65 5 Acoustic Remote Sensing of Photosynthetic Activity in Seagrass Beds Jean-Pierre Hermand CONTENTS 5.1 Introduction 66 5.2 Inßuence of Photosynthesis on Acoustics 67 5.2.1 Bubbles in Seawater 67 5.2.2 Posidonia Photosynthetic Apparatus 69 5.2.3 Oxygen Production 69 5.2.4 Gas in Matte and Sediment 69 5.3 The USTICA 99 Experiment 69 5.3.1 Test Site 69 5.3.2 Experimental ConÞguration 71 5.3.3 Acoustic Measurements 72 5.3.3.1 Signal Transmission 72 5.3.3.2 Ambient Noise Recording 72 5.3.3.3 Transducer Calibration 72 5.3.3.4 Equalized Matched-Filter Processing 73 5.3.4 Oceanographic Measurements: CTD and Dissolved Oxygen Content 73 5.4 Multiscale Acoustic Effects 75 5.4.1 Time-Varying Medium Impulse Response 75 5.4.2 Propagation Channel Modeling 77 5.4.3 Energy Time Distribution of Medium Response 80 5.4.4 Non-Photosynthesis-Related Effects 81 5.4.4.1 Tide 81 5.4.4.2 Sea Surface Motion 82 5.4.4.3 Water Temperature ProÞle 83 5.5 Effects of Photosynthesis on Sound Propagation 83 5.5.1 Time Variation of Dissolved Oxygen 83 5.5.2 Effect of Photosynthetic Bubbles on Multipaths 84 5.5.3 Effect on Reverberation 88 5.5.4 Effect on Ambient Noise 88 5.5.4.1 Spectral Characteristics 88 5.5.4.2 Time-Frequency Characteristics 90 5.5.4.3 Directional Characteristics 91 5.5.4.4 Other Observations 91 5.5.5 Gaseous Interchange of the Leaf Blade 91 5.6 Conclusion 92 Acknowledgments 93 Appendix 5.A Comparison with Earlier Experiments 94 References 94 © 2004 by CRC Press LLC 66 Handbook of Scaling Methods in Aquatic Ecology: Measurement, Analysis, Simulation 5.1 Introduction To be able to prevent damage to marine and freshwater ecosystems, for example, to avert negative consequences for biodiversity, environmental surveillance and monitoring tools are required that produce data that are continuous in time and representative of extended areas of interest. In recent years, research on acoustic remote sensing of the ocean has evolved considerably, especially in studying physical and biological processes in shallow water environments. 1 Methods and systems have been developed that exploit, to different degrees, the complex nature of sound propagation to identify physical and biological markers (parameters) of the water column, its boundaries, and subbottom structures. Among these, sophisticated acoustic inversion techniques based on matched Þeld and matched waveform processing have proved effective and reliable in determining range-average physical properties of the water column and upper sediments. 2–4 This chapter focuses on the use of acoustics to remotely sense biological processes through an original case study: the photosynthesis by Posidonia oceanica (L.) Delile, an endemic marine phanerogam of the Mediterranean Sea. The organism settles most commonly on loose sediments but can develop on hard and rocky substrata and, when it encounters favorable conditions, colonizes vast areas of the sea bottom forming prairies, which extend from the surface to a depth of approximately 35 to 40 m. The prairies represent the most characteristic and, probably, most important ecosystem of the Mediterranean Sea covering an estimated surface area of 20,000 square miles. They are an important habitat for numerous Þsh species, marine animals, and other species of plants and algae. They create natural barriers that reduce coastal erosion. Posidonia is called the “green lung of the Mediterranean” for its important characteristic of producing large quantities of oxygen. Unfortunately, the plants are sensitive to environmental decay and have suffered marked regression over the last 40 years. The development of methods to assess their state of health efÞciently is of considerable interest as traditional direct methods, e.g., underwater diving for inspection and sampling, and indirect methods, e.g., mechanical and high-frequency echographic An exploratory study was started in 1995 to Þnd ways of monitoring in situ, and on the scale of a prairie, the response of Posidonia plants to environmental conditions. 10 To this end, the effects of photosynthesis on long-range propagation of low frequency sound were investigated under controlled experimental conditions. 11,12 Transmission measurements in the frequency range 100 Hz to 1.6 kHz showed daily changes of frequency-dependent propagation characteristics including attenuation and dispersion (pulse spreading). The diurnal variations were attributed in part to undissolved gas present on the leaf blades during phases of photosynthesis cycle. A previously unsuspected phenomenon of waveguide propagation of sound in a bottom bubble layer was discovered, and it was shown that the phenomenon could be exploited to determine the oxygen void fraction in that layer. The proposed acoustic sampling is not invasive; i.e., it does not affect the metabolism of the plant and, in particular, the gaseous exchange with the ambient medium in ways that, for example, an incubator enclosing the plant does. The method preserves the natural life condition allowing us to obtain qualitative information about the plant response to environmental variables such as photosynthetically active radiation (light), temperature, stirring, and nutrients. Furthermore, the method alleviates the problem of space and time aliasing associated with traditional spot measurements. The acoustic propagation data “integrate” the acoustic effect of a great number of plants present along the source–receiver transect of arbitrary length within the prairie of interest. A static conÞguration of the transducers allows observation over long time periods (days to months) and at short time intervals (minutes to seconds), covering a great number of photo- synthesis cycles. In this chapter, we report and discuss results from a second experiment carried out under completely different conditions with respect to the Þrst experiment in terms of the measurement geometry, acoustic transmission, and environment. The major differences were the much shorter length of acoustic path, broader frequency range of transmission, much lower oxygen productivity of the plants, lower plant density, lesser homogeneity of the prairie, and acoustically harder rocky substratum. The experiment was conducted in September 1999 over a small Posidonia bed off the island of Ustica. Time series of calibrated measurements of acoustic transmission were obtained during 4 days using broadband chirp © 2004 by CRC Press LLC soundings, require considerable time and/or costly equipment; see, e.g., References 5 through 9. Acoustic Remote Sensing of Photosynthetic Activity in Seagrass Beds 67 signals emitted repeatedly from an underwater sound source and received on a pair of hydrophones, Þxed near the sea bottom. Contemporaneous depth proÞles and time series of dissolved oxygen and temperature in the water column were obtained with an oceanographic probe. Statistical analyses of the time-varying, medium impulse response allow to resolve marked changes in the propagation characteris- tics. Photosynthesis is seen to cause excess attenuation of multipaths, faster decay of reverberation, and lower level of ambient noise. There is a strong correlation with the release of oxygen in the water column measured with the dissolved oxygen probe. As for the Þrst experiment, the diurnal variations are ascribed in part to undissolved gases present on the leaf blades and at the roots during phases of photosynthesis cycle. The Posidonia plants form a water layer where the gas void fraction varies with the time of day. The photosynthesis-driven, absorptive, scattering, and dispersive bubble layer, with a sound speed lower than bubble-free water, modiÞes the interaction process of waterborne acoustic energy with the substra- tum of volcanic basalt. Multipaths with intermediate grazing angles are shown to be the most sensitive to photosynthesis. Section 5.2 brießy reviews the morphological features of Posidonia that are relevant to bubble acoustics. Section 5.3 describes the USTICA 99 experiment and data processing. In Section 5.4, the acoustical and environmental measurements are analyzed in detail. Section 5.5 focuses on the effects of photosynthesis on acoustic propagation including multipaths, reverberation, and ambient noise and provides an interpretation of the observed acoustic variations in terms of the gas transport in the seagrass. of the two experiments are compared. In the conÞnes of a single chapter we Þnd it necessary to omit or pass quickly over certain notions of ocean acoustics and signal theory. Interested readers are referred to the referenced textbooks. 5.2 Inßuence of Photosynthesis on Acoustics In coastal waters, the gas content in dissolved and bubble forms is determined by air–sea ßux and speciÞc environmental and biomass conditions including photosynthesis of aquatic plants, life processes of animals, and decomposition of organic materials. 5.2.1 Bubbles in Seawater It is well established that the presence of gas bubbles in seawater inßuences sound propagation in a way that depends on the resonant frequency of the bubbles. 13,14 In coastal waters, the presence of bubbles of many sizes, each with (sound) scattering and absorption cross sections, * cause frequency-dependent scatter, attenuation, and dispersion. Bubble radii are in the range 10 to 500 mm with a peak density typically somewhere in the range 10 to 15 mm. Recently published observations, using laser holography near the ocean surface, have shown that the densities of 10 to 15 mm radius bubbles can be as high as 10 6 m –3 mm –1 increment within 3 m of the surface of calm seas. 15 The density and distribution of bubble radius vary with depth, time of day, season, wind, and sea biological processes in the volume and on the seaßoor, which are quite speciÞc to each environment such as photosynthesis considered in this chapter. Typically, bubbles form only a very small percentage, by volume, of the sea in which they occur. Nevertheless, because air, or more generally gas, has a markedly different density and compressibility than seawater, and because of the resonant characteristics of bubbles, the suspended gas content has a profound effect on underwater sound. At frequencies of resonance, gas bubbles pulsate radially in response to a signal frequency dependent on bubble radius. For a spherical air bubble in water a simpliÞed ** expression of the resonant (breathing) frequency is as follows: * Ratio of the scattered power, referred to a unit distance, to the intensity incident on a unit area (or unit volume). ** i.e., no surface tension, adiabatic gas oscillation and no energy absorption. © 2004 by CRC Press LLC state (see, e.g., References 16 and 17). The bubble population is sensitive to the physical, chemical, and Section 5.6 concludes the chapter. In Appendix 5.A, the acoustic parameters and environmental conditions 68 Handbook of Scaling Methods in Aquatic Ecology: Measurement, Analysis, Simulation (5.1) where a is the bubble radius in mm, z is the depth in m, and k is the wavenumber. * For example, a bubble of radius 100 mm near the sea surface resonates at a frequency of ª32.5 kHz. The extinction (scattering plus absorption) cross section has a maximum at the resonant frequency and falls off with frequency away from the resonance. Well below the resonance the cross section increases as f 4 . Bubbles of near- resonant size extract a large amount of energy from the incident sound wave through scattering in all directions and conversion to heat. Also, in the vicinity of resonance, large changes in sound speed take place. Hence, over the range of resonance frequencies the medium is highly attenuative and dispersive. 18 By contrast, at high frequencies well beyond the resonant frequency of the smallest bubble present in the mixture, the effect of suspended gas content is negligible. At frequencies below resonance, the mixture of bubbles increases the compressibility of the water medium thereby reducing the sound speed below that obtained from pressure, temperature, and salinity measurements alone. ** When gas is dissolved FIGURE 5.1 Posidonia oceanica leaves. (A) Adult and intermediate leaves covered by epiphytes and encrustation. (B) Juvenile leaves and rhizome. (Underwater photographs taken during USTICA 99 experiments). FIGURE 5.2 Photosynthesis apparatus of P. oceanica. Leaf blade cross sections. (A, B) Monolayered epidermidis and mesophyll with large cells and small intercellular spaces (¥320 and ¥200). PC: phenolic cell. (C) Detail of the porous region under the cuticle (¥1100). (From P. Colombo, N. Rascio, and F. Cinelli, Posidonia oceanica (L.) Delile: a structural study of the photosynthetic apparatus, Mar. Ecol., 4(2), 133–145, 1983. With permission.) * k = 2p/l [radians/m] where l [m] is the wavelength. The wavenumber and angular frequency w = 2pf [radians/s] are related through the equation k = w/c where c [m/s] is the speed of sound. ** Sound speed is related to density and compressibility and, in the ocean, density is related to static pressure, salinity, and temperature. Sound speed is an increasing function of temperature, salinity, and pressure, with the latter a function of depth. It is customary to express sound speed c as an empirical function of three independent variables: temperature T in °C, salinity S in parts per thousand, and depth z in m. A simpliÞed expression for this dependence is c = 1449.2 + 4.6T – 0.055T 2 + 0.00029T 3 + (1.34 – 0.01T)(S – 35) + 0.016z. 19 A BC f a zka r ª ¥ + () << 325 10 101 1 6 12 . . for © 2004 by CRC Press LLC Acoustic Remote Sensing of Photosynthetic Activity in Seagrass Beds 69 in water the effect on sound speed is completely negligible, even when the water is completely saturated with gas. 20 5.2.2 Posidonia Photosynthetic Apparatus Posidonia oceanica (L.) Delile is an endemic phanerogam of the Mediterranean Sea. Its long ribbon- shaped leaves are grouped in shoots, which develop on various substrates in 1 to 50 m water depths medium. The leaf blade consists of a monolayered epidermis and a three- to four-layered mesophyll 21 The major site of photosynthesis is the epidermis where chloroplasts are densely arranged in small radially elongated cells. The outer wall of epidermidal cells is formed by an outer continuous layer (cuticle) and an underlying much thicker ( ª20 mm) porous region with irregularly shaped cavities. The lacunar system is constituted of connected air channels within the mesophyll. The particularly small dimensions of the lacunar system is a distinctive feature of P. oceanica. Photosynthesis is the major driving force for exchange of gases among seawater, the epidermal cells, the lacunar system. Unlike other aquatic plants, gaseous exchanges with seawater are effected by molecular diffusion as there are no stomata. The processes of oxygen uptake for respiration and release of photosynthetic oxygen are constrained by the diffusion boundary (unstirred) layer, and to a lesser extent, by the cuticle and cell wall. 5.2.3 Oxygen Production Photosynthesis by seagrass substantially increases the quantity of oxygen in dissolved and bubble forms in the water column. For Posidonia, a productivity of 5 to 10 g of Þxed carbon m –2 day –1 was reported. 22 Values of up to 14 l m –2 day –1 of produced oxygen have been reported for prairies of the Tuscan Arcipelago. 23 Specialized surveys showed that the time variation of oxygen concentration in the water column was determined principally by the daily cycle of oxygen productivity, with depth and seasonal dependence, including the possible occurrence of supersaturation conditions below sea surface. 11,24 5.2.4 Gas in Matte and Sediment The Posidonia matte is formed by the intertwining of various strata of rhyzomes, roots, and trapped sediments. 25 Typically, the sediments are made of poorly sorted sands, primarily organogenous. The geoacoustic properties of the matte are virtually unknown. Attenuation is known to be high as acoustic energy of a boomer hardly penetrates the matte layer owing to scattering and absorption. Sound speed is expected to be low due to the uneven nature of water-saturated loose sediments and to the presence of slow materials (rhyzomes and roots) and gas from the decomposition of organic material. For signal frequencies below the bubble resonances the bulk material properties of the matte is expected to dominate its mechanical behavior, producing an acoustic response equivalent to a monophasic material of low sound speed. Comparable conditions are encountered with soft porous sediments with high gas 5.3 The USTICA 99 Experiment 5.3.1 Test Site The experiment was conducted over a Posidonia bed off the island of Ustica in September 1999 of 65 km from Palermo (13°10 ¢ E, 38°42¢ N). It represents the relict of a vast submarine volcanic system of the Pleistocene age, which emerged 2000 m above the seabottom. 27,28 The island is characterized by © 2004 by CRC Press LLC (Figure 5.1). Leaf morphology allows for maximum release of photosynthetic oxygen to the ambient and the lacunar system. Respiratory activity is nearly an order of magnitude lower and largely involves (Figure 5.2). The blade width and thickness are respectively ª1 cm and ª180 mm. (Figure 5.3A). The island lies in the southern Tyrrhenian Sea, off the northern coast of Sicily, at a distance content (see, e.g., Reference 26). 70 Handbook of Scaling Methods in Aquatic Ecology: Measurement, Analysis, Simulation pillow-shaped outcrops of lava emerging from the sea surface. It has an area of 8 km 2 , a coastal perimeter of 12 km, and a summit elevation of 248 m. The coast is irregular and fretted, forming little inlets like the one where the experiment was conducted (Figure 5.3B). The island is surrounded by notably clear waters, which are subject to intense renewal, and seabeds abundant with marine ßora and fauna in an ecosystem still practically intact and now protected. * The seabed is settled by benthic communities typical of hard substrata. Marine vegetation includes surface FIGURE 5.3 Test site. (A) Geological marine map of Ustica island showing the location of the investigated Posidonia bed. (B) Sediments, biocenosis, and stratigraphy at the test site. The thick black line shows the position of the acoustic transect. S: source. R: receivers. (Adapted from Reference 48.) * Since 1986, a marine reserve has been established, covering an area of 3 miles from the coast. © 2004 by CRC Press LLC Acoustic Remote Sensing of Photosynthetic Activity in Seagrass Beds 71 formations, hard calcareous algae, and various species of the seaweed Cystoseira distributed over the water depths 0 to 35 m. The most euphotic sandy and subhorizontal bottoms are carpeted by the seagrass P. oceanica (0 to 30 m). The deep rocky seabed, which is washed by intense currents, is capped with dense oceanic settlements of Laminaria rodriguezi (50 to 70 m). The marine fauna is very rich and can be deemed as representative of the Central Mediterranean basin, with a notable host of subtropical forms. The richness of the encrusting biocoenoses is the most noticeable feature of the island’s seascape. 5.3.2 Experimental Configuration A sound source (S) and a pair of receivers (R1, R2) were deployed on the seaßoor (Figure 5.4). The positions were chosen to minimize bathymetric variations between S and R and the acoustical effects of nearby rock scatterers and coastal reßectors. The S–R horizontal distance was R = 53 m and the water depth, d, in the vertical section varied in the range 8 to 8.8 m. The source was a broadband piezoelectric transducer mounted in a ballasted tower and positioned at a height H S = 1.55 m above the seaßoor (Figure 5.5). The monopole-like source has a frequency range FIGURE 5.4 Experimental conÞguration for the acoustic remote sensing of undissolved oxygen produced by Posidonia photosynthesis. The positions of the underwater sound source (S) and hydrophones (R1, R2) are indicated. Eigenray diagram: The lines are the acoustic rays joining S and R1. Ray groups 1 through 10 are displayed. The black lines are the early arrivals of groups 1 and 2. The thin black line is one of the four paths belonging to group 10: nine reßections at each of the boundaries. S: surface; B: bottom; R: reßected; M: multiple. Horizontal scale 1:400. Vertical scale 1:200. FIGURE 5.5 Acoustic instrumentation deployed on the seagrass bed. (A) Sound-source tower, rear view. (B) Two-hydrophone vertical pole, front view. AB © 2004 by CRC Press LLC 72 Handbook of Scaling Methods in Aquatic Ecology: Measurement, Analysis, Simulation of 200 Hz to 20 kHz and is omnidirectional up to 2 kHz. It was cable-connected through an impedance transformer to a power driving ampliÞer and signal generator in the laboratory, located on shore, several hundred meters uphill. The receivers were two calibrated hydrophones mounted on a rigid pole and decoupled mechanically. A hydrophone (R1) was positioned within the Posidonia leaf layer and the other (R2) in the water layer, at respective heights of H R1 = 0.3 m and H R2 = 1.7 m above the seaßoor. The hydrophone signals were ampliÞed and bandpass-Þltered with a high-pass RC Þlter and third-order Bessel Þlter f –3dB = 500 Hz and a low-pass eight-order linear phase Þlter f –3dB = 16.7 kHz. The signals, carried by analog symmetric lines, were recorded by a portable data acquisition unit. The acoustic instrumentation was deployed by two divers with the support of a local Þshing boat and a work boat. 5.3.3 Acoustic Measurements 5.3.3.1 Signal Transmission — The coded signal, s(t), transmitted to measure the band-limited impulse response of the acoustic channel, g(t), consisted of a low-power, long duration, linearly frequency modulated (LFM) waveform: (5.2) where f 0 = 8.1 kHz, Df = 15.8 kHz, and Dt = 15.8 s (5.3) Re stands for real part, rect is the rectangular function, f 0 is the carrier frequency, Df is the bandwidth, and Dt is the duration. The frequency range is from f 1 = 200 Hz to f 2 = 16 kHz. Pulse compression was achieved through the use of a correlation receiver or matched Þlter (MF) whose impulse response is the same as the waveform of the signal emitted by the source, reversed in time. The large time-bandwidth product, DtDf ª 2.5·10 5 , permitted to resolve closely spaced multipath arrivals with a sufÞcient ratio of peak to ambient noise in spite of the limited power of the sound source, 180 dB mPa –1 re 1 m at resonance (940 Hz). The pulse repetition rate was Þxed at 1 ppm to obtain sufÞcient statistics in sampling the physical and biological processes over the timescales of interest. About 3 · 10 3 probe signals were trans- mitted over a 4-day period. The reader is referred to the original paper 29 for a conceptual description of the coded signal and its matched Þlter, to, e.g., References 30 through 32 for related theory of signal detection and estimation and optimum Þltering, and to, e.g., References 33 and 34 for aspects of digital signal processing that are relevant to this chapter. Further details are found in References 4, 35, and 36 that deal speciÞcally with the application of broadband, LFM-coded signals and MF receivers to inverse problems including the geoacoustic characterization of Þne-grained sediments in shallow water. 5.3.3.2 Ambient Noise Recording — Physical and biological sounds were recorded during the “silent” intervals of the acoustic transmissions. 5.3.3.3 Transducer Calibration — The transducers and electronics of the S and R chains were calibrated in situ after the experiments. A pole-mounted hydrophone was repositioned on the source axis at a distance R 0 = 1.93 m and the probe signal was retransmitted. requirement for precise measurements of the forward acoustic propagation. The Þrst bottom and surface bounces are recognized at time delays t = 1 ms and t = 7 ms. The surface-reßected signal displayed variability due to surface motion. The ª20-dB attenuation is somewhat larger than the spherical spreading loss calculated from the calibration geometry, i.e., 20log(R/R 0 ) = 16.4 dB where R = 13.05 m, due to the frequency-dependent source directivity and surface scattering loss. The bottom-reßected signal was strongly attenuated (>>5.4 dB geometrical loss) since the grazing angle q = 58° was beyond the expected critical angle of the basalt interface st t t j ft t j ft () = () () () [] Re exp exprect DDDpp 2 0 2 © 2004 by CRC Press LLC Figure 5.6 shows that the transmitted waveform was perfectly reproducible, which was an important Acoustic Remote Sensing of Photosynthetic Activity in Seagrass Beds 73 and there was a two-way, excess attenuation due to photosynthesis in the intervening seagrass layer as discussed subsequently. 5.3.3.4 Equalized Matched-Filter Processing — was used to design the reference signal of an MF receiver that compensated for amplitude and phase distortion of the source. 1. Hanning windowing was applied to the MF waveform to reduce the local bottom and surface echoes. 2. The transmitting sensitivity response, measured on the radiation axis (Figure 5.7B) was equalized for ßat spectrum. An inverse, Þnite impulse response (IFIR) Þlter was designed on the basis of a frequency-decimated version of the source spectrum magnitude. 3. The source waveform was convolved with the IFIR Þlter, which, in the frequency domain, is equivalent to multiplying by the IFIR squared magnitude with zero-phase distortion. 4. The resulting reference signal, time reversed, was convolved with the received signals. In Figure 5.7C, raw and equalized MF (EMF) outputs are compared for one realization of the received signal. The Þrst multipath arrivals, which were not identiÞed in the MF output, were perfectly resolved in the EMF output, recovering the time resolution limit, 1/Df = 63 ms. The EMF output represents the convolution of the transmitted autocorrelation function (sinc function) with the actual impulse response of the medium. For the encountered conditions of limited source peak-power and high-level background noise, the achieved processing gain allowed estimation of (the coherent part of) the medium response as if the source transmitted an ideal high-energy pulse. 5.3.4 Oceanographic Measurements: CTD and Dissolved Oxygen Content multiparameter, oceanographic probe Idromar IM51-201. Depth proÞles and time series were alternated to obtain vertical and temporal sampling of the water column. For most time series, the probe sensors concentration at the sea surface and seaßoor. The conductivity (salinity) was nearly homogeneous over the whole water column with mild time variability, S = 37.8 – 38 ppt. The probe was deployed at a short FIGURE 5.6 the stability of the transmission. The hydrophone was placed at an axial distance R 0 = 1.93 m which largely satisÞed the far Þeld condition: R 0 > pa 2 /l 2 = 0.46 m, where a = 0.12 m is the radius of the circular piston source, c = 1538.5 m/s is the sound speed near the bottom and l 2 = c/f 2 = 9.6 cm is the shortest transmitted wavelength. The dotted lines indicate the delays of the Þrst bottom and surface echoes, calculated from the geometry. 0 1 5 7 10 15 –1 0 1 Time (ms) Amplitude 43 Time series overlaid © 2004 by CRC Press LLC Matched, Þltered pressure signal measured in front of the source. The overlaid, 43 signal realizations show The emitted pressure waveform (Figure 5.7A) were positioned just above the Posidonia leaves. Figure 5.8 shows the depth proÞles of temperature and Physical and chemical conditions of seawater were monitored during the acoustic transmissions with a oxygen concentration at different times of day. Figure 5.9 shows time series of temperature and oxygen 74 Handbook of Scaling Methods in Aquatic Ecology: Measurement, Analysis, Simulation FIGURE 5.7 Equalized MF processing. (A) Raw MF transmitted signal (gray line) and Hanning (dashed) windowed version (black). (B) Spectrum magnitude in dB (thick, right scale); ideal (thin, left scale), and designed (dashed) inverse Þlter response. (C) Comparison of raw (gray line) and equalized (black) MF signal received on hydrophone R1. D, SR, and BR stand for direct, surface- and bottom-reßected paths, respectively. FIGURE 5.8 Depth proÞles of (A) temperature and (B) oxygen concentration at different times of day. Gray circles: raw data; solid lines: smoothed data; dotted lines: references at T = 25.5°C and C O = 6 mg/l for visual appraisal of the depth and time variations. The arrows indicate the direction of oxygen variation near the bottom (time-series part of the proÞles). 0 5 10 15 –1 0 1 Time (ms) Amplitude 10 2 10 3 10 4 –70 –60 –50 –40 –30 –20 –10 0 Frequency (Hz) Magnitude (dB) 0 1 Magnitude –1 0 1 2 3 4 5 –1 0 1 Time (ms) Amplitude D BR SR AB C 08:04 08:37 09:04 09:37 09:58 10:24 15:44 16:15 17:37 18:11 19:16 19:49 20:12 20:51 21:07 21:37 06:18 06:49 10:06 10:34 12:20 12:38 12:39 13:09 15:08 15:32 17:48 18:33 19:41 20:18 0 5 10 06:58 07:34 D (m) 25 25.5 26 20:34 20:41 T ( 5.5 6 6.5 O 2 (mg/l) 0 5 10 D (m) Sept. 23, 1999 Sept. 24, 1999 A B © 2004 by CRC Press LLC [...]... were larger © 20 04 by CRC Press LLC 90 Handbook of Scaling Methods in Aquatic Ecology: Measurement, Analysis, Simulation 20 18 16 14 12 10 Frequency (kHz) 8 6 4 2 A 3 2. 5 2 1.5 1 0.5 0 2 4 6 8 10 Time (s) 12 14 16 18 B FIGURE 5 .21 Spectrogram of ambient noise showing the transmitted chirp signal, biological transients, and breakingwave events (A) Normalized energy spectral density gray-coded in the range... technical document, Department of Ocean Engineering, Massachusetts Institute of Technology, Cambridge, MA, 1999 40 R.J Urick, Principles of Underwater Sound McGraw-Hill, New York, 1983 © 20 04 by CRC Press LLC 96 Handbook of Scaling Methods in Aquatic Ecology: Measurement, Analysis, Simulation 41 L Cohen, Time-Frequency Analysis Prentice-Hall, Englewood Cliffs, NJ, 1995 42 A Wood, A Textbook of Sound Macmillan,... geotime Magnitude of the analytical signal calculated by Hilbert transform *** More precisely, the coherent component of the coded-signal energy transmitted through the medium ** © 20 04 by CRC Press LLC 76 Handbook of Scaling Methods in Aquatic Ecology: Measurement, Analysis, Simulation 18 00 23 Sept 1999 06 12 Geotime (h) 18 00 24 Sept 1999 06 12 18 00 25 Sept 1999 06 12 –0.5 0 0.5 1 1.5 2 2.5 3 3.5 Time... Scaling Methods in Aquatic Ecology: Measurement, Analysis, Simulation 1 Energy (dB) 2 1 Energy (dB) 2 0 –1 0 –1 2 2 –3 –3 25 .2 25.4 25 .6 25 .8 26 Temperature (°C) [water column] A 5.4 5.6 5.8 6 6 .2 O2 (mg/l) [bottom] 6.4 B FIGURE 5.19 Acoustic vs environmental data The acoustic data are the energy propagated along intermediate grazingangle paths (group 7) shown in Figure 5.18A (A) Energy vs temperature in. .. measurements performed with an infrared gas analyzer 97 © 20 04 by CRC Press LLC 98 Handbook of Scaling Methods in Aquatic Ecology: Measurement, Analysis, Simulation FIGURE 6.1 (A) The whole system during a light incubation in the Seine estuary (B) Portable container with circuit of CO2 analysis (a pump, a drying column, a ßowmeter, an infrared gas analyzer) and data logger In this study, spatial variability... 300 25 0 05:16 06 :28 07:40 08: 52 Universal Time 10:04 11:16 FIGURE 6.3 Example of variations of CO2 concentration during successive light incubations under different irradiances from dawn to zenith; the last incubation is a dark one (Seine estuary, sandy mud, August 20 01) © 20 04 by CRC Press LLC Handbook of Scaling Methods in Aquatic Ecology: Measurement, Analysis, Simulation P (mgC m 2 h–1 ) 100 P =... exist in bubble-free water where sound speed depends only on temperature, salinity, and pressure © 20 04 by CRC Press LLC 86 Handbook of Scaling Methods in Aquatic Ecology: Measurement, Analysis, Simulation FIGURE 5.17 (Color Þgure follows p 3 32. ) Time distribution of multipath energy vs geotime The contour lines are from –4 dB to +1 dB in 1-dB steps The levels are referenced to the three-night median-average... steel, which is pushed into the substrate to a depth of 10 cm, enclosing a volume of 24 . 92 l and a surface area of 0. 126 m2 A pump (Brailsford and Co., TD-2SA) maintained an airßow of about 2 l min–1 through the closed circuit Variations of CO2 concentration were measured with an infrared gas analyzer (LiCor Li- 625 1) and PAR (photosynthetically active radiations, 400 to 700 nm) inside the chamber was...Acoustic Remote Sensing of Photosynthetic Activity in Seagrass Beds 75 26 .2 Surface T (°C) 26 25 .8 25 .6 25 .4 25 .2 Bottom 18 00 06 12 18 00 06 12 18 00 06 12 A –3 Acoustic O2 Probe 2 –1 6 0 Energy (dB) O2 (mg/l) 6.5 +1 Bottom +2 5.5 18 00 06 12 18 00 06 12 Geotime (h) 18 00 06 12 B FIGURE 5.9 Time series of (A) temperature and (B) oxygen concentration The small... scattering, increasing with frequency and extending to lower and higher grazing angles Figure 5. 12 shows the 4-day average of the medium impulse response measured between S and R1 The logarithm of the envelope is displayed because of the large dynamic range The leading part of the response * Ray-based models lack accuracy in predicting the low-frequency part of the propagation because of the inherent . (time -of- ßights). f mc dcc m w wb 0 2 12 05 21 = - - [] (.) (/) / |()|gt 2 egtt t = Ú () 2 d d |()|gt 2 Egtt= Ú () 2 d m t ttgtt== () Ú 2 d sm tt tgtttt 22 2 2 2 =- =- Ú ()()d · Ò t 2 ·Òt © 20 04. coherent component of the coded-signal energy transmitted through the medium. 18 00 06 12 18 00 06 12 18 00 06 12 25 .2 25.4 25 .6 25 .8 26 26 .2 T (° C) 18 00 06 12 18 00 06 12 18 00 06 12 5.5 6 6.5 Geotime. spreading. The matte and phase shift curves are independent of frequency. Referring to Figure 5.4, energy that propagates 78 Handbook of Scaling Methods in Aquatic Ecology: Measurement, Analysis, Simulation FIGURE