Spatio-temporal variations in sediment phosphorus dynamics in a large shallow lake: Mechanisms and impacts of redox-related internal phosphorus loading

The role of redox-related sediment phosphorus (P) release in shallow polymictic lakes remains poorly understood. Our previous studies in large and shallow Lake Peipsi suggested the importance of the redox-related P release in internal P loading. In the current study, we explored the validity of this hypothesis by also considering organic sediment P (Org-P). We analysed spatio-temporal changes in diffusive P flux and sediment P forms determined by P fractionation and nuclear magnetic resonance (NMR) spectroscopy in summer 2021. Using 1997–2021 data, we computed internal P load (IL) by two methods and studied their relationships with several water quality variables. Anoxia of sediment surfaces and P release progressed with an increase in water tem- perature during summer. In the long-term, IL estimates by two methods were similar (mean values: 315 and 346 mg/m2/year) and correlated with the predicted anoxia of sediment surfaces (AApred). A contribution of sediment iron-bound P (Fe-P) to P release was indicated by the positive correlation of Fe-P with orthophosphate (NMR) in the short-term studies. No similar evidence was found for Org-P, which contradicts the common tendency to attribute internal P loads largely to Org-P in eutrophic lakes. Nevertheless, organic matter seemed to support reductive dissolution, because seasonal changes in sediment Org-P correlated with those in Fe-P, and organic matter content and diffusive P flux were negatively correlated over different sites. Complex bottom morphology and hydrology affected spatial distribution of the sediment P forms and masked the relationships between sediment P variables and P release. Finally, the importance of redox-related release was reflected in significant relationships between AApred and associated IL with Secchi depth transparency, chlorophyll a concentration, and the biomass of phytoplankton and cyanobacteria. To our knowledge, this is the first time when such direct ev- idence was provided for a large polymictic lake.This study was funded by the Estonian Research Council grant PRG1167 and Estonian Ministry of Environment 4-1/21/76. Also, this project has received funding from the European Union’s Horizon 2020 research and innovation programme under grant agreement No 951963.This study was funded by the Estonian Research Council grant PRG1167 and Estonian Ministry of Environment 4-1/21/76. Also, this project has received funding from the European Union’s Horizon 2020 research and innovation programme under grant agreement No 951963

Dynamics of phytoplankton pigments in water and surface sediments of a large shallow lake

Our aim was to find out to which extent fossil phytoplankton pigments in the large shallow and turbid Lake Võrtsjärv carry information on the history of phytoplankton communities. For this purpose we examined how the changes in the pigment composition of surface sediments follow their changes in the water column. Depth-integrated lake water and surface sediment samples were collected weekly in May–October 2007. Considering cyanobacterial and diatom dominance in phytoplankton, we analysed fucoxanthin, diadinoxanthin and diatoxanthin as marker pigments for diatoms, zeaxanthin as a marker pigment for total cyanobacteria and canthaxanthin as a marker pigment for colonial cyanobacteria. Chlorophyll a and its derivative pheophytin a were applied as indicators for total phytoplankton. The dynamics of phytoplankton pigments in surface sediments generally did not follow their dynamics in the water column, possibly due to intensive resuspension and a high sedimentation rate in a large and shallow lake. It was noticed that the surface sediment carries information on pigment degradation intensity and on weight and size characteristics of phytoplankton cells, which affect their sinking and floating velocities. Higher pigment contents of sediment in spring were presumably caused by lower resuspension due to high water level and slower degradation in cold water. Pheophytin a and the marker pigments of cyanobacteria were found to be persistent against degradation in upper sediment layers, which makes them useful indicators for tracking the historical changes in phytoplankton communities also in a shallow lake. Sharp decrease in chemically unstable pigment contents between the sediment surface and deeper layers indicates that only the uppermost sediment surface is resuspended in Lake Võrtsjärv. The transformation of the diatom marker carotenoid diadinoxanthin to diatoxanthin was found to occur mainly in sediments and not in the water column, and the process is not induced by excess light

Use of sedimentary algal pigment analyses to infer past lake-water total phosphorus concentrations

AbstractWe tested the feasibility of using sedimentary algal pigment analyses by spectral deconvolution to infer past lake-water total phosphorus concentrations. We established equations that link lake-water nutrient concentrations and sediment pigment concentrations, using a combination of calibration in both space and time, with a training set of 31 Swedish lakes. The calibration dataset yielded a significant positive relationship between total carotenoid concentrations and lake-water total phosphorus concentrations. We also compared sediment-pigment-based nutrient inferences with time series of water column monitoring data to evaluate whether temporal changes in total phosphorus concentrations are well captured by analysis of sedimentary pigments. We found that changes in pigment preservation through time can alter the relationship between concentrations of lake-water nutrients and sedimentary pigments, thus limiting the reliability of historical ecological conditions inferred from pigments in the sediment. Our data suggested that ratios of Chlorophyll derivatives to total carotenoids (CD/TC ratio) and Chlorophyll a to Chlorophyll derivatives (CPI) can be used as proxies for pigment preservation. Using our approach, inferred temporal changes in water-column total phosphorus concentrations in lakes are promising, but require further development, specifically with respect to the influence of pigment degradation in both the water column and sediments, as well as the factors that control such degradation.</jats:p

Siseveekogud : õpik kõrgkoolidele

Inimeste kõige tavalisemad seosed siseveekogudega on matkamine, kalapüük, suplemine, janu kustutamine ja taimede kastmine. Et veekogude ääres viibimine mõjub paljudele rahustavalt, on jõgedel-järvedel miljonivaadete kaudu kindel koht ka kinnisvaraäris. Veekogudeta ei saa läbi sportlased (purjetajad, sõudjad ja motohuvilised). Leidub selliseidki indiviide, keda meelitavad mittesöödavad või koguni palja silmaga nähtamatud vee-elanikud. Eesti on väike madal maa, millel on pikk mererand, aga kus leidub ka palju siseveekogusid. Eriline on kahe suure järve – Peipsi ja Võrtsjärve – asumine lähestikku. Seisuvete pindala osakaalu järgi kogu riigi pindalast on Eesti Euroopas pärast Soomet ja Rootsit koos Norraga kolmandal-neljandal kohal. Eesti ja tema ümbrus on puhta veega seni niisiis hästi varustatud, kuid see rikkus ühtlasi kohustab veekogusid heaperemehelikult ja jätkusuutlikult majandama. Ka Eestis on muresid nii veevarude, veekogude kui nende seisundiga. Sisevete uurimine on Eestis kestnud juba üle 100 aasta. Seda on süstemaatiliselt korraldanud nii Looduseuurijate Selts, Tartu Riiklik Ülikool, Teaduste Akadeemia kui Maaülikool. Suurte järvede kõrval pole unustatud väikesi järvi ega vooluveekogusid. Uurida vee ja veekogude omadusi, arendada ja kasvatada nende spetsialiste ongi mõistlik seal, kus on, mida tundma õppida. Üha enam leitakse seoseid looduslike ja inimtekkeliste mõjurite ning ökosüsteemide vastuste vahel. Ühtlasi ühendatakse neid seoseid sotsiaalmajanduslike küsimuste ja looduskaitsega. Eesti siseveekogude kohta on peale arvukate ja enamasti võõrkeelsete teadusartiklite ilmunud ka eestikeelseid raamatuid. Siin neist väike loetelu: väikejärved (Eesti järved, 1968; Mäemets, 1977; Laarmaa jt 2019); Võrtsjärv (1973, 2003); Peipsi (1999, 2008), vooluveed (Järvekülg jt 2001; Timm jt 2019). Kalaraamatuid esindavad Mikelsaar (1984) ning Hunt (2019), veetaimi „Eesti taimede määraja“ (2010). Silmaga nähtavate veeselgrootute ülevaate pakub Timm (2015). Ülevaatlikku eestikeelset õpikut siseveekogude ning nende talitlemise kohta seni polnud. Eesti ülikoolides on kõigil kolmel õppetasandil (bakalaureuse-, magistri- ja doktoriõpe) õppekavasid, kus vajatakse teadmisi siseveekogudest. Võõrkeelseid eeskujusid leidub päris mitu, kuid need käsitlevad enamasti kas ainult hüdrobioloogiat või limnoloogiat. Esimene on elustiku-, teine keskkonna-alase suunitlusega. Uus õpik sisaldab mõlemaid ning sobib loodetavasti paljudele loodusteaduslikele ja looduskaitselistele kursustele, eriti bakalaureusetasemel. Õpik koosneb kolmest suurest alajaotusest: (1) siseveekogude füüsikalis-keemiline iseloomustus, levik ja teke; (2) elupaigad veekogudes, olulised elustikurühmad ning nendevahelised suhted; (3) siseveekogude majandamine, kaitse ja tervendamine. Peamiselt vaadeldakse Eesti siseveekogusid, aga seda kogu maailma taustal. Koostajad loodavad, et raamat annab lugejatele nii vastuseid küsimustele kui ka süvendab huvi sisevete kui kaunite, põnevate ning inimestele eluliselt oluliste loodusobjektide suhtes.Õpik on valminud riikliku programmi „Eestikeelsete kõrgkooliõpikute koostamine ja väljaandmine (2008–2012)“ raames ning Eesti Maaülikooli ja Sihtasutuse Archimedes osalisel toel

Pigment-based chemotaxonomy--a quick alternative to determine algal assemblages in large shallow eutrophic lake?

Pigment-based chemotaxonomy and CHEMTAX software have proven to be a valuable phytoplankton monitoring tool in marine environments, but are yet underdeveloped to determine algal assemblages in freshwater ecosystems. The main objectives of this study were (1) to compare the results of direct microscopy and CHEMTAX in describing phytoplankton community composition dynamics in a large, shallow and eutrophic lake; (2) to analyze the efficiency of the pigment-based method to detect changes in phytoplankton seasonal dynamics and during rapid bloom periods; (3) to assess the suitability of specific marker pigments and available marker pigment:chlorophyll a ratios to follow seasonal changes in eutrophic freshwater environment. A 5-year (2009-2013) parallel phytoplankton assessment by direct microscopy and by CHEMTAX was conducted using published marker pigment:chlorophyll a ratios. Despite displaying some differences from microscopy results, the pigment-based method successfully described the overall pattern of phytoplankton community dynamics during seasonal cycle in a eutrophic lake. Good agreement between the methods was achieved for most phytoplankton groups - cyanobacteria, chlorophytes, diatoms and cryptophytes. The agreement was poor in case of chrysophytes and dinoflagellates. Our study shows clearly that published marker pigment:chlorophyll a ratios can be used to describe algal class abundances, but they need to be calibrated for specific freshwater environment. Broader use of this method would enable to expand monitoring networks and increase measurement frequencies of freshwater ecosystems to meet the goals of the Water Framework Directive

Abrupt rise in the contribution of CH₄-derived carbon to benthic secondary production of a shallow hemiboreal/boreal lake

Understanding long-term carbon flows through aquatic food webs is essential to assess impact of climatic and environmental changes on lake ecosystems. We reconstructed temporal changes in carbon sources fuelling chironomid biomass in a small and shallow hemiboreal/boreal lake over the last 11 000 years. Results were compared with temporal dynamics of known potential controlling factors: summer air temperature, human activities, phytoplankton assemblages and organic matter composition. We report an abrupt rise in the contribution of methane-derived carbon to chironomid biomass. However, this sudden increase was not the consequence of drastic changes in the composition of sedimentary organic matter, mean air temperature, lake trophic state or human activities in the catchment area. Our results suggest that shallowing of the lake caused by sedimentary infilling processes was a possible driver of the expansion of anoxic zones, enhancing CH 4 cycling in lake sediments. However, complementary studies are needed to better understand the mechanisms by which lake shallowing could affect carbon flows to aquatic consumers, and then better assess the natural dynamics of past carbon processing in the benthic food web of shallow lakes.Financial support was provided by institutional research grant IUT21-2, IUT1-8 from the Estonian Science Agency and by Estonian University of Life Sciences, project No. 8M160101PKLJ, and ‘Estophilus Grant’ from the Ministry of Culture of Estonia. We are grateful to Christian Hossann (INRA, Nancy, Champenoux) for assistance with the stable isotope analysis of carbon. The PTEF facility is supported by the French National Research Agency through the Laboratory of Excellence ARBRE (ANR-11-LABX-0002-01).Financial support was provided by institutional research grant IUT21-2, IUT1-8 from the Estonian Science Agency and by Estonian University of Life Sciences, project No. 8M160101PKLJ, and ‘Estophilus Grant’ from the Ministry of Culture of Estonia. We are grateful to Christian Hossann (INRA, Nancy, Champenoux) for assistance with the stable isotope analysis of carbon. The PTEF facility is supported by the French National Research Agency through the Laboratory of Excellence ARBRE (ANR-11-LABX-0002-01)

Long-term effects of climate change on carbon flows through benthic secondary production in small lakes

1. A palaeolimnological study, covering the last c. 12,000 years, was conducted in a small subalpine lake located in the Alps to study climate change impacts on car- bon flows through food webs in small lakes. We used analysis of sedimentary pigments and carbon stable isotopic composition of chironomid remains (d13 C HC) to reconstruct past dynamics of phytoplankton community and carbon sources sustaining benthic consumers. 2. Chironomid biomass was sustained by a combination of allochthonous, auto- chthonous and CH 4 -derived organic matters, and their relative contributions were correlated to changes in temperature. Relatively high terrestrial contribu- tions to chironomid biomass were observed during period of the Holocene when in-lake production was low. Relatively high incorporation of CH 4 -derived carbon to chironomid biomass was found during anoxic events co-occurring with the Holocene thermal maximum. 3. Results were then compared with those collected in a small boreal lake in Esto- nia. We tested the hypothesis that responses in carbon flows through benthic food web to past climate change would be similar between these lakes. We found a negative correlation between d13 CHC values of both lakes and inferred air temperature, suggesting that temperature was the major driver to different food sources being incorporated into chironomid biomass. 4. Our study demonstrated that air temperature was the principal driver of the energy flows through benthic food web in the studied small lakes. We conjec- tured that carbon cycling in food webs of small lakes might be strongly sensitive to climate change.Financial support was provided by institutional research grants IUT1- 8 and IUT21-2 from Estonian Science Agency and by Estonian University of Life Sciences, project no. M160101, as well as from NEXTDATA project and the Park Paneveggio-Pale di S. Martino, Italy. The authors express their sincere thanks to W. Goedkoop for his help with the English language. We are grateful to Christian Hos- sann (INRA Nancy, Champenoux) for assistance in the stable isotope analysis of carbon. The PTEF facility is supported by the French National Research Agency through the Laboratory of Excellence ARBRE (ANR-11-LABX-0002-01). The authors gratefully acknowl- edge the two anonymous reviewers and Alan Steinman for their con- structive comments on this manuscript.Financial support was provided by institutional research grants IUT1- 8 and IUT21-2 from Estonian Science Agency and by Estonian University of Life Sciences, project no. M160101, as well as from NEXTDATA project and the Park Paneveggio-Pale di S. Martino, Italy. The authors express their sincere thanks to W. Goedkoop for his help with the English language. We are grateful to Christian Hos- sann (INRA Nancy, Champenoux) for assistance in the stable isotope analysis of carbon. The PTEF facility is supported by the French National Research Agency through the Laboratory of Excellence ARBRE (ANR-11-LABX-0002-01). The authors gratefully acknowl- edge the two anonymous reviewers and Alan Steinman for their con- structive comments on this manuscript

Algal Diet of Small-Bodied Crustacean Zooplankton in a Cyanobacteria-Dominated Eutrophic Lake.

Small-bodied cladocerans and cyclopoid copepods are becoming increasingly dominant over large crustacean zooplankton in eutrophic waters where they often coexist with cyanobacterial blooms. However, relatively little is known about their algal diet preferences. We studied grazing selectivity of small crustaceans (the cyclopoid copepods Mesocyclops leuckarti, Thermocyclops oithonoides, Cyclops kolensis, and the cladocerans Daphnia cucullata, Chydorus sphaericus, Bosmina spp.) by liquid chromatographic analyses of phytoplankton marker pigments in the shallow, highly eutrophic Lake Võrtsjärv (Estonia) during a seasonal cycle. Copepods (mainly C. kolensis) preferably consumed cryptophytes (identified by the marker pigment alloxanthin in gut contents) during colder periods, while they preferred small non-filamentous diatoms and green algae (identified mainly by diatoxanthin and lutein, respectively) from May to September. All studied cladoceran species showed highest selectivity towards colonial cyanobacteria (identified by canthaxanthin). For small C. sphaericus, commonly occuring in the pelagic zone of eutrophic lakes, colonial cyanobacteria can be their major food source, supporting their coexistence with cyanobacterial blooms. Pigments characteristic of filamentous cyanobacteria and diatoms (zeaxanthin and fucoxanthin, respectively), algae dominating in Võrtsjärv, were also found in the grazers' diet but were generally avoided by the crustaceans commonly dominating the zooplankton assemblage. Together these results suggest that the co-occurring small-bodied cyclopoid and cladoceran species have markedly different algal diets and that the cladocera represent the main trophic link transferring cyanobacterial carbon to the food web in a highly eutrophic lake