Reconstruction of trophic state shifts over the past 90 years in a eutrophicated lake in western Switzerland, inferred from the sedimentary record of photosynthetic pigments

Abstract

Anthropogenic eutrophication can initiate vast and persistent ecosystem state changes in lakes. Such changes may be characterized by increased phytoplankton taxa variability, which can affect the effectiveness and time of lake recovery mechanisms. Lake Morat in Switzerland has undergone intense eutrophication in the 20 th century (phosphorous concentrations up to 150 μg L-1) caused by excessive nutrient loadings from agricultural intensification and urbanization. Phosphorous reduction measures since 1986, such as the ban of phosphates in detergents and decreased use of fertilizers in agriculture, have resulted in total phosphorous concentrations up to 20 μg L-1 today. Despite this drastic reduction of total phosphorous, total biomass production stays high. We investigate historical changes in the phytoplankton community during the eutrophication and re-oligotrophication periods by comparing historical limnological data with sediment pigment concentrations measured by HPLC and other geochemical proxies in a radiodated sediment core. For the last 90 years, we identified four major trophic state changes in Lake Morat. The first period (AD 1924-1937) is characterized by low pigment concentrations and nutrient inputs, with good oxygen conditions. This section represents trophic conditions before the intensive eutrophication phase. The second period (AD 1937-1970) revealed an abrupt increase in pigment concentrations with higher primary production, cyanobacteria dominance and reduced oxygen levels. Oscillaxanthin indicated a Planktothrix rubescens dominance (AD 1954-1970) with concentrations up to 800 nmol g-1 OM. Their decline after AD 1970 suggests the initiation of an intense eutrophication phase (AD 1970-1983) associated with the dominance of other cyanobacteria species, higher total phosphorous inputs, and intense anoxia. In the restoration period (AD 1983-2014), there was a shift in the phototrophic community from cyanobacteria to green algae dominance, yet some cyanobacteria species remain present. Rapid phytoplankton community changes were identified in the studied period, yet overall primary production response was low. Limnological data revealed a delay of phosphorous reduction due to phosphorous recycling from the sediments. The observation of complex lake ecosystem reactions to prolonged eutrophication and subsequent reoligotrophication, as shown by the paleolimnological and limnological data in this study, emphasize the importance of careful lake management to revert eutrophication back to historical reference biomass values.

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

“…RABD 673 (spectral region 590-730 nm) was used to detect chlorophyll a and b and their diagenetic products (T Chl) and served as a proxy for aquatic primary production (Leavitt and Hodgson, 2001). RABD 845 (spectral region 790-895 nm) was used to detect total bacteriopheophytin a and b (Bphe) (Butz et al, 2015(Butz et al, , 2016, which is a proxy for anoxia and meromixis as described in Makri et al (2020). Bphe a and b is produced by anoxygenic phototrophic purple sulfur and nonsulfur bacteria that proliferate in illuminated anoxic habitats (Yurkov and Beatty, 1998;Madigan and Jung, 2009).…”

“…This is most relevant for lake management or restoration. Although recent anoxia and eutrophication have been very well studied and understood (Naeher et al, 2013;Friedrich et al, 2014;Jenny et al, 2016a), less is known about the onset, cessation and specific conditions of these changes in the past due to the lack of effective and easily measurable proxies (Friedrich et al, 2014;Makri et al, 2020). More specifically, to assess lake trophy and/or bottom-water oxygenation, proxies such as sedimentary pigments (Lami et al, 2000;Leavitt and Hodgson, 2001; Guilizzoni and ; lipid biomarkers (Naeher et al, 2012); diatom (Bennion and Simpson, 2011) and chironomid (Little et al, 2000) records; stable isotopes (Pearson and Coplen, 1978); and redox-sensitive elements such as Fe, Mn, Mo, V and U (Naeher et al, 2013;Wirth et al, 2013;Costa et al, 2015) have been extensively used so far.…”

“…HPLC-inferred pigments have a coarser temporal resolution due to laborious sample preparation and time-consuming HPLC measurements. Scanning hyperspectral imaging (HSI), a novel non-destructive method to quantitatively infer the abundance of algal (T Chl is chlorophyll a and b, and their derivatives) and bacterial (Bphe is bacteriopheophytin a and b) pigments, offers insight into past trophic and oxygen conditions at an unprecedented micrometer-scale (sub-seasonal) resolution (Butz et al, 2015;Makri et al, 2020) but with lower speciation sensitivity.…”

“…Rising mean global temperature can potentially worsen lake anoxia by enhancing water stratification and algal blooms (Adrian et al, 2009;Woolway and Merchant, 2019). Higher lake trophy and reducing conditions in anoxic bottom waters can have diverse and profound negative effects on lake ecosystems, such as toxic algal blooms, fish kills, biodiversity loss (Smol, 2010;Battarbee and Bennion, 2012;Makri et al, 2019), and nutrient recycling into the water column from the sediment through redox processes (Gächter, 1987;Tu et al, 2019). Hence, the global spread of hypoxia is growing into a major environmental concern (Diaz and Rosenberg, 2008).…”

A database of chlorophyll and water chemistry in freshwater lakes

“…Alterations in biological and chemical lake processes can affect how and when freshwater resources can be used. Particularly, increases in lake chlorophyll levels can impact water quality through alterations in colour and odor 7 , dissolved oxygen availability 8 , and overall lake production 9 .…”