Contrasting catchment soil pH and Fe concentrations influence DOM distribution and nutrient dynamics in freshwater systems.

Organic matter (OM) quantity, quality, and nutrient dynamics within twelve shallow lakes in the Czech Republic were assessed in the context of catchment soil pH and iron (Fe) concentration. The catchments of the lakes were classified into two categories: (i) slightly acidic (soil pH = 5.1-6.3) with Fe-rich soils (H_Fe; Fe = 315-344 mg kg-1 in Mehlich 3 extract); and (ii) neutral (soil pH = 6.8-7.6) with Fe-poor soils (L_Fe; Fe = 126-259 mg kg-1 in Mehlich 3 extract). The quality of OM in the two lake types was characterized using a combination of spectroscopic techniques (UV-Vis, fluorescence, and Fourier Transform Infrared spectroscopy). We show that dissolved nutrient and dissolved organic carbon (DOC) concentrations, as well as the amount of aromatic and protein-like compounds in the water column and sediment porewater were significantly (p < 0.01) lower in the H_Fe lakes compared to the waterbodies located within L_Fe catchments. The FTIR analyses of the H_Fe sediments contained higher relative concentrations of aromatic compounds with hydroxyl-containing functional groups and carbohydrates, while more aliphatic and oxidised OM was found in the L_Fe lake sediments. These results suggest that the pH value of catchment soils and, particularly, their Fe content have profound geochemical effects on the mobility of OM and nutrients in the sediments of recipient waters. Because the OM-Fe association stabilises OM in sediments, waterbodies within L_Fe catchments are likely more vulnerable to increasing eutrophication and oxygen depletion compared to those in H_Fe catchments and this has important implications for water quality management, risk assessment, and predictions of aquatic ecosystem vulnerability under conditions of accelerating climate change.

Photochemical degradation of dissolved organic matter reduces the availability of phosphorus for aquatic primary producers.

In situ experiments were done to determine the effects of the photochemical degradation of dissolved organic matter (DOM) and subsequent formation of particulate matter on dissolved phosphorus (P) concentrations in surface waters. Filtered (1.2 or 0.4 µm) headwaters (DOM of 8.1-26 mg L-1; P of 22-43 µg L-1) were exposed to solar radiation in quartz bottles located 5 cm below the water surface for 7-10 days. Dark controls were wrapped in aluminum foil. After incubation, particulate organic carbon (POC) and particulate phosphorus (PP) were determined in both the filtrate and newly formed particles. The results revealed increasing concentrations of PP and POC in exposed samples with increasing exposure time (cumulative irradiation energy). At the end of experiments, PP concentrations were from 5 to 20 µg L-1 in the exposed samples. Based on an enumeration of bacteria in the samples, we estimated the contribution of biotic and abiotic processes to the PP production. The abiotic PP formation ranged from 56 to 83% and 50-95% of the total PP in the exposed and control samples, respectively. The remainder was assumed to be bacterial P uptake. Despite the overlapping intervals, biotic and abiotic PP productions were usually higher in exposed samples than in controls. The PP and POC production was affected by the properties of DOM, such as its humic content and freshness index. We hypothesize that the observed immobilization of dissolved P in bacteria and on photochemically-formed particles can contribute to a P limitation of primary production in headwater environments that receive waters rich in soil DOM.

Trends in riverine element fluxes: A chronicle of regional socio-economic changes.

We show how concentrations of water solutes in the Vltava River (Czech Republic) and their riverine outputs from the catchment were modified by socio-economic changes, land use, and hydrology between 1960 and 2015. In the early 1960s, HCO3 and Ca were the dominant ions. During 1960-1989 (a period of planned economy with an over-use of synthetic fertilizers, excessive draining of agricultural land and little environmental protection), the riverine concentrations of strong acid anions (SAAs: SO4, NO3, and Cl) increased 2-4-fold and their leaching was accompanied for by a 1.4-1.8-fold increase in concentrations of Ca, Mg, K, and Na. SAAs mostly originated from diffuse agricultural sources (synthetic fertilizers and mineralization of organic matter in freshly drained and deeply tilled agricultural land) and their annual average concentrations (as well as those of Ca, Mg, and K) were positively correlated with discharge. During 1990-2015 (a period of a re-established market economy, reduced fertilization, ceased drainage, partial conversion of arable land to pastures, and increasing environmental protection), concentrations of SO4 and NO3 significantly decreased due to reduced agricultural production and atmospheric pollution, and their positive correlations with discharge disappeared. In contrast, Na and Cl concentrations increased due to more intensive road de-icing, and their concentrations became negatively correlated with discharge. Trends in phosphorus concentrations reflected changes in its input by both diffuse (fertilizers) and point (wastewater) sources and were discharge independent.

Photochemical cleaving of allochthonous organic-metal complexes contributes to phosphorus immobilization in surface waters.

The photochemical transformation of terrestrial dissolved organic carbon (DOC) in surface waters exposed to UV radiation causes the precipitation of metal (Al and Fe) bearing complexes with high phosphorus sorption capacities. To better elucidate this process, a series of laboratory experiments was performed with stream and river waters with pH range from 3.5 to 8.2 and concentrations of dissolved reactive phosphorus from 2 to 142 µg L-1. Samples were filtered (0.4 µm) and UV (350 nm) irradiated for 24 h at 68 W m-2, i.e. under conditions equivalent to ∼2 summer days of natural solar radiation. Irradiated samples and dark controls were then spiked with 33P-phosphate and the kinetics of P adsorption on freshly formed particles was determined after separation by ultracentrifugation. Up to 68% of the added P was removed from the solution within 48 h of the spike. The P sorption was pH dependent, with the maximum sorption ability at pHs of 6-7. We hypothesize that this process can importantly contribute to the immobilization and lower bioavailability of P in the inlet areas of (especially circum-neutral) lakes due to the intensive photochemical degradation of allochthonous DOC-metal complexes.