperiods and contributes to desalination. Lakes affected by wastewater inputs or surrounded by volcanic lithology showed phosphorus enrichment in both water and surface sediments. Planktonic respiration rates in these cases closely correlated with photosynthesis, suggesting the utilization of algal-derived dissolved organic matter. Conversely, wastewater-free lakes, mainly fed by runoff, accumulated uncolored, likely recalcitrant dissolved organic carbon (DOC). These lakes exhibited a better-preserved condition, characterized by higher salinity, moderate metabolic rates, and lower production/respiration ratios compared to the previous state, implying a greater dependence on allochthonous organic matter. Enhancement of management strategies, which should consider salinity, volcanic lake vulnerability, and the multifaceted impacts of wastewater, will prove more effective in the conservation and restoration of these unique and fragile ecosystems.