Abstract
During the 20th century, many lakes in the Northern Hemisphere were affected by increasing human population and urbanization along their shorelines and catchment, resulting in aquatic eutrophication. Ecosystem monitoring commenced only after the changes became apparent, precluding any examination of timing and dynamics of initial community change in the past and comparison of pre- and postimpact communities. Peri-Alpine Lake Constance (Germany) underwent a mid-century period of eutrophication followed by re-oligotrophication since the 1980s and is now experiencing warm temperatures. We extended the period for which monitoring data of indicator organisms exist by analysing historical environmental DNA (eDNA) from a sediment core dating back some 110years. Using three metabarcoding markers-for microbial eukaryotes, diatoms and cyanobacteria-we revealed two major breakpoints of community change, in the 1930s and the mid-1990s. In our core, the latest response was exhibited by diatoms, which are classically used as palaeo-bioindicators for the trophic state of lakes. Following re-oligotrophication, overall diversity values reverted to similar ones of the early 20th century, but multivariate analysis indicated that the present community is substantially dissimilar. Community changes of all three groups were strongly correlated to phosphorus concentration changes, whereas significant relationships to temperature were only observed when we did not account for temporal autocorrelation. Our results indicate that each microbial group analysed exhibited a unique response, highlighting the particular strength of multimarker analysis of eDNA, which is not limited to organisms with visible remains and can therefore discover yet unknown responses and abiotic-biotic relationships.
Highlights
Since the beginning of the 20th century, many freshwater lakes were subjected to anthropogenic perturbations resulting commonly in aquatic eutrophication, due to the faster pace of human population growth and urbanization
Even for lakes with a history of monitoring reaching back for decades, there is a lack of knowledge about (a) the lake's biota diversity and composition in the pre-eutrophication state, (b) the exact timing and dynamics of community change in response to rising phosphorus load and (c) the trajectories of the communities during re-oligotrophication when pre-eutrophication communities are considered as a reference
Molecular inventories obtained from a Lake Constance sediment core spanning the last century showed a high degree of congruency across the three groups microbial eukaryotes, diatoms and cyanobacteria, but with temporal offsets
Summary
Since the beginning of the 20th century, many freshwater lakes were subjected to anthropogenic perturbations resulting commonly in aquatic eutrophication, due to the faster pace of human population growth and urbanization. The richness, diversity and abundance of planktonic organisms are well-established bioindicators because they quickly respond to changes in nutrient load due to their fast generation times, high growth rates and high dispersal potential (Smith et al, 2016; Sommer, 1986) They are commonly used for long-term palaeoecological studies, where organismal remains retrieved from dated sediment cores are used to reconstruct the historical anthropogenic impacts on ecosystems (Battarbee et al, 2001; Catalan et al, 2013; Flower & Battarbee, 1983; Willis et al, 2010; Zillén et al, 2008). At a high temporal resolution and using available phosphorus concentration and air temperature data, we investigated (a) the timing of group-specific community responses, with special attention to the early 20th century without monitoring data, (b) how resilient, responsive and reversible the freshwater communities have been to 20th century's environmental perturbations, (c) the group-specific sensitivity of the microbial lineages to the anthropogenic modifications and their utility as eDNA bioindicators and (d) the role of changing phosphorus loads and rising air temperatures in lake biodiversity change
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