Variability in the diel vertical migration timing of euphausiids in Saanich Inlet, British Columbia, Canada, is quantified using 2 years of echosounder data from a cabled observatory. The continuous and high-resolution nature of the observations allows examination of second-order seasonal variability in migration timing relative to civil twilight times. Early dusk ascent and late dawn descent occur during spring–fall, while late dusk ascent and early dawn descent occur during winter. Ascent timing appears to be regulated by (1) light availability at the daytime depth of the euphausiids, which is modulated by phytoplankton bloom shadowing, and (2) euphausiid size-dependent visual predation risk. Because (1) does not apply at dawn, descent timing appears to be regulated by (2). During the pre-spawning period, higher energy demand for reproduction may cause earlier dusk ascent and later dawn descent to maximize energy gain, even with larger body size. Instead of the traditional view of diel vertical migration timing, correlated solely with civil twilight, our data suggest that euphausiids also adapt their migration timing to accommodate changes in environmental cues as well as their growth.
Microstructure and acoustic profile time series were collected near Ocean Station P in the eastern subarctic North Pacific and in Saanich Inlet at the south end of Vancouver Island, British Columbia, Canada, to examine production of turbulent dissipation by swimming marine organisms. At Ocean Station P, although a number of zooplankton species are large enough to generate turbulence with Reynolds numbers Re > 1000, biomass densities are typically less than 103 individuals per cubic meter ( less than 0.01% by volume), and turbulent kinetic energy dissipation rates ε were better correlated with 16-m vertical shear than acoustic backscatter layers. In Saanich Inlet, where krill densities are up to 104 individuals per cubic meter (0.1% by volume), no dramatic elevation of dissipation rates ε was associated with dusk and dawn vertical migrations of the acoustic backscatter layer. Dissipation rates are a factor of 2 higher [‹ε› = 1.4 × 10-8 W kg-1, corresponding to buoyancy Re = ‹e›/(νN2) ∼ 140] in acoustic backscatter layers than in acoustically quiet waters, regardless of whether they are vertically migrating. The O(1 m) thick turbulence patches have vertical wavenumber spectra for microscale shear commensurate with the Nasmyth model turbulence spectrum. However, the turbulence bursts of O(10-5 W kg-1) proposed to occur in such dense swarms appear to be rare. Thus far, intense turbulent bursts have been found infrequently, even in very dense aggregations O(104 individuals per cubic meter) characteristic of coastal and high-latitude environs. Based on sampling to date, this corresponds to a frequency of occurrence of less than 4%, suggesting that turbulence production by the marine biosphere is not efficient.
This study described the phenology and patterns of variability of forage fish and mesozooplankton populations near the Columbia River plume. Our objective was to identify the timing of the seasonal appearance of forage fish and to characterize the temporal patterns of abundance in relation to ocean conditions including zooplankton availability. Observations were collected at 2 stations in 2008 and 2009 using 200 kHz bio-acoustic moorings, and with bi-weekly net sampling surveys conducted at nearby stations to measure total fish density and relative species composition. Acoustic time series results revealed that the seasonal timing of acoustic schools, representing the forage fishes northern anchovy Engraulis mordax, whitebait smelt Allosmerus elongatus, and Pacific sardine Sardinops sagax, occurred abruptly, with specific dates of appearance identified at each station in mid-May in both years. Both 2008 and 2009 represented very similar years for the timing and strength of wind-driven coastal upwelling. The timing of forage fish appearances in these years was linked with ocean temperature and salinity data collected at a nearby oceanographic buoy but was poorly correlated with mesozooplankton abundance, which was highly variable and fluctuating over a period of hours to days. Fluctuations in zooplankton and forage fish populations drive many trophic interactions, including juvenile salmon, seabirds, and large predators (e.g. adult salmon) that rely on the timing and abundances of these populations and have significant implications for ecosystem based management.
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.
Micronekton in deep-scattering layers around the Hawaiian Islands undergo diel migrations with both vertical and horizontal components. We sought to determine whether resource availability provides an adaptive explanation for this migration. We simultaneously measured the spatio-temporal patterns of micronekton, using acoustics and imaging optics, and of their potential zooplankton prey, using net tows, acoustics, and optics. Zooplankton biomass, density, and total abundance were higher at night than prior to sunset at nearshore sites, whereas relatively little diel variation was observed offshore. All measures of zooplankton availability were 5 to 6 times higher nearshore than offshore during nighttime hours when migrating micronekton species were nearshore. There was no significant nearshore-offshore gradient in zooplankton prior to sunset, leading to 2 possible explanations for the day-night patterns in zooplankton: benthic emergence and vertical migration coupled with horizontal motion. Analysis of taxonomic patterns from net tows did not support the benthic emergence hypothesis. All 3 zooplankton assessment techniques supported the conclusion that zooplankton distribution could favor horizontal migration by micronekton given the pressures for micronketon to be in deep water during daylight to avoid predators. Recently published work has shown that small animals (2 to 10 cm in length) in scattering layers comprised of micronekton travel distances of at least 11 km roundtrip each night, often against currents, to obtain these increased food resources. The length and likely cost of the journey provides some insight about the importance of the potential feeding gains.
Upward-looking acoustic Doppler current profilers (300 kHz) and echo sounders (125 kHz) were deployed on moorings on- and off-shelf to the northwest of South Georgia between 14 October 2002 and 29 December 2005 to measure density of Antarctic krill and environmental parameters continuously. A distinct seasonal pattern in krill density, recurring consistently over all three years, was detected. Krill densities in winter were predominantly low (mean = 18.7 g m(-2), SD = 24.3), but had risen substantially by summer in each year (mean = 89.5 g m(-2), SD = 64.2). A sinusoidal regression model (period = 52 weeks) with time as the independent variable explained 64% of the observed week-to-week variation. Estimates of krill density from moored instruments were not statistically different (P > 0.05) from estimates derived from standard ship-based krill surveys in adjacent time periods, suggesting that the point estimates from moored instruments were representative of krill density in a wider spatial context (ship surveys cover c. 100 x 100 km). Data from moored instruments were used to explore whether high-frequency temporal variation (i.e. within-year) could have led to the perceived between-year variation in krill density arising from previous summer surveys in the South Georgia western core box region between 1990 and 2005. Comparison of these 'snap-shot' ship survey estimates with the observed pattern of within-year variability showed that some of the apparent 'year-to-year' variation could simply be attributed to sampling on different dates of the year (e.g. November cf. February). However, there were some survey estimates that were significantly different (P less than 0.01) from the regression-predicted within-year variation. Years that stand out for markedly low krill density (i.e. densities below the range expected due to intra-annual variation) were 1993/94, 1998/99 and 1999/2000. Moored instruments provide valuable data that could be important for ecosystem-based management at South Georgia because, for example, they will enable predator-prey functional responses to be explored there for the first time at appropriate temporal scales, and will enable hypotheses relating variation in krill abundance to physical oceanographic variability to be tested.
Models and laboratory experiments show that zooplankton may locate food more easily in turbulent waters, but whether plankton seek or avoid turbulence in the ocean is an open question. It is difficult to measure turbulence and plankton simultaneously and with the necessary spatial resolution using traditional methods (nets and airfoil shear sensors). Acoustics is commonly used to survey zooplankton abundance and recent studies have shown that stratified turbulence can also be a significant source of sound scatter. This may seem like more of a complication than a boon for those aiming to use acoustics to observe plankton in turbulence. We present acoustic data, however, that show that zooplankton and turbulence can be observed simultaneously with a single 307 kHz sounder. The different natures of the two targets (discrete targets versus a volume effect) allow them to be distinguished. The key is sampling the same targets at multiple ranges. The volume scattering strength of a discrete target will increase as the target nears the sounder, because the volume sampled decreases. Turbulence, as a volume scattering effect, has little range dependence to its volume scattering strength.
This study evaluates the effectiveness of a 200-kHz inverted echo sounder for monitoring the abundance and behavior of near-surface zooplankton and fish. Data from both oceanic and littoral environments are examined: first from an 81-day deployment at Ocean Station Papa (OSP) in the northeast Pacific Ocean during the spring of 1996, and second from an 8-day deployment in the southern Strait of Georgia in September 1998. The analysis combines calibrated backscatter intensity, echo-amplitude statistics, and acoustic-scattering models to produce estimates of mean scatterer size and abundance. The identity of the various scatterer classes is deduced from local net trawls and reference to previous studies. At the OSP site the dominant scatterers were found to be euphausiids, pteropods, and myctophid fishes, with mean lengths of 15, 1.5, and 28 mm, respectively. At the Strait of Georgia site three fish size classes were identified: juvenile herring with mean length near 10 cm, juvenile salmon with mean length of 20 cm, and there was weak evidence for an adult salmon class. Overall, the acoustically derived abundance estimates were in reasonable agreement with the local net trawls and results from previous studies. The usefulness of sustained monitoring over diurnal and seasonal time scales is demonstrated with the OSP zooplankton data.
While acoustic scatter from oceanic turbulence is sensitive to temperature–salinity covariations, there are unfortunately no published measurements of the turbulent temperature–salinity co-spectrum. Several models have been proposed for the form of the co-spectrum of two scalars in turbulence, but they all produce unsatisfactory results when applied to the turbulent scattering equations (either predicting negative scattering cross-sections in some regimes or predicting implausible levels of correlation between temperature and salinity at some scales). A new model is proposed and shown to give physically plausible scattering predictions in all density regimes. High-frequency acoustic data illustrate the importance of the co-spectrum for acoustic scattering, but were collected in a density regime where there is little difference between the co-spectrum models.
Co-located measurements of acoustic backscatter and temperature/velocity microstructure are used to confirm theoretical predictions of sound scatter from oceanic turbulence. The data were collected with a torpedo-shaped vehicle carrying four shear probes and two thermistors on its nose, and forward-looking 44.7 and 307 kilohertz echosounders (mounted 20 centimetres below the turbulence sensors). The vehicle was towed through the stratified turbulence that forms tidally over the lee side of a sill in a British Columbia fjord. Conventional downward-looking echosounder measurements were also made with a 100 kilohertz sounder mounted in the ship’s hull. Populations of amphipods, euphausiids, copepods and gastropods were present in the fjord (sampled with 335-micrometre mesh vertical net-hauls) and could be seen in the sounder data. These plankton net-hauls indicated that there were too few zooplankton in the turbulent regions to account for the scattering intensity. At both 44.7 and 307 kilohertz, scatter that is unambiguously correlated with turbulence was observed. Turbulent scatter is much stronger at the higher frequency, illustrating the mportance of salinity microstructure—long neglected in turbulent scattering models—and shedding some light on the form of the turbulent temperature-salinity co-spectrum. The turbulent temperature-salinity co-spectrum has never been measured directly. Although several models have been proposed for the form of the co-spectrum, they all produce unsatisfactory results when applied to the turbulent scattering equations (either predicting negative scattering cross-sections in some density regimes or predicting implausible levels of correlation between temperature and salinity at some scales). A new co-spectrum model is proposed and shown to be not only physically plausible in all density regimes, but also in reasonable agreement with the scattering data. At 307 kilohertz, the backscatter is mostly from salinity microstructure and, depending on the strength of the stratification, can be as strong as—or stronger than—the signal from a zooplankton scattering layer. This could easily confound zooplankton biomass estimates in turbulent regions. The two targets’ different natures (discrete targets versus a volume effect) often allow them to be distinguished even when they occur simultaneously. The key is sampling the same targets at multiple ranges. At long-range, discrete targets have a constant volume scattering strength proportional to their number density. The sampling volume, however, decreases as the targets approach the sounder. At some range there will be only one (or no) target in the sampling volume and the volume scattering strength will increase (or disappear) as the target continues to near the sounder. Turbulence, as a volume scattering effect, has no range dependence to its volume scattering strength. Thus, by examining the scattering nature at close range we can distinguish discrete targets (like zooplankton) from turbulence.