Zooplankton Layers Research Articles

Integrated measurements of acoustical and optical thin layers I: Vertical scales of association

This study combined measurements from multiple platforms with acoustic instruments on moorings and on a ship and optics on a profiler and an autonomous underwater vehicle (AUV) to examine the relationships between fluorescent, bioluminescent, and acoustically scattering layers in Monterey Bay during nighttime hours in July and August of 2006 and May of 2008. We identified thin bioluminescent layers that were strongly correlated with acoustic scattering at the same depth but were part of vertically broad acoustic features, suggesting layers of unique composition inside larger biomass features. These compositional thin layers nested inside larger biomass features may be a common ecosystem component and are likely to have significant ecological impacts but are extremely difficult to identify as most approaches capable of the vertical scales of measurement necessary for the identification of sub-meter scale patterns assess bulk properties rather than specific layer composition. Measurements of multiple types of thin layers showed that the depth offset between thin phytoplankton and zooplankton layers was highly variable with some layers found at the same depth but others found up to 16 m apart. The vertical offset between phytoplankton and zooplankton thin layers was strongly predicted by the fraction of the water column fluorescence contained within a thin phytoplankton layer. Thin zooplankton layers were only vertically associated with thin phytoplankton layers when the phytoplankton in a layer accounted for more than about 18–20% of the water column chlorophyll. Trophic interactions were likely occurring between phytoplankton and zooplankton thin layers but phytoplankton thin layers were exploited by zooplankton only when they represented a large fraction of the available phytoplankton, suggesting zooplankton have some knowledge of the available food over the entire water column. The horizontal extent of phytoplankton layers, discussed in the second paper in this series, is likely an important factor contributing to this selective exploitation by zooplankton. The pattern of vertical offset between phytoplankton and zooplankton layers was consistent between studies in different years and using different combinations of platforms, indicating the importance of the relationship between zooplankton layers and the fraction of phytoplankton within a layer at night within Monterey Bay. These results highlight the value of integrating measurements of various types of organisms to understand thin layers processes and the importance of assessing ecological interactions in plankton thin layers within the context of the properties of the entire water column, like the animals themselves do.

Edge gradients provide evidence of ecological interactions in planktonic thin layers

Physical, optical, and acoustical data from Monterey Bay, California, USA, describing the distributions of phytoplankton, zooplankton, and small fish as well as gradients in the physical habitat were used to examine the interactions between vertically compressed plankton structures (thin layers), their consumers, and the local physical forces they experience. The relationship between upper and lower edge gradients of a layer were used to define layer ‘shape'. The steepness of the vertical gradient on the top vs. the bottom of the plankton layer was correlated to the difference in the relative abundance of consumers above and below the layer. Phytoplankton layer gradients were steeper when more zooplankton were present on one side of the layer vs. the other while zooplankton layers were more diffuse when a greater number of fish were adjacent to one side of the layer than the other. Both layer types showed nearly symmetrical gradients when predators were in low abundance or absent. Predator‐associated differences in phytoplankton and zooplankton layer shape were not correlated with vertical gradients in shear or mixing potential surrounding layers. In the absence of strong physical gradients, grazers can play an important role in structuring plankton thin layers. These observations likely represent one end of a continuum of biological and physical forcing responsible for formation and maintenance of thin plankton layers.

High-frequency acoustics and bio-optics in ecosystems research

Abstract Holliday, D. V., Donaghay, P. L., Greenlaw, C. F., Napp, J. M., and Sullivan, J. M. 2009. High-frequency acoustics and bio-optics in ecosystems research. – ICES Journal of Marine Science, 66: 974–980. The propagation of light and sound in the ocean’s interior is modified by the presence of phytoplankton, zooplankton, fish, gas bubbles, and dissolved and suspended material. Information is encoded in the levels and spectral characteristics of acoustic and optical scattering and absorption. Using acoustics and optics allows us to study the distribution of marine life and learn about ecosystem-relevant processes. Two studies are highlighted. In the first, multifrequency, upward-looking echosounders deployed near autonomous, bio-optical profilers were used to track vertical migration and the formation and size structure of <1 m thick, zooplankton layers in relation to the biomass and size structure of thin phytoplankton layers. In the second, a multifrequency sonar was used to track the temporal (seasonal) evolution of zooplankton biomass and size structure in the Bering Sea at intervals of 20 min. This paper focuses on how advanced technologies are being used to observe processes, distributions, and behaviour of marine life that have, until now, been hidden, as it were, from biological oceanographers.

Acoustical and optical measurements provide evidence of ecological interactions in planktonic thin layers.

Coincident physical, optical, and acoustical data were obtained in Monterey Bay, CA during an extensive study of planktonic layers. These concurrent data describing the distributions of phytoplankton, zooplankton, and small fish as well as gradients in the physical habitat were used to examine the interactions between vertically compressed plankton structures (thin layers), their consumers, and the local physical forces they experience. The steepness of the vertical gradient on the top versus the bottom of the plankton layer was correlated to the difference in the relative abundance of consumers above and below the layer. Phytoplankton layer gradients were steeper when more zooplankton were present on one side of the layer versus the other while zooplankton layers were more diffuse with a greater number of fish adjacent to one side of the layer than the other. Both layer types showed nearly symmetrical gradients when predators were in low abundance or absent. Differences in phytoplankton and zooplankton layer “shape” were not correlated with vertical gradients in shear or mixing potential surrounding layers. The consequences of trophic interactions within aggregations can be detected and assessed, and in the absence of strong physical gradients, grazers can play an important role in structuring plankton thin layers.

The influence of midwater hypoxia on nekton vertical migration

AbstractParker-Stetter, S. L., Horne, J. K., and Langness, M. M. 2009. The influence of midwater hypoxia on nekton vertical migration. – ICES Journal of Marine Science, 66: 1296–1302. Hypoxia affects pelagic nekton, fish and large zooplankton, distributions in marine and fresh-water ecosystems. Bottom hypoxia is common, but midwater oxygen minimum layers (OMLs) may also affect nekton that undergo diel vertical migration (DVM). This study examined the response of pelagic nekton to an OML in a temperate fjord (Hood Canal, WA, USA). A 2006 study suggested that the OML created a prey refuge for zooplankton. Using acoustics (38 and 120 kHz), the 2007 night DVM patterns of nekton were quantified before (June, August) and during (September) an OML. All months had similar precrepuscular distributions (>50-m depth) of fish and invertebrates. During the September evening crepuscular period, a zooplankton layer migrated upwards (>1.5 m min−1), but the layer's rate of ascent slowed to <0.5 m min−1 when it reached the lower edge of the OML. The bottom edge of the layer then moved below the OML and remained there for 13 minutes before moving through the OML at >1.0 m min−1. As in June and August, fish in September followed the upward migration of the zooplankton layer to the surface, crossing through the OML. Our results suggest that the 2007 OML did not affect zooplankton or fish vertical distributions.

Influence of the Columbia River plume on cross‐shelf transport of zooplankton

A high‐resolution sampling strategy along a cross‐shelf transect near the mouth of the Columbia River was used to determine the distribution, abundance, and spatial relationship between plankton and a tidally modulated river plume. During a strong ebb tide, zooplankton‐sized particles were concentrated on the seaward side of the plume front and transported 15 km across the shelf at a rate of 38 cm s−1, roughly fivefold faster than typical wind‐driven transport in the region. Physical processes associated with the developing plume vertically depressed relatively dense layers of phytoplankton and zooplankton an average of 7 m deeper into the water column beneath the plume and up to 10 km beyond the extent of the plume front. The enhanced cross‐shelf transport by the Columbia River plume is unique for this region of the Northern California Current. Its impact on the vertical and horizontal distribution of zooplankton shifts prey availability for higher trophic levels, such as larval fish, that primarily feed in the upper water column.

Three-dimensional structure of thin zooplankton layers is impacted by foraging fish

The role of predation by fish in the formation and persistence of zooplankton thin layers was assessed from three-dimensional observations using a multibeam sonar attached to a mechanical rotator. Zooplankton in Monterey Bay, CA, USA were found in intense layers with vertical scales of 0.5-2.5 m at night. These features comprised more than 90% of the zooplankton biomass and had volume scattering strengths of up to -35 dB. These thin zooplankton layers showed complex structure with significant though gradual undulations in their depth, thickness, and intensity. Fish spent significantly more time within zooplankton layers than was expected by chance, suggesting concentrated foraging activity. Sonar tracks of individual fish showed them diving through zooplankton layers. These dives were correlated with a decrease in the intensity of zooplankton scattering at the scale of tens of centimeters, resulting in the appearance of holes in the layer. Continued observation of layers revealed that these voids slowly filled in with zooplankton after the fish's departure. At the level of foraging observed, fish can have a significant effect on the distribution of zooplankton layers, however, the layers are resilient to apparent foraging by fish with reformation of the distinct layer often occurring post-predation.

The effects of vertical sampling resolution on estimates of plankton biomass and rate calculations in stratified water columns

The effects of vertical sampling resolution on estimates of plankton biomass and grazing calculations were examined using data collected in two different areas with vertically stratified water columns. Data were collected from one site in the upwelling region off Oregon and from four sites in the Northern Gulf of Mexico, three within the Mississippi River plume and one in adjacent oceanic waters. Plankton were found to be concentrated in discrete layers with sharp vertical gradients at all the stations. Phytoplankton distributions were correlated with gradients in temperature and salinity, but microzooplankton and mesozooplankton distributions were not. Layers of zooplankton were sometimes collocated with layers of phytoplankton, but this was not always the case. Simulated calculations demonstrate that when averages are taken over the water column, or coarser scale vertical sampling resolution is used, biomass and mesozooplankton grazing and filtration rates can be greatly underestimated. This has important implications for understanding the ecological significance of discrete layers of plankton and for assessing rates of grazing and production in stratified water columns.

Effects of mesoscale physical processes on thin zooplankton layers at four sites along the west coast of the U.S.

Within the coastal marine environment, populations of phytoplankton, zooplankton, bacteria, viruses, and aggregations of marine snow are frequently concentrated beneath the surface in discrete, vertically thin layers. Thin layers range in vertical dimension from a few centimeters to three meters, and have been observed to extend horizontally for kilometers. They appear in the water column episodically and can persist for days. We present some of the results of an investigation of four coastal sites along the west coast of the United States to assess frequency of occurrence of thin layers of zooplankton. Our study sites included coastal sites near East Sound, Washington; Cape Perpetua, Oregon; Monterey Bay, California; and Santa Barbara, California. At each site, we collected several, weeks-long time series of hydrography, current velocity, and acoustic backscatter due to mesozooplankton. Our results show that thin layers were common features at all four sites. Across all study sites, a change in the predominant physical regime, usually precipitated by a change in the wind pattern, corresponded with an absence of thin zooplankton layers. In order to make a first-order prediction about when thin layers have the possibility of occurring in a coastal environment, we found it useful to examine regional wind and circulation patterns and to determine how they affect stratification in each local environment.

Surface patterns of zooplankton spatial variability detected by high frequency sampling in the NW Mediterranean. Role of density fronts

High frequency sampling was performed in daylight hours along a 35 km transect in the Ligurian Sea to investigate the upper layer zooplankton distribution during the spring phytoplankton bloom. The results show detailed spatial structure and biomass of key zooplankton functional groups, copepods, salps and krill larvae, within the different water masses characterizing this region. Although observed values of total copepod biomass distribution were rather constant along the transect, species-specific patterns were observed in the copepod spatial distribution. The larger species Calanus helgolandicus, as well as Centropages typicus, Oithona spp., and Oncaea spp., were associated with the frontal zone. However, Acartia spp. had a scattered distribution, and Clausocalanus/Paracalanus did not have a clear pattern. In addition, krill larvae were concentrated in the frontal area and salps had a scattered pattern. The cross-shore zooplankton distribution appeared strongly influenced by both the Northern Ligurian current governing inshore waters, which acts as a major flushing forcing, and the Ligurian front, which governs offshore waters and may act as retention area for zooplankton.

Spatial and temporal variance of walleye pollock (Theragra chalcogramma) in the eastern Bering Sea

Mobile acoustic surveys attempt to map and count aquatic organisms without biasing abundance estimates. Horizontal and vertical movements by target species may influence density measurements and net samples during acoustic surveys. To investigate the influence of fish movement on density data, we compared temporal and spatial variability of walleye pollock (Theragra chalcogramma) in three sets (2 night, 1 day) of 14.8-km transects in the eastern Bering Sea. Walleye pollock density distributions were also compared with those in the five nearest daytime survey transects. We found that horizontal density distributions did not change at temporal scales ≤4 h and that spatial variance remained consistent at scales ≤2.5 km. Spatial variance density patterns were similar in transects sampled during the day compared with those sampled at night and were also similar in along-shore compared with cross-shore transects. Transects that contained two biological scattering layers could be vertically separated into zooplankton and fish. Spatial variance patterns in the upper zooplankton layer mimicked those of passive tracers, while patterns in the lower layer were consistent with those previously observed for mobile nekton. Current sampling resolution of acoustic surveys adequately captures horizontal spatial variance of walleye pollock in the Bering Sea.

Effects of physical processes on structure and transport of thin zooplankton layers in the coastal ocean

Thin layers of plankton are recurrent features in a variety of coastal systems. These layers range in thickness from a few centimeters to a few meters. They can extend horizontally for kilometers and have been observed to persist for days. Densities of organisms found within thin layers are far greater than those above or below the layer, and as a result, thin layers may play an important role in the marine ecosystem. The paramount objective of this study was to understand the physical processes that govern the dynamics of thin layers of zooplankton in the coastal ocean. We deployed instruments to measure physical processes and zooplankton distribution in northern Mon- terey Bay; during an 11 d period of persistent upwelling-favorable winds, 7 thin zooplankton layers were observed. These zooplankton layers persisted throughout daylight hours, but were observed to dissipate during evening hours. These layers had an average vertical thickness of 1.01 m. No layers were found in regions where the Richardson number was <0.25. In general, when the Richardson number is <0.25 the water column is unstable, and incapable of supporting thin layers. Thin zoo- plankton layers were also located in regions of reduced flow. In addition, our observations show that the vertical depth distribution of thin zooplankton layers is modulated by high-frequency internal waves, with periods of 18 to 20 min. Results from this study clearly show an association between phys- ical structure, physical processes and the presence of thin zooplankton layers in Monterey Bay. With this new understanding we may identify other coastal regions that have a high probability of support- ing thin layers.

Diel changes in vertical distribution patterns of zooplankton and walleye pollock (Theragra chalcogramma) off the Pacific coast of eastern Hokkaido, Japan, estimated by the volume back scattering strength (Sv) difference method

AbstractIn this study, we tried to accurately discriminate between zooplankton and walleye pollock using a quantitative echosounder at two frequencies (38 and 120 kHz). To estimate the abundance of walleye pollock and zooplankton (mainly Euphausia pacifica), an acoustic and trawling survey with bongo net sampling and CTD (X‐CTD) was conducted during June 2001 off the Pacific coast of eastern Hokkaido, Japan. Neocalanus copepods and E. pacifica were predominant in almost all the net samples. One size class of Neocalanus copepods (mean TL ± SD = 2.0 ± 1.1 mm, N = 1 044 080) and two size classes of E. pacifica (mean TL ± SD = 6.1 ± 2.2 mm, N = 13,099; mean TL ± SD = 15.4 ± 3.6 mm, N = 7628) were collected. The distribution patterns of these two classes and of walleye pollock were identified using the volume back scattering strength (Sv) difference method. On the continental shelf, the walleye pollock schools (small walleye pollock: mainly age 1 and 2) overlapped with almost all zooplankton layers during both day (near the sea bottom and mid‐water) and night (mid‐water and sea surface). Near and offshore of the continental shelf break, during day, the walleye pollock schools (large walleye pollock: mainly older than age 3) overlapped with almost all zooplankton layers near the sea bottom, but during night, the large walleye pollock schools (which were mainly distributed near the sea bottom) showed little overlap with most zooplankton layers (which were mainly distributed between mid‐water and sea surface). This result suggests differences in feeding strategies between small and large walleye.

Ocean acoustic backscattering: When you can ignore acoustic scatter from turbulence and when you can’t

While models predicting measurable levels of acoustic backscattering from oceanic turbulence have been around for decades, they have proven notoriously hard to confirm. This is, in part, because potential turbulent scattering layers often coincide with zooplankton layers. Therefore, as zooplankton are known to cause measurable acoustic scatter, the source of scatter is obscured. Furthermore, estimates of zooplankton abundance from acoustic scattering measurements have been shown to agree with independent measures under a number of circumstances. This gives circumstantial evidence to fuel the belief that scatter from turbulence is negligible. In addition, even if the turbulent scattering theory is correct, it predicts that over most of the ocean the turbulent intensities and/or stratifications are too weak to give turbulent scatter of a similar magnitude as is observed from zooplankton layers. Yet, despite all this, here data are presented that show circumstances when scatter from turbulence is significant. The areas of the ocean (mostly coastal) that one might have to be concerned about turbulent scatter are discussed, as well as the intriguing idea of using technology already developed for zooplankton observation to measure turbulence. This technology could facilitate the in situ study of biophysical interactions between zooplankton and turbulence.

Critical scales for understanding the structure, dynamics, and impacts of zooplankton patches

The development and application of high resolution acoustic systems by Holliday and Greenlaw has played a key role in demonstrating that zooplankton could can form highly concentrated layers ranging in thickness from a few decimeters to a few meters. These thin layers are remarkable in that they persist longer and have higher spatial coherence than multi-meter thick patches that can be sampled by nets and conventional acoustics. These results are inconsistent with the classical theory that thicker patches are more persistent and thus more important. Simultaneous measurements of temporal and spatial changes in the finescale physical, optical, chemical structure have provided important insights into the mechanisms controlling not only the dynamics of thin zooplankton layers but also their impacts on marine ecosystems and the transmission of sound in the ocean. [Work supported by grants from the ONR Biology and Chemistry program.]

Some aspects of sound extinction by zooplankton.

To address the importance of sound extinction (or the shadowing effect) in the presence of a densely aggregated zooplankton layer, acoustic scattering by weakly scattering fluid objects is studied theoretically. An analytical expression for the extinction cross section is obtained based on the forward scattering theorem and the analytical formula for the forward scattering amplitude is derived from the Modal Based-Deformed Cylinder Model (MB-DCM). The validity of the MB-DCM solution for the forward scattering amplitude and its sensitivity to geometrical and physical parameters is studied. Comparison with the PC-DWBA (Phase-Compensated-Distorted Wave Born Approximation) shows a reasonable agreement between the two models. The extinction cross section is shown to be proportional to (ka)2 and the induced attenuation is predicted for various species of zooplankton over a wide frequency range. It is shown that, under certain realistic conditions, such as the presence of a swarm of aggregated krill, sound extinction by zooplankton could influence the acoustic measurements significantly, as much as an 85% reduction in acoustic intensity for Euphausia superba with n = 5000 ind./m3 over a range of 50 m.

Mesoscale subduction at the Almeria–Oran front: Part 2. Biophysical interactions

The Almeria–Oran front forms where surface waters of Atlantic and Mediterranean origin meet at the eastern end of the Alboran Sea. A multidisciplinary field experiment on RRS Discovery in December 1996, in the second observational phase of the EU-funded Observations and Modelling of Eddy scale Geostrophic and Ageostrophic motions (OMEGA) project, observed the biological impact of mesoscale frontal instability of the Almeria–Oran frontal jet. It is concluded that periodic vertical velocities of ∼20 m/day, associated with the propagation of wave-like meanders along the front, have a significant effect on the vertical distribution of zooplankton across the front despite their ability to migrate at greater speeds. Observations of a layer of fluorescence coincident with subducted surface waters indicated that phytoplankton were drawn down and along isopycnals, by cross-front ageostrophic motion, to depths of 200 m. From the study of sound-scattering layers (SSL) identified in acoustic backscatter data, a layer of zooplankton was found coincident with the drawn-down phytoplankton. This layer persisted during and despite diel vertical migration. High-resolution optical plankton counter (OPC) data showed smaller zooplankton, which did not undertake diel vertical migration, remained concentrated near the surface in the fast-flowing frontal jet.

Biophysical interactions at the Almeria–Oran front

The Almeria–Oran front forms where Atlantic origin waters meet Mediterranean surface waters at the eastern end of the Alboran Sea. Following a multidisciplinary cruise on RRS Discovery in December 1996, analysis of temperature and salinity on density surfaces at the front showed that periodic subduction of Mediterranean surface waters (MSW) influenced the distribution of phytoplankton and zooplankton in the frontal region. A layer of fluorescence coincident with the subducted MSW indicated that phytoplankton were drawn down and along isopycnals to depths of 200 m. The study of scattering layers, identified with acoustic backscatter data from the shipboard ADCP and a SIMRAD EK500 echosounder, revealed that a layer of zooplankton was also found coincident with the drawn-down phytoplankton. This layer persisted during and despite diel vertical migration. Smaller zooplankton, not undertaking diel vertical migration, were concentrated in the fast-flowing frontal jet as seen in high-resolution OPC data. EK500 target-strength data indicate that the subducted communities may constitute different size classes to those in the surrounding waters. Samples from Longhurst Hardy plankton recorder tows across the front provide additional ground truthing for our acoustic observations. [Work supported by EU MAST, NERC, and DERA.]

Volume acoustic backscattering measurements with a 68-kHz cylindrical array

Most sensors designed to measure volume acoustic backscatter over the entire water column have limited horizontal coverage. The 68-kHz cylindrical arrays of the U.S. Navy’s Toroidal Volume Search Sonar (TVSS), towed 80 m below the sea surface in 200-m water depth, have been used to measure volume backscatter over the entire water column with about 4.5 deg of angular resolution over a swath roughly 300 m wide. The data were analyzed to determine the volume acoustic backscattering strengths and cross sections, and the target strengths of (1) near-surface bubble clouds in the tow ship wake; (2) midcolumn zooplankton layers; and (3) near-bottom fish schools. Unlike previous acoustic studies of ship wakes, these results are obtained from data which sample the bubble field at a constant distance from the ship. Echo integration over a midcolumn scattering layer is used to assess the patchiness of zooplankton in the region. Target strengths of near-bottom fish schools are easily distinguished from the surrounding seafloor backscattering strengths. [Work sponsored by ONR-NRL (Contract No. N00014-96-1-G913).]

Diel vertical migration of zooplankton and mesopelagic fish in the Arabian Sea

Acoustic (153 kHz ADCP and 12 kHz hull-mounted transducers) data and MOCNESS (MOC01 and MOC10) net tow samples collected in the Arabian Sea during the Spring Intermonsoon (April/May) and Southwest Monsoon (August) in 1995 documented substantial diel migrations of fish and zooplankton despite the year-round presence of an oxygen minimum (<0.2 ml l −1 at 125–150 m). Fish and zooplankton layers were distinguished by comparing 12 kHz sonar and 153 kHz ADCP backscatter data, which indicated that the strongly migrating layers were predominantly composed of fishes. Fish vertical migration speeds were independently estimated from the slopes of the volume scattering layers and from the vertical velocity components of the ADCP, yielding average speeds of 4 and 3 cm s −1 and maximum speeds of 13 and 10 cm s −1, respectively. A few migrating zooplankton layers were identified with an average speed of about 2 cm s −1 and maximum speeds as high as 8 cm s −1. Migration depths for both zooplankton and fish differed somewhat amongst stations and appeared to be related to local hydrographic conditions (principally the vertical gradients in DO and water temperature). Zooplankton displacement volumes at individual sites suggested that zooplankton biomass during the Southwest Monsoon could be as much as fivefold greater than during the Spring Intermonsoon. This observation was confirmed for the region in general by first deriving a relationship between ADCP backscatter intensity and daytime zooplankton biomass and then comparing the latter between cruises using daytime ADCP data taken along a 1500 km transect that extended from the coast of Somalia to the center of the northern basin.