Nancy J. Copley

Determining dominant scatterers of sound in mixed zooplankton populations

High-frequency acoustic scattering techniques have been used to investigate dominant scatterers in mixed zooplankton populations. Volume backscattering was measured in the Gulf of Maine at 43, 120, 200, and 420 kHz. Zooplankton composition and size were determined using net and video sampling techniques, and water properties were determined using conductivity, temperature, and depth sensors. Dominant scatterers have been identified using recently developed scattering models for zooplankton and microstructure. Microstructure generally did not contribute to the scattering. At certain locations, gas-bearing zooplankton, that account for a small fraction of the total abundance and biomass, dominated the scattering at all frequencies. At these locations, acoustically inferred size agreed well with size determined from the net samples. Significant differences between the acoustic, net, and video estimates of abundance for these zooplankton are most likely due to limitations of the net and video techniques. No other type of biological scatterer ever dominated the scattering at all frequencies. Copepods, fluid-like zooplankton that account for most of the abundance and biomass, dominated at select locations only at the highest frequencies. At these locations, acoustically inferred abundance agreed well with net and video estimates. A general approach for the difficult problem of interpreting high-frequency acoustic scattering in mixed zooplankton populations is described.

Acoustical study of the spatial distribution of plankton on Georges Bank and the relationship between volume backscattering strength and the taxonomic composition of the plankton

Abstract High frequency (420 kHz) sound was used to study the volume backscattering from plankton and micronekton over Georges Bank as part of a study designed to determine the correlation length scales of plankton spatial patterns in relation to physical structure and to intercompare different kinds of sampling and remote-sensing instrumentation. Two physically distinct areas were studied: a well-mixed area in a shallow portion of the Bank and a stratified area on the deeper southern flank of the Bank. A submersible echo sounder with a down-looking transducer was mounted in a towed V-fin. Volume backscattering data were collected from near the sea surface to the bottom (40–80 m). Vertical and horizontal volume backscattering structure in the stratified region differed from that in the well-mixed area in both mean and variance, providing evidence that physical forcing of the pattern varied significantly between the two areas. Internal waves appeared to modulate the depth of dense mid-depth volume scattering layers in the stratified sites. In the mixed area, there was little horizontal layering or coarse-scale horizontal structure. However, fine-scale vertical lineations were evident with horizontal length scales on the order of the depth of the water column. One hypothesis to explain these vertical lineations in the well-mixed areas involves the development of secondary vertical circulation cells associated with the tidal flows over a rough bottom. Although volume backscattering at the stratified sites was 4–7 times higher than at the mixed site, there was no significant difference in MOCNESS (Multiple Opening/Closing Net and Environmental Sensing System) collected biovolumes between these locations. The difference in volume backscattering was due to differences in both the acoustic scattering properties of zooplankton taxa and the taxonomic composition of the plankton between the sites. Correlations between taxon abundance and volume scattering were positive and significant only for pteropods and euphausiid larvae. The abundances of copepods, chaetognaths, fish larvae, and amphipods were not significantly correlated with volume scattering. When taxon-specific model predictions of acoustic backscattering cross-section, developed by Stanton et al . ( ICES Journal of Marine Science , 51 (1994) 505–512), were used with field collected individual size and abundance data to predict measured volume backscattering data, good agreement was found between observed and predicted volume backscattering strengths.

Coarse-scale horizontal patchiness and vertical migration of zooplankton in Gulf Stream warm-core ring 82-H

Abstract A 1-m 2 MOCNESS with 20 nets was used to make a series of tows in Gulf Stream meander/ring 82-H (September/October 1982) including two 0–100 m undulating “towyos”. One towyo, made at dusk in the core of 82-H (of Sargasso Sea/Gulf Stream origin) permitted study of the effect of diel migration on the spatial variability of copepod and euphausiid species abundance, and species composition in a region of low physical variability. The other towyo taken across a front on the outer edge of 82-H (a mixture of Gulf Stream, Shelf and Slope Water), allowed comparison of spatial variability of the same biological properties in a region of strong physical variability. A sharp transition in euphausiid species composition occurred in the ring core after sunset as dielly migrating euphausiids moved into the surface waters. A similar, but less extreme change took place in copepod species composition because a smaller proportion of these migrated. All copepod migrants also entered surface waters after sunset with species living deeper in the water column during the day arriving in the surface waters later than those living shallower. Enright s (1977, Limnology and Oceanography , 22 , 856–872) hypothesis for the metabolic advantages available through diel vertical migration does not account for the observed behavior of the migrating copepods and euphausiids at this time and place. Estimated swimming speeds (typically 50–200 m h −1 ) of migrating copepods and euphausiids were similar in spite of large differences in body size between the two groups. Variations in species composition were substantially larger at the edge of the ring where species proportions changed radically in concert with changes in water-mass properties. There were also large differences in species composition between the samples from the ring core and the front which equaled those which occurred across the front. Hydrographic differences were stronger than diel changes due to migration for copepods but not for euphausiids. Streamers of surface water which originated within the frontal region and spiraled into the ring core could provide colonizers of many species not present at the time of ring formation.

Video Plankton Recorder estimates of copepod, pteropod and larvacean distributions from a stratified region of Georges Bank with comparative measurements from a MOCNESS sampler

Abstract A two-vessel exercise was conducted over the southern flank of Georges Bank during the onset of vernal stratification in May 1992. The Video Plankton Recorder (VPR), a towed video system, was used to map out the fine-scale distributions of zooplankton to a depth of 70 m along a trackline which described a regular grid (3.5 × 4.5 km) in Lagrangian space. A second vessel following a parallel course conducted Multiple Opening/Closing Net and Environmental Sensing System (MOCNESS) sampling during the last section of the grid, which provided an opportunity to compare data from the two systems. Both the VPR and the MOCNESS provided similar data on the taxonomic composition of the plankton which was numerically dominated by copepods ( Calanus, Pseudocalanus, Oithona ), pteropods ( Limacina ) and larvaceans ( Oikopleura ). The absence of rare ( −3 ) species from the VPR dataset was a consequence of the small volume sampled (0.0694 m 3 ) by the high magnification camera, while fragile gelatinous taxa were undersampled by the MOCNESS. Estimates of copepod and pteropod concentrations were comparable for the two gear types. While the species composition of the plankton did not change statistically along the grid, abundances of the dominant taxa varied along the transect and each taxon demonstrated pronounced fine-scale vertical patterns that appeared to be related to hydrographic features. The VPR represents a powerful tool for rapid surveys of the micro- to fine-scale structure of zooplankton assemblages either alone, or in conjunction with other sampling techniques.

Deep-water zooplankton of the Guaymas Basin hydrothermal vent field

Abstract Zooplankton from the Guaymas Basin deep-sea vent field were collected with a 1 m 2 MOCNESS to examine the distribution of total standing stock, taxonomic composition, size-frequency distribution of zooplankton, and the species composition of calanoid copepods. Low altitude (∼ 100 m above the bottom) horizontal tows along and across the axis of the basins southern trough, and oblique tows from the bottom of the basin (∼ 2000 m) to the surface were made. Total biomass in near-bottom samples (range: 13–46 cc/1000 m 3 ) was only about a factor of 10 lower than in the upper 100 m. However, there was little or no evidence for enrichment of biomass in the ∼ 100 m zone above the vent site relative to biomass at the same depth horizon over non-vent areas. Total numbers of individuals ranged between 2600 and 4800/1000 m 3 . Calanoid copepods consistently ranked first in abundance of counts of the taxa, followed by cyclopoid copepods, ostracods, chaetognaths, and amphipods. Other less abundant taxa, but in some cases important contributors to total biomass, were coelenterates (siphonophores, medusae), decapod shrimp, and polychaetes. Size-frequency analysis of individuals from each taxon indicated that the biomass and abundance spectra do not fit the theoretically expected spectra based on weight-dependent metabolism and growth. The pyramid of biomass was substantially different from the pyramid of numbers in this deep-sea community. Of the 67 species of copepods identified in two samples taken on low altitude tows, only 15 co-occurred in both samples. Many of the species in this relatively diverse community remain to be described. Larval and post-larval forms of benthic clams, gastropods, polychaetes, and crustaceans associated with the vents were collected 100–200 m above the southern trough, indicating the post-larvae may play an active role in dispersal of hydrothermal vent species.

DNA Barcoding of Marine Copepods: Assessment of Analytical Approaches to Species Identification

More than 2,500 species of copepods (Class Maxillopoda; Subclass Copepoda) occur in the marine planktonic environment. The exceptional morphological conservation of the group, with numerous sibling species groups, makes the identification of species challenging, even for expert taxonomists. Molecular approaches to species identification have allowed rapid detection, discrimination, and identification of species based on DNA sequencing of single specimens and environmental samples. Despite the recent development of diverse genetic and genomic markers, the barcode region of the mitochondrial cytochrome c oxidase subunit I (COI) gene remains a useful and – in some cases – unequaled diagnostic character for species-level identification of copepods. This study reports 800 new barcode sequences for 63 copepod species not included in any previous study and examines the reliability and resolution of diverse statistical approaches to species identification based upon a dataset of 1,381 barcode sequences for 195 copepod species. We explore the impact of missing data (i.e., species not represented in the barcode database) on the accuracy and reliability of species identifications. Among the tested approaches, the best close match analysis resulted in accurate identification of all individuals to species, with no errors (false positives), and out-performed automated tree-based or BLAST based analyses. This comparative analysis yields new understanding of the strengths and weaknesses of DNA barcoding and confirms the value of DNA barcodes for species identification of copepods, including both individual specimens and bulk samples. Continued integrative morphological-molecular taxonomic analysis is needed to produce a taxonomically-comprehensive database of barcode sequences for all species of marine copepods.

Assessing the distribution and abundance of zooplankton : a comparison of acoustic and net-sampling methods with D-BAD MOCNESS

Abstract Results are described from the first field study with the D-BAD MOCNESS (Dual-Beam Acoustics Deployed on a Multiple Opening/Closing Net and Environmental Sensing System), an instrument designed to collect acoustic data and net samples simultaneously from the same portion of the water column. Our primary objective was to evaluate the advantages and disadvantages of integrating in a single instrument these two very distinctive methods for assessing the distribution and abundance of zooplankton. In this context, we required a means of comparison that would enable us to groundtruth acoustic remote-sensing data with net sample data. The approach chosen, referred to as the forward-problem approach, compares the acoustic volume backscattering coefficients observed in situ with those predicted from net sample data and acoustic scattering models. The results from this study show that the observed acoustic volume backscattering data are generally consistent with the forward-problem predictions. This consistency is true in terms of both total acoustic volume backscattering as well as that portion of the volume backscattering contributed by each of the dominant sound scatterer types. The results also provide two examples of situations in which inconsistencies between the observed and predicted acoustic volume backscattering can be used to detect potential methodological problems. D-BAD MOCNESS appears to be a useful instrument for groundtruthing acoustic data; however, due to its slow towing speed, it is not a suitable instrument for large-scale, acoustic survey work.

Integrated biochemical, molecular genetic, and bioacoustical analysis of mesoscale variability of the euphausiid Nematoscelis difficilis in the California Current

Integrated assessment of the euphausiid Nematoscelis difficilis (Crustacea; Euphausiacea) and the zooplankton assemblage of the California Current was designed to investigate individual,population,and community responses to mesoscale variability in biological and physical characters of the ocean. Zooplankton samples and observational data were collected along a cross-shelf transect of the California Current in association with the California Cooperative Fisheries Investigations (CalCOFI) Survey during October 1996. The transect crossed three domains defined by temperature and salinity: nearshore,mid-Current,and offshore. Individual N. difficilis differed in physiological condition along the transect,with higher size-corrected concentrations of four central metabolic enzymes (citrate synthetase,hexokinase,lactate dehydrogenase (LDH),and phosphoglucose isomerase (PGI)) for euphausiids collected in nearshore waters than in mid-Current and offshore waters. There was little variation in the DNA sequences of the genes encoding PGI and LDH (all DNA changes were either silent or heterozygous base substitutions),suggesting that differences in enzyme concentration did not result from underlying molecular genetic variation. The population genetic makeup of N. difficilis varied from sample to sample based on haplotype frequencies of mitochondrial cytochrome oxidase I (mtCOI; P ¼ 0:029). There were significant differences between pooled nearshore and offshore samples,based on allele frequencies at two sites of common substitutions in the mtCOI sequence (P ¼ 0:020 and 0:026). Silhouette and bioacoustical backscattering measurements of the zooplankton assemblage of the top 100 m showed marked diel vertical migration of the scattering layer,of which euphausiids were a small but significant fraction. The biochemical and molecular assays are used as indices of complex physiological (i.e.,growth and condition) and genetic (i.e., mortality) processes; the bioacoustical observations provide insight into the ecosystem context for the single-species measurements. All data are intended for integration into predictive models of secondary production and biomass concentration in the ocean. r 2002 Elsevier Science Ltd. All rights reserved.

Evidence for vertical circulation cells in the well-mixed area of Georges Bank and their biological implications

Two surveys were conducted in the well-mixed region of Georges Bank to look for secondary vertical circulation cells, the first in 1996 and the second in 1997. Each survey collected high-frequency acoustic, temperature, and fluorescence data along a 1-n.mile square grid. Concurrent ADCP measurements also were made in the second year. MOCNESS and pump samples from both years caught large amounts of sand and organisms typical of this regions such as copepods and hydroids. However, forward problem calculations suggest that the acoustic scattering was dominated by post-larval bivalves. Sand and copepods also accounted for significant amounts of the estimated backscatter. The acoustic data from both surveys contained near-surface vertical bands of high-volume backscatter. The frequency and intensity of these bands was strongly correlated with the magnitude of the current velocity. Significant upwelling and downwelling were observed in the ADCP records, and the acoustic bands often co-occurred in the downwelling zones. Simulations of particle distributions within idealized circulation cells, consistent with the acoustic and ADCP data, suggest that the acoustic bands are caused by aggregations of positively buoyant or upward-swimming scatterers. The circulation cells proposed could have an important effect on the ecology of the well-mixed region by aggregating upward-swimming fish and zooplankton in near-surface patches.

Material properties of North Atlantic cod eggs and early-stage larvae and their influence on acoustic scattering

Chu, D., Wiebe, P. H., Copley, N. J., Lawson, G. L., and Puvanendran, V. 2003. Material properties of North Atlantic cod eggs and early-stage larvae and their influence on acoustic scattering. – ICES Journal of Marine Science, 60: 508–515. To study the acoustic signatures of Atlantic cod (Gadus morhua) at different biological stages from eggs to early-stage larvae (<37 days post-hatch), we conducted a series of experiments to estimate their sound-speed and density contrasts. A laboratory version of the ‘‘Acoustic Properties of Zooplankton’’ system was used. Sound speed was estimated by means of travel time between two transducers using a broadband-acoustic signal (� 300– 600 kHz). Density was estimated using a dual-density method in which two fluids of different densities were employed. It was found that the density contrasts of cod eggs and early-stage larvae were nearly all slightly less than unity (0.969–0.998), while the effective sound-speed contrasts were only slightly greater than unity (1.017–1.024) for eggs and yolk-sac stage larvae (<5 days post-hatch), and increased significantly ([1.130) for larvae older than 16 days. This change in sound-speed contrast reflected the transition of the swimbladder from an uninflated state to an inflated state. The regression relation between estimated target strength at 500 kHz and larval length in centimetres was found to be TS ¼ 176:1 log10L � 82:4 ðdBÞ. The inflation ratio of the swimbladder for early-stage larvae was an exponential function of time. The predicted period of time until full swimbladder inflation was 43.3 days.