title: 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
Creators: name: Tu, Luyao
Publisher: name: Universität Bern
Year: 2021
Subjects: subject: 540 Chemistry
subject: 550 Earth sciences &amp; geology

description: 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.