Acoustic Detection of Small Mesoplankton Such As Copepod Nauplii in the Marine Environment
Total Page：16
File Type：pdf, Size：1020Kb
TECHNICAL NOTE Acoustic Detection of Small Mesoplankton Such as Copepod Nauplii in the Marine Environment AUTHOR ABSTRACT John A. Fornshell The feasibility of using acoustic surveying techniques to study the distribution of plank- National Museum of Natural History tonic copepod nauplii in the marine environment is investigated using a computer simulation Department of Invertebrate Zoology model. The models simulate the effects of sound frequency, density of sea water and target organisms and celerity on the acoustic signature of copepod nauplii using the polyarthran Coullana canadensis as a model. The advantage is that the acoustic sampling can be for Introduction much greater lengths than those for net sampling episodes before the net becomes clogged (in less than 200 meters towing distance). The volume of water sampled for a given tow he use of acoustic monitoring tech- length would be about the same as conventional net sampling methods. Acoustic sampling niques in the study of zooplankton in the and enumeration of planktonic copepod nauplii is shown to be feasible, but has limitations Tmarine environment is steadily increasing. which must be considered in planning a field survey. Studies of zooplankton distribution in relationship to physical oceanographic features, where time constraints are an The purpose of this modeling study is to the animal to celerity of the sea water are important factor, have been greatly fa- investigate the feasibility of using acoustic within a few percent of unity. Stanton et al. cilitated by the use of acoustic monitoring back scattering as a surveying technique for (2000) modeled these properties from 1% methods (McClatchie et al., 2004). In some copepod nauplii (Figure 1). Nauplii are lar- to 6%. In order to calculate the maximum cases, high frequency acoustic methods vae with the following characteristics: only reasonably expected acoustic signal, a dif- allow the identification of major taxa of three pairs of well developed swimming ference of 6% is used in this study. zooplankton (Wiebe et al., 1996). High appendages, from anterior first antennae, In this study, the effects of naupliar frequency acoustic sampling methods are second antennae and mandibles; the mouth morphology and size and the acoustic also applicable for the study of invertebrate is located ventrally with a large exoskeletal properties of the environment and larvae, larvae (Haney et al., 1990; Wiebe et al., fold of tissue called the labium; the dorsal including density and sound velocity as 1996). In coastal waters, acoustic systems surface is covered with a cephalic shield well as wave length, will be evaluated using employing multiple frequency sonars have (Ferrari and Dahms, 2007). Although computer models. The ortho-nauplius or been used to differentiate between zoo- many marine crustaceans are known to N1 stage is used in this study. This will al- plankton and turbulence features (Sutor et develop free-living nauplii—including the low us to evaluate the potential of acoustic al., 2005; Stanton et al., 1998). Acoustic Cephalocarida, Branchiopoda, Ostracoda, monitoring of these very abundant mem- survey techniques were also found to be Mystacocarida, Copepoda, Cirripedia, bers of the zooplankton. useful in fresh water environments (Megard Ascothoracida, Rhizocephala, Facetotecta, et al., 1997). Euphausiacea, and Penaeidea—the bulk of As a prerequisite to experimental and the nauplii encountered in the marine en- FIGURE 1 field investigations employing acous- vironment are copepod nauplii (Dahms et Polyarthran copepod nauplius of Coullana tic sampling methodologies, computer al., 2005). Copepod nauplii may represent canadensis. simulations are often employed to aid in up to 90% of the zooplankton in tropical quantitatively identifying the acoustic coastal environments (Fornshell, 1994). back scattering produced by individual Because of their predominance in the zoo- plankters. This may include field survey plankton, copepod nauplii are the focus design to facilitate interpretation of data of this study. Copepod nauplii are tens of from sampling episodes (Kalikhman et al., microns to hundreds of microns in width 1997). Models may also be useful in identi- and length giving them cross-sectional areas fying the quantitative variations in acoustic of hundreds to tens of thousands of square backscatter resulting from variations in microns. The acoustic properties of nauplii the size, shape and acoustic properties of are the ratio of the density of the animal zooplankters (Stanton et al., 2000). to the density of sea water and celerity in 62 Marine Technology Society Journal Modeling Equations TS = 20Log[(Lbs)/1 m] The effect of the orientation of the nauplii and their morphology on their The acoustic detection of a Polyar- Where thran copepod nauplius was modeled in Target Strength was investigated calculat- the Java programming language. It was Lbs =a(ka)^2 ing the Target Strength as the larvae are assumed that the morphology of the 6 rotated 360 degrees. The results are shown in Figure 3. Figure 1 shows the animal with nauplius larvae would significantly affect with “k” being the wave number and “a” the an orientation angle of zero. The Target the target strength. The size, morphology, acoustic cross-section (Medwin, 2005). and physical and acoustic properties were Strength varied from -97 to -102.5 dB. modeled, using a computer model of the This would represent a four-fold variation nauplius body generated from a set of Results in the intensity of the signal. polygons to represent the body, antennules, The target strength as a function of A side scan sonar such as the one mod- antenna, mandibles and their attendant frequency was investigated for frequencies eled in this study, working at 0.5 to 20 setae. Following Jaffe (2005) the Target ranging from 0.50 MHZ to 40 MHZ. At MHZ, would have a relatively short range. Strength is: frequencies ranging from 0.5 MHZ to 5 The detection threshold is calculated from the following equation: (TS = 10Log(sigma/4 Pi) or MHZ backscatter was found to be in the 10Log(Iref/Iinc)). form of Rayleigh scattering . A plot of DT = SL - 2TL + TS -((NL - DI)/RL) Target Strength vs. ka gives a slope on the Where sigma is the acoustical cross sec- order of 81. For frequencies ranging from Where SL is the source level; TL is the tional area and Iref is the intensity of the 5 MHZ to 40 MHZ the backscatter was transmission loss; NL is the noise level; DI reflected signal and Iinc in the intensity of observed to be in the form of Geometric is the directivity index; RL is the reverber- the incident signal. The acoustic properties Scattering, and the slope was on the order ation level; TS is the target strength; and of the nauplii are assumed to be within of 5. The results of these calculations are DT is the detection threshold (Anon.). six percent of the ambient medium. High summarized in Figure 2. The dividing line For a 0.50 MHZ signal with an intensity frequencies from 0.50 MHZ to 40 MHZ between the two backscattering regimes was of 40 dB the maximum range of detection are considered. For modeling purposes, it found to be at ka equal to unity. These were would be on the order of tens of meters. was assumed that side scan sonar would be the same results as those reported by Stan- This however is for Rayleigh backscatter- used. Detection is assumed to be far field. ton et al. (2001) for crustacean plankters ing and would only apply to the detection Transmission loss is calculated assuming such as copepods and euphausids, which of unrealistically dense populations of that it is frequency dependant. The target are several orders of magnitude larger in nauplii on the order of 10^6 per cubic strength as a function of the frequency is terms of their acoustic cross-section but meter. At frequencies between 5 MHZ calculated using the equation: have the same density. and 40 MHZ and 40 dB for the signal FIGURE 2 FIGURE 3 Backscatter vs ka for frequencies from 0.50 MHz to 40.o MHz. Where k is the Target Strength vs Orientation Angle. wave number and a is the acoustic cross section. Summer 2008 Volume 42, Number 2 63 intensity the detection threshold would References Stanton, T.K., J.D. Warren, P. Wiebe, M.C. be achieved only for nauplii within 0.9 Anon. 1968. Naval Operations Analysis An- Benfield and C.H. Green. 1998. Contributions m of the transponder. napolis. Maryland: U. S. Naval Institute Press. of the Turbulence Field and zooplankton to 327 pp. acoustic backscattering by an internal wave. IOS/WHOI/ONR Internal Solitary Wave Discussion Dahms, H-U., J.A. Fornshell and b. Fornshell. Workshop. Victoria British Columbia, Canada. 2005. Key to the identification of crustacean The acoustic detection of copepod nauplii. Org Divers Evol. 6:47-56. Sutor, M.M., T.J. Cowles, W.T. Peterson, nauplii is technically feasible using off- S.D. Pierce. 2005. Acoustic observations of the-shelf acoustic devices. The range of the Fornshell, J.A. 1994. Copepod nauplii from finescale zooplankton distributions in the Target Strength was relatively small when the barrier reef of Belize. Oregon upwelling region. Deep-Sea Res Pt II. compared to the variability of the Target Hydrobiologia 292/293: 295-301. 52:109-121. Strength for copepods and euphausiids Ferrari, F.D., H-U. Dahms. 2007. Post- reported by Stanton et al. (2001). The Wiebe, P.H., D.G. Mountain, T.K. Stanton, embryonic Development of the Copepoda. acoustic cross-section, however, for nauplii C.H. Greene, G. Lough, S. Kaartvedt, Crustacean Monographs # 8, in press. Leiden: used in this study did not vary nearly as J. Dawson and N. Copley. 1996. Acoustical Brill Publishing. much as it does for the adults in the earlier study of the spatial distribution of Plankton modeling studies by Wiebe et al.