Sand-capping stabilizes muddy sediment and improves benthic light conditions in eutrophic estuaries: laboratory verification and the potential for recovery of eelgrass (Zostera marina)

Decades of eutrophication have increased water turbidity in Danish estuaries and led to light limitation of eelgrass (Zostera marina) growth. Former eelgrass areas are now denuded and consist of organic-rich muddy sediment with frequent resuspension events that maintain a high turbidity state. In addition, low anchoring capacity of eelgrass in the soft organic-rich sediments has contributed to eelgrass loss. When navigation channels in Danish estuaries are dredged, large amounts (~100.000 m3) of sandy sediment are shipped to remote dumping sites. Instead, we suggest that the dredged sand is used to consolidate adjacent muddy areas. We demonstrate in the present study that capping of fluid muddy sediment with 10 cm of sand is feasible without any vertical mixing and that this marine restoration approach can significantly lower the magnitude and frequency of resuspension events. Erosion of suspended solids change from 5 g m-2 min-1 in muddy areas to about 0.2 g m-2 min-1 in sand-capped areas, implying that sand-capping can significantly improve light conditions. Moreover, erosion thresholds increase from about 10-12 cm s-1 for mud to 40 cm s-1 for sand-capped mud. In conclusion, improved benthic light and increased anchoring capacity by sand-capping, a marine restoration practice, has the potential to facilitate restoration of otherwise lost eelgrass habitats.

A Conflict between the Legacy of Eutrophication and Cultural Oligotrophication in Hiroshima Bay

Although the water quality in Hiroshima Bay has improved due to government measures, nutrient reduction has sharply decreased fisheries production. The law was revised in 2015, where the nutrient effluents from the sewage treatment plants were relaxed, yet no increase in fishery production was observed. Herein, we investigate the distribution of C, N, S, and P within Hiroshima Bay. Material loads from land and oyster farming activity influenced the C and S distributions in the bay sediments, respectively. Natural denitrification caused N reduction in areas by the river mouths and the landlocked areas whose sediments are reductive. The P content was high in the areas under aerobic conditions, suggesting metal oxide-bound P contributes to P accumulation. However, it was low in the areas with reducing conditions, indicating P is released from the sediments when reacting with H2S. In such reductive sediments, liberated H2S also consumes dissolved oxygen causing hypoxia in the bottom layer. It was estimated that 0.28 km3 of muddy sediment and 1.8 × 105 ton of P accumulated in Hiroshima Bay. There remains conflict between the ‘Legacy of Eutrophication’ in the sediment and ‘Cultural Oligotrophication’ in the surface water due to 40 years of reduction policies.

Influence of anisotropy in sediment mechanical properties on CH4 bubble growth topology and migration pattern in muddy aquatic sediments

<p>Gas-charged sediments of shallow water bodies are significant sources of atmospheric methane, an important greenhouse gas. Past accounts of gas bubbles developed in shallow aquatic sediments (and in their surrogates) have reported a controversial occurrence of vertical as well as horizontal bubbles topologies. Within the framework of tensile fracturing of muddy sediment produced by the growing bubbles, the vertical orientation of bubbles is well understood, however factors controlling horizontal bubble growth are largely unclear. This study is conducted by employing a mechanical/reaction–transport numerical model, which couples diffusion-led expansion of gas bubble and elastic-fracture mechanical response of sediment to its growth. Muddy sediment is assumed to exhibit a transverse anisotropy in fracture toughness (a property describing an easiness of breaking the inter particle bonds), attributed to partial or full alignment of plate-like clay particles. Our results demonstrate that bubbles growing in isotropic sediment develop a vertically oriented topology and start their ascent once reaching their mature sizes. Under an increasing measure of anisotropy, the bubbles grow horizontally at the initial stages, however at later stages they start evolving in vertical direction as well, under influence of gravity, and eventually initiate their vertical ascent as well. Our results suggest an explanation of apparent conundrum about preferred orientations of bubbles in muddy sediments. Laterally growing bubbles produced in anisotropic sediment are able to coalesce with neighboring ones and form interconnected permeable horizontal gas networks, as observed in some lab experiments. For the first time, our results reveal that anisotropy-led initial lateral bubble growth can also play a crucial role in accumulating gas reserve from long distances around large and small scale seeps and outlets, at continental margins and inland water bodies sediments. Additionally, horizontal bubbles tend to be stationary (in contrast to the vertical bubbles) thus being responsible for high gas storage (or retention) capability of aquatic sediments.</p>

Seasonal cycle of benthic denitrification and DNRA in the aphotic coastal zone, northern Baltic Sea

Current knowledge on the seasonality of benthic nitrate reduction pathways in the aphotic, density stratified coastal zone of the Baltic Sea is largely based on data from muddy sediments, neglecting the potential contribution of sandy sediments. To gain a more comprehensive understanding of seasonality in this part of the Baltic Sea coast, we measured rates of benthic denitrification, anammox and dissimilatory nitrate reduction to ammonium (DNRA) monthly in the ice-free period of 2016 in both sandy and muddy aphotic sediments, northwestern Gulf of Finland. No anammox was observed. The seasonal cycle of denitrification in both sediment types was related to the hydrography-driven development of bottom water temperature. The seasonal cycle of DNRA was less clear and likely connected to a combination of bottom water temperature, carbon to nitrogen ratio, and substrate competition with denitrification. Denitrification and DNRA rates were 50-80 and 20% lower in the sandy than in the muddy sediment. The share of DNRA in total nitrate reduction, however, was higher in the sandy than in the muddy sediment, being (by ~50%) the highest DNRA share in sandy sediments so far measured. Our data add to the small pool of published studies showing significant DNRA in both cold and/or sandy sediments and suggest that DNRA is currently underestimated in the Baltic coastal nitrogen filter. Our results furthermore emphasize that the various environmental conditions of a coastal habitat (light regime, hydrography, and geomorphology) affect biogeochemical element cycling and thus need to be considered in data interpretation.

FIELD OBSERVATION OF FINE SEDIMENT TRANSPORT PROCESS AROUND RIVER MOUTHS IN NORTH WESTERN JAVA ISLAND, INDONESIA

Understanding of transport processes of discharged sediment through the river in coastal area is crucial for prediction of coastal bathymetry evolution. In case that fine sediment is dominant in sedimentary system, the transport process becomes complicate with the formation of fluid mud, which may be an important role on the transport and distribution of muddy sediment. The aim of this study is to elucidate the fine sediment dynamics, which may have a key role on the deformation of the deltaic topography around the estuary, in a tropical climate environment where experiences apparent seasonal variation of river discharge due to the wet and dry weather condition.

Morphological variations of the shell of the bivalve Lucina pectinata (Gmelin, 1791)

In Martinique, the species Lucina pectinata (Gmelin, 1791) is called "mud clam, white clam or mangrove clam" by bivalve fishermen depending on the harvesting environment. Indeed, the individuals collected have differences as regards the shape and colour of the shell. The hypothesis is that the shape of the shell of L. pectinata (P. pectinatus) shows significant variations from one population to another. This paper intends to verify this hypothesis by means of a simple morphometric study. The comparison of the shape of the shell of individuals from different populations was done based on samples taken at four different sites. The standard measurements (length (L), width or thickness (E - épaisseur) and height (H)) were taken and the morphometric indices (L/H; L/E; E/H) were established. These indices of shape differ significantly among the various populations. This intraspecific polymorphism of the shape of the shell of P. pectinatus could be related to the nature of the sediment (granulometry, density, hardness) and/or the predation. The shells are significantly more elongated in a loose muddy sediment than in a hard muddy sediment or one rich in clay. They are significantly more convex in brackish environments and this is probably due to the presence of more specialised predators or of more muddy sediments.