Sediment organic matter (SOM) differs in composition and function in different size fractions in relation to material and energy flows. The hydrodynamic effects on a lake’s autochthonous (terrestrial plants) and allochthonous (microalgae, aquatic plants and bacteria) inputs and transformation, and the components of SOM are still far from clear. To elucidate the SOM composition as driven by the hydrodynamic conditions associated with sedimentary grain size, we performed an ecosystem-level investigation in the shallow lake Izunuma, Japan, measuring source-specific fatty acids and the current velocity. High organic matter concentrations occurred in the finer fractions. Bacteria were dominant in the <32 µm fraction, microalgae were mainly present in the <32 and 63-125 µm fractions, aquatic plants appeared to be evenly distributed in all size fractions and terrestrial plants mainly comprised the >125 µm fraction. Although the linear correlation between current velocity and sedimentary total organic carbon (TOC) was not statistically significant, the relationship between current velocity frequency and sedimentary TOC was significant. The highest Pearson coefficient occurred at a current velocity of 6 cm/s in sand. The relationship between allochthonous inputs and current velocity frequency was similar to that of TOC. Autochthonous organics had the greatest coefficient with two peaks at 4.5 and 8 cm/s. The threshold for sand resuspension varied with organic sources, while this phenomenon was absent in cohesive mud. Hydrodynamic forces affected the grain size of sediments and drove the gradient distribution of the SOM sources, which should be considered when managing freshwater lakes in light of future climate change.

Site description

The typical small lake, Lake Izunuma, was selected to facilitate whole-ecosystem and high-resolution observations of current velocity and SOM dynamics. The lake, located in the Miyagi prefecture, Japan, is a shallow eutrophic lake with an area of 3.69 km2 and an mean depth of 0.76 m. The water body is well mixed throughout the year, and the oxygen concentration is consistently above 4 mg/L. The lake is a famous wintering ground for geese and swans in Japan and East Asia. The lake has been designated as a Wetland of Importance, especially as a Waterfowl Habitat in the Ramsar Convention and a National Protection Region for Birds and Animals. The lake has been transformed into its present eutrophic state due to decades of nutrient discharge. Although phytoplankton exist in the water column, a large portion of the lake is dominated by emergent and floating-leaf plants, especially lotus, which has led to paludification.

Sample collection and analysis

Sampling was conducted from June 24 to July 3, 2013 at 14 sampling locations throughout the lake, with the site information being recorded by portable GPS. After the collection using the Ekman-Birge grab, sediment samples were classified into sandy or muddy types and homogenized and then stored in a cool incubator at <4°C. After rapid transportation to the laboratory, each sediment sample was separated into two parts: one was kept intact and the other was classified into one of the four size fractions (<32, 32-63, 63-125, and >125 µm by a wet sieving method). Sediment samples were gently shaken in a 1L breaker filled with ultrapure water. The weight of each fraction was carefully measured to estimate their relative mass contribution to the sediment. Lastly, all of the bulk and separated fractions were stored at -40°C and freeze-dried for 24 h until the analyses of fatty acids and TOC. For the TOC determination, each sample was ground to <5 μm and treated with 1 M HCl to remove carbonates. After dried to a constant weight at 60°C, sub-samples were measured by the TOC analyzer (SSM-5000A, Shimadzu). 

The direct linkage between water velocity and SOM source and distribution remains unclear since the episodic or intensive sedimentation and resuspension of sediment particles is difficult to analyze with sound methods. More distinguishable proxies for the SOM sources are needed to define these relationships. Fatty acid content was used as a molecular biomarker and was successfully applied in our previous investigation of eutrophic lakes. The one-step method was applied to extract and esterify lipids to fatty acid methyl esters (FAMEs), as described in the previous study. The FAMEs were separated and quantified by a gas chromatograph (GC-2014, Shimadzu) equipped with a split/splitless injector and a flame ionization detector. The concentrations and percentages of specific fatty acids were calculated by referring to the previous study. The validity of using fatty acid biomarkers for potential sources of organic matter (terrestrial plants, aquatic plants, microalgae, and bacteria) was identified in a previous study. Specifically, the individual and specific fatty acids, 18:3ω6 and 18:4ω3, 20:5ω3, and 22:6ω3 are introduced as biomarkers of microalgae, including cyanobacteria, green algae, diatoms, and dinoflagellates, etc. The 18:2ω6 and 18:3ω3 fatty acids were considered as indicators for aquatic plants. The long-chain fatty acids (>24 carbons) generally indicate the provenance of terrestrial plants. Fatty acids with odd-numbered carbon chains and branches, including Σ15, Σ17, and 18:1ω7, were considered as indicators for bacteria. All fatty acids originate from organisms, which means that their presence depends on the original organism, rather than the environmental media. Thus, allochthonous SOM tracked by fatty acids include OM migrated through all forms of media such as river transportation, groundwater seepage, and others. Although groundwater may be a source of allochthonous SOM going into the lake, the sedimentary fingerprint of many organisms can be detected by the fatty acid biomarkers.

Current velocity measurement

The settling method of current velocity was described in our previous observations of coastal ecosystems. Briefly, the two-dimensional velocity meter powered by electromagnetic force (Alec Electronics, compact-EM) was set at 5 cm above the sediment surface at every study site to measure current velocity. In the studied site, 1 Hz raw current data was a significant indicator of fine particles being at the sediment-water interface, and was considered to be related to sediment resuspension driven by waves and turbulence. The lake has hydraulic characteristics like many typical lakes, where the sediments may not only be affected by multiple inflowing rivers but also by annular, wind-driven, offshore, and other flow patterns influenced by the lake bed topography, rainfall, wind, and other physical features. Current velocity, as a integral parameter, can reflect the combined influence of these hydraulic factors on the sediment. Therefore, the linkages between current velocity and characteristics of SOM that were established in this study can be used to develop new insights for lacustrine management.