Participants: Marvin Blizard, Jack Green, Charles Flagg, Van Holliday (briefly), Mark Huntley (briefly)
A high priority was placed on developing multiple-frequency instruments that simultaneously sense several size classes of zooplankton. This resolution of body size is important given that zooplankton growth rates and predator-prey interactions are known to vary with body size. The ability to interpret acoustic backscattering profiles, to understand behavioral differences among species, and to distinguish changes in marine ecosystem structure also require resolution of acoustic backscattering into several size classes. Acoustic validation of population and ecosystem models will also require size class resolution.
| Representative Organisms and Size Classes of Interest | |||
|---|---|---|---|
| Length (mm) | ESD (mm) | ESR (mm) | Representative Organisms |
| 1.2 | 0.5 | 0.25 | Small copepods, Pseudocalanus, Acartia, Paracalanus, Calanus copepodites |
| 2.5 | 1.0 | 0.5 | Adult Calanus, Metridia |
| 5.0 | 2.0 | 1.0 | Adult Eucalanus, Neocalanus, Euchaeta, larval euphausiids |
| 10.0 | 4.0 | 2.0 | Juvenile euphausiids, mysids, amphipods |
| 20.0 | 8.0 | 4.0 | Adult euphausiids, mysids, amphipods |
The probable end points for transducer frequencies are ca 3 MHz for the smallest size class and ca. 100 kHz for the largest size class. Final selection of the target size classes and acoustic frequencies should be done by examination of existing zooplankton size frequency distributions from different ocean basins, and in consultation with acousticians (e.g., computer modeling). The instrument should be built in modular fashion so that different transducers may be substituted; the number of frequencies may be greater than the number of desired size classes, depending on the method(s) employed and cost considerations.
Fundamental to the design of the instrument is a low profile, versatile underwater package. The package must be capable of being deployed in the following ways:
Flexible post-collection data processing software should be developed in conjunction with the acoustic device. This software should permit data to be aggregated in variable bin sizes; means and standard errors computed by depth, time, or scan; contouring; and plotting of vertical profiles, sections, and time series plots. The software should have "open architecture" to allow other variables (e.g., fluorescence, CTD, or thermistor data) to be processed in a similar manner and should accommodate user customization (e.g., flexible database structure).
Procurement Considerations and Timing
The development and construction of an acoustic instrument with attributes similar to those discussed above were considered to be appropriate for inclusion within future "Calls for Proposals" from the GLOBEC program. In order to have a maximal impact on GLOBEC science, development of this instrument should strive for prototype completion within 18 months, including comparisons with pump and net zooplankton samples. Potential manufacturers for the production instrument should be identified as early as possible. The manufacturer(s) should be encouraged to vigorously pursue construction and marketing. The advantage of such an instrument to a manufacturer is the establishment of a "standard" instrument with a worldwide market. Two of the operational advantages of this instrument to the scientific community are the broad base of user knowledge and the ability to compare results between study sites.
Other considerations
We recognize that gelatinous zooplankton (e.g., salps, larvaceans, medusae, ctenophores) can dominate marine zooplankton assemblages. In some cases, they do so as transients in rapid population bursts. Better methods are needed to acoustically distinguish gelatinous organisms from non gelatinous. Experimental study of target strengths and unique acoustic signatures of these organisms are needed.
We recognize the need to sample eggs, nauplii, and juvenile stages of many species of zooplankton. We recommend that acoustic methods be compared to other methods for achieving this. If sensor costs imposed by the need to offset the extreme acoustical attenuation at high frequencies substantially increase the overall system cost, then sampling these smaller organisms might best be done with optical measurements (e.g., High Definition TV) in combination with the acoustic instrument described here.
We underscore the need for collection of pump and net samples to collect "ground truth" data, (e.g., species identification) for the acoustic instrument. This will necessitate accelerated development of rapid, automated means to enumerate and identify zooplankton samples.
The doppler shift due to swimming zooplankton can be obtained by Fast Fourier Transform methods (among others). Changes in the average doppler shift and the doppler spectrum width of volume reverberation from a volume of water may indicate the mean and extreme swimming speeds of a group of zooplankters. Relatively straightforward modifications to the Priority 1 instrument should permit the doppler parameters to be measured, combining measurements of zooplankton abundance with measurements of zooplankton swimming speeds.