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Estuarine systems, being situated at the interface between land and marine environments, are important sites for nitrate (NO3–) retention and processing due to large inputs, long retention time, and high biogeochemical activity. However, it remains uncertain how pelagic and benthic processes control NO3– cycling and how the relative importance of these processes is affected by seasonal changes in estuarine conditions. We measured the suite of processes governing NO3– cycling in the Curonian Lagoon (Southeast Baltic Sea) during two time periods representing spring and summer conditions. We show that in spring, benthic dissimilatory and assimilatory NO3– processes prevailed, while in summer, pelagic assimilatory processes dominated. During spring, warming temperatures and riverine nitrogen (N) inputs were associated with the onset of diatom blooms. N assimilation by diatoms resulted in the delivery of particulate organic N and organic matter to the benthos, resulting in greater denitrification in the sediments and a flux of NO3– from the water column to the sediments. In summer, phytoplankton blooms of buoyant cyanobacteria and high rates of assimilatory uptake dominated, resulting in greater particulate organic N export from the lagoon into the sea. Given the low dissolved inorganic N concentrations in summer, high uptake indicates that the pelagic community possessed a nutritional strategy to efficiently utilize multiple N forms at high rates. Overall, our findings show that diatom-dominated communities foster strong benthic-pelagic coupling, whereas cyanobacteria dominance is associated with pelagic-based N cycling. While this study sheds new light on mechanisms of NO3– retention in the Curonian Lagoon, further spatiotemporal resolution is recommended to better represent the variability in rates and to include other Baltic lagoons for a comprehensive understanding of N cycling in shallow estuarine systems. [ABSTRACT FROM AUTHOR]

Estuarine systems, being situated at the interface between land and marine environments, are important sites for nitrate (NO3-) retention and processing due to large inputs, long retention time, and high biogeochemical activity. However, it remains uncertain how pelagic and benthic processes control NO3- cycling and how these differ between contrasting seasons. In this study, we measured pelagic and benthic assimilatory and dissimilatory NO3- processes in a large lagoon (Curonian Lagoon, SE Baltic Sea) to understand changes in NO3- cycling in relation to variation in riverine inputs and shifts in phytoplankton community composition. We show that in spring, benthic dissimilatory and assimilatory NO3- processes were important, while in summer, pelagic assimilatory processes dominated. During spring, diatom blooms promote greater delivery of nitrogen (N) and labile organic matter to the benthos resulting in greater denitrification in the sediments and a net flux of NO3- from the water column to the sediments. In summer, phytoplankton blooms dominated by buoyant cyanobacteria exhibited high rates of assimilatory uptake and greater particulate organic N export to the sea, but low rates of sediment--water exchange. Cyanobacteria blooms were associated with higher absolute rates of NO3- uptake, as well as higher mass-specific rates compared to spring. Given the low dissolved inorganic N in summer, high uptake indicates that the pelagic community possessed a nutritional strategy to efficiently utilize multiple N forms. Overall, our findings show that the seasonal succession from diatom to cyanobacteria-dominated communities is associated with a shift from strong benthic-pelagic coupling to predominantly pelagic-based N cycling. [ABSTRACT FROM AUTHOR]

The vertical distribution of radiocarbon (14C) was examined in two bottom sediment cores, taken from Lake Drūkšiai which had been used as a cooling pond for the Ignalina nuclear power plant (INPP) with two RBMK type reactors. The aim of this work was to reconstruct 14C amounts in the lake ecosystem during an 8-year period after the INPP was closed, as any official monitoring of 14C in liquid releases from the INPP was not performed. The possibility of comparing the variation of the 14C specific activity in the corresponding layers of the same period of 3 different cores (one taken in 2013 and two in 2019) revealed the variability of the determined values of liquid radiocarbon discharges from the INPP into the lake. Cores taken in 2019 showed a permament14C release rate of 0.76±0.06 GBq/y all eight years after the closure of the INPP. The 14C release rate established from radiocarbon measurements in both cores did not differ by more than 0.8 GBq/y. However, including data from the core taken several years ago, the estimated radiocarbon release rate values varied within 1.3 GBq/y. [ABSTRACT FROM AUTHOR]

The carbon isotope distribution and its relationship with stable N and S isotope ratio values were investigated within a fish assemblage from the shallow lake Tapeliai, which is constantly affected by inflows of 14C depleted water from the surrounding watershed mires. The "conventional" radiocarbon age within the fish from this lake varied from 119 to 693 yr. The 14C/12C and δ13C values correlated significantly (r=0.85 p<0.001), which is not typical in lakes of the temperate zone. There were no observed statistical differences (Kruskal–Wallis ANOVA tests) in the 14C/12C values among different fish species. The radiocarbon dating values and 15N/14N measurements did not correlate. The radiocarbon measurement values also did not correlate with δ34S, however, the distribution of these isotopes in carp (119 yr and 1.3‰, respectively) and roach (344 yr and 4.5‰, respectively) indicated that fish may include allochthonous food sources in their diet. [ABSTRACT FROM AUTHOR]

The aim of this work was to evaluate the possibilities of using carbon isotope measurements to determine a strong climate signal of trees influenced by the pollutant stress as well as to examine the difference in reaction of several investigated species to the climate change. Eurasian aspen (Populus tremula), pedunculate oak (Quercus robur) and Scots pine (Pinus sylvestris) trees from Vilnius region were selected for the stable carbon isotope analysis. Our results showed that over the last 70 years (1940-2009) the climate during summer months was the most important factor for the variation of stable carbon isotope values in tree rings, but its influence on different tree species slightly differed. δ13C measurements in Eurasian aspen best corresponded to the air temperature and the precipitation signal in July-August. Oak was more sensitive to the temperature and relative humidity in June-July. A positive correlation coefficient with temperature in the period of transition from the previous vegetation to dormancy season (previous September-October) was specific to oak as well. Scots pine grown at a polluted site was sensitive only to precipitation in summer months of the growing season. The correlation between δ13C measurements and climate data was determined to be from low to moderate in all the investigated trees and did not exceed the value of |r| ≥ 0.71 suggested by McCarroll et al. (2003) as a necessary value for the reliable climate reconstruction. However, the highest correlation coefficient values of 0.65 and 0.61, obtained between the maximal air temperature throughout the hydrological year (previous September-current August) and δ13C in oak rings or between mean monthly sunshine hours throughout July-August and δ13C values in aspen rings, respectively, are comparable with those used in many palaeoclimatic reconstructions that were based even on weaker correlations. [ABSTRACT FROM AUTHOR]

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Background. The catchment of the Nevėžis River, the sixth longest river in Lithuania, is among the most affected by agricultural activities. Although seriously disturbed by land reclamation and agricultural pollution, the Nevėžis River still has natural, anthropogenically unaffected stretches. Fish species were studied at two sites of the upper part of the river with different hydrological regime, exhibiting clear differences in the trophic status of fish. The combined effect of hydrogeomorphic and water-chemistry variability on the stable isotope composition of fish assemblages in Lithuania has not been investigated. The presently reported study was intended to explore the possible effects of the riverbed morphology and agricultural activities on the chemistry and stable isotope composition of the three main fish species. Materials and methods. This study covered three species of freshwater fishes: the European perch, Perca fluviatilis Linnaeus, 1758; the roach, Rutilus rutilus (Linnaeus, 1758); and the northern pike, Esox lucius Linnaeus, 1758. The sampling was performed in both natural and regulated river stretches using backpack electrofishing equipment. Stable isotope composition of the samples was revealed by applying stable isotope analyses (SIA) with an elemental analyser connected to an isotope ratio mass spectrometer. To obtain more information about environmental variability and variation of stable isotopes d13C and d15N in fish, the entire data matrix was analysed using Principal Component Analysis (PCA). Results. In the study sites on the Nevėžis River, algal carbon isotope ratios were more 13C-depleted than those from terrestrial sources. The fishes were clearly dependent on terrestrial carbon sources in natural locations. However, the fishes collected from regulated sites had carbon isotope signatures more similar to algal than to terrestrial sources. Mean values of δ15N of the fishes from the natural sites were lower than those from the regulated ones. Conclusion. The presently reported study revealed impact of abiotic habitat variables indicating the importance of physicochemical and morphological characteristics on δ13C and δ15N isotopic values variance of fish at natural and regulated river sites. Our findings would help to evaluate and predict changes on river food webs that occur due to hydromorphological alterations determined by human activity and climate change. The results could be useful for fisheries, i.e., impact of environmental factors on fish nutrition and growth. [ABSTRACT FROM AUTHOR]

Signatures of ΔC and δC of total organic carbon in sediments as well as of total lipid extracts and phospholipid-derived fatty acid fractions isolated from the surface (0-3 cm) sediments collected in the Curonian Lagoon and in the open Baltic Sea were studied. An end-member mixing-model approach was applied to estimate relative contributions of the marine and terrestrial inputs to organic carbon in sediments, and to elucidate a possible leakage of chemical warfare agents at the Gotland Deep dumpsite. [ABSTRACT FROM AUTHOR]

This article investigates variations of Δ14C and δ13C of total organic carbon (TOC) in sediments as well as in humic acids, lipid, and phospholipid fractions isolated from the surface (0-3 cm) sediment samples collected in the Curonian Lagoon and in the Baltic Sea. This study was performed to estimate relative contributions of the marine and terrestrial inputs to organic carbon in sediments, to assess a possible effect of petroleum hydrocarbon contamination on radiocarbon signatures, and to elucidate a possible leakage of chemical warfare agents (CWA) at the Gotland Deep dumpsite. Depleted Δ14C values of the TOC (down to -453) and of the total lipid extracts (down to -812.4) were detected at the CWA dumpsite. Application of the compound-specific method indicated a possible effect of CWA on depleted Δ14C and δ13C values in the investigated organic carbon fractions. The obtained results have indicated the different origin and behavior of lipids and TOC at the CWA dumpsite as compared to the area affected by the terrestrial-freshwater OC input. The Δ14C data of the TOC and total lipid extracts showed that recent sediments at the CWA dumpsite contain an excess of fossil carbon capable of influencing the 14C dating at the site. [ABSTRACT FROM AUTHOR]

The Ignalina Nuclear Power Plant (INPP) in Lithuania is a rare case when lake water is used instead of river or sea water for cooling. Lake Drūkšiai with water residence time of 3–4 year and undisturbed sediment layers is a unique system to assess the impact of a nuclear facility on the aquatic ecosystem with a sufficiently high temporal resolution. We constructed a model of radiocarbon cycling processes in lake ecosystem which evaluates the 14C specific activity vertical distribution in two organic sediment fractions: alkali-soluble and alkali-insoluble. Model calculations proved that during the first 15 years of operation since 1983, 14C annual aqueous releases from the INPP were in water dissolved inorganic carbon form and varied in the range of 2.4 ÷ 3.7 × 108 Bq/year. The results of the modeling of hypothetic scenarios also showed that there was the only one episode of elevated releases from the INPP in 2000–2001, which changed the interaction between the two organic sediment fractions for the period of 2000–2006. It was caused most probably by released chemicals from INPP but not by 14C contamination. Interaction processes between both sediment fractions recovered to its original state after 2006, indicating that the released additional chemical compounds lake ecosystem have been cleaned-up. [ABSTRACT FROM AUTHOR]