Unmixing the Mixing: A 2000-Year-Long Perspective on Human-Environment Relationships
by Maurycy Żarczyński
Breathe in and dive
Being unable to breathe is a rather unpleasant feeling. A lot of aquatic organisms feel the same way. Availability of oxygen dissolved in water plays a vital role in aquatic ecosystems [1]; be it respiration (breathing) of living organisms, or complex biogeochemical processes involving water and sediments (Fig. 1). In lakes, oxygen is distributed in the water column through mixing – a complex process controlled by different physical and chemical processes that occur across time and space. Simply put, either the entire water column can mix or only certain portions. Whenever the water column does not mix entirely (a state called meromixis), bottom waters become oxygen-depleted. Unfortunately, human impact, global climate change, land use changes and nutrient availability are impacting oxygen levels, causing hypoxia (a state of low oxygen concentrations) in lakes across the globe [2]. Currently, the transition to meromixis is mostly attributed to human impact [3], although this interpretation is not always easy, as for some lakes this state is natural. As we aim for better understanding, we need to reach back in time.
Fig. 1: Exaggerated difference between lakes with sufficient and insufficient levels of dissolved oxygen (symbols not up to scale, https://doi.org/10.6084/m9.figshare.11302547.v1)
One of the goals of my PhD thesis research, in collaboration with some wonderful scientists, was to test selected proxies and then understand how past land use changes affected Lake Żabińskie, northeastern Poland (Fig. 2). Together, we studied a 2000-year-long, varved sediment core from Lake Żabińskie. To read in detail what varves are and what they can tell us, see this blog post by Alicja Bonk. The current state of Lake Żabińskie and some aspects of its functioning (particularly erosion in the catchment) over the last 1000 years are already well-studied [5–7]. These results suggest that Lake Żabińskie may have a longer, more intricate history of human influence, especially on the lake mixing regime. Therefore, we investigated human-induced land use changes over the last 2000 years to better understand how their activities may have impacted lake mixing patterns. The exceptional age control [8] based on varved sediments allowed a high-resolution reconstruction of the influence of human impact on the lake catchment and the intensity of the lake mixing [9].
In a multiproxy approach, research is based on a wide array of data and techniques, which together lead to a more complete identification of the underlying and ever-changing processes that affect lake environments. However, most of the proxies that are used in paleoenvironmental research need some kind of validation and quality assessment. Even though it might be tempting to go with the well-established interpretation in the literature. Yet, each site should be treated with caution and data should be tested. As proxies could be controlled by different processes, they might effectively tell us a different story. Dissolved oxygen concentrations control many intertwined biogeochemical cycles. Some elements are prone to alter their solubility and mobility depending on oxygen concentrations. Among these, iron (Fe) and manganese (Mn), and their ratio (Fe/Mn: lower values mean better oxygen availability; higher values mean worse oxygen availability) are the most common indicators of changing oxygen conditions. Both elements could potentially be used as proxies for water mixing intensity. However, other environmental factors such as erosion, i.e. delivery of terrestrial material from the catchment into a lake, can influence concentrations of Fe and Mn, rendering them virtually useless as mixing intensity proxies. Fortunately, we can trace erosion intensity with other elements, such as titanium (Ti) or potassium (K), which allows us to decipher whether Fe and Mn changes are controlled by oxygen availability. Our results showed that indeed, they are!
The combination of sediment structure data, geochemistry and pollen data, with some archaeological background, led to the paleoenvironmental reconstruction of both human impact and lake mixing patterns (Fig. 3). At the beginning of the Common Era 2000 years ago, parts of NE Poland commonly known as the Masurian Lakeland were sparsely inhabited by the Bogaczewo settlers (Period 1, Fig. 3). These settlers were western Balts who had strong influence on the Masurian Lakeland until around the beginning of the 5th century CE. They partially deforested this region and the Lake Żabińskie catchment, as seen in the pollen data. Deforestation led to enhanced wind stress on the lake surface, causing intense water mixing and higher oxygen concentrations at the lake bottom. In presence of oxygen, portions of Fe and Mn circulating in lake waters were permanently deposited in sediments, leading to low values of Fe/Mn ratio (Period 1, Fig 3). From the 5th century (the end of the Migration Period) until the start of the ca. 17th century CE (Period 2, Fig 3), the lake vicinity was abandoned. With no human activity, the surrounding woodlands were restored, effectively sheltering the lake. As a result, water mixing was short-lasting and less efficient, leading to a stable meromixis. This is seen in the very simple sediment structure with only two distinct layers forming a varve, almost no Fe and Mn deposition (high Fe/Mn ratio), and no signs of high erosional input (Ti and K).
During the 17th century until the end of 19th century CE, the Masurian Lakeland was reclaimed by man, specifically the Teutonic Order, then Duchy and Kingdom of Prussia, which substantially changed land use (Period 3, Fig. 3). Once again, deforestation and agriculture led to an opening of the landscape, which enhanced wind stress on the lake, allowing deeper and longer-lasting water mixing. Higher accumulation of both Fe and Mn (low Fe/Mn ratio) in the sediments accompany this, suggesting better oxygen availability at the lake bottom. However, higher erosional input seen in Ti and K suggests that the clastic input could control fraction of the observed Fe and Mn accumulation. Still, human activity in the area led to better oxygen availability in the lake. However, other water traits have worsened - especially lake trophy, i.e. the rate at which organic matter is supplied to or produced in the given ecosystem. During Period 3, production and preservation of biomass in the lake started to grow. In the last period (Period 4, Fig. 3), the lake catchment became partially reforested as local fields were abandoned. At the same time, agriculture mechanization and the use of fertilizers, especially after World War II, led to both higher crop yields and nutrient inputs into the lake. Recovery of the woodlands has weakened wind stress on the lake, however, ongoing climate change continues to affect lake functioning. Specifically, with higher air temperatures, lakes are becoming warmer, which in turn promotes deeper lake stratification (development of water layers of different characteristics, especially temperature) and hinders lake mixing. This has led to very complicated water mixing patterns, as monitoring studies show [5]. Sediments reflect changes in lake mixing patterns, its thermal structure and rising production of the biomass, as deposition patterns and chemical properties became very complex in Period 4.
If you have questions or comments concerning Maurycy's post, please leave a comment below, or send him an email. You can also follow his research on ResearchGate or Twitter.
_____________________________________________________
Breathe in and dive
Being unable to breathe is a rather unpleasant feeling. A lot of aquatic organisms feel the same way. Availability of oxygen dissolved in water plays a vital role in aquatic ecosystems [1]; be it respiration (breathing) of living organisms, or complex biogeochemical processes involving water and sediments (Fig. 1). In lakes, oxygen is distributed in the water column through mixing – a complex process controlled by different physical and chemical processes that occur across time and space. Simply put, either the entire water column can mix or only certain portions. Whenever the water column does not mix entirely (a state called meromixis), bottom waters become oxygen-depleted. Unfortunately, human impact, global climate change, land use changes and nutrient availability are impacting oxygen levels, causing hypoxia (a state of low oxygen concentrations) in lakes across the globe [2]. Currently, the transition to meromixis is mostly attributed to human impact [3], although this interpretation is not always easy, as for some lakes this state is natural. As we aim for better understanding, we need to reach back in time.
Fig. 1: Exaggerated difference between lakes with sufficient and insufficient levels of dissolved oxygen (symbols not up to scale, https://doi.org/10.6084/m9.figshare.11302547.v1)
Hold your breath
By studying lake sediments, paleolimnology reveals past variability of lacustrine environments and their surroundings, allowing meaningful insights into the main drivers of ecosystems transitions and their possible future [4]. A lake’s catchment, especially the immediate vicinity of a lake, plays an important role in shaping lacustrine processes, which respond to local, regional and higher-scales changes. Land use changes can significantly impact lake environments and have changed over time with different human civilizations and cultures. Land use changes are effectively traced with pollen analysis, for example cereals (pollen grains from cultivated crops), accurately show different phases of past human activity.One of the goals of my PhD thesis research, in collaboration with some wonderful scientists, was to test selected proxies and then understand how past land use changes affected Lake Żabińskie, northeastern Poland (Fig. 2). Together, we studied a 2000-year-long, varved sediment core from Lake Żabińskie. To read in detail what varves are and what they can tell us, see this blog post by Alicja Bonk. The current state of Lake Żabińskie and some aspects of its functioning (particularly erosion in the catchment) over the last 1000 years are already well-studied [5–7]. These results suggest that Lake Żabińskie may have a longer, more intricate history of human influence, especially on the lake mixing regime. Therefore, we investigated human-induced land use changes over the last 2000 years to better understand how their activities may have impacted lake mixing patterns. The exceptional age control [8] based on varved sediments allowed a high-resolution reconstruction of the influence of human impact on the lake catchment and the intensity of the lake mixing [9].
 |
| Fig. 2. Springtime at Lake Żabińskie. (A) Site location in Europe and (B) in Poland |
In a multiproxy approach, research is based on a wide array of data and techniques, which together lead to a more complete identification of the underlying and ever-changing processes that affect lake environments. However, most of the proxies that are used in paleoenvironmental research need some kind of validation and quality assessment. Even though it might be tempting to go with the well-established interpretation in the literature. Yet, each site should be treated with caution and data should be tested. As proxies could be controlled by different processes, they might effectively tell us a different story. Dissolved oxygen concentrations control many intertwined biogeochemical cycles. Some elements are prone to alter their solubility and mobility depending on oxygen concentrations. Among these, iron (Fe) and manganese (Mn), and their ratio (Fe/Mn: lower values mean better oxygen availability; higher values mean worse oxygen availability) are the most common indicators of changing oxygen conditions. Both elements could potentially be used as proxies for water mixing intensity. However, other environmental factors such as erosion, i.e. delivery of terrestrial material from the catchment into a lake, can influence concentrations of Fe and Mn, rendering them virtually useless as mixing intensity proxies. Fortunately, we can trace erosion intensity with other elements, such as titanium (Ti) or potassium (K), which allows us to decipher whether Fe and Mn changes are controlled by oxygen availability. Our results showed that indeed, they are!
The combination of sediment structure data, geochemistry and pollen data, with some archaeological background, led to the paleoenvironmental reconstruction of both human impact and lake mixing patterns (Fig. 3). At the beginning of the Common Era 2000 years ago, parts of NE Poland commonly known as the Masurian Lakeland were sparsely inhabited by the Bogaczewo settlers (Period 1, Fig. 3). These settlers were western Balts who had strong influence on the Masurian Lakeland until around the beginning of the 5th century CE. They partially deforested this region and the Lake Żabińskie catchment, as seen in the pollen data. Deforestation led to enhanced wind stress on the lake surface, causing intense water mixing and higher oxygen concentrations at the lake bottom. In presence of oxygen, portions of Fe and Mn circulating in lake waters were permanently deposited in sediments, leading to low values of Fe/Mn ratio (Period 1, Fig 3). From the 5th century (the end of the Migration Period) until the start of the ca. 17th century CE (Period 2, Fig 3), the lake vicinity was abandoned. With no human activity, the surrounding woodlands were restored, effectively sheltering the lake. As a result, water mixing was short-lasting and less efficient, leading to a stable meromixis. This is seen in the very simple sediment structure with only two distinct layers forming a varve, almost no Fe and Mn deposition (high Fe/Mn ratio), and no signs of high erosional input (Ti and K).
 |
| Fig. 3. Main changes in Lake Żabińskie and its catchment during the last 2000 years (modified after https://doi.org/10.6084/m9.figshare.8362094.v2) |
During the 17th century until the end of 19th century CE, the Masurian Lakeland was reclaimed by man, specifically the Teutonic Order, then Duchy and Kingdom of Prussia, which substantially changed land use (Period 3, Fig. 3). Once again, deforestation and agriculture led to an opening of the landscape, which enhanced wind stress on the lake, allowing deeper and longer-lasting water mixing. Higher accumulation of both Fe and Mn (low Fe/Mn ratio) in the sediments accompany this, suggesting better oxygen availability at the lake bottom. However, higher erosional input seen in Ti and K suggests that the clastic input could control fraction of the observed Fe and Mn accumulation. Still, human activity in the area led to better oxygen availability in the lake. However, other water traits have worsened - especially lake trophy, i.e. the rate at which organic matter is supplied to or produced in the given ecosystem. During Period 3, production and preservation of biomass in the lake started to grow. In the last period (Period 4, Fig. 3), the lake catchment became partially reforested as local fields were abandoned. At the same time, agriculture mechanization and the use of fertilizers, especially after World War II, led to both higher crop yields and nutrient inputs into the lake. Recovery of the woodlands has weakened wind stress on the lake, however, ongoing climate change continues to affect lake functioning. Specifically, with higher air temperatures, lakes are becoming warmer, which in turn promotes deeper lake stratification (development of water layers of different characteristics, especially temperature) and hinders lake mixing. This has led to very complicated water mixing patterns, as monitoring studies show [5]. Sediments reflect changes in lake mixing patterns, its thermal structure and rising production of the biomass, as deposition patterns and chemical properties became very complex in Period 4.
Exhale and conclude
In the case of Lake Żabińskie, meromixis should be considered its natural state given that it persisted for over 1100 years in Period 2 (Fig. 3). It is not surprising for such a small and deep lake. Human activity in the lake catchment was an indirect yet driving force behind changes in the past lake mixing. This shows that early human societies did have a significant impact on the environment. Our results demonstrate that early human activity led to better deep-water oxygenation in Lake Żabińskie through deforestation. Certainly, climate change and human activities specific to this site (tourism, agriculture) will continue to heavily impact lake functioning. Meromixis is expected to be longer and more stable in the coming years, while lake trophy will remain on the rise – a dire projection for this and numerous other lakes.
Maurycy Żarczyński, PhD
Postdoctoral fellow, Institute of Geography, University of Gdańsk
Postdoctoral fellow, Institute of Geography, University of Gdańsk
If you have questions or comments concerning Maurycy's post, please leave a comment below, or send him an email. You can also follow his research on ResearchGate or Twitter.
_____________________________________________________
References:
[1] Wetzel, R. G. Oxygen. in Limnology (Third Edition) (ed. Wetzel, R. G.) 151–168 (Academic Press, 2001). doi:https://doi.org/10.1016/B978-0-08-057439-4.50013-7.
[2] Jenny, J.-P. et al. Global spread of hypoxia in freshwater ecosystems during the last three centuries is caused by rising local human pressure. Glob. Chang. Biol. 22, 1481–1489 (2016).
[3] Jenny, J. et al. Urban point sources of nutrients were the leading cause for the historical spread of hypoxia across European lakes. Proc. Natl. Acad. Sci. (2016) doi:10.1073/pnas.1605480113.
[4] Cohen, A. S. Paleolimnology: the history and evolution of lake systems. (Oxfrod University Press, 2003).
[5] Bonk, A., Tylmann, W., Amann, B., Enters, D. & Grosjean, M. Modern limnology, sediment accumulation and varve formation processes in Lake Żabińskie, northeastern Poland: comprehensive process studies as a key to understand the sediment record. J. Limnol. 74, 358–370 (2015).
[6] Hernández-Almeida, I. et al. Resilience, rapid transitions and regime shifts: Fingerprinting the responses of Lake Zabinskie (NE Poland) to climate variability and human disturbance since AD 1000. The Holocene 27, 258–270 (2017).
[7] Bonk, A. et al. Sedimentological and geochemical responses of Lake Żabińskie (north-eastern Poland) to erosion changes during the last millennium. J. Paleolimnol. 56, 239–252 (2016).
[8] Żarczyński, M., Tylmann, W. & Goslar, T. Multiple varve chronologies for the last 2000 years from the sediments of Lake Żabińskie (northeastern Poland) – Comparison of strategies for varve counting and uncertainty estimations. Quat. Geochronol. 47, 107–119 (2018).
[9] Żarczyński, M., Wacnik, A. & Tylmann, W. Tracing lake mixing and oxygenation regime using the Fe/Mn ratio in varved sediments: 2000 years-long record of human-induced changes from Lake Żabińskie (NE Poland). Sci. Total Environ. 657, 585–596 (2019).
[1] Wetzel, R. G. Oxygen. in Limnology (Third Edition) (ed. Wetzel, R. G.) 151–168 (Academic Press, 2001). doi:https://doi.org/10.1016/B978-0-08-057439-4.50013-7.
[2] Jenny, J.-P. et al. Global spread of hypoxia in freshwater ecosystems during the last three centuries is caused by rising local human pressure. Glob. Chang. Biol. 22, 1481–1489 (2016).
[3] Jenny, J. et al. Urban point sources of nutrients were the leading cause for the historical spread of hypoxia across European lakes. Proc. Natl. Acad. Sci. (2016) doi:10.1073/pnas.1605480113.
[4] Cohen, A. S. Paleolimnology: the history and evolution of lake systems. (Oxfrod University Press, 2003).
[5] Bonk, A., Tylmann, W., Amann, B., Enters, D. & Grosjean, M. Modern limnology, sediment accumulation and varve formation processes in Lake Żabińskie, northeastern Poland: comprehensive process studies as a key to understand the sediment record. J. Limnol. 74, 358–370 (2015).
[6] Hernández-Almeida, I. et al. Resilience, rapid transitions and regime shifts: Fingerprinting the responses of Lake Zabinskie (NE Poland) to climate variability and human disturbance since AD 1000. The Holocene 27, 258–270 (2017).
[7] Bonk, A. et al. Sedimentological and geochemical responses of Lake Żabińskie (north-eastern Poland) to erosion changes during the last millennium. J. Paleolimnol. 56, 239–252 (2016).
[8] Żarczyński, M., Tylmann, W. & Goslar, T. Multiple varve chronologies for the last 2000 years from the sediments of Lake Żabińskie (northeastern Poland) – Comparison of strategies for varve counting and uncertainty estimations. Quat. Geochronol. 47, 107–119 (2018).
[9] Żarczyński, M., Wacnik, A. & Tylmann, W. Tracing lake mixing and oxygenation regime using the Fe/Mn ratio in varved sediments: 2000 years-long record of human-induced changes from Lake Żabińskie (NE Poland). Sci. Total Environ. 657, 585–596 (2019).
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