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We observed phosphorus (P) compound groups in sediment in Lake Kasumigaura in winter (February and March) and summer (July and August) to identify how their composition differs between the seasons. The concentration of soluble unreactive P in sediment pore water (SUP) was significantly higher in winter than in summer, whereas the concentration of soluble reactive P (SRP) was significantly higher in summer than in winter. In summer, when the concentration of SRP was high, the concentration of orthophosphate was greatest (~80 %) among the P compound groups. The concentration of orthophosphate diesters had a significant negative correlation with SUP concentration and was significantly lower in winter (high SUP concentration) than in summer. Such relative abundance of P compound groups in sediment could have resulted from adsorption of orthophosphate in summer and degradation of orthophosphate diesters in winter; these seasonal processes could contribute significantly to the changes in the concentrations of SRP and SUP, possibly influencing the P concentrations in the water column in Lake Kasumigaura. [ABSTRACT FROM AUTHOR]

We tested the hypothesis that in shallow, eutrophic Lake Kasumigaura, the concentration of particulate phosphorus (PP) is controlled by biogenic P (P in living or dead phytoplankton and bacterial cells), rather than by resuspension of inorganic P in sediment. Increases in wind velocity and turbidity were associated with bottom shear stress exceeding the critical value for the lake (τ c = 0.15 N m − 2 ); this increased turbidity was due to sediment resuspension. However, concentrations of PP; HCl-extractable, reactive P in PP (P-rP); and HCl-extractable, non-reactive P in PP (P-nrP) were not correlated with wind velocity (PP vs. wind velocity: r = 0.40, p > 0.05). Rather, the P-nrP concentration accounted for approximately 79% of PP, and the concentrations of PP, P-rP, and P-nrP were correlated with the particulate organic carbon (POC) concentration (POC vs. PP: r = 0.90, p < 0.01; POC vs. P-rP: r = 0.82, p < 0.01; POC vs. P-nrP: r = 0.86, p < 0.01). In our 31 P nuclear magnetic resonance spectroscopy results, mononucleotides accounted for the largest proportion among the detected P compound classes. In addition, concentrations of mononucleotides, orthophosphate, and pyrophosphate were significantly higher in samples with high POC concentrations, whereas the DNA-P concentration was not. These results suggest that biogenic P affects PP concentrations more strongly than does sediment resuspension, and the production of biogenic P creates a pool of mononucleotides, a class of easily degradable P, even in shallow, eutrophic Lake Kasumigaura. [ABSTRACT FROM AUTHOR]

The chlorophyll a-specific absorption coefficient ( $$ a_{\text{ph}}^{*} \left( \lambda \right) $$) in a highly eutrophic lake can show characteristics distinct from that in the ocean due to the differences in the structure and composition of phytoplankton. In this study, investigated the variation of $$ a_{\text{ph}}^{*} \left( \lambda \right) $$ in Lake Kasumigaura, a highly eutrophic lake in Japan, in association with the package effect and the effect of accessory pigments, and carried out the parameterization of $$ a_{\text{ph}}^{*} \left( \lambda \right) $$. Although $$ a_{\text{ph}}^{*} \left( \lambda \right) $$ did not vary spatially, it did show significant temporal variation, with a particularly high value after spring-bloom. This high $$ a_{\text{ph}}^{*} \left( \lambda \right) $$ in spring was attributed to a lower package effect and a higher proportion of carotenoid than the other samples. Although the value of $$ a_{\text{ph}}^{*} \left( \lambda \right) $$ was correlated with the concentration of chlorophyll- a (Chl- a), the correlation coefficient was lower than those reported in the ocean. Some lake-water samples showed variations of the package effect and the effect of accessory pigments that were independent of the concentration of Chl- a, and these independent variations resulted in the weak correlation between $$ a_{\text{ph}}^{*} \left( \lambda \right) $$ and the concentration of Chl- a. Together, these results suggest that the factors controlling $$ a_{\text{ph}}^{*} \left( \lambda \right) $$ in highly eutrophic lakes are distinct from that in ocean samples. [ABSTRACT FROM AUTHOR]

Abstract: Sediment core samples from the center of Lake Kasumigaura, Japan, were collected from 1979 to 2007 at intervals of 1month to 5yr. We evaluated the degree of modification in N isotope composition during sedimentation and diagenesis. We estimated the degree of isotope discrimination during diagenesis by comparing historical changes in N isotope composition of the surface sediment (top 2cm) against the vertical profile of the isotope composition of sediment core samples (15cm depth). The degree of 15N enrichment during sedimentation appeared to be significant under the preferential N decomposition that occurred in the periods with low C/N ratio values of suspended particulate organic matter. We documented 15N depletion in sediment deeper than approximately 3cm during diagenesis. The contrasting directions of N isotope discrimination during sedimentation and diagenesis suggest changing mechanisms of isotopic shift across an oxidation–reduction boundary. [Copyright &y& Elsevier]

Abstract: In an attempt to better characterize dissolved organic carbon (DOC) in several specific sources to Lake Kasumigaura, such as sewage-treatment-plant effluent (STPE), domestic sewage (DS) and forest stream (FS), we analyzed radiocarbon (14C) and stable carbon isotopic compositions (13C) of the DOCs. The measurements of 14C for DOC were performed by an accelerator mass spectrometer (AMS) at the National Institute for Environmental Studies (NIES-TERRA) in Japan. The Δ14C and δ13C values of the DOCs in several sources to Lake Kasumigaura, have low carbon isotopic values, ranging from −470‰ to −79‰ and from −27.9‰ to −24.2‰, respectively. These carbon isotopic values are substantially different from those of Lake Kasumigaura. These results imply different origins for the DOC in Lake Kasumigaura. The 14C and 13C analyses of DOC led to a useful classification for DOCs in Lake Kasumigaura, Japan. [Copyright &y& Elsevier]

Incubation experiments using filtered waters from Lake Kasumigaura were conducted to examine bacterial contribution to a dissolved organic carbon (DOC) pool. Bacterial abundance, bacterial production, concentrations of DOC, total dissolved amino acids (TDAA), and total dissolved neutral sugars (TDNS) were monitored during the experiments. Bacterial production during the first few days was very high (20 to 35 μg C liter-1 day-1), accounting for 40 to 70% of primary production. The total bacterial production accounted for 34 to 55% of the DOC loss during the experiment, indicating high bacterial activities in Lake Kasumigaura. The DOC degradation was only 12 to 15%, whereas the degradation of TDAA and TDNS ranged from 30 to 50%, suggesting the preferential usage of TDAA and TDNS. The contribution of bacterially derived carbon to a DOC pool in Lake Kasumigaura was estimated using D-amino acids as bacterial biomarkers and accounted for 30 to 50% of the lake DOC. These values were much higher than those estimated for the open ocean (20 to 30%). The ratio of bacterially derived carbon to bulk carbon increased slightly with time, suggesting that the bacterially derived carbon is more resistant to microbial degradation than bulk carbon. This is the first study to estimate the bacterial contribution to a DOC pool in freshwater environments. These results indicate that bacteria play even more important roles in carbon cycles in freshwater environments than in open oceans and also suggests that recent increases in recalcitrant DOC in various lakes could be attributed to bacterially derived carbon. The potential differences in bacterial contributions to dissolved organic matter (DOM) between freshwater and marine environments are discussed. [ABSTRACT FROM AUTHOR]