Diurnal signals in vertical motions on the Hebridean Shelf

Transcription

1 Diurnal signals in vertical motions on the Hebridean Shelf Tom P. Rippeth and John H. Simpson School of Ocean Sciences, University of Wales Bangor, Menai Bridge, Anglesey, LL59 5EY, United Kingdom Abstract In this paper, we consider the interpretation of measurements of vertical velocity and backscatter signal intensity, made using an acoustic doppler current profiler (ADCP) mounted on the seabed, at a position on the Hebridean Continental Shelf. The existence of vertically migrating scatterers is inferred from both the backscatter signal intensity and vertical velocity data, which indicate migration rates of 2-3 cm s-l, and shows consistency between the displacement deduced from the vertical velocity and the observed movement of scatterers from the near bed to the near-surface regions, evident in temporal variations in the backscatter signal intensity. Independent evidence that the observed vertical velocities are largely due to movements of the acoustic scatterers relative to the water is obtained through comparison of the ADCP vertical velocity data with vertical velocities inferred from the movement of the thermocline. The close phase locking between the motion and sunset and sunrise times has led us to interpret the signal in terms of the diel vertical migration of zooplankton. Although no biological samples were collected during this study, previous surveys of the area have identified species of copepod and euphausiid that are known to migrate. The 12 d of data available show an initial period, when there was a strong vertical migration signal, and a later period, during which the signal was not clear. By combining the observed vertical and horizontal velocity components in a particle tracking model, it is demonstrated that the change in the migration signal may be a result of the advection of the patch of migrating zooplankton away from the ADCP, during the later part of the observational period. The use of ADCPs has become central to the study of biological and physical oceanography. Acoustics has long provided biological data for marine scientists (for example, McCartney 1976). Greenlaw (1979) describes the relationship between acoustic backscatter and the size and abundance of zooplankton. Flagg and Smith (1989) and Heywood et al. (1991) correlated the intensity of the acoustic backscatter from a shipborne ADCP, with zooplankton biomass inferred from net samples, and demonstrated a relationship between backscatter and biomass. Acoustic backscatter records from ADCPs have been used to characterize diel migration patterns (Plueddemann and Pinkel 1989; Roe and Griffiths 1993) and patchy distribution of zooplankton in the deep ocean (Roe and Griffiths 1993). Plueddemann and Pinkel (1989) presented ADCP measurements of acoustic backscatter intensity and vertical velocity, both of which provided convincing evidence of diel vertical migration of the sound scattering layers that continued over many days, although they had no supporting biological data. Buchholz et al. (1995) studied the environmental influences on diel migration of the euphausiid, Meganyctiphanes norvegica, using an ADCP and net samples. Relating acoustic backscatter from the ADCP to biological communities is, however, nonspecific as the ADCP is single frequency. Various multifrequency techniques are described in the literature, which relate acoustic backscatter intensity to the size Acknowledgments The data presented here were collected as part of the U.K. NERC LOIS-SES community project. The ADCP was deployed and recovered from the RRS Challenger by Alan Harrison and his team from the Proudman Oceanographic Laboratory, and the data were supplied to us by the British Oceanographic Data Center. We also thank Jack Matthews and Geraint Tarling (Scottish Association for Marine Science) and Tony Walne (Plymouth Marine Laboratory) for their assistance in locating the biological data and abundance of plankton (Mitson et al. 1996) and the orientation of euphausiids (Cochrane et al. 1994) and provide acoustic classification of zooplankton (Martin et al. 1996; Stanton et al. 1996). The multifrequency studies illustrate the considerable problems associated with the estimation of biomass from acoustic backscatter measurements and indicate that the single frequency ADCP measurements are only of qualitative use in assessing zooplankton behavior. In this paper, we present observations of vertical velocity and acoustic backscatter intensity, made using an ADCP located on the seabed at a shelf sea location. The observations provide consistent and convincing evidence that the acoustic scatterers were performing diel vertical migration. Independent evidence that the scatterers were moving relative to the water is provided by comparing the vertical velocity measured by the ADCP with that inferred from the movement of the thermocline. Finally, the weakening of the migration signal halfway through the time series is discussed. The most convincing explanation is that the weakening of the signal is a result of the advection of the patch of migrating zooplankton away from the ADCP. Methods The data reported here were obtained from a bottommounted ADCP and thermistor chain deployed on the Hebridean Shelf, at a location 50 km landward of the shelf edge (56 28'N, 08 11'W). The instruments were located in a water depth of 140 m between 12 and 24 July 1996 (days ) as part of the recent U.K. LOIS-SES measurement campaign (LOIS 1992). The thermistor chain collected data. at 10 levels, the uppermost of which was 15 m below the sea surface and the lower of which was 83 m below the sea surface. Radiation data were obtained for the period of the observations from the Meteorological Station at Stornaway, approximately 100 mi. northeast of the mooring position.

2 Diurnal signals 1691 ADCPs use the change in the observed pitch of sound resulting from a relative motion between a source and a receiver to infer a velocity. The RDI Instruments ADCP used here has four transducers, inclined at 30 from the vertical, from which both the vertical and horizontal current components can be obtained. Sound pulses sent out by a transmitter are scattered by particles, which are typically zooplankton having sizes of the order millimeters (Gordon 1996), with some of the sound energy reflected back to the receiver. The Doppler shift is then used to estimate the currents, on the assumption that the scatterers are drifting freely with the flow. By collecting the returned signal into successive time bins, a vertical profile of the current is obtained. In addition to the Doppler shift, we may also utilize the backscattered signal intensity, which is dependent on the strength of the transmitted signal, and the energy lost through radial spreading, attenuation of the signal, and the effective cross-sectional area of the reflecting particles. The radial spreading and attenuation losses are range dependent, but, at a fixed range, variations in the backscattered signal intensity are dependent only on the concentration and scattering efficiency of the scatterers (Plueddemann and Pinkel 1989). The ADCP used in this study was a 150-kHz broadband model configured to sample with a 2-min time ensemble (At) over 5-m depth bins (AZ). The center of the first bin was located 12.5 m above the seabed, and the last good-data bin was located 12.5 m below the sea surface, so that observations covered >80% of the water column. Roe and Griffiths (1993) give an estimate of the error in the observed vertical velocity as 0.01 m s-l. The theoretical precision of the vertical velocity, estimated using the variance lower bound theory given by Theriault (1986), was in this case of the order m s-l. Plueddemann and Pinkel (1989) suggest that this value should be doubled in reality for a shipborne ADCP because of a decrease in coherence between velocity components far from the ADCP and increasing signal-tonoise ratio. Using the ADCP mounted on the seabed, as is the case here, eliminates errors associated with contamination of the vertical velocity signal by horizontal components resulting from the motion of the ship. Acoustic backscatter intensity- Measurements of the intensity of sound scattered from the ocean are generally reported in terms of the volume scattering strength (Urick 1983); (1) where Lcot is the intensity of sound scattered by 1 m3 of water, and Iinc is the incident plane wave intensity. Using the volume scattering strength serves as a normalization of the received intensity level, accounting for attenuation and spreading losses. A profile of Sv is sensitive to changes in the abundance and acoustic cross-section of scatterers as a function of depth. At present, no procedures are available for the absolute calibration of broadband ADCP backscatter intensity observations (Gordon 1996), but they are useful as relative measurements to demonstrate temporal changes in scatterer density at a given depth. For the consideration of temporal changes, it is useful to 0-25 Time - day number Fig. 1. Temperature structure as derived from the thermistor chain deployed at position 56 28'N and 8 11'W on the Hebridean Shelf. normalize the backscatter signal intensity, thereby giving a measure of scattering strength. To achieve this, we follow Plueddemann and Pinkel(l989) and use log intensity anomaly, which is defined as the difference between the log intensity at a given depth and time and the long-term mean log intensity profile (taken over the full time series). If the instantaneous returned intensity from depth z at time t is An(z, t), then the log intensity anomaly is given by Eq. 2: where, and j,, is the mean of Z,,(z, t) over the full sampling interval. For discrete data sampled at time intervals At and depth bins of thickness AZ: Although I,; is relative to an arbitrary reference, it is of value as a comparative measure of the deviation from the longterm mean. Results During the period of the observations, the water column was stability stratified with warm surface layer water ( C) overlying cooler (< 10.5 C) deep water. The difference in salinity between the two layers was small (<0.2), implying that the density structure was largely determined by the temperature field at that time. The thermistor chain data (Fig. 1) indicate vertical displacement of the isotherms by m, with an approximately semidiurnal periodicity, indicative of the presence of an internal tide. The irradiance data showed days 195 and 196 were slightly overcast, with a midday maximum of 600 W m-2, and days were clear, with a midday irradiance maxima of 820 W m-2. During the remainder of the observational period, the weather (2)

3 1692 Rippeth and Simpson Fig. 2. Vertical velocity estimated from the movement of isotherms at a depth of 37.5 m (solid line) and the vertical velocity observed at 37.5 m using the ADCP (dashed line). was overcast, with midday irradiance levels between 200 and 400 W m-2. The vertical velocity was estimated from the observed vertical gradient (X/az) and temporal derivative (Xlat) of temperature in the thermocline region using Eq. 3: (3) The vertical velocities estimated using Eq. 3 for a depth of 37.5 m (the region where the vertical temperature gradient was strongest), together with the vertical velocity observed at that level using the ADCP, are given in Fig. 2. Generally, both the observed and estimated vertical velocities were <0.005 m s-1. There were, however, major discrepancies between the observed and estimated vertical velocities in the form of diurnal pulses in the observed velocity of up to 0.02 m s-l. The difference between the estimated and observed vertical velocities suggests that during the diurnal pulses, the scatterer motion is independent of the motion of the water. Further evidence to support this statement was found in the fact that the diurnal pulses in the vertical velocity were not correlated with the observed horizontal velocity components. The vertical velocity observed by the ADCP at three levels (near surface, mid-water column, and near bed) are given in Fig. 3. The vertical velocities at 27.5 m below the surface reached 0.01 m s-l (Fig. 3a). The dominant signal until day 200 was a diurnal variation, with an upward velocity of 0.01 m s-1, 2 h before midnight, and a downward velocity of 0.01 m s-l, 3.5 h after midnight. During the day, the vertical velocities were much smaller. After day 201, the observed vertical velocity was generally <0.003 m s-l. At 77.5 m (Fig. 3b), the diurnal pulsing around midnight was more prominent (velocities were >0.02 m s-l on several days), occurring approximately 2.3 h before midnight, and the maximum downward velocities occurring 3.5 h after midnight, during the first 5 d of the time series. For the rest of each of these days and for the remainder of the time series, the vertical velocities were similar to those observed nearer to the surface. In the time series taken at m (Fig. 3c), the diurnal pulsing was only distinguishable during the first 3 d of measurements. It is weak in comparison with the currents observed at the other levels (<0.01 m s-l), and the Fig. 3. Vertical velocity observed using the ADCP at: (a) a depth of 27.5 m, (b) a depth of 77.5 m, and (c) a depth of pulses occur sooner in the evening (2.5 h before midnight) and later in the early morning (4 h after midnight). For the remainder of the time series, the vertical velocities were small. The pulses of maximum vertical velocity were therefore observed to occur progressively later in the evening, the higher up the water column one moved, and progressively later in the morning, the further down the water column one moved. The implication is that the scatterers were moving up the water column in the evening and back down the water column in the early hours of the following morning. The vertical velocities measured at 77.5 m are plotted together with the times of sunset and sunrise (at this position and time of the year, the sun sets at about 2200 h Greenwich Mean Time [GMT] and rises at 0420 h GMT) in Fig. 4. We see that the large diurnal velocity signal is strongly phase locked to solar time, with the largest upward velocities observed around sunset and the largest sinking velocities around sunrise. The evidence would therefore suggest that the large vertical velocities observed during the first half of the ADCP deployment were a consequence of the scatterers performing diel vertical migration.

4 Diurnal.riynn/s 1693 Fig. 4. Venial velocity meawred at a depth of 77.5 m and local sunset and sunrise times (the dashed line indicates the period between sunset and wnrisc). Fig. 5. Composite plot of the particle track (broken line), calculated from the observed vertical velocity, plotted together with COIICOU~F of the log (intensity anomaly) (The units are arbitraryi The contour plot of log intensity anomaly (Fig. 5) indicates a strong diurnal variation in backscatter intensity at most depths. This is particularly apparent during the first 5 d of the ADCP deployment. At depths >loo m, there was a strong positive anomaly during the day, with a negative anomaly between 25 and 50 m. At night, there was a negative anomaly lower in the water column, with a positive anomaly nearer the surface. After day 200, the anomaly became more negative throughout the water column, indicating a decrease in the number of scattering particles. To demonstrate consistency between the observed vertical velocity and backscatter intensity, a particle tracking model was set up. in which a particle was assumed to move up and down the water column with the observed vertical velocities (starting from the seabed at the beginning of the time series). The resultant path is shown as a dashed line in Fig. 5. The consistency between the vertical velocity and backscatter intensity provides further evidence that we have observed die1 vertical migration of the acoustic scatterers. It is of interest to note that the depth of migration was largest (about 100 m) during the first few days of the observations, when IIIgration rates were fastest (>0.02 m s-l), with smaller migration depths (about 50 m) toward the end of the time series, when migration rates were comparable to the inferred vertical velocity (Fig. 2). To check for conservation of scatterer density at a particular depth, the daily mean backscatter intensities have been calculated (Fig. 6). Two distinctive groups of profiles are evident, those for days prior to day 201 and those for days following day 201. The consistency between the profiles for the two periods suggests that the number of particles at each depth was conserved during each period. There was a significant fall in the daily mean intensities for the days following 201 at depths <30 m and >80 m. implying a fall in the number of scatterers at these depths. There was, however, little change in the middle of the water column (between 10 and 80 III). f Log (daily mean intensity) Fig. 6. Daily mean backscatter intensity protiles (the unils are arbitrary). The sohd line indicates the mean profiles lor each day before day 201, and the broken line mdxates the mean profile- fo each dav following day 201

5 1694 Rippeth and Simpson Discussion We have demonstrated that the vertical velocities observed at this continental shelf site, using a moored ADCP, were considerably larger than those estimated from the movement of the thermocline. The vertical movement of scatterers has been inferred from variations in backscatter signal intensity at different depths. Comparison of the predicted vertical track of a particle, assumed to be moving at the observed vertical velocities, with the backscatter signal intensity, indicates consistency between the observed vertical velocities and the inferred movement of the scatterers. This consistency, together with the precise phase locking of the migration to solar time, has led us to conclude that, up until day 201, a significant proportion of the scatterers were performing diel migration. The observed migration rate was about 0.02 m s-l, which is in agreement with estimates made from echo sounder measurements (McCartney 1976), other ADCP measurements (Plueddemann and Pinkel 1989; Roe and Griffiths 1993), and with speeds inferred from the results of zooplankton trawls (Enright 1977; Hays 1996). It should be noted, however, that the vertical velocity estimated for a particular depth bin, by the ADCP is the mean value for most of the scatterer population in that bin, including nonmigrating scatterers but excluding any individuals with migrations so large (>5-6 cm s-l) they did not lie within the spectral window for the analysis. Given the likely presence of some nonmigrating scatterers, we would expect the actual rate of vertical movement of individual migrating zooplankton to be slightly larger than that observed. Although no biological measurements were made at the time of the mooring deployment, previous surveys of the area indicate that the most likely acoustic scatterers of sufficient size to be detected by a 150 khz ADCP are copepods and euphausiids (OLE 1973). The Hebridean shelf is dominated by relatively small copepods, and observations have suggested that only a small proportion of the community (10-24%) perform diel migration (Hays 1996). The dominant species of copepods at this shelf sea location in the summer were Calanus finmarchicus, Pleuromamma robusta, Metridia lucens, and Euchaeta novegica (OLE 1973), all of which are known to be capable of migrating over several hundreds of meters and attaining vertical velocities approaching 0.01 m s-1 (Longhurst and Williams 1979; Roe 1984; Hays 1995a). The main species of euphausiids previously identified were M. norvegica, Thysanoessa longicaudata, Nyctiphanes couchi, and Euphausia krohni (OLE 1973). M. norvegica has been observed, by means of an ADCP, to migrate from depths of 150 m to the surface at speeds of m s-l (Buchholz et al. 1995), while Frank and Widder (1997) report observing M. norvegica migrating at rates in excess of 0.1 m s-1. It has also been reported that both N. couchi (Williams and Fragopoulu 1985) and E. krohni (Roe et al. 1984) have distinct migration cycles and are able to move vertically with speeds of m s-1, although Williams and Lindley (1982) found no evidence of large-scale migration of T. longicaudata. The copepod species are numerically dominant but are smaller (2-5 mm) than the euphausiid Fig. 7. Predicted horizontal advection of a particle using the observed ADCP velocity field. The particle is released from the origin at the beginning of the time series. l indicates the position at midnight on a particular day (day numbers are marked where important). (a) Track of a vertically migrating particle, calculated using the observed horizontal velocity at the level of the particle. (b) Track of a particle that remains in the surface layer. species (10-20 mm), so it is not clear whether either class will be the dominant scatterer at 150 khz. However, the high vertical migration rates observed would suggest euphausiids are the most likely candidate in this case. During the period of our observations, the migration signal was very pronounced before day 201, after which time the vertical motion was greatly reduced and more variable, thus suggesting that most of the zooplankton were no longer undergoing organized vertical migration. The switch implies either a change in the behavior of the zooplankton scatterers or a change in the composition of the scatterer population to nonmigrating zooplankton or sediment particles. The change in both near-surface and deep-water daily mean backscatter intensities, shown in Fig. 6, indicated a decrease in the total number of scatterers in the water column around day 201. The implication of this result is that a good proportion of the migrating zooplankton had moved away from the ADCP scattering volume. The change in the vertical velocity and backscatter signal intensity could have been in response to some environmental change, which resulted in most of the zooplankton ceasing to migrate and moving vertically out of the ADCP range. Longhurst (1976) suggests that there are many benefits gained by zooplankton performing vertical migration. These

6 Diurnal signals 1695 are related to efficient utilization of resources or to the avoidance of mortality due to predation. Some evidence supports the idea that vertical migration is a method for avoiding visual predation, with the zooplankton residing deeper in the water column, where light levels are low, during the day, and rising to feed in the phytoplankton-rich surface layers at night (Gliwicz 1986; Neill 1990; Hays 199%). A significant change in light conditions might therefore have been responsible for the change in vertical velocity and backscatter signal intensity. It may therefore be more than coincidental that the weather changed from being clear to overcast on day 201. Regulation of zooplankton migration by light appears to be species dependent. Enright and Honeggar (1977) showed the occurrence and timing of migration of the copepod Calanus helgolandicus was not sensitive to a reduction in light level resulting from overcast weather conditions. In contrast, Buchholz et al. (1995) showed that M. norvegica were well dispersed with depth on overcast days and were concentrated into discrete bands on clear days. A similar conclusion was drawn by Roe (1983) following a study of euphausiids in the northeastern Atlantic. Such a change in behavior does not, however, explain the change in the daily mean backscatter on day 201 (Fig. 6). Although the decrease in nearsurface and deep-water daily mean backscatter intensity is consistent with the zooplankton becoming more dispersed with depth, the necessary increase in the daily mean backscatter intensity in the middle of the water column was not observed. The change in light levels do not therefore appear to be responsible for the change observed on day 201. An alternative hypothesis is that diel migration is a mechanism for horizontal transport (Lough and Trites 1989; Hill 1991, 1995). The horizontal transport is achieved through the exploitation of the vertical shear in the flow and is the result of the interaction between migration and the vertically sheared tidal flow. To test this hypothesis, a particle tracking model was set up in which the vertically migrating particle was advected using the horizontal velocities measured by the ADCP. The movement of a migrating particle is shown in Fig. 7a. The track indicates that prior to day 201, the particle stayed within 6 km from the starting position. After day 201, the particle was advected at approximately 10 km d-l to the east. An interpretation of this result is that the interaction between advection and vertical migration resulted in the movement of the main migrating zooplankton patch away from the ADCP, hence explaining both the weakening of the diurnal migration signal in the observed vertical velocity (Fig. 3) and the decrease in the daily mean intensity (Fig. 6) after day 201. If the zooplankton had remained near the surface, it would have stayed within 10 km of the starting position throughout the period of observations (Fig. 7b). However, as a result of the diel migration, the zooplankton was able to take advantage of the deep-water easterly flow. The size of euphausiid patches is generally 30s of kilometers (Sameoto 1983), so the horizontal advection achieved through migration would move a patch of zooplankton into a new crop of phytoplankton every few days. In summary, we have demonstrated how acoustic backscatter intensity and vertical velocity measurements made with an ADCP can be used to provide some quantitative information about vertical migration timing and speed. The usefulness of the simultaneous measurements of the temperature structure in separating out the migration signal from the internal wave motion has also been demonstrated. By combining the migration signal with the observed horizontal velocities, we have provided convincing evidence that the weakening of the migration signal, on day 201, was a result of advection of the migrating patch away from the ADCP. Clearly, future studies should include a focused zooplankton sampling scheme, using both nets and vessel-based acoustics, which will provide details of species, sizes, and spatial distribution and allow an estimate of biomass, along with direct measurements of the local optical irradiance. References BUCHHOLZ, E, C. BUCHHOLZ, J. REPPIN, AND J. FISCHER Diel vertical migration of Meganyctiphanes norvegica in the Kattegat: Comparison of net catches and measurements with acoustic doppler current profilers. Helgol. Meeresunters. 49: COCHRANE, N. A., D. D. SAMEOTO, AND D. J. BELLIVEAU Temporal variability of euphausiid concentrations in a Nova Scotia shelf basin using a bottom-mounted acoustic doppler current profiler. Mar. Ecol. Prog. Ser. 107: ENRIGHT, J. T Diurnal vertical migration: Adaptive significance and timing. Part 1, Selective advantage: A metabolic model. Limnol. Oceanogr. 22: AND W. W. HONEGGAR Diurnal vertical migration: Adaptive significance and timing. Part 2. Test of the model: Details of timing. Limnol. Oceanogr. 22: FLAGG, C. N., AND S. L. SMITH On the use of the acoustic doppler current profiler to measure zooplankton abundance. Deep-Sea Res. 36: FRANK, T. M., AND E. A. WIDDER The correlation of downwelling irradiance and staggered vertical migration patterns of zooplankton in Wilkinson Basin, Gulf of Maine. J. Plankton Res. 5: GLIWICZ, M. Z Predation and the evolution of vertical migration by zooplankton. Nature 320: GORDON, R. L ADCP principles of operation: A practical primer, 2nd ed. for Broad Band ADCPs, RDI Instruments. GREENLAW, C. F Acoustic estimation of zooplankton populations. Limnol. Oceanogr. 24: HAYS, G. 1995a. Diel vertical migration behavior of Calanus hyperboreus at temperate latitudes. Mar. Ecol. Prog. Ser. 127: b. Zooplankton avoidance activity. Nature 376: Large scale patterns of diel migration in the North Atlantic. Deep-Sea Res. 43: HEYWOOD, K. J., S. SCROPE-HOWE, AND E. D. BARTON. 1991, Estimation of zooplankton abundance from ship borne ADCP backscatter. Deep-Sea Res. 38: HILL, A. E A mechanism for horizontal zooplankton transport by vertical migration in tidal currents. Mar. Biol. 111: The kinematic principles governing horizontal transport induced by vertical migration in tidal flows. J. Mar. Biol. Assoc. U.K. 75: [LOIS] LAND OCEAN INTERACTION STUDY Science plan for a community research project. Natural Environment Research Council, LONGHURST, A. R Vertical migration, p In D. H.

7 1696 Rippeth and Simpson Cushing and J. J. Walsh [eds.], The ecology of the seas, Blackwell Scientific. AND R. WILLIAMS Materials for modeling: Vertical distribution of Atlantic zooplankton in summer. J. Plankton Res. 5: LOUGH, R. G., AND R. W. TRITES Chaetognaths and oceanography on Georges Bank. J. Mar. Res. 47: MARTIN, L. V., T. K. STANTON, P. H. WIEBE, AND J. F. LYNCH Acoustic classification of zooplankton. ICES J. Mar. Sci. 53: MCCARTNEY, B. S Comparison of the acoustic and biological sampling of the sonic scattering layers: RRS Discovery SOND cruise, J. Mar. Biol. Assoc. U.K. 56: MITSON, R. B., Y. SIMARD, AND C. Goss Use of a twofrequency algorithm to determine size and abundance of plankton in three widely spaced locations. ICES J. Mar. Sci. 53: NEILL, W. E Induced vertical migration in copepods as a defence against invertebrate predation. Nature 345: [OLE] OCEANOGRAPHIC LABORATORY, EDINBURGH Continuous plankton records: A plankton atlas of the North Atlantic and North Sea. Bull. Mar. Ecol. 7: PLUEDDEMANN, A. J., AND R. PINKEL Characterization of the patterns of diel migration using a doppler sonar. Deep-Sea Res. 36: ROE, H. S. J Vertical distribution of euphausiids and fish in relation to light intensity in the northeastern Atlantic. Mar. Biol. 77: The diel migration and distribution within a mesopelagic community in the northeast Atlantic. 4. The copepods. Prog. Oceanogr. 13: AND G. GRIFFITHS Biological information from an Acoustic Doppler Current Profiler. Mar. Biol. 115: P T. JAMES, AND M. H. THURSTON The diel migration and distribution within a mesopelagic community in the northeast Atlantic. 6. Medusae, Ctenophores, Amphipods and Euphausiids. Prog. Oceanogr. 13: SAMEOTO, D. D Euphausiid distribution in acoustic scattering layers and its significance to surface swarms. J. Plankton Res. 5: STANTON, T. K., D. CHU, AND P. H. WIEBE Acoustic scattering characteristics of several zooplankton groups. ICES J. Mar. Sci. 53: THERIAULT, K. B Incoherent multibeam doppler current profiler performance: Part l-estimate variance. IEEE J. Oceanogr. Eng. OE-11: URICK, R. J Principles of underwater sound. McGraw-Hill. WILLIAMS, R., AND N. FRAGOPOULU Vertical distribution and nocturnal migration of Nyctiphanes couchi in relation to the summer thermocline in the Celtic Sea. Mar. Biol. 89: AND J. A. LINDLEY Variability in abundance, vertical distribution and ontogenetic migrations of Thysanoessa longicauduta in the northeastern Atlantic Ocean. Mar. Biol. 69: Received: 13 November 1997 Accepted: 29 June 1998