Aquatic ecosystem shifts in response to early human land-use changes in two low-elevation lakes in northwestern Europe
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Growing problems related to eutrophication of (freshwater) aquatic systems, e.g. loss of biodiversity, availability of clean water and problems with its recreational and food services, highlight the necessity for management and conservation of such systems. For this, adequate understanding of the causes and consequences of the shifts in nutrient availability is required, as well as proper reference baselines. Therefore, this study aimed to reconstruct pre-human impact baselines for two low-elevation lakes in northwestern Europe, Hijkermeer and Llangorse Lake, with the use of trophic state transfer functions based on diatoms. Also, the ecosystem’s response to these changes was analysed and an attempt was made to delineate natural variability from human impact. Over ca. 3400 yrs the inferred trophic state of Hijkermeer shifted from oligotrophic (12 μg TP/L) to mesotrophic (32 μg TP/L), coinciding with continuously increasing anthropogenic activity in the catchment. The reconstructed trophic state of the Llangorse Lake increased from a eutrophic (54 μg TP/L) to hypereutrophic (168 μg TP/L) state between ca. 5000 and 2500 cal. yr BP and subsequently the trophic state returned again to former levels. This appeared to be related to a simultaneous temporary increase in agriculture as indicated by the pollen record. A remarkable observation was that the relation between the trophic state and diversity seemed to be unimodally distributed and the ecosystem of Llangorse Lake appeared to show a recovery of diversity following the phase of increased nutrient availability and anthropogenic activity. This showcases the resilience of the ecosystem and could be used as a model for (on-going) management and conservation of the present eutrophication problems.