ALBERT

Description: Three-dimensional (3D) effects can profoundly influence underwater sound propagation in shallow-water environments, hence, affecting the underwater soundscape. Various geological features and coastal oceanographic processes can cause horizontal reflection, refraction, and diffraction of underwater sound. In this work, the ability of a parabolic equation (PE) model to simulate sound propagation in the extremely complicated shallow water environment of Long Island Sound (United States east coast) is investigated. First, the 2D and 3D versions of the PE model are compared with state-of-the-art normal mode and beam tracing models for two idealized cases representing the local environment in the Sound: (i) a 2D 50-m flat bottom and (ii) a 3D shallow water wedge. After that, the PE model is utilized to model sound propagation in three realistic local scenarios in the Sound. Frequencies of 500 and 1500 Hz are considered in all the simulations. In general, transmission loss (TL) results provided by the PE, normal mode and beam tracing models tend to agree with each other. Differences found emerge with (1) increasing the bathymetry complexity, (2) expanding the propagation range, and (3) approaching the limits of model applicability. The TL results from 3D PE simulations indicate that sound propagating along sand bars can experience significant 3D effects. Indeed, for the complex shallow bathymetry found in some areas of Long Island Sound, it is challenging for the models to track the interference effects in the sound pattern. Results emphasize that when choosing an underwater sound propagation model for practical applications in a complex shallow-water environment, a compromise will be made between the numerical model accuracy, computational time, and validity.

Description: Virus-like particles (VLPs) are thought to increase the dissolved organic carbon by releasing the contents of the host cell, which, in turn, can affect bacterial growth in natural aquatic environments. Yet, experimental tests have shown that the effect of VLPs on the bacterial growth rate at different depths has seldom been studied. Bacteria–VLP interaction and the effect of VLPs on bacterial growth rate in the sunlit surface (3 m) and dark, deep ocean (130 m) environments were first explored at a test site in the southern East China Sea of the northwest Pacific. Our experimental results indicated that bacterial and virus-like particle (VLP) abundance decreased with depth from 0.8 ± 0.3 × 105 cells mL−1 and 1.8 ± 0.4 × 106 VLPs mL−1 at 3 m to 0.4 ± 0.1 × 105 cells mL−1 and 1.4 ± 0.3 × 106 VLPs mL−1 at 130 m. We found that the abundance of VLPs to Bacteria Ratio (VBR) in the dark deep ocean (VBR = 35.0 ± 5.6) was higher than in the sunlit surface environment (VBR = 22.5 ± 2.1). The most interesting finding is that in the dark, deep ocean region the bacterial growth rate in the presence of VLPs was higher (0.05 h−1) than that in virus-diluted treatments (0.01 h−1). However, there was no significant difference in the bacterial growth rates between the treatments in the sunlit surface ocean region. Deep-sea ecosystems are dark and extreme environments that lack primary photosynthetic production, and our estimates imply that the contribution of recycled carbon by viral lysis is highly significant for bacterial growth in the dark, deep ocean environment. Further work for more study sites is needed to identify the relationship of VLPs and their hosts to enable us to understand the role of VLPs at different depths in the East China Sea.