Petr Porcal

Element fluxes in watershed-lake ecosystems recovering from acidification: Plešné Lake, the Bohemian Forest, 2001–2005

Fluxes of major ions and nutrients were measured in the watershed-lake ecosystem of a strongly acidified lake, Plešné jezero (Plešné Lake), in the Czech Republic in hydrological years from 2001 through 2005. The lake is situated in a Norway spruce forest and has a steep watershed between elevations of 1090 and 1378 m. The average water input and output from the ecosystem was 1372 mm and 1157 mm (37 L km−2 s−1), respectively, and the water residence time averaged 306 days. Despite ecosystem recovery from acidification occurring since the late 1980s, the Plešné watershed was an average net source of 25 mmol SO42− m−2 yr−1. Nitrogen saturation of the watershed caused low retention of the deposited inorganic N (< 44% on average) before 2004. Then, the watershed became a net source of 28–32 mmol m−2 yr−1 of inorganic N in the form of NO3− due to climatic effects (a dry summer in 2003 and a cold winter in 2004) and forest dieback caused by a bark beetle attack in 2004. Nitrogen transformations and SO42− release were the dominant terrestrial sources of H+ (72 and 49 mmol m−2 yr−1, respectively) and the watershed was a net source of 24 mmol H+ m−2 yr−1. Ionic composition of surface inlets showed seasonal variations, with the most pronounced changes in NO3−, ionic Al (Ali), and DOC concentrations, while the composition of subsurface inlets was more stable. The in-lake biogeochemical processes reduced on average 59% of the incoming H+ (251 mmol H+ m−2 yr−1 on a lake-area basis). NO3− assimilation and denitrification, photochemical and microbial decomposition of allochthonous organic acids, and SO42− reduction in the sediments were the most important aquatic H+ consuming processes (358, 121, and 59 mmol H+ m−2 yr−1, respectively), while hydrolysis of Ali was the dominant in-lake H+ generating process (233 mmol H+ m−2 yr−1). Photochemical liberation from organic complexes was an additional in-lake source of Ali. The net in-lake retention or removal of total phosphorus, total nitrogen, and silica were on average 50%, 27%, and 23%, respectively. The lake was a net source of NH4+ due to a cease in nitrification (pH < 5) and from NH4+ production by dissimilation exceeding its removal by assimilation.

Trends in aluminium export from a mountainous area to surface waters, from deglaciation to the recent: Effects of vegetation and soil development, atmospheric acidification, and nitrogen-saturation

We reconstructed the history of terrestrial export of aluminium (Al) to Plesné Lake (Czech Republic) since the lake origin approximately 12,600 year BC, and predicted Al export for 2010-2050 on the basis of previously published and new data on mass budget studies, palaeolimnological data, and MAGIC modelling. We focused on three major Al forms; ionic Al (Al(i)), organically-bound Al (Al(o)), and particulate Al hydroxide [Al(OH)(3)]. In early post-glacial time, Plesné Lake received high terrestrial export of Al, but with a minor proportion of Al(OH)(3) (4-25 microM), and concentrations of Al(i) and Al(o) were negligible. Since the forest and soil development ( approximately 9900-9000 year BC), erosion has declined and soil organic acids increased export of Al(o) from soils. The terrestrial Al(o) leaching ( approximately 7.5 microM) persisted throughout the Holocene until the industrial period. Then, Al(i) concentrations continuously increased (up to 28 microM in the mid-1980s) due to atmospheric acidification; the Al(i) leaching was mostly associated with sulphate. The proportion of Al(i) associated with nitrate has been increasing since the beginning of lake recovery from acidification after approximately 1990 due to reduction in sulphur deposition and nitrogen-saturation of the catchment, leading to persistent nitrate leaching. Currently, nitrate has become the dominant strong acid anion and the major Al(i) carrier. Al(o) (5.5 microM) is predicted to dominate Al concentrations around 2050, but the predicted Al(i) concentrations ( approximately 4 microM) are uncertain because of uncertainty associated with the future nitrate leaching and its effect on soils.

Seasonal and photochemical changes of DOM in an acidified forest lake and its tributaries

Abstract.The concentration, composition, and photochemical changes of dissolved organic matter (DOM) were evaluated in surface and bottom layers, and surface and subsurface tributaries of Plešné Lake, Bohemian Forest, Czech Republic, during one hydrological year. DOM was fractionated using the Amberlite XAD-8 and ion exchange resins into five fractions: hydrophilic neutrals, hydrophilic acids, hydrophilic and hydrophobic bases, hydrophobic acids, and hydrophobic neutrals. Photochemical transformations of DOM by photosynthetically active radiation (PAR) were tested with in-situ experiments using water from the surface tributary. The fraction composition of DOM was dominated by hydrophobic acids in all samples, with annual averages of 54–58% of DOM. The surface tributary differed from the subsurface one in higher and lower percentages of hydrophobic neutrals and hydrophilic neutrals, respectively. The surface and bottom water of the lake did not significantly differ in the composition of DOM fractions despite significantly higher DOM concentrations at the bottom. The results of in-situ photochemical experiments showed the significance of PAR in photochemical as well as biological transformations of allochthonous DOM in this lake. The net rates of photo-production of DIC and photochemical plus biological degradation of DOC ranged from 0.3 to 0.6 and 0.6 to 4.3 μmol L–1 day–1, respectively, with maximum values in late spring. The hydrophobic acids fraction and the remaining DOM fractions were decomposed biologically (in dark bottles) and by combined photochemical and biological degradation (in transparent bottles) at similar rates, resulting in unchanged fraction composition of DOM in the experiments.

Photochemical release of humic and fulvic acid-bound metals from simulated soil and streamwater

This study demonstrates the strong impact of photochemical degradation of soil dissolved organic matter (DOM) on its metal complexing capacity. The role of light in the fate of organically-bound metals transported from soils to surface waters was studied in laboratory experiments. We studied four humic and one fulvic acid isolates from different soil horizons in the Bohemian Forest (Czech Republic). Different concentrations of aluminium (Al) and iron (Fe) salts were added to the solutions of organic acids (initial dissolved organic carbon (DOC) concentration 0.5 mmol L(-1)), and the samples were irradiated in a reactor equipped with 350 nm irradiation lamps for 0 to 120 min. Aliquots of irradiated samples and dark controls were analyzed for DOC, ionic and organically-bound Al and Fe (Al(i), Fe(i), and Al(o), Fe(o), respectively), pH, and UV-VIS spectra. The initial Fe(o) concentrations in the samples (2.09 to 5.66 micromol L(-1)) decreased from 21 to 52% during irradiation, while the initial Al(o) concentrations (2.28 to 5.37 micromol L(-1)) decreased from 7 to 41%. The greatest decrease in the organically-bound metal concentrations occurred for the fulvic acid, and the smallest decrease occurred for the humic acid from the deepest soil horizon. The extrapolation of laboratory experiments to in situ conditions suggested that the DOMs ability to bind metals changes greatly within the first few hours after groundwater enters the stream. The rapid degradation of organically-bound Al and Fe can be an important process in first and second-order streams, and lake epilimnia.

Photochemical production and decomposition of particulate organic carbon in a freshwater stream

Irradiation of dissolved organic carbon (DOC) of terrestrial origin that is freshly released to surface waters can produce particulate organic carbon (POC). Laboratory experiments with stream water were conducted to determine POC and particulate metals formation during exposure to artificial solar radiation comparable to surface intensity. The results showed that decreases in DOC concentration were accompanied initially by increasing and later by decreasing POC concentrations. Data were fit to first order kinetics models to compare rates of POC formation and loss with DOC and absorbance loss, and to better understand how metals might be involved in photochemical POC formation. This abiotic mechanism may be important for the transfer of allochthonous DOC and metals to sediments in temperate aquatic systems.

Temperature Dependence of Photodegradation of Dissolved Organic Matter to Dissolved Inorganic Carbon and Particulate Organic Carbon.

Photochemical transformation of dissolved organic matter (DOM) has been studied for more than two decades. Usually, laboratory or “in-situ” experiments are used to determine photodegradation variables. A common problem with these experiments is that the photodegradation experiments are done at higher than ambient temperature. Five laboratory experiments were done to determine the effect of temperature on photochemical degradation of DOM. Experimental results showed strong dependence of photodegradation on temperature. Mathematical modeling of processes revealed that two different pathways engaged in photochemical transformation of DOM to dissolved inorganic carbon (DIC) strongly depend on temperature. Direct oxidation of DOM to DIC dominated at low temperatures while conversion of DOM to intermediate particulate organic carbon (POC) prior to oxidation to DIC dominated at high temperatures. It is necessary to consider this strong dependence when the results of laboratory experiments are interpreted in regard to natural processes. Photodegradation experiments done at higher than ambient temperature will necessitate correction of rate constants.

Distinct Optical Chemistry of Dissolved Organic Matter in Urban Pond Ecosystems

Urbanization has the potential to dramatically alter the biogeochemistry of receiving freshwater ecosystems. We examined the optical chemistry of dissolved organic matter (DOM) in forty-five urban ponds across southern Ontario, Canada to examine whether optical characteristics in these relatively new ecosystems are distinct from other freshwater systems. Dissolved organic carbon (DOC) concentrations ranged from 2 to 16 mg C L-1 across the ponds with an average value of 5.3 mg C L-1. Excitation-emission matrix (EEM) spectroscopy and parallel factor analysis (PARAFAC) modelling showed urban pond DOM to be characterized by microbial-like and, less importantly, by terrestrial derived humic-like components. The relatively transparent, non-humic DOM in urban ponds was more similar to that found in open water, lake ecosystems than to rivers or wetlands. After irradiation equivalent to 1.7 days of natural solar radiation, DOC concentrations, on average, decreased by 38% and UV absorbance decreased by 25%. Irradiation decreased the relative abundances of terrestrial humic-like components and increased protein-like aspects of the DOM pool. These findings suggest that high internal production and/or prolonged exposure to sunlight exerts a distinct and significant influence on the chemistry of urban pond DOM, which likely reduces its chemical similarity with upstream sources. These properties of urban pond DOM may alter its biogeochemical role in these relatively novel aquatic ecosystems.

Sulphate leaching from diffuse agricultural and forest sources in a large central European catchment during 1900–2010

Using dynamic, mass budget, and empirical models, we quantified sulphate-sulphur (SO4-S) leaching from soils in a large central European catchment (upper Vltava river, Czech Republic) over a 110-year period (1900-2010). SO4-S inputs to soils with synthetic fertilisers and atmospheric deposition increased in the 1950s-1980s, then rapidly decreased (~80%), and remained low since the middle 1990s. The proportion of drained agricultural land rapidly increased from 4 to 43% between the 1950s and 1990s; then the draining ability of the system slowly decreased due to its ageing. Sulphate concentrations in the Vltava exhibited similar trends as the external SO4-S inputs, suggesting that they could be explained by changes in atmospheric and fertiliser S inputs. The available data and modelling, however, showed that (i) internal SO4-S sources (mineralization of soil organic S in the drained agricultural land), (ii) a hysteresis in SO4-S leaching from forest soils (a net S retention at the high S inputs and then a net release at the lowered inputs), and (iii) hydrology must be taken into account. An empirical model was then employed, based on parameters representing hydrology (discharge), external SO4-S sources (inputs by synthetic fertilisers and atmospheric deposition), and internal SO4-S sources (mineralization related to soil drainage). The model explained 84% of the observed variability in annual SO4-S concentrations in the Vltava river during 1900-2010 and showed that forest soils were a net sink (105 kg ha(-1)) while agricultural land was a net source (55 kg ha(-1)) of SO4-S during 1960-2010. In the late 1980s, forest soils changed from a sink to a source of S, and the present release of SO4-S accumulated in forest soils thus delays recovery of surface waters from acidification, while S losses from agricultural soils increase the risk of future S deficiency in S-demanding crops.

Changes in surface water chemistry caused by natural forest dieback in an unmanaged mountain catchment

Ionic and nutrient compositions of throughfall, tributaries and lake outlet were analysed in the Plešné catchment-lake system (an unmanaged mountain forest in Central Europe) from 1997 to 2016. The aim was to evaluate changes in surface water chemistry after natural forest dieback. In the 2004-2008, 93% of the Norway spruce trees were killed by bark beetle outbreak, and all dead biomass remained in the catchment. Forest dieback changed the chemistry of all water fluxes, and the magnitude, timing, and duration of these changes differed for individual water constituents. The most pronounced decreases in throughfall concentrations occurred for K+, dissolved organic carbon (DOC), Ca2+ and Mg2+, i.e. elements mostly originating from canopy leaching, while concentrations of NH4+ and soluble reactive phosphorus (SRP) remained almost unaffected. In tributaries, the most rapid changes were increases in NO3-, K+, H+ and ionic aluminium (Ali) concentrations, while terrestrial export of DOC and P forms started more slowly. Immediately after the forest dieback, increase in NO3- concentrations was delayed by elevated DOC availability in soils. NO3- became the dominant anion, with maximum concentrations up to 346μeqL-1 within 5-7years after the bark beetle outbreak, and then started to decrease. Terrestrial exports of Ali, K+, H+, Mg2+, and Ca2+ accompanied NO3- leaching, but their trends differed due to their different sources. Elevated losses of SRP, DOC, and dissolved organic nitrogen continued until the end of the study. In the lake, microbial processes significantly decreased concentrations of NO3-, organic acid anions, H+ and Ali, and confounded the chemical trends observed in tributaries. Our results suggest that terrestrial losses of elements and the deterioration of waters after forest dieback are less pronounced in unmanaged than managed (clear-cut) catchments.

Catchment biogeochemistry modifies long-term effects of acidic deposition on chemistry of mountain lakes

Abstract Using relationships between the composition of precipitation and emission rates of sulphur and nitrogen compounds and dust from 1978 to 2012, we modelled concentrations of sulphate, nitrate, ammonium, chloride, and base cations and pH of precipitation in the Tatra Mountains (central Europe) back to 1900. The modelled precipitation chemistry exhibited a high degree of temporal coherence with the lake water chemistry in 1937 and during the period 1984–2014. The ionic composition of lake water reflected progress in their acidification until the late 1980s, and then a rapid recovery. The response of lake water chemistry to changes in precipitation chemistry differed for individual ions, nutrients, and among lakes, predominantly reflecting the proportions of soil in their catchments. Important differences occurred in nutrient concentrations. In-lake concentrations of dissolved organic carbon (DOC) and total organic nitrogen (TON) exhibited inverse patterns to nitrate. DOC and TON were higher in lakes with a higher proportion of soil in the catchments, and increased in many lakes during recovery from acidification, while nitrate concentrations were higher and decreased most steeply in the catchments with sparse soils. Lake water concentrations of total phosphorus (TP) were spatially similar to that of DOC and TON, but increased most steeply during recovery in lakes with a high proportion of till soils. Our results indicate that chemical recovery of mountain ecosystems is seriously modified by catchment biogeochemistry and may result not only in elevated DOC leaching, but also in an increase of terrestrial export of TON and TP to the receiving surface waters.