Methods in Oceanography最新文献

Developing and validating data records from operational ocean color satellite instruments requires substantial volumes of high quality in situ data. In the absence of broad, institutionally supported field programs, organizations such as the NASA Ocean Biology Processing Group seek opportunistic datasets for use in their operational satellite calibration and validation activities. The publicly available, global biogeochemical dataset collected as part of the two and a half year Tara Oceans expedition provides one such opportunity. We showed how the inline measurements of hyperspectral absorption and attenuation coefficients collected onboard the R/V Tara can be used to evaluate near-surface estimates of chlorophyll-a, spectral particulate backscattering coefficients, particulate organic carbon, and particle size classes derived from the NASA Moderate Resolution Imaging Spectroradiometer onboard Aqua (MODISA). The predominant strength of such flow-through measurements is their sampling rate—the 375 days of measurements resulted in 165 viable MODISA-to-in situ match-ups, compared to 13 from discrete water sampling. While the need to apply bio-optical models to estimate biogeochemical quantities of interest from spectroscopy remains a weakness, we demonstrated how discrete samples can be used in combination with flow-through measurements to create data records of sufficient quality to conduct first order evaluations of satellite-derived data products. Given an emerging agency desire to rapidly evaluate new satellite missions, our results have significant implications on how calibration and validation teams for these missions will be constructed.

A global dataset of in situ particulate absorption spectra has been decomposed into component functions representing absorption by phytoplankton pigments and non-algal particles. The magnitudes of component Gaussian functions, used to represent absorption by individual or groups of pigments, are well correlated with pigment concentrations determined using High Performance Liquid Chromatography. We are able to predict the presence of chlorophylls $a,b$, and $c$, as well as two different groups of summed carotenoid pigments with percent errors between 30% and 57%. Existing methods of analysis of particulate absorption spectra measured in situ provide for only chlorophyll $a$; the method presented here, using high spectral resolution particulate absorption, shows the ability to obtain the concentrations of additional pigments, allowing for more detailed studies of phytoplankton ecology than currently possible with in-situ spectroscopy.

Measurements of particle area, organic suspended mass, and mineral suspended mass were collected at 9 sites on the west and south coasts of Great Britain. Multiple linear regression of particle area on organic suspended mass and mineral suspended mass was used to estimate the area-to-mass ratios of organic and mineral matter. Statistically, the null hypothesis that the organic area-to-mass ratio was 2 times the mineral ratio could not be rejected. Failure to reject this hypothesis may indicate that component particle composition is not correlated with the packing geometry of aggregated particles or the size of component particles that comprise the aggregates. Alternatively, correlations between particle parameters may exist, but they offset one another, thereby producing an organic area-to-mass ratio that is not significantly different from two times the mineral area-to-mass ratio.

We present a method for measuring one-dimensional surface wave spectra using a ship-mounted ultrasonic altimeter in combination with a motion correction device. The instruments are mounted at the bow of the ship and provide high-resolution, local, wave information. We present results from three recent field studies. The results are compared with data from a conventional waverider buoy and, when in-situ observations are not available, with wave model analyses and satellite altimetry. We find good agreement with regard to integrated parameters such as significant wave height and mean period. Comparison with a waverider demonstrates fair agreement with regard to spectral shape, but the representation of the low frequency part depends on the quality of the motion correction data.

A simple statistical clustering method is demonstrated to aid the identification of spatially repeating or persistent Lagrangian circulation patterns inherent in ensembles of drifting buoy trajectories. The technique is applied to the regional oceanic circulation about Japan and to the mesoscale circulation off Fukushima on Japan’s east coast. The drifter trajectories form a highly irregular data set in both space and time, with very different locations, configurations, cumulative travel distances (tens of metres to thousands of km), travel times (hours to years), and start and end positions. The interpolation technique of Kim, Kim, Ho, and Chu, Journal of Climate 24 (2) (2011), is found suitable to transform the spatially complex buoy data into a form suitable for statistical clustering. To resolve and give context to the very different spatial scales encountered in oceanic circulations the methods are applied in a hierarchical fashion to progressively smaller areas (120–180°E, 20–50°N; 140–160°E, 32–44°N; 140–145°E, 35–40°N). A winding number method is devised to identify and distinguish between clockwise and anti-clockwise sense of circulation. The analysis techniques provide what can be regarded as a spatial decomposition of Lagrangian flow fields. The methods do not require curve fitting or modelling, feature analysis, curve alignment, or spatial gridding, binning, and averaging, and are not dependent on density of observations. The methodology forms a useful data exploration technique for examination of trajectory data in general.

This paper explores the use of broadband acoustics to differentiate between biological scattering layers using observatory-based acoustic observations with minimal supporting biological observations. Targets and layer assemblages were classified based on 85–155 kHz acoustic data collected on the VENUS observatory in Saanich Inlet, B.C. between March 2008 and February 2010 using a clustering algorithm and different broadband acoustic data descriptors. First, a 6-h segment of data, for which there were coincident depth-resolved net-tow data, was analyzed. Clustering based on the calibrated spectrum of volume scattering strength for each target resulted in clusters that were distributed just as those resulting from clustering based on 120 kHz narrowband data because the clustering was dominated by the scattering level, rather than the spectral shape. When the target spectra were normalized, the clustering results were consistent with the different taxa found in the net samples, but often could not distinguish taxonomic groups. However, layers with distinct species assemblages had different distributions of target classifications, suggesting the assemblages could be distinguished using frequency-dependent scattering information. Ensemble-averaging the scattering observations and converting the spectral data to a 3-descriptor acoustic color representation prior to clustering was (1) more effective at distinguishing the dominant scattering layers based on their assemblages and (2) much more efficient in terms of computational cost. Clustering two years of acoustic-color data identified 4 main groups (diel migrating euphausiids and chaetognaths, fish, and a mix of pteropods and bottom-to-oxycline migrating amphipods) that were consistent with contemporaneous and historical observations of zooplankton in the inlet. A wider frequency band might be effective in better distinguishing individual zooplankton targets.

Environmental conditions within deep-sea ecosystems such as cold-seep provinces or deep-water coral reefs vary temporally and spatially over a range of scales. To date, short periods of intense ship-borne activity or low resolution, fixed location studies by Lander systems have been the main investigative methods used to investigate such sites.

Cabled research infrastructures now enable sensor packages to receive power and transmit data from the deep-sea in real-time. By attaching mobile research platforms to these cabled networks, the investigation of spatial and temporal variability in environmental conditions and/or faunal behaviour across the deep sea seafloor is now a possibility.

Here we describe one such mobile platform: a tracked Deep Sea Crawler, controlled in real-time via the Internet from any computer worldwide. The Crawler has been extensively used on the NEPTUNE Canada cabled observatory network at a cold-seep site at ∼890 m depth in the Barkley Canyon, NE Pacific. We present both the technical overview of the Crawler development and give examples of scientific results achieved.

A portable Particle Erosion Simulator (PES) device, also referred to as the BEAST (Benthic Environmental Assessment Sediment Tool) (Walker et al., 2008) has been re-designed for quantifying erosion in natural intact sediment cores. The BEAST was re-configured from an older design (Tsai and Lick, 1986), which had uncalibrated flow characteristics and was limited to viewing resuspension. In addition to calibrating friction velocity at the sediment–water interface, we employ a combination of real-time turbidity monitoring (via measurement of % transmission which decreases proportionally to suspended solid concentration) to quantify erosion threshold and calculate erosion rate, as well as digital imaging to document sequences of erosion and particle size response of resuspended material. The BEAST consists of a clear acrylic Plexiglas™ core liner with a perforated disc oscillating vertically in a piston motion. Performance of the device was calibrated by (a) comparing predicted to observed friction velocity as a function of motor speed, (b) using a hot film anemometer in the chamber to measure shear velocity, (c) verifying the applicability of anemometric calibration by relating the power of the grid stroke to stress dissipation, and (d) comparing measured critical stress of foundry sand to predictions from a validated model. Measurements indicate the friction velocity is uniform over >50% of the radial distance from the core center. Bottom stress is highly sensitive to the final height of piston down-stroke, a variable that can be altered to control the range of friction velocities. A plot of piston motor RPM vs. predicted $u_{\ast }$ was identical to the regression fit through the observed data. We verified that the proportionality between power input and thermistor heat dissipation corresponds to the scaling of $u_{\ast }$ and RPM, consistent with our calibration using the stress sensor. An example of an erosion sequence is demonstrated from a field core obtained in the Beaufort Sea in which two erosion stages were clearly indicated in the combined results from measurements of % transmission (to determine turbidity), particle size, and erosion rate. Our studies confirm that the BEAST has predictable flow characteristics expected from first principles, and that applied shear stress causes erosion in a way quantitatively similar to horizontal shear. In addition, the predicted erosion threshold of sand-sized particles corresponds to within 3%–18% of measured values made using the device. These multiple sources of BEAST validation demonstrate its practical capability to provide quantitative field measureme