The influence of hydrology and sediment grain-size on the spatial distribution of macroinvertebrate communities in two submerged dunes from the Danube Delta (Romania)The present study focused on the ecological preferences of benthic macroinvertebrates regarding water flow and sediment characteristics in two submerged dunes from Danube Delta (Romania). Three hydraulic microhabitats, the stoss, trough and crest areas were sampled, along with measurements of water hydraulics, hydrology, sediment grain-size, and organic content. The results showed that the slope angles between stoss-crest and crest-trough areas are crucial in modulating local flows, sediment structure, organic content, macroinvertebrate communities, and taxonomic richness and density. As such, the stoss microhabitats are considered zones with the highest turbulence, hence driving low taxonomic richness and density compared to crest and trough microhabitats. When local environmental conditions in trough microhabitats allow the accretion of fine and organically enriched sediments, the development of maximum density for certain macroinvertebrates groups is reached. However, the benthic assemblages did not show clear preferences for certain microhabitats, suggesting ubiquitous ecologic traits, crucial for the successful colonisation of dynamic habitats, such as the submerged dunes in large rivers. The results of this study offer a better understanding on the abiotic factors driving the spatial preferences, density, and diversity of benthic macroinvertebrates in these understudied hydrogeomorphological units from large rivers.
Abstract:In the Danube Delta, on the Sulina branch, the morphology, sediment, and bedform characteristics were investigated. Three-dimensional (3D) bathymetry, flow velocity, suspended-load concentration, and liquid and solid discharge data were acquired throughout several cross sections along the Sulina channel, in order to investigate the distribution of water and sediment discharges and their influence against the river bed. A single observation (in February 2007) was made regarding the geometry, sediment composition, and hydraulic conditions under which the dunes grew and degenerated. The investigation focuses here mostly on the geometrical parameters of these bedforms, such as height, length, as well as grain size characteristic of the sediment and water dynamics. Based on in-site measurements, different hydraulic parameters were calculated, such as bed shear stresses and Reynolds number. During the field campaign, the measured water mean velocity was from v = 0.22-1.13 m·s −1 . At the same time, the measured range of shear stresses within the dune field formation was from τ 0 = 2.86 N·m −2 (on the cutoffs) to 8.62 N·m −2 (on the main channel). It was found that the correlation between height (H) and length (L) of the Sulina branch dunes describes the formula: H = 0.093L 0.5268 . The bedforms of the Sulina channel are, in general, developed in fine sand (D 50 between 0.06 and 0.35 mm).
Since the 1980s intensive anthropogenic disturbances have affected the channel of the St. George branch, the southern distributary of the Danube River. The meander cutoff program since [1984][1985][1986][1987][1988] induced different hydrosedimentary impacts on the local distribution of river flow velocities, discharge, and sediment fluxes between the former meanders and the man-made canals (Ichim and Radoane, 1986;Popa, 1997;Panin, 2003). This paper selects three large cutoff meander reaches of the St. George branch (the Mahmudia,
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