In headwater streams, many aquatic insects rely on terrestrial detritus, while their emergence from streams often subsidizes riparian generalist predators. However, spatial variations in such ...reciprocal trophic linkages remain poorly understood. The present study, conducted in a northern Japanese stream and the surrounding forest, showed that pool-riffle structure brought about heterogeneous distributions of detritus deposits and benthic aquatic insects. The resulting variations in aquatic insect emergence influenced the distributions of riparian web-building spiders. Pools with slow current stored greater amounts of detritus than riffles, allowing more benthic aquatic insects to develop in pools. The greater larval biomass in pools and greater tendency for riffle insects to drift into pools at metamorphosis resulted in an emergence rate of aquatic insects from pools that was some four to five times greater than from riffles. In the riparian forest, web-building spiders (Tetragnathidae and Linyphiidae) were distributed in accordance with the emergence rates of aquatic insects, upon which both spider groups heavily depended. Consequently, the riparian strips bordering pools had a density of tetragnathid spiders that was twice as high as that of the riparian strips adjacent to riffles. Moreover, although limitations of vegetation structure prevented the aggregation of linyphiid spiders around pools, linyphiid density normalized by shrub density was higher in habitats adjacent to pools than those adjacent to riffles. The results indicated that stream geomorphology, which affects the storage of terrestrial organic material and the export of such material to riparian forests via aquatic insect emergence, plays a role in determining the strength of terrestrial-aquatic linkages in headwater ecosystems.
Several studies have demonstrated the importance of the bioavailability of particulate phosphorus (PP) exported from various watersheds, especially agricultural land. However, few studies have ...documented the influence of hydrological conditions on the sources and bioavailability of PP exported from watersheds with multiple land uses. In this study, to assess the sources and bioavailability of PP under different hydrological conditions, we conducted synoptic monitoring of compositional changes in the chemical fractions of PP exported from the Yasu River Watershed, which is covered with a mixture of forest (57.7%) and rice paddy (20.5%) areas. In this river, total phosphorus was composed mainly of PP at all monitoring sites and time points. The sequential extraction of PP revealed that a fraction bound to redox‐sensitive iron and manganese, regarded as bioavailable, was specific to the drainage waters of rice paddies, while a fraction bound to organic matter, regarded as less bioavailable, was specific to a forested stream. In the downstream of the Yasu River covering the whole watershed, PP exported under the base flow and small rainfall conditions was composed mainly of the rice paddy‐derived fraction, while most of the PP exported under heavy rainfall conditions was the forest‐derived fraction. These results suggest that the sources and bioavailability of PP can vary dynamically, depending on the hydrological regimes and land use pattern in the watershed. Even taking into account the fact that the large amount of PP exported during heavy rainfall is considered to be forest‐derived PP with low bioavailability, the monitoring results suggested that the annual amount of bioavailable PP exported is larger than that of dissolved phosphorus, and can have a large impact on phosphorus recycling in the downstream lake ecosystem through sedimentation and regeneration.
We assessed the influence of hydrological conditions on the sources and bioavailability of particulate phosphorus (PP) exported from watersheds with multiple land uses. In the downstream of the Yasu River covering the whole watershed, PP exported under the base flow and small rainfall conditions was composed mainly of the rice paddy‐derived fraction, while most of the PP exported under heavy rainfall conditions was the forest‐derived fraction. These results suggest that the sources and bioavailability of PP can vary dynamically, depending on the hydrological regimes and land use pattern in the watershed.
Methanogenesis in freshwater lakes has classically been considered to arise from anaerobic methanogens in oxygen-depleted sediments. However, the accumulation of supersaturated methane in fully ...oxygenated water columns is commonly observed in many lakes, and factors responsible for the formation of the subsurface methane maximum (SMM) remain largely unknown. The present study conducted in 14 Japanese freshwater lakes showed that the SMM formation during the summer stratification period is a common feature in large and deep oligotrophic lakes. The seasonal survey of a deep oligotrophic lake revealed that SMM formation may be uncoupled with the dissolution of atmospheric methane, as well as with the transport of methane from tributary rivers, littoral sediments, and hypolimnetic anoxic sources, suggesting the contribution of in situ methane production. In fact, batch-culture experiments confirmed that bacterioplankton present in lake subsurface waters produce methane aerobically through the decomposition of methylphosphonic acid. Moreover, the development of SMM was closely associated with the seasonal dynamics of planktonic cyanobacteria such as Synechococcus, which may carry the enzyme to catabolize organophosphonate compounds. Therefore, we suggest that the predominance of Synechococcus during the thermal stratification period plays a significant role in SMM formation, and likely the methane flux from lakes to the atmosphere.
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•The study examined the subsurface CH4 maximum (SMM) formation in oxic lake waters.•The SMM found in deep lakes was associated with the dynamics of cyanobacteria.•Lake P depletion may cause cyanobacterial CH4 production by phosphonate metabolism.•Enhanced oligotrophy by mixing regime alteration could promote CH4 flux from lakes.
Planktonic sulfur oxidizers are important constituents of ecosystems in stratified water bodies, and contribute to sulfide detoxification. In contrast to marine environments, taxonomic identities of ...major planktonic sulfur oxidizers in freshwater lakes still remain largely unknown. Bacterioplankton community structure was analyzed in a stratified freshwater lake, Lake Mizugaki in Japan. In the clone libraries of 16S rRNA gene, clones very closely related to a sulfur oxidizer isolated from this lake, Sulfuritalea hydrogenivorans, were detected in deep anoxic water, and occupied up to 12.5% in each library of different water depth. Assemblages of planktonic sulfur oxidizers were specifically analyzed by constructing clone libraries of genes involved in sulfur oxidation, aprA, dsrA, soxB and sqr. In the libraries, clones related to betaproteobacteria were detected with high frequencies, including the close relatives of Sulfuritalea hydrogenivorans.
Methane oxidation coupled to denitrification is mediated by ‘Candidatus Methylomirabilis oxyfera’, which belongs to the candidate phylum NC10. The distribution of putative denitrifying ...methane-oxidizing bacteria related to “M. oxyfera” was investigated in a freshwater lake, Lake Biwa, Japan. In the surface layer of the sediment from a profundal site, a phylotype closely related to “M. oxyfera” was most frequently detected among NC10 bacteria in PCR analysis of the 16S rRNA gene. In the sediment, sequences related to “M. oxyfera” were also detected in a pmoA gene library. The presence of NC10 bacteria was also confirmed by catalyzed reporter deposition fluorescence in situ hybridization (CARD-FISH). Denaturing gradient gel electrophoresis (DGGE) and quantitative real-time PCR indicated that the abundance of the “M. oxyfera”-related phylotype was higher in the upper layers of the profundal sediment. The horizontal distribution of the putative methanotrophs in lake sediment was also analyzed by DGGE, which revealed that their occurrence was restricted to deep water areas. These results agreed with those in a previous study of another freshwater lake, and suggested that the upper layer of the profundal sediments is the main habitat for denitrifying methanotrophs.
Since temperature is a key factor affecting photosynthetic and respiration rates, the rates of gross primary production (GPP) and ecosystem respiration (ER) are expected to be lower for rivers at ...higher latitudes, while the net ecosystem production (NEP) rate likely decrease in rivers at lower latitude due to higher sensitivity of ER to temperature compared with GPP. To examine these possibilities, we estimated the ecosystem metabolism of 30 rivers located from 43.03°N to 32.38°N in Japan during summer using a Bayesian model with hourly changes in dissolved oxygen concentrations. In addition, we examined latitudinal trends of GPP, ER and NEP in a global scale by compiling and analyzing river metabolic data estimated in previous studies. Our analysis showed that both GPP and ER tended to increase with latitude, although these rates were positively related to water temperature in Japanese rivers. Global dataset of GPP and ER also showed increasing trend towards higher latitude. In addition, contrary to our initial expectations, NEP decreased with latitude and most rivers were net heterotrophic at both regional (Japanese rivers) and global scales. These results imply that the latitudinal temperature effect on river metabolism is masked by other factors not examined in this study, such as land use in the watershed, which play pivotal roles in explaining the latitudinal variation of river metabolism.
The middle and lower reaches of the Fuji River system, Central Japan
The increase in turbidity over the last decade is considered a major cause of habitat degradation of aquatic organisms in the Fuji ...River and the adjacent coastal area of Suruga Bay. This study identifies the source and fate of suspended solids (SS) in the Fuji River system based on an investigation of the isotopic composition of strontium (87Sr/86Sr) in river water, SS, and fluvial sediments.
The Sr isotope mass-balance model revealed that the strong turbidity (SS > 500 mg L−1) observed in a major tributary system—the Hayakawa River—accounted for 65 % of riverine SS in the Fuji River; however, the Hayakawa River water contributed only 24 % to the total river water flow in the Fuji River. In contrast, riverbed sediments in the Fuji River contained only 11 % of the sediments derived from the Hayakawa River system. These results suggest that the Hayakawa River SS could be a major source of turbidity in the Fuji River but most of it flowing into Suruga Bay without significant sedimentation. The present Sr isotope ratio, depends on host rock types in geologically heterogeneous catchments, can be used for fingerprinting of fluvial SS and improving ecosystem management planning in watersheds susceptible to frequent soil erosion and landslides.
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•The sediment transport in the Fuji River system was examined by 87Sr/86Sr.•Fingerprinting using the Sr isotopic composition revealed a major turbidity source.•Riverine-suspended solids mostly flowed into the coastal sea without sedimentation.•The 87Sr/86Sr can be a robust tracer in geologically heterogeneous catchments.
The effect of resource subsidies on recipient food webs has received much recent attention. The purpose of this study was to measure the effects of significant seasonal seaweed deposition events, ...caused by hurricanes and other storms, on species inhabiting subtropical islands. The seaweed represents a pulsed resource subsidy that is consumed by amphipods and flies, which are eaten by lizards and predatory arthropods, which in turn consume terrestrial herbivores. Additionally, seaweed decomposes directly into the soil under plants. We added seaweed to six shoreline plots and removed seaweed from six other plots for three months; all plots were repeatedly monitored for 12 months after the initial manipulation. Lizard density (
Anolis sagrei
) responded rapidly, and the overall average was 63% higher in subsidized than in removal plots. Stable-isotope analysis revealed a shift in lizard diet composition toward more marine-based prey in subsidized plots. Leaf damage was 70% higher in subsidized than in removal plots after eight months, but subsequent damage was about the same in the two treatments. Foliage growth rate was 70% higher in subsidized plots after 12 months. Results of a complementary study on the relationship between natural variation in marine subsidies and island food web components were consistent with the experimental results. We suggest two causal pathways for the effects of marine subsidies on terrestrial plants: (1) the "fertilization effect" in which seaweed adds nutrients to plants, increasing their growth rate, and (2) the "predator diet shift effect" in which lizards shift from eating local prey (including terrestrial herbivores) to eating mostly marine detritivores.
Particulate organic phosphorus (P) compounds were examined in ultraoligotrophic Lake Saiko, Japan. A cartridge filter was used to collect sufficient amount of suspended particles for analysis by a ...two-dimensional NMR (
1
H-
31
P heteronuclear multiple bond correlation). 2-Aminoethylphosphonic acid (2-AEP), a phosphonate, was detected in suspended particles in Lake Saiko. The identity of the phosphonate was confirmed by comparison with a commercially available compound. Because 2-AEP is bioavailable, microorganisms can store and use this compound under extremely P-limited conditions. This is the first study to detect 2-AEP in an ultra-oligotrophic environment.
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