Sedimentary phosphorus dynamics in response to lake trophy and mixing regime changes during the Late-Glacial, Holocene and the Anthropocene: three case studies from deep lakes in Switzerland

Phosphorus (P) released from sediments into surface waters, termed internal P loadings, has been widely recognized as a key P source contributing to delayed recovery from human-induced eutrophication in lakes where external P loadings have been reduced. It is critically important to evaluate the availability of sediment-P and its release risks into lake water by studying P fractions in sediment profiles. However, previous studies focused only on well-mixed shallow lakes and did not examine the relationships between sediment records of P fractions and lake trophic status in seasonally stratified, deep lakes. Additionally, records of P fractions spanning longer than a few decades have not been studied. Hypolimnetic waters in temperate eutrophic deep lakes tend to become anoxic during stratification periods, which can lead to high internal P loadings and sustain eutrophication. It is not yet fully understood whether, and how, lake trophic levels and hypolimnetic redox conditions can influence the long-term behavior of sedimentary P fractions and potentials of internal P loadings in deep lakes. Our research questions were: (i) How have P fractions in sediment profiles and potentials of internal P loadings varied with different lake trophic levels over the past few decades? (ii) What was the role of hypolimnetic anoxia and lake trophic state in affecting P cycling, sediment P fractions stratigraphy, and potentials of internal P loadings in the last century prior to cultural eutrophication? (iii) Is the P cycling under natural or pre-anthropogenic anoxic conditions comparable with the P cycling under anthropogenic eutrophication and anoxia in recent times? In order to address these questions, we selected three deep lakes in Switzerland: the Ponte Tresa basin of Lake Lugano, Lake Burgäschi, and Soppensee. We investigated sediment cores from these lakes to produce records of P fractions in sediments, lake trophy, and anoxia history covering the past few decades, more than one century, and the Late-glacial and Holocene periods, respectively. The Ponte Tresa basin was selected because it is one of several deep lakes in Switzerland that have not yet recovered from human-induced eutrophication after large reductions of external P loadings, and the lake’s eutrophication history since the mid-20th century is already well documented. Lake Burgäschi was selected because there were substantial changes in trophic levels and possibly lake mixing regimes during the last century, and the lake has exceptionally long historical and limnological survey data available for most of the last 50 years. We selected Soppensee because it is a deep, eutrophic lake featuring a varved sediment record, its sediments have an exceptionally good chronology, and it has a record of diatom-inferred epilimnetic total P (DI-TP) concentrations available for the entire Holocene. Hyperspectral imaging (HSI) was applied for high-resolution analysis of sedimentary pigments, combined with geochemical analyses, which allows reconstructions of lake primary production (eutrophication history) and hypolimnetic redox conditions. XRF core scanning was used to determine the elemental composition of sediments and these geochemical variables were related to in-lake and catchment processes (e.g. XRF-Mn and Fe/Mn ratios as redox proxies). In this study, different P forms in sediments were characterized by P-fractionation schemes, mainly by sequential P extraction procedures and the standards, measurements and testing (SMT) protocol. The results of P-fractionation showed that, in each lake, labile P fractions (mainly the redox sensitive Fe-bound P or Fe/Al metal oxides bound P) were the dominant P form in anoxic sediments during most of the periods studied. This phenomenon suggests high potentials of internal P loadings in the three lakes. Hypolimnetic redox conditions appear to control contents of redox-sensitive Fe and Mn in sediments, which in turn influences P retention in sediment profiles of the lakes. In the Ponte Tresa basin of Lake Lugano, we find that net burial rates of total P and the labile P fraction (mainly redox sensitive Fe-bound P) in sediments showed significant decreasing trends from 1959 to 2017, when the lake underwent higher eutrophic levels and severe anoxia. This finding suggests that, in the Ponte Tresa basin, higher eutrophication conditions increased internal P loadings, thus reducing net burial rates of P in sediments. This case study highlights the concern that eutrophication restoration might be hindered in deep, seasonally stratified lakes by extensive internal P cycling and reduced capacity of P-trapping in surface sediments. In Lake Burgäschi, the results highlight the importance of hypolimnetic redox conditions in controlling the long-term P cycling and P retention in sediments since the 1900s. We found relatively high total P and labile P fractions in Fe- and Mn enriched layers when the hypolimnion was seasonally oxygenated. The results also imply that hypolimnetic water withdrawal in Lake Burgäschi can effectively reduce P retention in sediments and potentials of internal P loadings. In Soppensee, we estimated long-term qualitative internal P loadings by comparing the Holocene record of DI-TP concentrations with the labile P fraction (Fe/Al-P) concentrations in sediments under changing trophic, redox, and lake mixing regimes. The results demonstrate that enhanced internal P loadings acted as a positive feedback to promote and maintain natural eutrophication process in Soppensee from ~9000 to 6000 cal BP. However, such a positive feedback was not inferred for other eutrophic phases. For example, ferromagnetic minerals from magnetotactic bacteria preserved in sediments from ~6000 to 2000 cal BP and Fe-rich layers formed from ~2000 to 200 cal BP appear to have prevented internal P loadings in these two periods, resulting in high labile P fraction in sediments. In summary, this project provides new insights about the influence of lake primary production and hypolimnetic redox conditions on P cycling and the record of sediment P fractions on short and long timescales in seasonally stratified, deep lakes. We conclude that, in stratified deep lakes, sedimentary total P and P fractions may not reflect lake trophic evolution and the history of external P loadings. However, comparisons of the lake trophic history and the record of sediment P fractions can shed light on in-lake P cycling in the past

The influences of historic lake trophy and mixing regime changes on long-term phosphorus fraction retention in sediments of deep eutrophic lakes: a case study from Lake Burgäschi, Switzerland

Hypolimnetic anoxia in eutrophic lakes can delay lake recovery to lower trophic states via the release of sediment phosphorus (P) to surface waters on short timescales in shallow lakes. However, the long-term effects of hypolimnetic redox conditions and trophic state on sedimentary P fraction retention in deep lakes are not clear yet. Hypolimnetic withdrawal of P-rich water is predicted to diminish sedimentary P and seasonal P recycling from the lake hypolimnion. Nevertheless, there is a lack of evidence from well-dated sediment cores, in particular from deep lakes, about the long-term impact of hypolimnetic withdrawal on sedimentary P retention. In this study, long-term sedimentary P fraction data since the early 1900s from Lake Burgäschi provide information on benthic P retention under the influence of increasing lake primary productivity (sedimentary green-pigment proxy), variable hypolimnion oxygenation regimes (Fe∕Mn ratio proxy), and hypolimnetic withdrawal since 1977. Results show that before hypolimnetic withdrawal (during the early 1900s to 1977), the redox-sensitive Fe∕Mn-P fraction comprised ∼50 % of total P (TP) in the sediment profile. Meanwhile, long-term retention of total P and labile P fractions in sediments was predominantly affected by past hypolimnetic redox conditions, and P retention increased in sedimentary Fe- and Mn-enriched layers when the sediment-overlaying water was seasonally oxic. However, from 1977 to 2017, due to eutrophication-induced persistent anoxic conditions in the hypolimnion and to hypolimnetic water withdrawal increasing the P export out of the lake, net burial rates of total and labile P fractions decreased considerably in surface sediments. By contrast, refractory Ca–P fraction retention was primarily related to lake primary production. Due to lake restoration since 1977, the Ca–P fraction became the primary P fraction in sediments (representing ∼39 % of total P), indicating a lower P bioavailability of surface sediments. Our study implies that in seasonally stratified eutrophic deep lakes (like Lake Burgäschi), hypolimnetic withdrawal can effectively reduce P retention in sediments and potential for sediment P release (internal P loads). However, after more than 40 years of hypolimnetic syphoning, the lake trophic state has not improved nor has lake productivity decreased. Furthermore, this restoration has not enhanced water column mixing and oxygenation in hypolimnetic waters. The findings of this study are relevant regarding the management of deep eutrophic lakes with mixing regimes typical for temperate zones

Anthropogenic modification of phosphorus sequestration in lake sediments during the Holocene: A global perspective

Human activity has fundamentally altered the global phosphorus (P) cycle. Yet our understanding of when and how humans influenced the P cycle has been limited by the scarcity of long-term P sequestration records, particularly outside Europe and North America. Lake sediments provide a unique archive of past P burial rates and allow the human-mediated disruption of the global P cycle to be examined. We compiled the first global-scale and continentally resolved reconstruction of lake-wide Holocene P burial rates using 108 lakes from around the world. In Europe, lake P burial rates started to increase noticeably after ∼4000 calendar years before 1950 CE (cal BP), whereas the increase occurred later in China (∼2000 cal BP) and in North America (∼550 cal BP), which is most likely related to different histories of population growth, land-use and associated soil erosion intensities. Anthropogenic soil erosion explains ∼86% of the observed changes in global lake P burial rates in pre-industrial times. We also provide the first long-term estimates of the global lake P sink over the Holocene (∼2686 Tg P). We estimate that the global mean lake sediment P sequestration since 1850 CE (100 cal BP) is ∼1.54 Tg P yr−1, representing approximately a six-fold increase above the mean pre-industrial value (∼0.24 Tg P yr−1; 11,500 to 100 cal BP) and around a ten-fold increase above the Early-Middle Holocene low-disturbance baseline of 0.16 Tg P yr−1. This study suggests that human activities have been affecting the global P cycle for millennia, with substantial alteration after industrial times (1850 CE)

Anthropogenic Activities Generate High-Refractory Black Carbon along the Yangtze River Continuum

12 pages, 7 figuresCombustion-driven particulate black carbon (PBC) is a crucial slow-cycling pool in the organic carbon flux from rivers to oceans. Since the refractoriness of PBC stems from the association of non-homologous char and soot, the composition and source of char and soot must be considered when investigating riverine PBC. Samples along the Yangtze River continuum during different hydrological periods were collected in this study to investigate the association and asynchronous combustion drive of char and soot in PBC. The results revealed that PBC in the Yangtze River, with higher refractory nature, accounts for 13.73 ± 6.89% of particulate organic carbon, and soot occupies 37.53 ± 11.00% of PBC. The preponderant contribution of fossil fuel combustion to soot (92.57 ± 3.20%) compared to char (27.55 ± 5.92%), suggested that fossil fuel combustion is a crucial driver for PBC with high soot percentage. Redundancy analysis and structural equation modeling confirmed that the fossil fuel energy used by anthropogenic activities promoting soot is the crucial reason for high-refractory PBC. We estimated that the Yangtze River transported 0.15–0.23 Tg of soot and 0.15–0.25 Tg of char to the ocean annually, and the export of large higher refractory PBC to the ocean can form a long-term sink and prolong the residence time of terrigenous carbonThis study was supported by grants from the National Natural Science Foundation of China (nos. 42277214, 42207256, and 41971286), major programs of the National Social Science Foundation of China (grant nos. 22&ZD136), the Special Science and Technology Innovation Program for Carbon Peak and Carbon Neutralization of Jiangsu Province (grant no. BE2022612)Peer reviewe

Phosphorus fractions in sediments and their relevance for historical lake eutrophication in the Ponte Tresa basin (Lake Lugano, Switzerland) since 1959

Lake Lugano is one of several deep lakes in Switzerland that have not yet recovered from eutrophication after large reductions of external phosphorus (P) loadings. Persistent eutrophication has been attributed mainly to internal P loadings from sediments. To achieve the restoration goals, it is critically important to evaluate the sediment P availability and release risk in this lake. In this study, we combined sequential P extraction (four fractions) with enzyme hydrolysis to assess distribution characteristics of P forms and potential bioavailability of organic P in an anoxic sediment profile from the Ponte Tresa basin of Lake Lugano, southern Switzerland. Labile P forms, i.e. mostly redox-sensitive iron bound P and metal oxides bound P (Al/Fe-P), comprised ~70% of total P in the sediment profile (1959–2017 CE), suggesting a high potential for P release from the anoxic sediment. Potentially bioavailable organic P forms (determined by addition of substrate specific enzymes) were considerably higher in the surface sediments (top 5 cm), which is very likely to release P in the near future with early diagenesis. The net burial rates (NBR) of redox sensitive Fe-P fraction and total P in sediments both showed significant decreasing trends from 1959 to 2017 CE, when trophic levels of the lake increased from mesotrophic to hypertrophic status. We suggest that, in the Ponte Tresa basin, higher eutrophication conditions led to enhanced sediment P release (mainly from redox sensitive Fe-P fraction), thus reducing P-NBR in sediments. This study highlights the concern that in deep monomictic lakes, eutrophication restoration might be hindered by extensive internal P cycling and reduced capacity of sediment P-trapping

Holocene phototrophic community and anoxia dynamics in meromictic Lake Jaczno (NE Poland) using high-resolution hyperspectral imaging and HPLC data

Global spread of hypoxia and less frequent mixing in lakes is a growing major environmental concern. Climate change and human impact are expected to increasingly deteriorate aquatic ecosystems. The study of processes and drivers of such changes in the past provides a great asset for prevention and remediation in the future. We used a multi-proxy approach combining high-resolution bulk pigment data measured by Hyperspectral Imaging (HSI), with lower resolution specific chlorophylls and carotenoids measured by HPLC to examine Holocene trophic state changes and anoxia evolution in meromictic Lake Jaczno, NE Poland. A redundancy analysis RDA including pollen-inferred vegetation cover, temperature and human impacts provides insight into specific conditions and drivers of changing trophic and redox states in the lake. Anoxic and sulfidic conditions established in Lake Jaczno after initial basin infilling 9500 years ago. Until 6700 cal BP, lake trophy was relatively low, water turbidity was high, and green sulfur bacteria (GSB) were abundant within the phototrophic community, suggesting a deep oxic–anoxic boundary and weak stratification. The period between 6700–500 cal BP is characterized by constantly increasing lake production and a gradual shift from GSB to purple sulfur bacteria (PSB), suggesting a shallower oxic–anoxic boundary and pronounced stratification. Yet, the presence of spheroidene and speroidenone in the sediments indicates intermittent anoxia. After 500 cal BP, increasing human impact, deforestation and intensive agriculture promoted lake eutrophication, with a shift to PSB dominance and establishment of permanent anoxia and meromixis. Our study unambiguously documents the legacy of human impact on processes determining eutrophication and anoxia

Geochemical parameters determined in sediment cores from Lake Burgäschi, Switzerland

Sediment phosphorus (P) released to surface water (a.k.a. internal P loading) is the key factor delaying lake recovery from eutrophication in many lakes. The sediment P availability and release risk into lake water can be largely evaluated by P fractions in sediments. In deep eutrophic lakes, hypolimnetic anoxia is suggested to prompt sediment-P mobility and potentially increase internal P loading on short term. However, the long-term effects hypolimnetic redox conditions and trophic state on sedimentary P-fraction retention in deep lakes are not clear. In this study we investigated two sediment cores retrieved from the deepest part of Lake Burgäschi, Switzerland. The age-depth profile based on 210Pb and 137Cs chronology dates back to the early 1900s. Sedimentary green-pigments (chlorophylls and diagenetic products) inferred from hyperspectral imaging (HSI) scanning and X-ray fluorescence (XRF)-inferred Fe/Mn ratios primarily reflect lake trophic state evolution (aquatic primary productivity) and hypolimnetic oxygenation, respectively. A sequential P-extraction with five P fractions was performed to uncover P fractionation in sediment profiles. In addition, we measured the sediment bulk elements of carbon, nitrogen and sulfur. The temporal records of P fractions concentrations and net burial rates (NBR) in sediments were investigated