Climate-related soil saturation and peatland development may have conditioned surface water brownification at a central European lake for millennia
Graphical abstract
Introduction
Water brownification, i.e. increased brown colouration, stems mainly from heightened terrestrial input of humic substances measured as dissolved organic carbon (DOC) (Roulet and Moore, 2006; but see Kritzberg and Ekström, 2012). Brownification has gripped an attention of the scientific community, as fresh waters across the northern hemisphere have been getting browner for the last 30 years (Kritzberg, 2017; Meyer-Jacob et al., 2019). Proposed factors driving higher levels of DOC in surface waters involve recent climate change (Creed et al., 2018; Fee et al., 1996), land-use (Mattsson et al., 2009; Meyer-Jacob et al., 2015; Sankar et al., 2020) and/or recovery from anthropogenic acid deposition from the atmosphere (Hruška et al., 2009; Monteith et al., 2007). Although recovery from acid deposition cannot explain the hemispherical extent of recent surface water brownification (Clark et al., 2010; Kritzberg, 2017), the recovery-based hypothesis builds on the important point that many freshwater bodies were probably brown naturally before the period of anthropogenic surface water acidification. Lacustrine sedimentary records offer the potential to study the progress of water brownification over these millennial time-scales. Such long-term records are needed to disentangle the differing mechanisms attributed to brownification, since recovery from anthropogenic acidification can bias correlation between spatial and decadal-scaled studies (Gavin et al., 2018; Stetler et al., 2021).
In low productivity (oligotrophic) lakes, the heightened concentration of DOC triggers a series of interconnected environmental shifts with direct impact such as (i) lower pH caused by the dominant proportion of humic acids in DOC (Thurman, 1985) and (ii) effective light attenuation (Vincent and Pienitz, 1996; Wetzel, 2001). Those direct impacts can result in further indirect changes producing (iii) water column thermal stratification by enhanced heating of surface layer (Fee et al., 1996; Snucins and Gunn, 2000), (iv) anoxic bottom conditions sustained by thermal stratification limiting oxygen replenishment (Brothers et al., 2014; Nürnberg and Shaw, 1999) and (v) altered nutrient availability (Corman et al., 2018; Nürnberg and Shaw, 1999; Sanders et al., 2015). Through this complex suite of stressors, water brownification can produce severe impacts on aquatic organisms (Karlsson et al., 2009; Solomon et al., 2015; Vasconcelos et al., 2016), which can amplify the effects of brownification by biotic processes (Brothers et al., 2014). Conversely, promotion of growth of aquatic biota by allochthonous input of humic compounds has also been documented (Daggett et al., 2015; Kissman et al., 2013; Pienitz and Vincent, 2000). A hypothesis of a unimodal relationship between primary production and DOC links these contradicting observations and depicts the responses to brownification as a trade-off between positive response to input of nutrients from dissolved organic matter (DOM) and limitation by reduced light availability (Jones, 1992; Kelly et al., 2018). The degree of fertilizing ability of DOM varies with differences in the stoichiometry between nutrients contained in the organic matter (Tipping et al., 2016) or change in co-export mechanism of DOM and soil-bounded nutrients during leaching (Kopáček et al., 2011). The capacity of soils to bound DOM alters substantially with redox changes on the gradient of soil saturation (Olivie-Lauquet et al., 2001; Possinger et al., 2020). Spatial scales appear to also matter, with positive correlations observed between concentrations of DOC in fresh waters and extent of wetlands in catchments (Kortelainen, 1993; Laudon et al., 2011; Rantala et al., 2016) and rainfall/run-off (Brothers et al., 2014; Ejarque et al., 2018).
The inter-linked impacts of precipitation dynamics and the processes of soil development, e.g., podzolization and paludification, could be traced on the millennial scale and further elucidate mechanisms of natural brownification. In the Post-glacial, vegetational succession mediated the effects of climate on brownification by providing sources of organic matter and interacting with soil biogeochemistry and hydrology (Engstrom et al., 2000; Huvane and Whitehead, 1996; Korsman et al., 1994; Pienitz et al., 1999). Soil formation and subsequent hardening of soil horizons during podzolization made dissolved organic matter (DOM) available for a transport into lakes (Engstrom et al., 2000; Steinberg, 1991). Humic acids potentially played an important role in acidification on deglaciated base-poor bedrocks (Ampel et al., 2015; Ford, 1990), besides the prominence of depletion of base cations in the initial phases of glacial lake ontogeny (Boyle, 2007; Boyle et al., 2013, Boyle et al., 2013). Steps in the progress of millenial-scale brownification often occurred in phase with pulses of climate humidity (Brodin, 1986). Whereas the cold and dry Younger Dryas stadial (~12.9–11.7 ky BP) interrupted water brownification in some lakes due to temperatures limiting vegetation growth and freezing soil waters (Ampel et al., 2015; Norton et al., 2011), a lack of leaching water locally during the dry Mid-Holocene “thermal maximum” (~8–5 cal. ky BP) may have reduced brownification for some catchments (Huvane and Whitehead, 1996; Itkonen et al., 1999).
A regional shift to a wetter climate after ~6 cal. ky BP, known as Mid-Holocene Climate Transition (Magny et al., 2006; Wanner et al., 2008), appears to have accelerated brownification in many boreal catchments (Myrstener et al., 2021; Pienitz et al., 1999; Solovieva and Jones, 2002). This pronounced climatic threshold has been linked to the inception and expansion of peatlands (paludification) (Bauer et al., 2003; Le Stum-Boivin et al., 2019; Myrstener et al., 2021), that further supplied headwaters with DOC (Belyea and Malmer, 2004). The extent to which the formation of impermeable soil horizons (podzolization) conditioned this regional paludification remains the subject of debate (Payette et al., 2012; Schaffhauser et al., 2017). Similarly, uncertainties accompany understanding of the progress of brownification outside the well-studied boreal regions of northern Europe and the North America in the Holocene. In particular, new paleolimnological records from mid-latitude Europe could alter the picture of Holocene brownification, given both the differences in the Holocene trajectory of climate humidity compared to northern Europe (Florescu et al., 2019; Mauri et al., 2015) and the acidic base-line of these catchments prior to anthropogenic acidification (Jüttner et al., 1997; Sienkiewicz, 2016). Unfortunately, studies from central Europe tracking the progress and possible controls over brownification in mountain lakes through the Holocene are rare (e.g., Steinberg, 1991).
DOC levels in prehistory can be reconstructed for lake sediments through diatom-based transfer functions (Pienitz and Vincent, 2000) or by measuring total organic carbon concentration in sediments (Meyer-Jacob et al., 2017; Russell et al., 2019). However, an increase of humic content in boreal lakes during the Holocene was most often detected indirectly in paleorecords as a decrease in ecosystem productivity or water pH, hence the terms “dystrophication” and “natural acidification” remain rooted in paleolimnological studies. The various scale and complexity of feedback mechanisms during brownification invite the application of a multi-proxy approach (Birks and Birks, 2006) to reveal particular driving factors behind and impacts on aquatic biota during millennial-scaled brownification. Paleolimnological studies can employ numerous complementary geochemical and biological indicators that can disentangle sensitive and interwoven responses to the impacts of brownification, including:
- i)
Acidification by humic acids can be detected using changes in diatom communities (Battarbee et al., 2010; Birks and Simpson, 2013; Curtis et al., 2009).
- ii)
Shading of the water column creates imbalance in primary production between planktonic and benthic communities (Karlsson et al., 2009).
- iii)
Thermal stratification imprints on dynamics of floating-dependent diatom species (Reynolds, 2006; Rühland et al., 2015) and possibly to some extent on changes in the ratio between chrysophycean cysts and diatom valves - C:D ratio (Werner and Smol, 2005).
- iv)
Anoxic bottom water conditions affect chironomid indicators and concentrations of their remains (e.g., Quinlan and Smol, 2001; Ursenbacher et al., 2020) and can be reflected in redox-sensitive element ratios such as Mn/Fe, Mn/Ti and Fe/Ti (Davison, 1993; Kylander et al., 2013; Makri et al., 2021).
- v)
Altered nutrient availability affects the taxonomic composition of both diatom (Hall and Smol, 2010; Rivera-Rondón and Catalan, 2020) and chironomid communities (Brodersen and Quinlan, 2006; Lindegaard, 1995) and the C:D ratio (Smol, 1985).
Here, we assess the evidence and explore the range of causal factors for lake water brownification using the sediments of a small mountain lake (Prášilské jezero) located in the Bohemian Forest of central Europe. This type of lake, with poorly-buffered soils and boreal-type vegetation, holds the catchment properties constant providing the opportunity to compare brownification in central Europe with that encountered in the more widely studied boreal regions. The data generated facilitate inferring trends in pH using diatom-based transfer functions, estimating the intensity of light limitation of algal life-forms in the water by separate assessment of periphytic and euplanktonic diatoms, using chironomid assemblage composition and trends redox-sensitive elements (e.g., Mn/Ti and Fe/Ti) to reveal phases of anoxia, and exploring the nutrient dynamics during brownification episodes using changes in diatom-based transfer functions (e.g., total phosphorus) and sediment geochemistry (e.g., Al, Si, P). Specifically, we: (i) track the onset and extent of brownification in what appears to be a naturally humic and small catchment lake, (ii) reconstruct the impacts of brownification on the lacustrine ecosystem using remains of aquatic organisms (i.e., diatoms and chironomids) and sediment geochemistry, and (iii) explore the main factors causing brownification during the Holocene and contrast the ecosystem functioning of this central European lake with those across wider boreal regions.
Section snippets
Study site
Eight glacial lakes are situated along the Czechia-Germany-Austria border in the Bohemian Forest (central Europe, Fig. 1) and analyses of their sediments document the last phases of the deglaciation in this low mountain range (Mentlík et al., 2013; Vondrák et al., 2019a; Vondrák et al., 2021). Shallow soils, siliceous bedrock (gneiss, mica-schist, granite, quartzite) and presence of Norway spruce (Picea abies) as the dominant tree taxon, have enhanced the sensitivity of lake waters to
Zonation
In total, four zones were determined based on the cluster analyses for diatom and chironomid relative abundances: Zone 1a, Zone 1b, Zone 2, and Zone 3 (see Fig. 2, Fig. 3 and A.2). Zones 1, 2 and 3 were supported by significant clustering of diatom assemblages, based on the whole diatom assemblage (Zone 2/Zone 3; depth of 1566.25 cm) and periphytic diatom assemblage (Zone 1/Zone 2; depth of 1585.25 cm). Zone 1 was subdivided further based on the first non-significant chironomid splitting
Ecosystem functioning thresholds at Prášilské jezero
One hundred and fifty years of hydrobiological research of Bohemian Forest lakes have documented complex changes in their water chemistry, including changes in water colour and transparency (e.g., Veselý, 1994; Vrba et al., 2000). However, that longer-term perspective on millennial-scale water changes in brownification had remained elusive before the application of paleolimnological methods (e.g., Vondrák et al. (2019b) and Moravcová et al. (2021)). Here, we focus on more detailed history and
Conclusions
Browning of surface waters at Prášilské jezero occurred as three successive steps and varied in scale of impacts and probable driving factors. The lake experienced elevated input of humic compounds from formation at the onset of the Holocene. The first step in browning appeared ~10.7 cal. ky BP and probably originated from soil stabilization with the catchment afforestation. We propose that browning supported thermal stratification and bottom anoxia of the lake, but shading effects were
CRediT authorship contribution statement
Anna Tichá: Investigation, Formal analysis, Visualization, Writing – original draft. Daniel Vondrák: Investigation, Data curation, Writing – review & editing. Alice Moravcová: Investigation, Data curation, Writing – review & editing. Richard Chiverrell: Investigation, Data curation, Writing – review & editing. Petr Kuneš: Writing – review & editing, Project administration, Funding acquisition.
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
The authors would like to thank Šumava National Park authorities for a fieldwork permission and support of the project. We are also grateful to Jiří Kopáček, Jaroslav Vrba and Petra Zahajská for inspiring discussion about interpretation of data and three anonymous reviewers for substantial improvement of the original manuscript.
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