Estimation of fluxes of nutrients and global warming gases from sediments

Principal Investigator (Contact person)

Ayato KOZU (kohzu[at] "[at] is replaced by @"

  • Overall



   Outbreaks of blue-green algae and aquatic macrophyte have been a serious problem in eutrophic lakes worldwide. Nutrient return from the sediment surface to the water column has been suggested as a primary cause of this because lake sediment is one of the most deteriorated parts of inland ecosystems. However, no reliable tools have been available to analyze the physical structures within the sediment such as benthic tubes and methane bubbles.
In this study, we succeeded in obtaining 3D structures of chironomid tubes within the sediment using MRI analysis. A comparison of the MRI pictures of sediment cores and chloride ion concentrations in pore water suggested an enhancement of the inter-layer exchange of pore water at the depth where many tubes could be found.
We also analyzed the methane production processes in the sediment using measurements of the carbon isotope ratios (δ13C) of methane and dissolved inorganic carbon (DIC). Low values of δ13C of methane (ca. -70‰) and high values of13C of DIC (up to +6.9‰) suggested that reduction of carbon dioxide was the main methane production process within the sediment of Lake Kasumigaura. We found that methane production was enhanced at the depth where the 13C values of both methane and DIC were higher.

Backgrounds and aims

   Globally, many shallow inland waters have high nutrient concentrations and high primary production. Overproduction of particulate organic matter (POM) within the water column can lead to accumulation on the surface of the sediment, which can accelerate the sediment oxygen demand (SOD). Even in shallow lakes and ponds, increased SOD can reduce the utilization of the dissolved oxygen within the surface sediment, which can lead to the disappearance of large benthic organisms such as bivalves and chironomids. Chironomids are one of the primary benthic organisms and they contribute to the production of oxidative conditions within the surface sediment via tube structures. Chironomids larvae live within their tubes and digest filtrated POM derived from the water column. In the process of POM filtration, chironomids larvae intake fresh oxygenated water via the tube inlet, which helps maintain oxic conditions at the depth in the sediment where the chironomids tubes are distributed. Therefore, the disappearance of chironomids larvae would lead to an acceleration of anoxic conditions in the sediment surface and nutrient efflux into the water column. In addition, changes in the oxic/anoxic conditions would affect the flux potentials of global warming gases such as methane and CO2.
   However, there is little knowledge about the relationships between oxic/anoxic conditions, tube structures, and nutrient efflux from the sediment surface. The lack of techniques to visualize and quantify the tube structures within the sediment has hampered progress in this subject.
This study represented the first application of an MRI analysis to visualize the tube structures within the sediment. Following the optimization of the MRI analysis, we could estimate the maximum depth of the tube structures within the sediment cores and discuss the relationships with vertical profiles of pore water chloride concentration. As the second research issue concerned the analysis of the flux potentials of global warming gases, we developed apparatus to extract the methane or CO2 gas that was partly dissolved in the pore water. Then, we measured the stable carbon isotope ratios and the concentration of the extracted gas. Based on the MRI results of the sediment in Lake Kasumigaura, we discussed whether the tube structures affected the nutrient flux from the sediment. From the carbon isotope ratios of the methane and CO2 in the sediment, we established the primary methane production process and its activity.

Materials and methods

MRI conditions
   We used MRI (4.7 T, Agilent Corp.) device that is designed for human head scans (Fig. 1). We set the sediment cores (φ 110 mm, L 190 mm) at the center of the TEM coil. After optimizing the gradient echo (GE method) and spin echo (SE method) for the tube structures, we started the MRI analysis and obtained 3D sequences using the following parameters: TR = 400 ms, TE = 8 ms, FOV 19.2 × 19.2 × 19.2 (cm), matrix 256 × 256 × 256, two times integration. For the RF pulses, we adopted a square wave with a non-selective excited pulse (T = 160 μs). Duration of the MRI scanning was 15 hours for each core sample.
Gas extraction device
   The primary global warming gases produced from the sediment were either methane or CO2. Because methane gas is low in its solubility (Bunsen gas solubility coefficient: 0.034 at 20°C), both dissolved gas and gas in the bubble of sediment must be extracted for the estimation of gas concentration and its carbon isotope ratios. We designed a gas extraction device that can extract methane and CO2 (Fig. 3). The gas extraction procedure was as follows. 1) Sediment layers with 30-mm thickness were transferred into the device. 2) After the headspace was N2-purged, pure water (Milli-Q) that was also N2-purged was dripped onto the sediment. 3) After breaking down the sediment structures with the dripped Milli-Q within the sealed space, the gas extraction device was shaken laterally at 70 rpm for 2 hours to realize the gas-liquid equilibrium. 4) Liquid samples (ca. 22 ml) were extracted by syringe and preserved in sealed vial bottles after the injection of saturated HgCl2 to prevent respiration.


Fig. 1 Scene in MRI analysis (left), and self-advances apparatus to quanitify global warming gas in the sediment that was either gas or dissolved phase (right)

Primary results and its implication on the analysis of nutrient and gas efflux from sediment

Tube structures revealed by MRI analysis
   In Lake Kasumigaura, there are two chironomid species. We used Chironomus plumosus because they construct relatively stable tubes that are reinforced internally by their secretions. The MRI analysis enabled us to observe the 3D distribution of the tube structures of C. plumosus (Fig. 2). In the sediment cores with chironomid larvae (nine individuals, two weeks), we found more than five thick long tubes, whereas the average in the control cores was less than two tubes. The significant difference in the number of thick tubes suggests that MRI analysis is a valuable technique for the estimation of chironomid tube structures within the sediment.


Fig.2 Tube structure of chironomid larvae in lake sediment revealed by MRI analysis 

Relationships between tube structures and pore water quality
   Chironomids larvae live within their tubes and digest filtrated POM derived from the water column. In the process of POM filtration, chironomids larvae intake fresh oxygenated water from above the sediment surface via the tube inlet. Therefore, we should expect changes in pore water quality because of this exchange. In the case of a biologically non-reactive major ion such as the chloride ion, we succeeded in showing that the ion concentration was similar at the depth at which the tube structures were distributed (see Result 1 schematic figure). This fact suggests that the increase in the density of chironomids larvae would lower the vertical change in ion concentration in the pore water. If this were true for nutrient concentrations in the sediment surface, we should expect depletion in nutrient efflux from the sediment.

Stable carbon isotope ratios (δ13C) of methane and DIC within the sediment
   In Lake Kasumigaura, the values of δ13C of methane extracted from the sediment were -69.7 ± 4.5 ‰ (n = 50), which were typical of methane production via the reduction of DIC. In the sediment where methane production via the reduction of DIC proceeds, the DIC that is the source of the methane becomes enriched in 13C, while the produced methane is depleted in 13C. Thus, we should expect a 1:1 relationship between the δ13C of methane and DIC in the sediment, especially when there is little effect from methane oxidation, which results in the reverse sense of 13C enrichment. Our results showed that there was a significant 1:1 relationship between the δ13C of methane and DIC in the sediment, suggesting methane production via the reduction of DIC with little methane oxidation (Fig. 3). In addition, we found a significant positive relationship between methane concentration and the δ13C of DIC. This fact showed that methane production activity could be estimated from both the δ13C of methane and DIC.

Fig.3 Methane production processese from the analysis of the stable carbon isotope ratios (δ13C) in methane and dissolved inorganic carbon (DIC) in sediment.

   Methane concentration was highest at around the depth of 10 cm below the surface. As for the effect of sediment temperature, methane production was high at incubation temperatures above 20°C and low at 4°C.Further study considering the effects of tube structure on methane production would be required to enable the forecasting of the efflux of global warming gases in Lake Kasumigaura.

Publications & Link