Broadband Acoustic Characterization of Thecosome Pteropods in the Context of Ocean Acidification
Andone Lavery, Applied Ocean Physics & Engineering
Timothy Stanton, Applied Ocean Physics & Engineering
Peter Wiebe, Biology
AbstractThecosome pteropods are a widely distributed group of shelled zooplankton that are important members of pelagic ecosystems as they constitute key prey for a variety of commerciallyexploited fish species as well as other zooplankton and top predators, such as whales and birds. Though they are generally small contributors to global zooplankton abundance and biomass, they have been observed at very high densities at some times and locations. As a result of their highly variable distributions, or patchiness, and often inadequate sampling techniques, relatively little is known about their distribution. Ocean acidification will result in a seawater environment that is progressively more corrosive to their aragonite shells, with the effects being more pronounced at depth. It is speculated that ocean acidification will have significant impacts on both their behavior and shell condition. In order to understand the impact of ocean acidification on these shelled zooplankton, it is necessary to develop tools that can sample them on relevant spatial and temporal scales. Broadband acoustic scattering techniques are one such remote sensing tool. Thecosome pteropods are particularly well suited to characterization using acoustic scattering techniques as they secrete aragonite shells that make them highly efficient scatterers of sound relative to their size. However, these shells are thin, rough, and of complex shapes, and include an opercular opening, which represents a large discontinuity from the perspective of acoustic scattering. Understanding the complex scattering physics is key to using acoustics as a quantitative remote sensing tool for sampling these organisms.
We propose to perform high-frequency (120 kHz - 1 MHz) laboratory measurements of broadband acoustic backscattering from a variety of different pteropods shells in order to better understand the scattering physics, with the ultimate goals of allowing field measurements of volume backscattering to be quantitatively interpreted in terms of abundance and size. This project complements and augments an NSF-funded ocean acidification project (PIs Lawson, Wiebe, Lavery, Wang) involving cruises in the northwest Atlantic and northeast Pacific. The goal of the NSF project is to quantify the distribution, abundance, species composition, shell condition, and vertical migratory behavior of oceanic thecosome pteropods in the context of the depth distribution of aragonite saturation levels. Sampling involves traditional narrowband acoustic scattering techniques, together with in situ measurements of carbonate chemistry, zooplankton net tows and video plankton images. The use of broadband acoustic backscattering as a tool to remotely sample thecosome pteropods is being explored, with data already collected on one cruise, and with a range of overlapping frequencies to the ones we propose using in the laboratory experiments. The shell samples to be used in the proposed laboratory experiments were collected during the 2011 northwest Atlantic cruise, and additional samples will be collected during the upcoming 2012 northeast Pacific cruise. The impacts of shell condition on acoustic scattering due to the effects of ocean acidification and differences in the scattering characteristics of pteropods collected at these different locations will be investigated.