Plankton Layers Research Articles

Airborne Bioaerosol Observations Imply a Strong Terrestrial Source in the Summertime Arctic

AbstractPrimary biological aerosol (PBA) is a component of coarse mode aerosol which may affect climate and health. The possible climate impacts arise from interactions between PBA and water vapor, especially since some PBA nucleate ice at warm temperatures. The health impacts span from seasonal allergies to transmission of pathogens. Despite their potential importance, the emissions, transport, and atmospheric distribution of PBA are poorly understood, especially at high latitudes where cloud effects could be pronounced. Here we report summertime measurements of fluorescent aerosol (a proxy for PBA) over the Bering and Chukchi Seas using a Wide‐Band Integrated Bioaerosol Sensor aboard a Twin Otter, alongside a lidar which detected water column productivity. Most observations occurred at 300 m over the ocean with periodic excursions to 60 and 900 m. Loadings were always low in the marine boundary layer, despite the presence of subsurface plankton layers and in contrast to other recent reports, likely indicating low oceanic emissions during our study. Large variability was observed in PBA aloft, with higher concentrations approaching those observed over the Continental US. Back trajectory analysis showed that high loadings were associated with recent transit through the continental boundary layer and we estimate PBA emissions from the Arctic tundra of up to 300 m−2 s−1 at the warmest observed temperatures. On days with strong transport from land (∼50% of our flights), PBA accounts for 12% of supermicron number and 64% of supermicron volume, indicating potentially significant effects on the albedo, glaciation and lifetime of Arctic clouds.

Investigation of the impact of hydrodynamic conditions on sediment resuspension in shallow lakes

ABSTRACT In this study, we sampled the original sediment columns from three points located in the eastern, central, and western areas of Chaohu Lake, and then investigated sediment resuspension under hydraulic disturbances in the laboratory. The results showed that when the hydraulic disturbance is weak and sediment surface shear stress is small (<0.02 N/m²‌), sediment surface layer moves alone. When the hydraulic disturbance is moderate and sediment surface shear stress is within a threshold (3.7 N/m²‌), only a small proportion of the surface sediments begins to move. However, when the hydraulic disturbance is further strengthened and sediment surface shear stress is larger than the threshold value, surface sediments move smoothly. By the experimental results, we found that the plankton layer is about 1 mm (1.56% of the maximum content of suspended matter (SSCmax)), which is the major contribution of individual sediment initiation. The new sediment layer is about 19–22 mm (20.9% of the SSCmax), which considerably affects sediment resuspension in shallow lakes. The consolidated layer is about 22 mm below the bed surface, which requires high shear stress and is hard to be transferred to the water column. The quantified experimental observation results are further verified with the theoretical results.

Shipborne variable-FOV, dual-wavelength, polarized ocean lidar: design and measurements in the Western Pacific.

For the requirement of high-precision vertical profile of the polarization and optical properties of natural seawater, a ship-borne variable-FOV, dual-wavelength, polarized ocean lidar system is designed to obtain the volume linear depolarization ratio (VDR), color ratio and optical parameter profiles of seawater. With the high signal-to-noise ratio, which benefits from the high power (355 nm with 120 mJ, 532 nm with 200 mJ) solid-state laser and a photon counting recorder with a sampling rate of 1 GHz, the attenuated backscattered signal of seawater in the western Pacific campaign reaches to the depth of 50 m, where a plankton layer presents. The receiver of lidar is capable of switching to wide and narrow field of view (FOV), respectively, to obtain the lidar attenuation coefficient Klidar, which is in good agreement with the beam attenuation coefficient of seawater c with a narrow FOV and diffuse attenuation coefficient Kd with a wide FOV. Besides, the Klidar, and the VDR, at two wavelengths of 355 nm and 532 nm are compared to explore the possibility of multi-wavelength of laser application in the ocean lidar. The VDR and the color ratio profiles have a desirable correlation with the in-situ measurement of chlorophyll a (Chla) and chromophoric dissolved organic matter (CDOM) profiles, respectively. With the combination of the Klidar, the VDR and the color ratio profiles, measured in different regions and time periods during the campaign, the multi-wavelength and polarization lidar shows its potential to explore various ocean compositions, such as the ocean particles size shape, the species and vertical migration characteristics of planktons, and the profile distribution of the ocean compositions.

Airborne hyperspectral detection of underwater camouflaged targets and the effect of target shape and seafloor on image quality

The goal of the present work is to increase the detectivity of hyperspectral imaging systems, deployed on airborne marine search and rescue platforms (SAR), also the increase of the probability for camouflaged target detection and recognition through optimization of image quality. Hence, water column spectral characteristics will be simulated, bringing into consideration the effect of both air layer and plankton layer. This experimental simulation is based on hyperspectral imaging of camouflaged dummy targets of different shapes in a special aquarium designed for this purpose, where an argon laser source was used as multi-spectral line target illuminator and three different bottom backgrounds were tested.

Stratification, plankton layers, and mixing measured by airborne lidar in the Chukchi and Beaufort seas

A total of 4.9 million vertical profiles of optical backscattering were measured by airborne lidar in July of 2014 and July of 2017 in the Chukchi and Beaufort seas. We found very different ice conditions in the study area between July 2014 and July 2017, but the characteristics of subsurface plankton layers measured by the lidar and their dependence on ice cover were similar for the two years. In both years, the prevalence of subsurface plankton layers exponentially decreased with increasing ice cover. The average depths were similar for both years, with layers in open water deeper than those in the pack ice. The depths of subsurface plankton layers were consistent with mixed layer depth in areas where in situ density profiles were available. A noticeable difference in layer strength (defined as the ratio of the layer signal to the background) was likely caused by higher background phytoplankton concentrations in 2017. Differences in layer thickness were observed, which could be the result of higher current shears in 2017. Turbulent mixing of phytoplankton and zooplankton in Barrow Canyon was inferred from the power spectral density of lidar and acoustic scattering. Lidar measurements suggested that the level of turbulence and its vertical distribution were affected by local upwelling-favorable winds. The vertical distribution of acoustic scattering was different from that of the lidar, which we interpret as different vertical distributions of phytoplankton and zooplankton.

Detecting subsurface phytoplankton layer in Qiandao Lake using shipborne lidar.

Qiandao Lake is located in the northern edge of subtropics, and its water body is thermally stratified in summer. It is of great scientific significance to study the vertical physical and chemical indexes and phytoplankton characteristics of the Qiandao Lake to reveal the aquatic ecosystem structure of the thermally stratified lake. Conventional observation uses in-situ profile instruments, which is time consuming and labor intensive. In recent years, lidar has shown increasing oceanic applications; however, it has not yet been extensively applied in inland water. There are no studies using lidar for detecting subsurface plankton layer in Qiandao Lake. In this study, we investigated the applicability of this technology for identifying subsurface plankton layer. A simple and fast phytoplankton layer detection method was introduced. The lidar-detected layer was found to well correspond with that of the in-situ measured subsurface chlorophyll maximum layer (SCML) and phycocyanin maximum layer. Primary results show that lidar and our detection method are effective for subsurface phytoplankton layer detection. They can serve as a good monitoring tool for studying inland water stratification.

Influence of wind events on larval fish mortality rates in the southern California Current Ecosystem

Wind-induced mixing can affect the vertical distribution of plankton in the upper water column, influencing the prey available for larval fishes. The stable ocean hypothesis proposes that periods of calm winds facilitate the development of plankton layers at concentrations sufficient for successful larval foraging and increased survival. Conversely, storm events redistribute prey, leading to reduced foraging success. Here, we investigate this hypothesis by comparing larval fish mortality rates estimated from 37 years of ichthyoplankton data against metrics of wind events defined as storms and calm periods. Contrary to expectations, we found that mortality for Pacific hake (Merluccius productus) significantly decreased as storm events increased in the southern California Current Ecosystem. Mortality rates for northern anchovy (Engraulis mordax), Pacific sardine (Sardinops sagax), Pacific mackerel (Scomber japonicus), and jack mackerel (Trachurus symmetricus) had no relationship to storms, and no species’ mortality rates were related to the number of calm events. Our results highlight the differing sensitivities of larval survival among fishes in the region and indicate that responses to atmospheric processes are species-dependent.

Subsurface plankton layers observed from airborne lidar in Sanya Bay, South China Sea.

In recent years, airborne lidar has been used in a wide range of oceanic applications, including detection of bathymetry, bubbles, internal waves, and schools of fish. However, it has not yet been extensively applied in Chinese seas. For example, there have been no studies to detect subsurface plankton layers in the South China Sea (SCS) by airborne lidar. In this study, we investigated this technology's applicability for identifying subsurface plankton layers in Sanya Bay, SCS. Three airborne lidar flight experiments were carried out in March 2018 and in September 2017. Shipboard synchronous measurements were carried out in March 2018 to validate the lidar measurements. The method that is presented here can be used to detect a subsurface plankton layer, which is characterized by depth, thickness, and intensity. Compared with chlorophyll-a profile synchronously measured by shipborne fluorometer, there was a consistent relationship. The subsurface plankton layer depth error was less than 0.7 m. Next, the spatial distribution and seasonal variation of lidar measured subsurface plankton layers in Sanya Bay, SCS, was analyzed. The results showed that airborne lidar can potentially detect subsurface plankton layer within 50 meters deep in relatively clear water. This will enhance our understanding of biogeochemical processes in these optically complex aquatic systems.

Semi-analytic Monte Carlo radiative transfer model of laser propagation in inhomogeneous sea water within subsurface plankton layer

Current Monte Carlo simulation methods of laser propagation in sea water are mostly based on the assumption of homogeneous water. In this study, a semi-analytic Monte Carlo radiative transfer model is developed to study laser propagation in inhomogeneous sea water within subsurface plankton layer. It is based on analytical estimate of the probability of collection by a remote receiver of scattered or emitted photons; in particular, the water optical input parameter of vertical profile is no longer simply set as a constant. In the new transfer model, the input optical vertical profile can vary with a resolution as high as 0.11 m. Using this model, we study the influences of water optical properties, multiple scattering, wind-driven sea surface condition, and lidar incident angle. The effective lidar attenuation coefficient is found to depend on the field of view of the lidar system: it approaches attenuation coefficient with a narrow field of view, and approaches absorption coefficient with a wide-enough field of view. We also find the lidar transmittance through the air-sea interface decreases significantly as the wind speed increases to greater than 7 m/s, and the transmittance can even be reduced by half when the wind speed reaches 18 m/s. Finally, we compare the simulations by our new model with airborne lidar measurements. We find that the simulation curve is quite similar to that of the lidar measurements. The absolute error of normalized simulation signals compared with the measurements is within 0.05, suggesting the effectiveness and applicability of our approach for inhomogeneous water.

Larval fish collected from sound-scattering layers in an offshore tropical area.

The composition of the larval fish assemblage in the sound-scattering layer of the continental shelf waters off the coast of south-eastern Brazil (12 and 22° S), a research project that is part of the Brazilian programme Avaliação do Potencial Sustentável de Recursos Vivos na Zona Econômica Exclusiva (REVIZEE), is described. Samples were collected during daylight hours and at dusk at five oceanographic stations in the winter of 1999 using an Isaacs-Kidd Midwater Trawl (IKMT). The oceanographic stations were chosen based on the detection of plankton layers by acoustic observation. A total of 2192 larval fish were identified, comprising 52 families and 62 species. Maurolicus stehmanni (Sternoptychidae) was the most abundant species found within the study area, comprising 18·5% of all identified larvae, followed by Psilotris celsus (Gobiidae) at 10·9%.

Inversion of oceanographic profiling lidars by a perturbation to a linear regression.

We present a simple, robust inversion for airborne oceanographic lidar profiles. A linear regression to the logarithm of the return is followed by a perturbation to obtain a backscatter estimate. For typical thin plankton layer examples, errors are expected to be <10% over 90% of the ocean. The inversion was applied to lidar data off the coast of Florida, where the correlation between lidar backscatter at 5m and surface chlorophyll concentration from satellite ocean color measurements was 0.92.

Bio-optical model to describe remote sensing signals from a stratified ocean

We use a bio-optical model of the optical properties of natural seawater to investigate the effects of subsurface chlorophyll layers on passive and active remote sensors. A thin layer of enhanced chlorophyll concentration reduces the remote sensing reflectance in the blue, while having little effect in the green. As a result, the chlorophyll concentration inferred from ocean color instruments will fall between the background concentration and the concentration in the layer, depending on the concentrations and the depth of the layer. For lidar, an iterative inversion algorithm is described that can reproduce the chlorophyll profile within the limits of the model. The model is extended to estimate column-integrated primary productivity, demonstrating that layers can contribute significantly to overall productivity. This contribution also depends on the chlorophyll concentrations and the depth of the layer. Using passive remote sensing alone to estimate primary productivity can lead to significant underestimation in the presence of subsurface plankton layers. Active remote sensing is not affected by this bias.

Macroorganism Paleoecosystems During the Middle-Late Ediacaran Period in the Yangtze Block, South China

The evolution of paleoecosystems was often accompanied by the expansion of ecological niches; organismal habitats extended from the sediment surface to the water column, and then to the interior part of the sediment. A major step in ecosystem innovation was recorded in the macrobiota of the upper Doushantuo Formation during the middle-late Ediacaran Period, including the Miaohe biota from western Hubei and the Wenghui biota from northeastern Guizhou, in the Yangtze Block, South China. The macrobiota was dominated by branching and unbranching macroalgae, with abundant metazoans (including worms or vermiform animals, trilobozoans, poriferans, and medusiform animals) and ichnofossils. They lived in a warm and calm tropical ocean, with plentiful sunlight, oxygen, and nutrients. The habitats of the organisms can be subdivided into three layers of morphology and paleoecology: the epibenthic layer, the erect-benthic layer, and the plankton layer. In the case of the epibenthic and erect-benthic layers, the organisms lived on the depositional surface, or were fixed on the seafloor; in the plankton layer, the organisms floated in the water column or on the water surface. In middle-late Ediacaran macroorganism paleoecosystems, the following changes occurred: diversification, the intensification of competition between organisms, a reinforcement of the ability of organisms to change their surroundings, and the effective conversion of bioenergy. The macroorganism paleoecosystem was a key transition in the establishment of a complicated and multilayered ecological pyramid and increasing atmospheric oxygen, and a prelude to the Cambrian explosion.

Subsurface plankton layers in the Arctic Ocean

AbstractThe first synoptic measurements of subsurface plankton layers were made in the western Arctic Ocean in July 2014 using airborne lidar. Layers were detected in open water and in pack ice where up to 90% of the surface was covered by ice. Layers under the ice were less prevalent, weaker, and shallower than those in open water. Layers were more prevalent in the Chukchi Sea than in the Beaufort Sea. Three quarters of the layers observed were thinner than 5 m. The presence of these layers, which are not adequately captured in satellite data, will influence primary productivity, secondary productivity, fisheries recruitment, and carbon export to the benthos.

Airborne lidar detection and characterization of internal waves in a shallow fjord

A dual-polarization lidar and photography are used to sense internal waves in West Sound, Orcas Island, Washington, from a small aircraft. The airborne lidar detected a thin plankton layer at the bottom of the upper layer of the water, and this signal provides the depth of the upper layer, amplitude of the internal waves, and the propagation speed. The lidar is most effective when the polarization filter on the receiver is orthogonal to the transmitted light, but this does not depend significantly on whether the transmitted light is linearly or circularly polarized. The depolarization is greater with circular polarization, and our results are consistent with a single parameter Mueller scattering matrix. Photographs of the surface manifestation of the internal waves clearly show the propagation direction and width of the phase fronts of the internal waves, even though the contrast is low (2%). Combined with the lidar profile, the total energy of the internal wave packet was estimated to be 9 MJ.

BIOPHYSICS, BIOENERGETICS AND MECHANISTIC APPROACHES TO ECOLOGY

Biophysics – the study of how the structure of individual plants and animals contributes to their function – has rarely been considered on the ecological scale. However, Mark Denny from Stanford University, USA, has a broader vision. Because the mechanisms that underpin individual performance

Planktonic layers: New insights stimulated by Van Holliday

During the late 1980s and early 1990s, initial evidence for persistent thin layers of phytoplankton (less than 1 m in thickness) in the upper ocean was obtained with optical instrumentation. These observations were inconsistent with the existing understanding of small-scale vertical mixing processes and raised questions about the ecological consequences of such structures, should they prove to occur on a regular basis. Dr. D. Van Holliday quickly saw the need for new approaches in acoustical oceanography to address these important ecological questions. Over a period of a few years, Dr. Holliday developed new acoustical tools that could be deployed in concert with other sensors. His moorings with customized high-frequency transducers allowed acoustical resolution over scales of centimeters, sufficient to capture the acoustic signature of marine zooplankton and their spatial relationship to their food supply. The insights provided by Van's work, and his constant encouragement to his colleagues, have led to ...

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.

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.

Physical and biological controls on the maintenance and dissipation of a thin phytoplankton layer

During an 8.5 h overnight study in Monterey Bay in 2005, continuous, high-resolution vertical profiles of a thin layer of phytoplankton as well as physical properties of the water were mea- sured. The thin layer was populated by strong-swimming, vertically migrating dinoflagellates. This dataset provided a unique opportunity to apply an existing mathematical model that describes how competing convergence and divergence mechanisms contribute to the formation and structure of thin plankton layers. Compared to straining by shear, cell settling velocities, and plankton motility, vertical displacements caused by the passage of internal waves had the greatest influence on changes in layer thickness. Thus, for a model to accurately describe thin-layer dynamics in an envi- ronment with internal waves, the convergent and divergent effects of the vertical oscillations caused by internal waves must be considered.