Modeled changes in pseudo-first-order kinetics rate constants of photoammonification (<i>k₁</i>), subsequent oxidation of ammonia to N_x (<i>k₂</i>), direct N_org transformation to N_x (<i>k₃</i>), and NO₃⁻ reduction to N_x (<i>k₄</i>), respectively.

<p>Grey areas represent the snow cover periods. Error bars represent uncertainty propagation.</p

Seasonal trends in (A) initial DOC and CDOM concentrations (dashed line - calculated as the sum of absorbance from 250 to 450 nm according to [30]) in stream water, and (B) percent decrease in DOC and CDOM (dashed line) after 48 hours of irradiation.

<p>Grey areas represent the snow cover periods. Error bars represent 1 standard error of triplicate samples.</p

Spearman R correlations between pseudo-first-order photodegradation kinetics rate constants (<i>k₁</i> to <i>k₄</i>) and chemical and meteorological parameters (* p<0.05; ** p<0.01; *** p<0.001).

<p>Spearman R correlations between pseudo-first-order photodegradation kinetics rate constants (<i>k₁</i> to <i>k₄</i>) and chemical and meteorological parameters (* p<0.05; ** p<0.01; *** p<0.001).</p

The decrease in DOC concentrations in the same sample exposed to laboratory UV irradiation (Lab) and natural solar radiation (Sun).

<p>The X-axis representing days of exposure to solar radiation (May 2012) was adjusted to the daily measured solar insolation at the Římov station.</p

Seasonal changes in initial N_org (A), NH₄⁺ (B), NO₃⁻ (C) concentrations (white symbols) and percent change (black symbols) in N_org (A), NH₄⁺ (B), NO₃⁻ (C) concentrations after 48 hours of irradiation in stream water.

<p>C∶N molar ratios (D – no symbols), solid line represents C∶N ratio at the beginning and dashed line at the end of irradiation. Grey areas represent the snow cover periods. Error bars represent 1 standard error of triplicate samples.</p

The calculated changes in N_org, NH₄, NO₃, and N_x concentrations after one day of average solar insolation for given day.

<p>A - decrease in N_org concentration; B - increase in NH₄ concentration; C - decrease in NO₃ concentration; D - increase in other inorganic nitrogen. Error bars represent 1 standard error.</p

Initial chemical composition of stream samples (minimum, mean ±1 standard error, maximum).

<p>Units are µmol L⁻¹, except for DOC (mmol L⁻¹) and pH; B.D.L. - below detection limit.</p><p>Initial chemical composition of stream samples (minimum, mean ±1 standard error, maximum).</p

Concentrations and fluxes of N species in precipitation (direct atmospheric bulk deposition on the lake surface), watershed export (via tributaries), and output from Plešné Lake in the 2011 hydrological year.

<p>Uncertainty (±) of the fluxes was calculated using an error propagation method <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0116364#pone.0116364-Bevington1" target="_blank">[27]</a>.</p>1)<p>Ammonium (NH₄⁺), nitrate (NO₃⁻), dissolved organic N (N_org), particulate organic N (PN = total N_org−N_org), and total nitrogen (TN = NH₄⁺+NO₃⁻+N_org+PN).</p><p>Concentrations of nitrite were <0.1 µmol L⁻¹ and were neglected.</p>2)<p>Change in storage represents the average change in amount of N species in the lake during study period and was based on differences between their concentrations at 0.5, 4, 9, 14, and 17 m depths on 31 October 2011 and 20 October 2010, and volumes of the respective water layers <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0116364#pone.0116364-Kopek1" target="_blank">[21]</a>.</p>3)<p>Net in-lake production was calculated on an annual basis as described by Kopáček et al. <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0116364#pone.0116364-Kopek1" target="_blank">[21]</a> (production = output+change in storage−input by precipitation and tributaries), and then recalculated to daily fluxes given on a lake-area basis.</p><p>Concentrations and fluxes of N species in precipitation (direct atmospheric bulk deposition on the lake surface), watershed export (via tributaries), and output from Plešné Lake in the 2011 hydrological year.</p

A scheme of possible transformations between nitrogen species.

<p>N_x represents the sum of undetermined nitrogen species (nitrogen oxides, N₂, and NO₂⁻) possibly formed during the photodegradation of major N forms. Abbreviations <i>k₁</i> to <i>k₄</i> represent pseudo-first-order photodegradation kinetics rate constants of reactions 1 to 4, respectively.</p

Watershed of Plešné Lake, its location in Europe, and locations of lake tributaries (PL-I to PL-IV) and throughfall precipitation plots (PL-H and PL-L).

<p>Bulk precipitation was sampled ∼0.4 km east of PL-L.</p