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Deciphering long‐term records of natural variability and human impact as recorded in lake sediments: a palaeolimnological puzzle

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Global aquatic ecosystems are under increasing threat from anthropogenic activity, as well as being exposed to past (and projected) climate change, however, the nature of how climate and human impacts are recorded in lake sediments is often ambiguous. Natural and anthropogenic drivers can force a similar response in lake systems, yet the ability to attribute what change recorded in lake sediments is natural, from that which is anthropogenic, is increasingly important for understanding how lake systems have, and will continue to function when subjected to multiple stressors; an issue that is particularly acute when considering management options for aquatic ecosystems. The duration and timing of human impacts on lake systems varies geographically, with some regions of the world (such as Africa and South America) having a longer legacy of human impact than others (e.g., New Zealand). A wide array of techniques (biological, chemical, physical and statistical) is available to palaeolimnologists to allow the deciphering of complex sedimentary records. Lake sediments are an important archive of how drivers have changed through time, and how these impacts manifest in lake systems. With a paucity of ‘real‐time’ data pre‐dating human impact, palaeolimnological archives offer the only insight into both natural variability (i.e., that driven by climate and intrinsic lake processes) and the impact of people. While there is a need to acknowledge complexity, and temporal and spatial variability when deciphering change from sediment archives, a palaeolimnological approach is a powerful tool for better understanding and managing global aquatic resources. WIREs Water 2017, 4:e1195. doi: 10.1002/wat2.1195

Lake records from across eastern Africa, highlighting the importance of temporal‐scaling and the sensitivity of aquatic systems to climatic and anthropogenic drivers. Over long timescales (and often at a lower temporal resolution) many lake systems in eastern Africa record the dominant climate signal of wet versus dry phases (e.g., Lake Naivasha, Lake Edward and Lake Victoria; note the inverted y‐axis for Lakes Edward and Victoria). In smaller systems, such as Lake Nyamogusingiri (a crater lake in western Uganda), the lake appears to respond to the dominant climate driver over the last 1000 years (indicated by lake level fluctuations inferred from the planktonic diatom Nitzschia lancettula). The onset of intensive anthropogenic activity with the lake catchment c. 1900 CE, leads to an increase in sediment loading to the lake system (increase in MAR). This anthropogenic impact decouples the lake response from the climate system, and the lake records an increase in turbidity and nutrient enrichment (inferred from Cyclotella meneghiniana).
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Conceptual diagram of anthropogenically‐driven environmental forcings acting over long‐ to short‐ temporal scales alongside landscape characteristics (terrestrial, aquatic and subterranean) which shape limnological variables of lakes within a landscape. This serves to highlight that even with identical forcing, lakes can respond in different ways as a result of internal processes and legacy effects (see Figure (a) also).
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A summary diagram highlighting climatic and anthropogenic drivers, and the impacts they have on lake systems. It should be noted that both climate and humans can cause changes in lake systems that manifest in a similar fashion in the lake sediment archive, and that multiple lines of evidence used together are required to fully apportion the impacts of the different drivers (see Table also) (*these impacts are not covered in the text).
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The multiproxy record from Lake Parishan in the Zagros mountains of Iran shows evidence of catchment disturbance from pollen (Quercus) and charcoal records, and also evidence of grazing animals at these times from Sporormiella (a fungi associated with animal dung). The ostracod (Limnocythere inopinata) and oxygen isotope data (δ18Ocarb) indicate changes in lake hydrology and the organic content of the sediment (Organic C; sediment composition), some of which correlate with the evidence of catchment disturbance. The water composition (alkalinity: calcium) preferences of the ostracod species suggest that during periods of catchment disturbance groundwater and/or surface water input to the lake was reduced, this in turn led to a decrease in lake level and more positive oxygen isotope ratios. The multiproxy approach narrows the envelope of possible interpretations to explain the data recorded, making anthropogenic disturbance of the catchment the most likely driver of hydrological change for much of this 4000 year record. The scatter plots show data from core depths where all proxies were analyzed (there are two samples with no charcoal data). The correlation (r) and significance of the correlation (p‐value) are also given.
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The carbon accumulation rate (CAR) at Gudme Sø (on the island of Fuen, Denmark) showing how rates vary in a cultural landscape. Deforestation started during the Neolithic (c. 2700 BCE) and increased in the mid‐Bronze age (c. 1000 BCE). As well as clearance processes, the C AR reflects a range of other factors, including ecosystem nutrient availability, but most notably variable sediment focussing due to lake infilling. A shallow lake today (c. 0.5 m water depth), the lake would have been nearly 10‐m deep in the Bronze Age with more pronounced sediment focussing than today, as a result comparing rates directly can be problematic.
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Thickness versus maximum grainsize in Lake Anterne flood‐triggered deposits over the last 10,000 years. Black crosses highlight samples from Iron Age‐antiquity enhanced erosion period that is marked by drastic rise in thickness (i.e., the amount of solid flow) without any significant rise in grainsize (i.e., the intensity of the flood‐triggered stream flow). The human origin of this disturbance in the erosion‐climate relationship is suggested by the presence of a pasturing hut dated the same age close to the lake and the absence of any significant glacier advance in the Alps during this period. The first ever sediment DNA‐based reconstruction from the lake suggests the presence of domestic animals, from which pasturing activities are inferred. The sedimentary DNA profile provides an independent line of evidence attesting to the anthropogenic driver of observed changes in the sediment archive.
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Sediment accumulation rate curves from a series of selected lakes, with a global spread (Lake Erhai, Lake Anterne, Lake Bourget, Gudme Sø, Lake Salpetén, Lac D'Annecy, Lake Nyamogusingiri and Blelham Tarn). In all cases the accumulation rates (MAR) show an increase attributed to human activity. The differences in timescale should be noted, and highlight how human activity has impacted (and intensified) erosion rates during the Holocene.
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The pollen record from Lake Erhai, China, attests to the long‐term role of climate and anthropogenic impacts in shaping the response of catchment vegetation. The c. 13,000‐year record suggests climate was the dominant driver of change early in the lake's history, with a gradually warming and drying climate (following the Younger Dryas) driving changes in the vegetation composition, promoting the growth of broadleaf trees. The marked decline in broadleaf vegetation, concomitant with the increase in pine and grass pollen provides the first evidence of human impact c. 6000 cal. yr BP. The persistent increase of secondary pine forest indicates that human impacts in the catchment were a result of a sustained period of shifting agriculture. Catchment modification, linked to intensified agriculture and urbanization, increased c. 2000 cal. yr BP, as evidenced by an increase in the erosion indicator (XfD%). A sharp increase in broadleaf taxa towards the top of the record is attributed to a catchment management, and a recent phase of reforestation (c. last 25 years).
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(a) A flow diagram summarizing the complex interactions of drivers (both climatic and anthropogenic) that affect aquatic systems, and the associated impacts that may be recorded in the lake sediment archive, and how this may differ between unimpacted and impacted lake catchments, as the legacy of impacts often alters the response of an aquatic ecosystem to future stressors. (b) A flow diagram highlighting the complex interactions of anthropogenic drivers and their impacts on water quality as recognized in the palaeolimnological records of the Murray‐Darling Basin wetlands. These wetlands have been subjected to multiple stressors, often coincidentally, and is attributed to the causal co‐variation between several anthropogenic drivers and stressors in the Murray‐Darling Basin.
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A generalized schematic of major climatic episodes and historical periods of population expansion.
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