Järvede produktiivsuse ja toitainete peetuse sesoonsus, aastatevahelised erinevused ning sõltuvus järvede morfomeetriast

Despite the considerable percentage of inland waters in the temperate zone land cover, their role in the global carbon balance is not clear and the existing data suggest large type specific differences. Respiratory CO2 release from inland waters is a major process in the global carbon cycle, retaining more than half of the carbon flux from terrestrial sources that otherwise would reach the sea. The strongly lake-type-specific balance between primary production and respiration determines whether a lake acts regionally as a net sink or source of CO2. In the global change context even small changes in photosynthesis and respiration rates and/or in the loadings of organic matter or nutrients (e.g. phosphorus) may change the metabolic type of the lake and turn it from autotrophic to heterotrophic system or vice versa. Todays in situ sondes fitted with sensitive and accurate optical oxygen probes have become a useful tool for estimating lake ecosystem metabolism through high frequency continuous measurements. The sonde method provides an alternative to more traditional methods of measuring metabolism that only use discrete samples. Continuous sampling by sondes overcomes many of the limitations of discrete samples and makes it possible to measure metabolism in a wide variety of systems. Long-term observations across many years define the range of natural variability of ecological systems and provide a baseline from which to assess whether a system has changed significantly. Regular hydrological data on large and shallow (area 270 km2; mean depth 2.8 m) Lake Võrtsjärv have been collected for more than 80 years. Regular measurements of nutrients and ecosystem components started in 1960s. Lake Võrtsjärv was intensively studied for more than 50 years before the first monitoring buoy was deployed for high frequency measurements of physical and chemical parameters. Phytoplankton primary production in situ measurements with monthly frequency started in Võrtsjärv already in 1982 but the dataset has many gaps. Present thesis summarizes for Lake Võrtsjärv: the first data on seasonal changes of the ecosystem metabolism based on high frequency measurements; the reconstruction of long term series of phytoplankton primary production by bio-optical modelling; the incoming and outgoing fluxes of carbon, nitrogen, phosphorus, and silicon; and gives an overview of phosphorus retention in Lake Võrtsjärv compared with 54 lakes and reservoirs in different climate regions around the world.Hoolimata siseveekogude rohkusest parasvöötmes, mis on limnoloogiliselt enim uuritud piirkond Maal, ei ole meil senini selget ülevaadet nende veekogude rollist globaalses süsinikubilansis. Elusorganismide elutegevuse käigus toodetav süsihappegaas (CO2) vabaneb meid ümbritsevasse keskkonda. Magevetes hingamisel vabaneva CO2 kogus omab olulist rolli globaalses süsinikuringes. Tänu mageveekogudes toimuvale fotosünteesile tarbitakse esmatootjate (primaarprodutsentide) poolt ära rohkem kui pool kogu maismaal toodetavast süsihappegaasist, mis vastasel juhul jõuaks meid ümbritsevasse keskkonda. Veekogudes toimuva primaarproduktsiooni ja hingamise alusel saab järved jagada kahte erinevasse ainevahetuse tüüpi: CO2 tootvateks (heterotroofseteks) ja CO2 talletavateks (autotroofseteks). Globaalses kontekstis võib ka väikene nihe nii primaarproduktsiooni kui hingamise tasakaalus, põhjustatuna näiteks orgaanilise aine või toiteainete (näiteks fosforiühendite) sissevoolu muutustest, muuta järve kui terviku ainevahetuse tüüpi. Oma varasematele uurimistele ja teadmistele tuginedes oletasime, et Võrtsjärv võib vahetada oma metabolismi tüüpi jää tuleku ja minekuga. Jää- ja lumekatte all väheneb oluliselt vette jõudev valguse hulk, mis on üks kõige olulisem mõjur fotosünteesi käivitamisel ja läbiviimisel. Valguse ööpäevane ja aastaajaline muutlikkus ongi üheks peamiseks põhjuseks veekogude metabolismitüübi kujundamisel. Muutused erinevate tegurite osas, mis mõjutavad primaarproduktsiooni või hingamisprotsesse, võivad toimuda ajaliselt üsna kiiresti (tundide ja päevade jooksul), mistõttu oli neid seni kord kuus toimunud vaatluste abil väga raske jälgida. Võrtsjärve on regulaarseire raames uuritud juba rohkem kui 80 aastat. Primaarproduktsiooni mõõtmistega tehti algust 1982. aastal, kuid kogutud andmeridades esineb kahjuks mitmeid "auke". 2008. aastal alustati süsinikubilansi koostamiseks vajalikke uuringuid ka Võrtsjärve suurematest sissevooludes ning väljavoolus. Tänu Euroopa Liidu toetusfondidele ja riiklikult sihtfinantseeritavale teadusteemale soetati 2009. ja 2010. aastal mitmeid uurimisplatvorme ning ka järvepoi, millede abil said Võrtsjärvel alguse pidevmõõtmised. Käesolev töö: määratleb Võrtjärve kui suure ja madala eutroofse parasvöötme järve metabolismitüübi ja rolli nii regionaalses kui globaalses süsinikuringes; koondab Võrtsjärve primaarproduktsiooni (PP) pikaajalised muutused ning täidab PP andmestikus olevad lüngad modelleerimise abil; annab ülevaate Võrtsjärve sisse- ja väljavoolude süsinikuühendite ja toiteainete hulkadest; ning hindab fosfori kinnipidamist 54. erinevas maailma järves.The doctoral studies and the publication of the current thesis were supported by the Doctoral School of Ecology and Environmental Sciences created under the auspice of European Union Social Fund

The relevance of pelagic calcification in the global carbon budget of lakes and reservoirs

Calcite precipitation acts as a carbon sink in the sediments and a short-term source of carbon dioxide (CO2) to the atmosphere, as widely acknowledged in marine studies. However, pelagic calcite precipitation has received limited attention in lakes. Here we use the relationship between lake water alkalinity and reported calcification rates to provide the first global estimate of pelagic calcification in lakes. Global gross calcification rates amount to 0.03 Pg C yr-1 (0.01 - 0.07) comparable to rates of or­ganic carbon burial, whereas its related CO2 release is largely buffered by the carbonate equilibria. Calcification occurs at water alkalinity above 1 meq/L corresponding to 57 % of global lake and reservoir surface area. Pelagic calcification therefore is a prevalent process in lakes and reservoirs at the global scale, with a potentially relevant role as a sedimentary inorganic carbon sink, comparable in magnitude to the total calcite accumulation rates in ocean sediments

Major effects of alkalinity on the relationship between metabolism and dissolved inorganic carbon dynamics in lakes

Several findings suggest that CO2 emissions in lakes are not always directly linked to changes in metabolism but can be associated with interactions with the dissolved inorganic carbon equilibrium. Alkalinity has been described as a determining factor in regulating the relative contributions of biological and inorganic processes to carbon dynamics in lakes. Here we analyzed the relationship between metabolic changes in dissolved oxygen (DO) and dissolved inorganic carbon (DIC) at different timescales in eight lakes covering a wide range in alkalinity. We used high-frequency data from automatic monitoring stations to explore the sensitivity of DIC to metabolic changes inferred from oxygen. To overcome the problem of noisy data, commonly found in high-frequency measurements datasets, we used Singular Spectrum Analysis to enhance the diel signal-to-noise ratio. Our results suggest that in most of the studied lakes, a large part of the measured variability in DO and DIC reflects non-metabolic processes. Furthermore, at low alkalinity, DIC dynamics appear to be mostly driven by aquatic metabolism, but this relationship weakens with increasing alkalinity. The observed deviations from the metabolic 1:1 stoichiometry between DO and DIC were strongly correlated with the deviations expected to occur from calcite precipitation, with a stronger correlation when accounting also for the benthic contribution of calcite precipitation. This highlights the role of calcite precipitation as an important driver of CO2 supersaturation in lakes with alkalinity above 1 meq L−1, which represents 57% of the global area of lakes and reservoirs around the world

NETLAKE guidelines for automated monitoring system development. How to deploy a low cost option (Factsheet 003).

In this factsheet, we give an overview of one “low cost” platform system

NETLAKE guidelines for automated monitoring system development. Factsheet 007: Communication options.

The purpose of this fact sheet is to provide some advice on the available methods to communicate with and retrieve data from your automated monitoring system

NETLAKE guidelines for automated monitoring system development. Factsheet 001: options for buoy design.

In this factsheet, we describe some of the options that can be used to house an automatic monitoring station (AMS) on a lake

Eutrophication and geochemistry drive pelagic calcite precipitation in lakes

Pelagic calcification shapes the carbon budget of lakes and the sensitivity of dissolved inorganic carbon (DIC) responses to lake metabolism. This process, being tightly linked to primary production, needs to be understood within the context of summer eutrophication which is increasing due to human stressors and global change. Most lake carbon budget models do not account for calcification because the conditions necessary for its occurrence are not well constrained. This study aims at identifying ratios between calcification and primary production and the drivers that control these ratios in freshwater. Using in situ incubations in several European freshwater lakes, we identify a strong relationship between calcite saturation and the ratio between calcification and net ecosystem production (NEP) (p-value < 0.001, R2 = 0.95). NEP-induced calcification is a short-term process that is potentiated by the increase in calcite saturation occurring at longer time scales, usually reaching the highest levels in summer. The resulting summer calcification event has effects on the DIC equilibria, causing deviations from the metabolic 1:1 stoichiometry between DIC and dissolved oxygen (DO). The strong dependency of the ratio between NEP and calcification on calcite saturation can be used to develop a suitable parameterization to account for calcification in lake carbon budgets

An estimation of diel metabolic rates of eight limnological archetypes from Estonia using high-frequency measurements

We employed a Bayesian model to assess the metabolic state of 8 Estonian lakes representing the 8 lake types according to the European Union Water Framework Directive. We hypothesized that long-term averages of light-related variables would be better predictors of lake metabolism than nutrient-related variables. Model input parameters were in situ high-frequency measurements of dissolved oxygen, temperature, and irradiance. Model simulations were conducted for several (5&ndash;12) diel cycles for each lake during the summer season. Accounting for uncertainty, the results from the Bayesian model revealed that 2 lakes were autotrophic for the duration of the experiment, 1 was heterotrophic, and 5 were balanced or had an ambiguous metabolic state. Cross-comparison with a traditional bookkeeping model showed that the majority of lakes were in metabolic balance. A strong coupling between primary production and respiration was observed, with the share of autochthonous primary production respired by consumers increasing with light extinction and nutrient-related variables. Unlike gross primary production, community respiration was strongly related to light extinction, dissolved organic carbon (DOC) and total phosphorus. These findings suggest that a drastic decrease in light-limited primary production along the DOC gradient counter-balanced nutrient supply in the darker lakes and thus blurred the relationship between primary production and nutrients. Thus, contrary to our hypothesis, both light and nutrient-related variables seemed to be good predictors of lake respiration and its coupling to lake primary production