Denitrification of Permeable Sand Sediment in a Headwater River Is Mainly Influenced by Water Chemistry, Rather Than Sediment Particle Size and Heterogeneity

Sediment particle size and heterogeneity play an important role in sediment denitrification through direct and indirect effects on, for example, the material exchange rate, environmental gradients, microbial biomass, and grazing pressure. However, these effects have mostly been observed in impermeable sediments. On the other hand, the material exchange of permeable sediments is dominated by advection instead of diffusion, with the exchange or transport rates exceeding those of diffusion by two orders of magnitude relative to impermeable sediments. The impact of permeable sediment particle size and heterogeneity on denitrification remains poorly understood, especially at the millimeter scale. Here, we conducted an in situ control experiment in which we sorted sand sediment into four homogeneous-particle-sizes treatments and four heterogeneous treatments. Each treatment was deployed, in replicate, within the riffle in three different river reaches with contrasting physicochemical characteristics. After incubating for three months, sediment denitrifier communities (nirS, nirK, nosZ), denitrification gene abundances (nirS, nirK, nosZ), and denitrification rates in all treatments were measured. We found that most of the denitrifying microbes in permeable sediments were unclassified denitrifying microbes, and particle size and heterogeneity were not significantly correlated with the functional gene abundances or denitrification rates. Water chemistry was the key controlling factor for the denitrification of permeable sediments. Water NO3−-N directly regulated the denitrification rate of permeable sediments, instead of indirectly regulating the denitrification rate of sediments by affecting the chemical characteristics of the sediments. Our study fills a knowledge gap of denitrification in permeable sediment in a headwater river and highlights that particle size and heterogeneity are less important for permeable sediment denitrification.

Median bed-material sediment particle size across rivers in the contiguous U.S.

Bed-material sediment particle size data, particularly for the median sediment particle size (D50), are critical for understanding and modeling riverine sediment transport. However, sediment particle size observations are primarily available at individual sites. Large-scale modeling and assessment of riverine sediment transport are limited by the lack of continuous regional maps of bed-material sediment particle size. We hence present a map of D50 over the contiguous U.S. in a vector format that corresponds to millions of river segments (i.e., flowlines) in the National Hydrography Dataset Plus (NHDplus) dataset. We develop the map in four steps: 1) collect and process the observed D50 data from 2577 U.S. Geological Survey stations or U.S. Army Corps of Engineers sampling locations; 2) collocate these data with the NHDplus flowlines based on their geographic locations, resulting in 1691 flowlines with collocated D50 values; 3) develop a predictive model using the eXtreme Gradient Boosting (XGBoost) machine learning method based on the observed D50 data and the corresponding climate, hydrology, geology and other attributes retrieved from the NHDplus dataset; 4) estimate the D50 values for flowlines without observations using the XGBoost predictive model. We expect this map to be useful for various purposes such as research in large-scale river sediment transport using model- and data-driven approaches, teaching of environmental and earth system sciences, planning and managing floodplain zones, etc. The map is available at http://doi.org/10.5281/zenodo.4921987 (Li et al., 2021).

Body size affects the vertical movement of benthic amphipods through subsurface sediments in response to drying

This study aimed to experimentally examine how riverbed drying and different rates of water level reduction influenced the vertical movement of amphipods of various sizes into different subsurface sediment compositions. Using sediment columns (mesocosms) filled with different sized transparent substrates, we explored how varying speeds of drawdown affected vertical movement and stranding of individuals. We hypothesised that: (1) larger individuals would be less able to migrate within subsurface sediments compared to smaller ones; (2) smaller sediment particles would lead to more individuals becoming stranded and; (3) faster rates of water level drawdown would increase the likelihood of individuals becoming stranded above the waterline. Body size significantly influenced the final position of an individual, with smaller individuals accessing deeper sediments more readily. Larger amphipods were more likely to become stranded above the waterline. Amphipods migrated to greater depths during faster water level reduction rates with smaller individuals displaying greater overall movement. Sediment particle size did not influence the ability of amphipods to move vertically into subsurface sediments in response to water level reduction. The results indicate that subsurface sediments may serve as a refuge from surface drying but that both the size of individual invertebrates influences their ability to migrate vertically.

Distribution of Benthic Macroinvertebrates in Seafloor Northward of Pulau Indah, Klang

An assessment of community of benthic macroinvertebrates in waters northwards of Pulau Indah, Klang was made with the main objective to determine distribution and diversity of benthic macroinvertebrates as well as to establish possible correlation between the community’s distributions with environmental parameters. The sediments were obtained via Ponar Grab (0.023 m2 mouth area), followed by filtration (500 μm) and laboratory sorting in order to extract all specimens from sediments. The specimens were identified to the lowest taxonomic hierachy as possible. The physical parameters such as organic carbon (TOC) and grain size distributions were analysed. A total of 775 annelids, 15 arthropods, 12 echinoderms and 32 molluscs individuals were identified and recorded in six stations overall. Family Cirratulidae accounted to the highest numbers of the Annelids (n=358), whilst Cerithidae recorded as highest numbers of Molluscs. Station 5, 6 and 1 recorded the highest diversity index (H’=2.1845), evenness index (J’=0.6316) and richness index (Dmn=22.0454) respectively. Principal Component Analysis indicated sediment particle size as the major connector on all stations, with different station correlated to certain sizes of sediment particle. Pearson correlation analysis showed positive correlations between environmental parameters with eight benthic taxa in this study, with most correlations were on specific sediment particle size. As the study of benthic community is still inadequate in most of Malaysians waters, particularly in highly industrialised area such as waters in Port Klang, this study can serve as a starting point for any future studies concerning ecological disturbance affecting benthic community in Malaysian waters.