One possible justification for ignoring such targets is that they have been thought to make relatively small contributions to acoustic backscatter at commonly used frequencies. In contrast to sound, light is strongly attenuated by water, limiting the working range of underwater optical imaging. Trajectories of simulated zooplankters seeded into these flow fields were followed to quantify temporal patterns of velocity gradients and accelerations that individuals encountered. Because of their microscopic size, fragile composition, and patchy distribution, these targets have not been studied in underwater acoustical research and perhaps more importantly, in the interpretation of high-frequency acoustic survey data. Aquatic and fisheries ecologists have therefore favored acoustical methods over optical imaging to conduct large-scale biological surveys of zooplankton and fish assemblages Fernandes et al. Introduction There is a continued interest in understanding the sources of oceanic backscatter and the use of echosounders to estimate the abundances and distributions of marine organisms. The authors, however, could not refute the hypothesis that the high acoustic returns were caused by the presence of oxygen bubbles. Unlike the common TS measurement used for discrete-frequency, narrowband systems, the BTS represents the intensity of the returned energy weighted over the 1 MHz bandwidth of the chirp:
Aquatic and fisheries ecologists have therefore favored acoustical methods over optical imaging to conduct large-scale biological surveys of zooplankton and fish assemblages Fernandes et al. We found that such zooplankters are not subjected to steady velocities or velocity gradients, but rather encounter rapidly fluctuating accelerations and velocity gradients with peaks reaching several orders of magnitude above mean values and lasting fractions of a second, much shorter than the wave period. Because the goal has generally been to sample zooplankton, other organisms and particles are typically unaccounted for during traditional ground-truthing experiments, despite their conspicuous presence in rich, productive regions of the ocean. We focused on zooplankton in wavy turbulent boundary layer flow near benthic communities because such flow affects important processes, including larval settlement and prey capture by benthic zooplanktivores. Although several ground-truthing experiments i. Targets such as small gelatinous organisms, marine snow, and phytoplankton, e. Marine snow layers are common features of coastal waters Alldredge and Silver, ; Alldredge et al. Data from the latest generation of broadband sonars present new opportunities for acoustical investigations of plankton Lavery et al. One possible justification for ignoring such targets is that they have been thought to make relatively small contributions to acoustic backscatter at commonly used frequencies. Since then, there have been a few efforts to quantify the acoustic reflectance of phytoplankton Selivanovsky et al. However, little work has been conducted on acoustic estimates of these abundant and densely aggregated particles, though there are anecdotal references in the acoustical literature e. Ground-truthing exercises are by far the best and most direct method to aid in the interpretation of acoustic data. We calculated the proportion of time zooplankters spent affected e. With a few exceptions in higher latitudes, most marine ecosystems are species rich e. However, determining the identity of the targets insonified by such tools has been a major challenge. To explore the validity of this assumption, this work investigates the acoustic reflectivity of these often-ignored organisms and particles at the ultrasonic frequencies of 1. In contrast to sound, light is strongly attenuated by water, limiting the working range of underwater optical imaging. Though the sensing capability, fast acquisition, and almost real-time processing of acoustical data give acoustical methods an advantage over optical technologies, classification of acoustic target returns can be ambiguous. Their thickness ranges from tens of centimeters to a few meters as documented in a variety of marine environments e. However, no attempts have focused on the potential acoustic reflectance of phytoplankton and marine snow in the field. Using acoustic methods to understand the distributions of biological acoustic scatterers in the water column requires knowledge of the acoustic properties of the targets. Recently, however, Timmerman et al. Given that phytoplankton and marine snow layers are ubiquitous features of coastal regions; this works suggests that they should be considered as potential sources of backscatter in biological acoustic surveys. Because of their microscopic size, fragile composition, and patchy distribution, these targets have not been studied in underwater acoustical research and perhaps more importantly, in the interpretation of high-frequency acoustic survey data. Thin phytoplankton layers are ubiquitous features of coastal regions, extending over kilometers and persisting from hours to several days. However, while optical imaging methods lack long-range capabilities with the exception of oligotrophic waters with low turbidity they are capable of generating detailed images of targets, commonly allowing identification down to genus and often to species; coarse taxonomic identification e.
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