Refining the palaeoecology of lacustrine testate amoebae: insights from a plant macrofossil record from a eutrophic Scottish lake

Arcellinida (lobose testate amoebae) preserve well in lake sediments and have shown great potential as palaeolimnological indicators, but a limited understanding of their ecology prevents more in-depth interpretations of both contemporary and fossil assemblages. Loch Leven, Scotland, has a well-documented history of nutrient-enrichment and associated lake biological change, creating a temporal gradient along which testate amoeba ecology can be investigated. Plant macrofossil data from a dated sediment core (LEVE14) indicate that a transition from oligo-mesotrophic to eutrophic conditions occurred over at least the last 200 years, enabling direct comparisons to be drawn with fossil testate amoeba assemblages from the same core. Cluster analysis and principal components analysis show that testate amoebae and macrophytes responded synchronously to an early phase of nutrient-enrichment after ca. AD 1200, and to anthropogenic water-level lowering and further enrichment after ca. AD 1830. The results of indicator species analysis (IndVal) support the suggestion inferred previously that Cucurbitella tricuspis is an indicator of nutrient-enrichment, while the association of some less abundant taxa, specifically Difflugia amphora and Mediolus corona, with eutrophic environments is also corroborated. Overall, the testate amoeba assemblage response appears to reflect a complex interaction of factors which were ultimately driven by eutrophication. These include variations in microhabitat associated with changing plant structure and sedimentary conditions influenced by macrophyte community succession. In particular, a shift from an oligo-mesotrophic macrophyte assemblage composed of isoetid plants to one dominated by Potamogeton spp. and dense stands of Characeae is suggested to influence the relative proportions of sediment dwelling and plant-associated testate amoebae. The results highlight the utility of testate amoebae as benthic indicators of lake nutrient-enrichment and demonstrate the value of using multi-proxy palaeolimnological data to refine the ecology of lesser-studied palaeoecological groups.