A chronic problem impeding study of animals that live in marine sediments and muddy bottom waters is an inability to observe either the animals themselves or evidence of their activities directly over any area of appreciable size and at anything near real time. Sediments effectively absorb or scatter most kinds of energy. They are obviously opaque to light, but even X-rays have limited penetration. Acoustic imaging methods, from ultrasound to sidescan, have been applied in benthic settings, but ultrasound gives reasonable subsurface images only at high power and then over only small spatial ranges, whereas sidescan is tuned to provide accurate microtopographic images of only the sediment-water interface, nor does it resolve small but ecologically relevant structures like worm tubes. Somewhat lower acoustic frequencies penetrate further but lose the resolution required to form images of individual animals. They contain information nevertheless, just as do backscatter intensity measures from waters that contain zooplankton. In the spirit of "seeing" what biological information we can extract, we have examined propagation of sound from two different perspectives, i.e., how organisms influence acoustic propagation and how acoustics can be used to get information on activities of benthic organisms. The acoustic frequencies that we have been exploring for seabed applications are primarily in the 40 - 300 kHz range.
My acoustic interests began with a retrospective analysis of data collected for other purposes on the northern California shelf at 90 m in winter (STRESS/CODE site), wherein we saw evidence of putatively biogenic changes in 40-kHz backscatter from the seabed (Jumars et al. 1996). In July - September 1995, with ONR funding, we fielded a series of manipulative experiments within a sonar-ensonified circle at the bottom of West Sound, Orcas Island, WA. We first collected data at both 40 and 300 kHz without manipulation to create explicit time-series models of acoustic backscatter. After selected manipulations, the circle was ensonified periodically by both 40- and 300-kHz acoustic pulses to test whether the manipulations significantly affected acoustic backscatter. The sediments were not very reflective of sound, and the treatments that showed up the most clearly were emplacements of highly reflective bivalves (Self et al. 2001). Our colleagues at BAE SYSTEMS saw some of our manipulations in their side-scan mode.
Another phenomenon revealed clearly during the Orcas program by means of TAPS (Tracor Acoustic Profiling System, developed by Van Holliday and his colleagues and not renamed even though the corporate moniker has changed to BAE SYSTEMS) was the nightly emergence from sediments of a "shallow scattering layer." Subsequent field samples showed the emergers to be primarily mysids. Notably, TAPS is another non-imaging system that was originally built for getting size-specific information about zooplankton with high spatial and temporal resolution. It took us some time to develop more realistic inversions for mysid-shaped bodies, but now the size-frequency data from TAPS and emergence traps appear to agree (Kringel, Holliday and Jumars 2003).
Serendipity in the Orcas experiments led us to follow two parallel tracks in our continuing acoustics work. Emergence can be resolved with established technology, i.e., with TAPS, and the dramatic events already seen are providing a fertile bed for student research on the stimuli, extent and consequences of this phenomenon from individual to ecosystem scales. Work toward an acoustic capability with infauna is much less certain and direct, so it is being pursued primarily by PIs and technicians rather than students, whose projects must have more predictable durations and outcomes.
Along the emergence track, we have recently completed a multi-year study combining TAPS-6 (the suffix 6 indicating a unit with six acoustic frequencies spanning 265 kHz to 3 MHz in frequency), ADCPs and emergence traps to characterize emergence in Maine coastal regions and estuaries. This phenomenon appears ubiquitous, and we are currently studying its extent, timing and taxonomic composition in Maine estuaries and coastal regions, where (as at the Orcas site) it also appears to be dominated in macrofaunal size categories by mysids. Heather Abello's M.S. work (Abello et al. 2005) revealed that mysids in our region use the second time derivative of irradiance as a cue to emerge and alter their ascent speed depending on irradiance. Leslie Taylor's parallel M.S. work (Taylor et al. 2005) showed that the largest emergence event of the night occurs typically at the beginning of the first tidal speed deceleration after dark. Mei Sato's follow-up M.S. used spectral analysis to determine the relative importance of tidal and diel cycles in mysid emergence, and she documented large seasonal changes in behavior. In addition she found very different patterns of emergence in copepod size categories (Sato and Jumars 2008). Moreover, her work shows that our earlier work greatly underestimated total mysid abundance because it did not resolve the high concentrations of mysids very close to the bottom that were revealed only in a downward-looking mode of deployment.
Our findings in Puget Sound and the Damariscotta River estuary led me to ask how widespread the phenomenon of mysid emergence was. It proves to be univerally important in mid-latitude shallow waters but woefully underestimated because mysids are difficult to catch and live in places that are difficult to sample (Jumars 2007).
With ONR funding, I have secured a steerable, split-beam, bottom-mounted device that measures at 420 and 120 kHz. This device will be able to assess mysid and fish abundances and behaviors simultaneously. It has a much larger of field of view than a statically deployed TAPS.
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e-mail: jumars@maine.edu
