Sedimentary diatom and pigment-inferred recent anthropogenic accelerated eutrophication of a Mediterranean lake (Lake Dojran, Republic of North Macedonia / Greece)

Abstract

Lake eutrophication is recognised as a serious global challenge, and many regional legislative programmes are being made to attempt to relieve nutrient pollution and restore deteriorated lake ecological state. However, it is of primary importance to understand the degradation processes and reference conditions. The palaeolimnological approach allows us to use ecological evidences preserved in lake sediments to track the changes of lake trophic status under human impact. Diatoms, a proxy for ecological and limnological change, and pigments, a proxy for algal production and composition, were analysed on a short sediment sequence from Lake Dojran (Republic of North Macedonia and Greece), and their preservation qualities were evaluated before environmental interpretation. Good diatom preservation is inferred mainly from the consistent co-occurrence of robust, highly-silicified taxa and small taxa throughout the sequence. Pigment evaluation of the comparison between wet sediment samples in dark and cold storage and their corresponding dry sediment samples lyophilized immediately after the recovery reveals that sediment restoration conditions are critical for the accuracy of analysis. We show that the increased chlorophyll and xanthophyll pigment concentrations, particularly the siliceous-algae pigment fucoxanthin and diatoxanthin, together with the distinct increase in diatom concentration, indicate accelerated lake eutrophication and a major ecological shift linked to intensified water abstraction practice and agricultural expansion in the late 18th to early 19th century. Evidence of diatom assemblage composition is muted probably by the dominance of widely-tolerant small fragilaroid species in diatom composition and the better competitive ability of cyanobacteria and chlorophytes than diatoms for low light under eutrophic and turbid conditions. This study improves our understanding of recent human-induced environmental change and current ecological restoration target in this lake.