the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 20 Mar 2026
A high-resolution perspective on climate drivers of lake stratification and phototrophic community dynamics in Late Glacial Central Europe
Abstract. Predicting the trajectory of aquatic deoxygenation under global warming requires a mechanistic understanding of lacustrine responses to rapid climate shifts. We investigated how climate-driven changes in catchment vegetation and local iron-rich lithology regulated lake stratification and ecosystem resilience in the maar lake Holzmaar (Central Europe). We focused on the Late Glacial, specifically on transitions during Dansgaard-Oeschger Event 1 (DOE-1; ca. 14,690–11,700 cal yr BP), a period of rapid natural warming and cooling that serves as an analogue for future high amplitude climate variation and for modern Arctic lakes undergoing rapid climate-driven transitions. Combining non-destructive hyperspectral imaging (HSI) of sedimentary pigments with high-resolution XRF geochemistry, we resolved parts of the ecosystem trajectory during DOE-1.
Ecological succession progress from a pioneer community of cyanobacteria to a stable anoxic late-successional community characterized by planktonic diatom Stephanodiscus minutulus and anoxygenic purple sulphur bacteria (PSB) in the photic zone. While regional warming (mean summer temperature increased ~2.8 °C) provided the physical potential for lake stratification, our data suggest that intense anoxia was primarily triggered by the expansion of Betula in the watershed. This afforestation stabilized the water column through wind shielding. The termination of the anoxic phase coincided with the onset of the Younger Dryas cooling and increased aridity, which effectively destabilized the existing stratification. While the shift from Betula to Pinus forest may have caused a change in the terrestrial-aquatic linkage, the primary driver of the transition was the physical forcing (lake mixing) of the climatic shift (cooling).
Geochemically, the lake exhibited remarkable resilience. Unlike carbonate-dominated systems prone to internal phosphorus loading, Holzmaar efficiently sequesters nutrients via a dual mechanism of reactive iron binding (authigenic vivianite) and stable mineral burial. The phosphorous trap prevents nutrient release by permanently sequestering P in the sediment, allowing rapid ecosystem recovery without delay once the specific climate and vegetation drivers shift. Our findings demonstrate that in volcanic maar lakes, catchment vegetation characteristics and local lithology can modulate, and even override, the direct effects of climate warming on aquatic anoxia.
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Status: open (until 14 May 2026)
- CC1: 'Presentation of GDGT data', Paul Zander, 30 Mar 2026 reply
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CC2: 'Comment on egusphere-2026-1390', Brian F. Cumming, 01 May 2026
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Review of a high-resolution perspective on climate drivers of lake stratification and phototrophic community dynamics in Late Glacial Central Europe
This is an interesting and well-written paper that addresses an important question related to changes in lakes and rapid changes in climate. The issue of increasing anoxia in lakes due to changes in climate is an important issue, and studying past changes helps inform us of potential risks. The study lake is unique in terms of its size, but the findings are interesting and relevant, albeit not applicable to larger lakes where anoxia can be a problem to cold-water fish.
I would also categorize the study as of very high quality, in terms of the questions asked, the quality of the core over the time-frame of interest (both warming and cooling, defined by pollen assemblages), and the types and quality of the analyses (hyperspectral imaging, pigments, XRF, and diatoms). The paper is interesting and relevant.
General comment – I don’t particularly like the framing of assemblages as pioneering to late-successional. This concept assumes stationarity of limnological conditions and time to reach equilibrium. Paleolimnology informs us that patterns of succession can occur, but that there can be rapid reversals of algal assemblages due to changes in climate that can change lake conditions. This cannot be referred to as succession.
Comments by line, some of which reinforce the general comments above.
Line 18 – typo - change ‘phosphorous’ to ‘phosphorus’.
Line 74-78 – Questions asked – not sure you are answering these questions in your paper. The first questions I would keep as how phototrophic communities change over periods of rapid climate change over the D-O Event 1 from 14,690 to 11,700 cal BP, involving both warming and cooling events. You are inferring stratification, which is not the question.
Question 2 is more of a discussion point and not a central question.
Question 3 – not sure you can assess hysteresis and this isn’t discuss much in the paper. Hysteresis not properly defined in the paper.
Line 79 – ‘identify’ and ‘hypothesize’ are two similar but different concepts. I would not use ‘identify’.
Line 86 – change to 5.8 ha (we can all visualize this and conclude this is a but deep lake)
Line 87 – your site description needs to be better. You state that Holzmaar is predisposed to meromixis, but you never state that is or is not meromictic. This is important. More information is needed on this lake. Is it permanently meromictic. How does the monimolimnion mix. The thermocline and the chemocline are two separate things, and normally don’t co-occur. Do they in Holzmaar? This leads to great uncertainty later in your discussion.
Line 101 – don’t know what is meant by ‘parallel’ – Sediment cores taken within ‘x’ meters in the same location?
Line 146 – not sure you need ‘absolute’ before pigment ..
Maybe be more explicit on why hyperspectral and HPLC – to allow for inexpensive higher resolution analysis.
Line 185 – ‘neutral pH’ – distilled water is not neutral (pH of ~5.6).
Line 185 – reference for your modified evaporation methods. What evaporation method (Battarbee?) and how modified. If you have concentrations of diatoms, I would prefer this over the relative abundances presented so that you can relate to your Si from XRF.
Line 190 – subscript for HNO3
Line 220 – Only a selection of variables…. was used for further RDA… - please describe how you selected the variables you used.
Line 230 – state the ‘common’ resolution?
Line 236 -237 – I agree completely with the reduction of dimensionality, but how did you do this (see above). Did you adjust your permutation test for multiply comparison? Fig. 4 seems to have a lot of forward selected variables. Did you assess the VIFs.
3.2 Primary producer communities – see earlier – I would not characterize these as Pioneers, secondary colonizers and diatoms + PS. This has been done in the literature in Arctic systems, but high abundances the benthic diatoms you list in Fig. 3 in any lake dependent on the depth sampled. These taxa are likely ‘low’ light specialists (see Kingsbury et al. 2012, doi:10.1111/j.1365-2427.2012.02781.x; Gushalak et al. 2020, https://doi.org/10.1007/s10933-020-00146-w)
Line 257 – Agree that you have a pattern in Fig. 2, but all pigments inferring very low production and high erosion.
Fig. 2 – if you keep it, needs to be better labelled. I would much prefer seeing a plot of your pigments in terms of concentrations.
Line 263 – silicifiers is not a commonly used term – do you mean diatoms and chrysophytes or just diatoms. What do you mean by red-blooming algae – dinoflagellates or Rhodophyta). Likely best to specify the pigment as not to confuse the reader.
Line 264 – I didn’t read Vossel et al. (2015), but you should explain how they concluded that P. ocellata ‘thrives in oligotrophic waters with low phosphorus concentrations and a stratified water column. Vossel et al. from the tital seems like a taxonomic paper and not an ecological paper. In your Fig. 3, P. ocellata has similar %abundances during the B-A when S. minutulus and Chl a are high, so in your core, your interpretation is contradictory.
Line 268-269 – Likely the lake level likely increased, so this is likely not succession.
Line 277 – agree that production was low in the Pleniglacial. Please state why you believe that nitrogen was limiting.
Line 278 – if the driver is an increase in lake level, why would you refer to these taxa as late-successional. The environment is changing, succession needs stability.
Line 302 – would like a little more discussion on vivianite. Biogeochemists that I have discussed vivianite in the past believe that vivianite is post-depositional, requires anoxia and ferrous iron. Wouldn’t sequestering of phosphorus occur simply by the formation of meromixis.
Fig. 3 – you state ‘selected sedimentary pigments’. Why were these ones selected (higher concentrations?); do you have concentrations of diatoms – do the concentrations correspond to your spikes in Si from Fig. 1. At least in North America, P. ocellata is a planktonic taxa. I would not agree that it is benthic. It is a round rapheless centric.
Discussion
Line 324 – you represent your pollen as relative abundances. Pine can be transported a very long distance, so don’t over interpret high AP values.
Line 326 – totally disagree – this is not like ecological succession.
Line 332 – yes you have small amounts of many taxa including benthic diatoms. To me this infers low light and an unstable landscape. What is a generalist cyanobacteria??
Line 342 – agree with your rising lake levels and expansion of macrophytes and a more diverse benthic flora in the early Bolling, and then even higher water levels. In your HPLC data could you look at the ratio of chlorophyll a/pheophytin a as a proxy; I would hypothesize that if the lake got deeper this ratio would be lower and stable (more photodegradation on the way to the sediments), and when shallow and turbid the ratio would be high and potentially variable do to quick burial. Just a thought if you have this data.
Line 346 – the trend towards thermal stratification cannot be inferred by okenone. Okenone is normally associated with meromixis. Thermal stratification could occur in the upper water layer (mixolimnion) that could be categorized based on the Lewis Lake classification system (i.e. with respect to mixing patterns).
Line 354 – I would not use the term succession
Line 361 – I don’t see a paradox. Anaerobic bacteria and the S. minutulus occupy different regions of the lake and could easily vary with seasonality, even if the lake was chemically stratified. You do not need to imply ecological plasticity. Do you have any data on seasonality. Does this lake also thermally stratify in the monomolimnion?
Line 369 – Sound like you have some data on seasonality and distribution of diatoms. Please describe the data collected. However, upon examination of the references, any evidence is sparce. If you have any evidence that S. minutulus exists above the bacterial plate please let the reader know. This appears to be highly speculative but is written definitely. Your statement “….S. minutulus thrive in the lower epilimnion immediately above the bacterial plate…” make no sense. The epilimnion would be up in the monimolimnion, and the bacterial plate by the chemocline. The deep chlorophyll maximum normally occurs on the metalimnion in the monimolimnion, the chemocline is normally deeper in the lake. Spanbauer’s statement is correct, but she is referring to the lower epilimnion, were diatoms get stuck due to thermal density changes. If you have directly knowledge of cooccurrence of diatoms and anoxic bacteria. This whole section is not written well. Coming in at the end of the paragraph with new data on seasonal separation should have come earlier.
Line 392-394 – I would insert that the expansion of birch may have acted as a reinforcing factor.
Line 403 – yes, DOC may be important but without data is you are getting complex without a lot a support.
Line 412-415 – yes, may be a factor, but the cooling/aridity is likely the most likely factor. Did regional conditions show drops in lake levels? The drop in production is huge. When you say stable lake stratification, do you mean the breakdown of meromixis? or both. On Fig. 3, were diatoms simply not present but analyses attempted. Did lakes freeze over permanently? Do other records show such large changes?
Line 469 – internal phosphorus loading and hysteresis are not the same. Needs clarification.
Citation: https://doi.org/10.5194/egusphere-2026-1390-CC2 -
CC3: 'Reply on CC2', Brian F. Cumming, 01 May 2026
reply
Just noticed that I mistakenly used monimolimnion in place of mixolimnion in a few places in CC2. To clarify, in a meromictic lake, the mixolimnion sits on top of the chemocline with the monimolimnion below the chemocline. Apologies for the mistake.
Citation: https://doi.org/10.5194/egusphere-2026-1390-CC3
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CC3: 'Reply on CC2', Brian F. Cumming, 01 May 2026
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Dear Dr. Zahajská et al.,
Congratulations on this nice study - I found it very interesting to read about the changes in lake ecology during this period of rapid climate changes.
My comments pertain to the use of GDGT data from Holzmaar and nearby Auel Maar published in Zander et al., 2024 (Climate of the Past). As author of that study, I want to suggest a couple small changes to how you present the temperature reconstruction based on GDGTs.
1) I suggest to remove data points in the temperature reconstruction that were not included the final temperature reconstruction. These are the 3 youngest data points from Auel Maar, which were measured as a check on the overlap with Holzmaar, but were excluded due to the poor chronology of Auel Maar in this section and because there may have some influence of soil GDGTs as Auel Maar was very shallow transitioning to a floodplain during this time. The data points to exclude have ages
13128 b2k
13340 b2k
13875 b2k
In the PANGAEA data file, these are marked with a comment as not included in the final temperature reconstruction, but I can understand how this could easily be missed, and I plan to resubmit the dataset with these temperature estimates removed.
2) Please note in Figure 6 either on the axis label or in the caption that the GDGTs represent Temperatures of Months Above Freezing (TMAF).
3) It might be interesting to mention that the very high %GDGT-0 indicates a lot of methanogenic activity in Holzmaar, including the late glacial, and the presence brGDGT-IIIa'' also confirms anoxic conditions in the hypolimnion even in the oldest sample I analyzed at 14.2 ka BP.
Sincerely,
Paul Zander