Deep Scattering Layer Research Articles

A deep scattering layer under the North Pole pack ice

The 3.3 million km2 marine ecosystem around the North Pole, defined as the Central Arctic Ocean (CAO), is a blind spot on the map of the world’s fish stocks. The CAO essentially comprises the permanently ice-covered deep basins and ridges outside the continental shelves, and is only accessible by ice-breakers. Traditional trawling for assessing fish stocks is impossible under the thick pack ice, and coherent hydroacoustic surveys are unachievable due to ice-breaking noise. Consequently, nothing is known about the existence of any pelagic fish stocks in the CAO, although juveniles of Boreogadus saida richly occur at the surface associated with the sea ice and ice-associated Arctogadus glacialis has been reported as well. We here present a first indication of a possible mesopelagic fish stock in the CAO. We had the opportunity to analyse a geophysical hydroacoustic data set with 13 time windows of usable acoustic data over a transect from 84.4 °N in the Nansen Basin, across the North Pole (90.0 °N), to 82.4 °N in the Canada Basin. We discovered a deep scattering layer (DSL), suggesting the presence of zooplankton and fish, at 300–600 m of depth in the Atlantic water layer of the CAO. Maximum possible fish abundance and biomass was very low; values of ca. 2,000 individuals km−2 and ca. 50 kg km−2 were calculated for the DSL in the North-Pole area according to a model assuming that all acoustic backscatter represents 15-cm long B. saida and/or A. glacialis. The true abundance and biomass of fish is even lower than this, but cannot be quantified from this dataset due to possible backscatter originating from pneumatophores of physonect siphonophores that are known to occur in the area. Further studies on the DSL of the CAO should include sampling and identification of the backscattering organisms. From our study we can conclude that if the central Arctic DSL contains fish, their biomass is currently too low for any sustainable fishery.

Vertical Migration of Pelagic and Mesopelagic Scatterers From ADCP Backscatter Data in the Southern Norwegian Sea

Records of backscatter and vertical velocity obtained from moored Acoustic Doppler Current Profilers (ADCP) enabled new insights into the dynamics of deep scattering layers (DSLs) and diel vertical migration (DVM) of mesopelagic biomass between these deep layers and the near-surface photic zone in the southern Norwegian Sea. The DSL exhibits characteristic vertical movement on inter-monthly time scales, which is associated with undulations of the main pycnocline between the warm Atlantic water and the underlying colder water masses. Timing of the DVM is closely linked to the day-night light cycle—decent from the photic zone just before sunrise and ascent immediately after sunset. Seasonal variations are also evident, with the highest DVM activity and lowest depth averaged mean volume backscatter strength (MVBS) during spring. This suggests that both oceanographic and optical conditions are driving the complex dynamics of pelagic and mesopelagic activity in this region. We hypothesize that the increased abundance of calanoid copepods in the near-surface layer during spring increases the motivation for vertical migration of pelagic and mesopelagic species, which therefore can explain the increased DVM activity during this season.

Composition and distribution of the larval decapod community in the deep sea of the Western Mediterranean Sea Balearic Sub‐basin

AbstractMechanisms facilitating larvae transport from continental to more oceanic areas were investigated in the Balearic sub‐basin (BSB), Western Mediterranean Sea. The abundance, distribution, and development stages of decapod larvae were recorded for a transect of 28 stations crossing the central eddy of the BSB. Zooplankton samples were taken to 1,800 m depth by horizontal and oblique 500‐µm mesh size nets hauls towed near the benthic boundary (BBL) and in deep scattering layers (DSL). In total, 67 taxa belonging to Decapoda and one Stomatopoda were identified. Advanced development represented 75% of the individuals recorded. 75% of the species corresponded to adults of deep‐sea species, 9% were sergestids (mesopelagic species), and the remaining 16% corresponded to shelf and coastal species. Cluster assemblages formed were related to the hydrological conditions, water masses dynamics, and geomorphologic structures mainly associated with nepheloid layers. Advanced and juvenile specimens of commercial species such as Parapenaeus longirostris, Geryon longipes, and Aristeus antennatus were found close to seafloor BBL. The influence of trophic ecology should be considered as the priority factor of larvae concentrations in deep.

Sea-ice and water dynamics and moonlight impact the acoustic backscatter diurnal signal over the eastern Beaufort Sea continental slope

Abstract. A 2-year-long time series of currents and acoustic backscatter from an acoustic Doppler current profiler, moored over the eastern Beaufort Sea continental slope from October 2003 to September 2005, were used to assess the dynamics and variability of the sound-scattering layer. It has been shown that acoustic backscatter is dominated by a synchronized diel vertical migration (DVM) of zooplankton. Our results show that DVM timings (i) were synchronous with sunlight and (ii) were modified by moonlight and sea ice, which attenuates light transmission to the water column. Moreover, DVM is modified or completely disrupted during highly energetic current events. Thicker ice observed during winter–spring 2005 lowered the backscatter values but favored extending DVM toward the midnight sun. In contrast to many previous studies, DVM occurred through the intermediate water layer during the ice-free season of the midnight sun in 2004. In 2005, the midnight-sun DVM was likely impacted by a high acoustic scattering generated by suspended particles. During full moon at low cloud cover, the nighttime moonlight illuminance led to zooplankton avoidance of the subsurface layer, disrupting DVM. Moreover, DVM was disrupted by upwelling, downwelling, and eddy passing. We suggest that these deviations are consistent with DVM adjusting to avoid enhanced water dynamics. For upwelling and downwelling, zooplankton likely respond to the along-slope water dynamics dominated by surface- and depth-intensified flow, respectively. This drives zooplankton to adjust DVM by aggregating in the low or upper intermediate water layer for upwelling and downwelling, respectively. The baroclinic eddy reversed DVM below the eddy core.

The response of mesopelagic organisms to artificial lights

Though mesopelagic fish respond to natural light (e.g., diurnal vertical migration), few studies have looked at how they respond to artificial light and if artificial lights could be used in commercial operations to improve catchability of mesopelagic fish. Here we present a preliminary study on how mesopelagic organisms respond to blue and green spotlights, as well as red and white diffuse lights in Masfjorden (Norway; max depth of 480 m). The response of organisms in each of the three sound scattering layers (SSLs) was observed when a) artificial lights were positioned in a layer or b) moved with a constant speed (generally 0.03 ms−1) towards a layer. The artificial lights were attached to a rig with a self-contained echosounder, which recorded the vertical and horizontal avoidance of organisms in each SSL to different artificial lights. Net hauls (MIC-net) and video footage confirmed that Maurolicus muelleri and siphonophores were present in the upper layer (100–150 m), while Benthosema glaciale were present in the deeper layers (∼200 m and ∼300 m to seabed). Our findings suggest that M. muelleri (SSL1) horizontally avoid blue spotlight and white diffuse light, while B. glaciale (SSL2 and SSL3) mainly avoid the same lights downwards and can be herded downwards over 250 m. Though this study should be regarded as preliminary, the observed avoidance/herding response suggests artificial lights could be applied to improve existing fish capture methods for mesopelagic fish.

From coast to slope: Zooplankton communities shift in the Northern Alboran Sea

The zooplankton composition of the Sound Scattering Layer (SSL) detected at multiples frequencies, 18, 38, 120 and 200 kHz, along the continental shelf of the Northern Alboran Sea (Mediterranean Sea) has been identified in July 2013 and 2014 by means of different plankton nets and the SSL dominant scatterers have been identified using zooplankton scattering models.Three acoustic patterns, based on their distinctive relative frequency response (r(f)), have been detected along the continental shelf, matching with three zooplankton communities; in coastal areas (25–40 m bottom depth), a homogeneous community composed by Fluid-like zooplankton, mainly crustaceans, exhibiting a r(f) peak at 120 kHz, (ii) in intermediate areas (40–190 m) a heterogeneous community composed by Fluid-like (crustaceans, chaetognaths, larvaceans, doliolids, Calycophora siphonophores), Elastic-shelled (heteropods) and Gas-bearing (fish larvae) zooplankton, exhibiting a r(f) peak at 38 kHz (iii) in the continental shelf edge (180–250 m), a zooplankton community dominated by Maurolicus muelleri fish larvae, exhibiting a r(f) peak at 18 kHz. Scattering models results showed that stronger scatterers as fish larvae (Gas-bearing), although scarce in samples, dominated the backscattering energy at almost all frequencies and locations. Fluid-like zooplankton contribution was only significant in coastal areas at high frequencies (120 and 200 kHz).The application of these findings to all echograms collected over the study area made it possible to obtain, the macro scale spatial distribution of the zooplankton communities as well as the SSL backscatter energy all over the continental shelf, highlighting the suitability of the use of acoustic data as an indicator of secondary production.

Persistent Sampling of Vertically Migrating Biological Layers by an Autonomous Underwater Vehicle Within the Beam of a Seabed-Mounted Echosounder

Many pelagic animals, such as krill, lanternfish, and cephalopods, migrate to deep water at dawn to avoid visual predators during daylight hours and move up toward the sea surface at dusk to search for food. This behavior is termed “diel vertical migration.” Migrating animals graze on phytoplankton or zooplankton and in turn serve as food for higher trophic levels, hence providing a key mechanism for carbon export via this migration. These animals are often observed as sound-scattering layers by echosounders, but the animals causing the acoustic scattering are difficult to identify using acoustics alone. In a spring 2019 experiment in Monterey Bay, we deployed autonomous underwater and surface vehicles over a seabed-mounted upward-looking echosounder to collect environmental DNA (eDNA) with the goal of identifying the vertically migrating animals. The echosounder was installed at 890-m depth on the Monterey Accelerated Research System (MARS) seabed cabled ocean observatory, providing real-time data of acoustic backscatter from the full water column. One long-range autonomous underwater vehicle (LRAUV) carrying a Third-Generation Environmental Sample Processor (3G-ESP) acquired water samples from a sequence of layers from near surface down to ~ 290 m as directed by the distribution of animals observed by the echosounder. During the sampling of each layer, the LRAUV ran on a tight circular yo-yo trajectory directly above the echosounder, remaining in its beam by acoustically tracking a station-keeping Wave Glider on the sea surface marking the echosounder's latitude and longitude. The persistent and simultaneous acoustic observation and eDNA acquisition enables identification of animals at precise locations to better understand their vertical migration behaviors. We present the methods and the system performance in the experiment.

Paralarval and juvenile cephalopods within warm-core eddies in the North Atlantic

Many descriptions of paralarval and juvenile cephalopods are poor. By using DNA barcoding, a global bioidentification system for animals, along with morphological investigation, we can confirm species identifications. We have a better chance of eliminating misidentifications and, therefore, documenting the correct abundance and distribution of cephalopods within an area by combining morphological and molecular evidence. The central objectives of this study are to: (1) compare morphological vs molecular identification of cephalopods and (2) determine the occurrence of cephalopods within the deep scattering layer (DSL) within warm core eddies. The specimens reported here were collected between 2014 and 2016 during three transatlantic cruises from Galway, Ireland to St John's, Newfoundland, with a focus on assemblages in warm-core mesoscale eddies on the western part of the transect. Samples were collected from the DSL at multiple stations across mesoscale eddies. In total, 301 cephalopods belonging to 29 species were collected. Not only does our study increase the knowledge of abundance and diversity of pelagic cephalopods in this area, but it also provides sequences for species for which no comparative sequences were previously available. By examining the match/mismatch between morphological and molecular identifications, we highlight a need for revisions in some taxonomic groupings such as the family Cranchiidae.

Oceanographic Structure and Light Levels Drive Patterns of Sound Scattering Layers in a Low-Latitude Oceanic System

Several factors have been found to structure the spatial and temporal patterns of deep scattering layers (DSLs) including temperature, oxygen, salinity, light, and physical oceanographic conditions. We examine the variance in acoustically detected DSLs in the northern Gulf of Mexico and investigate the importance of multiple biotic and abiotic factors including mesoscale oceanographic conditions (e.g., Loop Current-origin water (LCOW), frontal boundaries (FB), and common water (CW)) in structuring DSLs. Results indicate heterogeneity in the vertical position and acoustic backscatter of DSLs relative to oceanographic conditions and light intensity. LCOW regions displayed consistent decreases (by a factor of two and four) in acoustic backscatter in the upper 200 m relative to FB and CW, respectively. DSLs had considerably greater backscatter at night in comparison to the day (25X for FB, 17X for LCOW, and 12X for CW). The importance of biotic (primary productivity) and abiotic (sea surface temperature, salinity) factors varied across oceanographic conditions and depth intervals, suggesting that the patterns in distribution and behavior of the mesopelagic assemblage in the northern Gulf of Mexico are highly dynamic.

Fine-scale vertical structure of sound-scattering layers over an east border upwelling system and its relationship to pelagic habitat characteristics

Abstract. Understanding the relationship between sound-scattering layers (SSLs) and pelagic habitat characteristics is a substantial step to apprehend ecosystem dynamics. SSLs are detected on echo sounders representing aggregated marine pelagic organisms. In this study, SSL characteristics of zooplankton and micronekton were identified during an upwelling event in two contrasting areas of the Senegalese continental shelf. Here a cold upwelling-influenced inshore area was sharply separated by a strong thermal boundary from a deeper, warmer, stratified offshore area. Mean SSL thickness and SSL vertical depth increased with the shelf depth. The thickest and deepest SSLs were observed in the offshore part of the shelf. Hence, zooplankton and micronekton seem to occur more frequently in stratified water conditions rather than in fresh upwelled water. Diel vertical and horizontal migrations of SSLs were observed in the study area. Diel period and physicochemical water characteristics influenced SSL depth and SSL thickness. Although chlorophyll-a concentration insignificantly affected SSL characteristics, the peak of chlorophyll a was always located above or in the middle of the SSLs, regularly matching with the peak of SSL biomass. Such observations indicate trophic relationships, suggesting SSLs to be mainly composed of phytoplanktivorous zooplankton and micronekton. Despite local hypoxia, below 30 m depth, distribution patterns of SSLs indicate no vertical migration boundary. The results increase the understanding of the spatial organization of mid-trophic species and migration patterns of zooplankton and micronekton, and they will also improve dispersal models for organisms in upwelling regions.

Mesopelagic fish composition and diets of three myctophid species with potential incidence of microplastics, across the southern tropical gyre

The species occurrence of mesopelagic fish collected in the scattering layers was studied across the Indian Ocean subtropical gyre during the dry season (2015). Ocean eddies can retain zooplankton and plastic debris, and determine biological composition through advection. The conditions inside the gyre were stabilised by low chlorophyll-a concentrations and a deep nutricline. A diverse assemblage of mesopelagic fish, dominated by myctophids, exhibited diel migrations to the upper water layers for feeding since the early night. Myctophids were concentrated ca. 100 m depth, where the deep chlorophyll maximum was located, and coinciding with the nutricline. Another group of mesopelagic fish remained in deeper waters during the daytime, with a higher occurrence of Stomiiformes (Phoshycthyidae and Sternoptychidae) at the 500–600 m Deep Scattering Layer. Moreover, the diets of 220 myctophid specimens belonging to the species Ceratoscopelus warmingii, Diaphus effulgens, and Symbolophorus evermanni, were investigated based on stomach content analysis. Calanoid copepods, mainly Pleuromamma abdominalis, made up 30% of the diet in the three myctophid species, with interspecific differences in the preferred second item: C. warmingii preferred ostracods, D. effulgens, non-calanoid copepods, and S. evermanni, euphausiids. Thus, partial dietary segregation was observed among the three species. We hypothesized that these myctophid diets differed between stations located at the gyre core and its outer edge. Any dietary pattern indicating aggregation of food resources, or an enhancement of the foraging opportunities for myctophids, was discerned as an effect of the gyre advection. However, prey composition diverged between the core and the outer edge stations regardless of the fish species, with a higher dietary representation of P. abdominalis and euphausiids (migratory zooplankton) in the individuals collected in the core-influenced area. A total of six plastic pieces were present in the stomachs of D. effulgens and S. evermanni, in contrast with the high presence of plastic particles found at the surface in most sampling stations (average number of plastic pieces was 41 per hour). The selected myctophid species did not appear to be important transport vectors for microplastics in comparison with species from other oceans.

Characterization of sound scattering layers in the Bay of Biscay using broadband acoustics, nets and video.

Sound scattering layers (SSLs) are observed over a broad range of spatio-temporal scales and geographical areas. SSLs represent a large biomass, likely involved in the biological carbon pump and the structure of marine trophic webs. Yet, the taxonomic composition remains largely unknown for many SSLs. To investigate the challenges of SSL sampling, we performed a survey in a small study area in the Northern Bay of Biscay (France) by combining broadband and narrowband acoustics, net sampling, imagery and video recordings. In order to identify organisms contributing to the observed SSLs, we compared measured frequency spectra to forward predicted spectra derived from biological data. Furthermore, to assess the confidence in SSL characterization, we evaluated uncertainties in modeling, acoustical and biological samplings. Here, we demonstrate for the first time that SSL backscattering intensity in the Bay of Biscay can be dominated in springtime by resonant gas bearing organisms below 100 kHz, namely siphonophores and juvenile fishes and by pteropods at higher frequencies. Thus, we demonstrate the importance of broadband acoustics combined to nets, imagery and video to characterize resonant backscatterers and mixed mesozooplankton assemblages.

Seasonal and spatial variability in the vertical distribution of pelagic forage fauna in the Southwest Pacific

Acoustic data are an invaluable source of information for characterizing the distribution and abundance of mid-trophic level organisms (MTLOs) in the ocean. These organisms play a key role in the ecosystem as prey of top predators and as predators of lower trophic level organisms, as well as in carbon export from the surface into deeper waters. This study used 38 kHz-EK60 acoustic echosounder data from six cruises spanning 2011–2017 to explore the seasonal and spatial variability in the vertical distribution of MTLOs’ from 10 to 600 m in the New Caledonian (South Pacific) Exclusive Economic Zone. A total of 16715 acoustic vertical profiles of acoustic backscattering strength were clustered into homogeneous groups. Two small shallow scattering layers (SSLs) between 0 and 100 m, and one large deep scattering layer (DSL) at around 550 m depth, characterized the mean vertical distribution of MTLOs. A machine-learning model (eXtreme Gradient tree Boosting algorithm, XGBoost) was fitted to explain the acoustic profile clusters with environmental variables as predictors. Sun inclination was the most important factor in structuring the vertical profile shapes due to the diel vertical migration signal, followed by the mean oxygen value of the top 600 m. Bathymetry, euphotic depth, 0–600 m mean temperature and SST were the next most significant variables. Isotherm depth, surface chlorophyll-a, wind, and mean salinity had a lower influence on the shape of the vertical profiles. The model was then used to construct vertical echograms at the scale of the New Caledonian EEZ, showing an accuracy up to 87% in cross validation. Across the EEZ, the shape of vertical acoustic profiles were comparable, though layer echo intensities varied spatially with a marked north-south gradient that remained relatively constant seasonally. The vertically-averaged acoustic values were characterized by a maximum to the south of the EEZ in summer, mainly driven by high oxygen values as well as shallow euphotic depth. We also estimated a migrant proportion between day DSL and night SSL of about 78%. Our methodology offers a promising approach for analyzing the control of the environment on the vertical distribution of MTLOs for other oceanic provinces, while also providing a framework to investigate the corresponding trophic interactions between MTLOs and their predators feeding at different depths and times. Moreover, our findings stress the need to consolidate knowledge on species composition to optimize acoustic data interpretation.

Mesopelagic predator-prey interactions revealed by joint passive and active acoustic observations

The ocean’s mesopelagic zone is one of the Earth’s largest habitats and contains large numbers of animals, playing important roles in aquatic food webs—notably as food for many marine mammal species. While these communities have been observed for decades on echosounders as sound scattering layers (SSLs), their ecological dynamics remain poorly understood. Using a broadband (0-128 kHz) hydrophone and an upward-looking echosounder (38 kHz) at a cabled observatory in Monterey Bay, CA in early 2019, we observed numerous occasions where SSLs abruptly increased their depth during bouts of echolocation clicks from odontocete whales. Clicking bouts occurred 6.1 times per day on average, mostly at night, lasting minutes to several hours. Pacific white-sided dolphins and Risso’s dolphins produced most of these clicks. Increases in clicking were significantly cross-correlated with increases in SSL depth. The deepest SSL, centered at 400–500 m depth, dove 15 m during an average clicking bout, with some excursions up to 100 m. Video surveys from a remotely operated vehicle identified juvenile Pacific hake, myctophids, and sergestid shrimp as likely constituents of these SSLs. Our results suggest these animals actively dive to avoid odontocetes, and that fear of predation can restructure mesopelagic ecosystems on short time scales.

Mapping the global prey-field: Combining acoustics, optics and net samples to reduce uncertainty in estimates of mesopelagic biomass

Mesopelagic (200–1000 m depth zone) biomass largely comprises zooplankton, jellyfish, squid and fish. Part of this biomass actively migrates between the surface and the mesopelagic zone on a daily basis, playing an important role in global biogeochemical cycling. When observed using echosounders, aggregated components of this community (largely made up of fish and siphonophores) are known as Deep Scattering Layers (DSLs). DSLs have been used to partition the Global Ocean into biogeographically distinct regions, based on DSL vertical distribution and echo energy, and estimates of mesopelagic biomass have stemmed from this work. These estimates are typically derived using a combination of biological sampling, optics, and active-acoustics, but are often biased (e.g., through trawl-based escapement and avoidance) and uncertain. We have developed a new method that combines multiple strands of observations, both from vessels and ocean probes, to minimise sampling bias and produce accurate estimates of mesopelagic biomass. We apply the method to the specific case of estimating mesopelagic fish biomass in the Tasman Sea, combining observations made from a vessel and a Profiling Lagrangian Acoustic Optical System. We compare our new biomass estimates to previous estimates and ecosystem model predictions. Our new method provides a robust framework for future ocean biomass estimates.

Mesopelagic Sound Scattering Layers of the High Arctic: Seasonal Variations in Biomass, Species Assemblage, and Trophic Relationships

Mesopelagic sound scattering layers (SSL) are ubiquitous in all oceans. Pelagic organisms within the SSL play important roles as prey for higher trophic levels and in climate regulation through the biological carbon pump. Yet, the biomass and species composition of SSL in the Arctic Ocean remain poorly documented, particularly in winter. A multifrequency echosounder detected a SSL north of Svalbard, from 79.8°N to 81.4°N, in January 2016, August 2016, and January 2017. Midwater trawl sampling confirmed that the SSL comprised zooplankton and pelagic fish of boreal and Arctic origins. Juvenile beaked redfish dominated the fish assemblage in January and Arctic cod in August. The macrozooplankton community mainly comprised the medusa Cyanea capillata, the amphipod Themisto libellula, and the euphausiids Meganyctiphanes norvegica in August and Thysanoessa inermis in January. The SSL was located in the Atlantic Water mass, between 200-700 m in August and between 50-500 m in January. In January, the SSL was shallower and weaker above the deeper basin, where less Atlantic Water penetrated. The energy content available in the form of lipids within the SSL was significantly higher in summer than winter. The biomass within the SSL was >12-fold higher in summer, and the diversity of fish was slightly higher than in winter (12 vs. 9 species). We suggest that these differences are mainly related to life history and ontogenetic changes resulting in a descent towards the seafloor, outside the mesopelagic layer, in winter. In addition, some fish species of boreal origin, such as the spotted barracudina, did not seem to survive the polar night when advected from the Atlantic into the Arctic. Others, mainly juvenile beaked redfish, were abundant in both summer and winter, implying that the species can survive the polar night and possibly extend its range into the high Arctic. Fatty-acid trophic markers revealed that Arctic cod mainly fed on calanoid copepods while juvenile beaked redfish targeted krill (Thysanoessa spp.). The relatively high biomass of Arctic cod in August and of redfish in January thus suggests a shift within the SSL, from a Calanus-based food web in summer to a krill-based food web during winter.

Seasonal patterns in deep acoustic backscatter layers near vent plumes in the northeastern Pacific Ocean

We used moored 75 kHz acoustic Doppler current profilers (ADCPs) to examine seasonal cycles in zooplankton deep scattering layers (DSLs) observed below 1300 m depth at Endeavour Ridge hydrothermal vents. DSLs are present year-round in the lower water column near vent plumes. Temporal variations suggest passive, flow-induced displacements superimposed on migratory movements. Although the strongest DSLs are shallower than the neutrally buoyant plumes (1900–2100 m), anomalies also occur at and below plume depth. Upward movement from plume depth in the main DSL is evident in late summer/fall, resulting in shallower DSLs in winter, consistent with the timing of adult diapause/reproduction in upper-ocean migratory copepods. Movement from the upper ocean to plume depth coincides with pre-adult migration to greater depths in spring. Synchronous 20–40 d cycles in DSLs may account for patchiness in space and time of above-plume zooplankton layers observed in summer during previous net-sampling surveys, and suggests lateral and vertical migratory movements to counter current drift away from plume-derived food sources. Persistent near-bottom DSLs move vertically between the spreading plume and seafloor. Historical net data suggests that these are deep, resident fauna. Unlike upper ocean fauna, they seem to be advected considerable distances from the ridge axis, where they are evident as remnant scattering layers.

Variation of Zooplankton Mean Volume Backscattering Strength from Moored and Mobile ADCP Instruments for Diel Vertical Migration Observation

Zooplankton can be detected by using acoustic Doppler current profiler (ADCP) instruments through acquiring the mean volume backscattering strength (MVBS) data. However, the precision of the backscattered signal measured by single ADCP measurement has a limitation in the MVBS variation of zooplankton. The objectives of this study were to analyze the MVBS and vertical velocity from ADCPs at the same time and location for zooplankton’s daily vertical migration (DVM) observation. Measurements were conducted in Lembeh Strait, North Sulawesi, Indonesia. Instruments used included a moored ADCP 750 kHz and a mobile ADCP 307.2 kHz. High MVBS value was found at 11.5–16 m depths and was identified as the sound scattering layer (SSL). The DVM patterns in the SSL displayed significant differences over time and had good relationships with the diurnal cycle. Theoretical target strength (TS) from the scattering models based on a distorted-wave Born approximation (DWBA) was estimated for Oithona sp. and Paracalanus sp.; the two dominant species found in the observed area. However, ΔMVBS and ΔTS proved that the dominant zooplankton species were not the main scatterers. The strong signal in SSL was instead caused by the schools of various zooplankton species.

Seamount effects on the diel vertical migration and spatial structure of micronekton

We examined the diel vertical migration (DVM) behavior and vertical spatial structure of sound-scattering layers (SLs) at two seamounts (Condor and Gigante) in the Azores and in surrounding open-waters. Active acoustic data were recorded day and night during nine cruises conducted in spring, summer and autumn between 2009 and 2011. SLs were permanent features with two main layers, shallow scattering layers (SSLs) and deep scattering layers (DSLs). Over seamount plateaus, SSLs aggregated close to the seafloor during the day and in slightly shallower waters at night. Backscatter intensity on plateaus varied little between day and night and was consistently higher than in SSLs above slopes or in open-waters. DSLs found over slopes and open-waters migrated towards the surface at dusk and returned to their daylight depths at dawn but an intense DSL persisted overnight in deep open-waters. Variograms showed that SSLs and DSLs were not uniformly distributed but amount of spatial heterogeneity varied between seamount plateaus, slopes and open-waters, and day-night periods. Taken together, these findings suggest that (1) Condor and Gigante Seamounts host a resident micronekton community, (2) there is an influx of vertically migrating organisms over the plateaus at night, and (3) seamount topography affects the DVM behavior of the DSL. Physical processes, such as Taylor cap effect, along with topographic blockage and lateral advection of migrating organisms, may play a role in transporting micronekton from open-waters to seamount plateaus. These findings are critical to understanding the ecology of seamount communities and the function of seamount habitats in oceanic ecosystems.

Changes in zooplankton communities from epipelagic to lower mesopelagic waters

Zooplankton form a trophic link between primary producers and higher trophic levels, and exert significant influence on the vertical transport of carbon through the water column (‘biological carbon pump’). Using a MultiNet we sampled and studied mesozooplankton communities (i.e. >0.2 mm) from six locations around Bermuda targeting four depth zones: ∼0–200 m, ∼200–400 m, ∼400–600 m (deep-scattering layer), and ∼600–800 m. Copepoda, our focal taxonomic group, consistently dominated samples (∼80% relative abundance). We report declines in zooplankton and copepod abundance with depth, concurrent with decreases in food availability. Taxonomic richness was lowest at depth and below the deep-scattering layer. In contrast, copepod diversity peaked at these depths, suggesting lower competitive displacement in these more food-limited waters. Finally, omnivory and carnivory, were the dominant trophic traits, each one affecting the biological carbon pump in a different way. This highlights the importance of incorporating data on zooplankton food web structure in future modelling of global ocean carbon cycling.