DOI: 10.5061/dryad.jwstqjq9s
title: Hydrodynamic-driven changes in the source and composition of sedimentary organic matter via grain size distribution in shallow lakes

description: 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 &lt;32 µm fraction, microalgae were mainly present in the
 &lt;32 and 63-125 µm fractions, aquatic plants appeared to be evenly
 distributed in all size fractions and terrestrial plants mainly comprised
 the &gt;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.
description: 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 &lt;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 (&lt;32, 32-63, 63-125, and
 &gt;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 &lt;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 (&gt;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.