OS53B-01 INVITED 13:40h
Extreme and Episodic Events: Their Roles in Physical-Biological Interactions
Biological and physical interactions occur on all time and space scales. However, the roles of extreme and episodic events have remained largely unknown because of observational constraints. Within the past few years,advanced bio-optical, biological, and physical instruments have enabled the collection of data at unprecedented rates under even the most severe oceanic conditions. Recent results, which suggest the important roles of phenomena such as hurricanes, typhoons, mesoscale eddies, and Rossby waves in affecting the optics, biology, ecology, biogeochemistry, and marine geology of the ocean, are reviewed.
http://www.opl.ucsb.edu
OS53B-02 INVITED 13:55h
Convection and the Seeding of the North Atlantic Bloom
The springtime North Atlantic bloom occurs when the reduction in vertical mixing and increased daylight allows overwintering diatoms to grow rapidly. However, these diatoms sink sufficiently fast during the winter to end up far below the springtime euphotic zone. How, then, is the spring bloom is seeded? This question is explored using vertical velocity data from neutrally buoyant floats. The large vertical velocities which occur in the deep wintertime convective layers can act to retain some of the diatoms near the surface as a seed population. This model is found to apply in the Labrador Sea, but not near the southern edge of the bloom near Bermuda. The depth of wintertime convection relative to cell sinking rate may thus limit the southern extent of the spring diatom bloom.
OS53B-03 INVITED 14:10h
Planar Laser Imaging Fluorometry In Situ: Microscale Distributions of Phytoplankton in Relation to Their Physical Environment
Using a Planar Laser Imaging Fluorometry (PLIF) system mounted on a free-falling platform, we have observed the spatial distribution patterns and size-frequency spectra of large fluorescent particles in the upper ocean. In drops of the system off San Diego, CA, we found that the size-frequency spectrum was strongly related to the total phytoplankton biomass (measured as chlorophyll a). Differences of the observed spectra from those predicted based on biomass showed regions suggesting aggregation and sinking of particles, systematic and sudden changes in spectra with depth, and associations of anomalously large particles with hydrographic features. The relative spatial distributions of particles in the images showed the particles to be non-randomly distributed on scales of a few cm, except in regions of high concentration (the subsurface chlorophyll maximum). Some images in every profile were found to be streaky, indicating that the water moved while the camera shutter was open. A careful analysis of the source of the streaking showed that the length and angle of the streaks, along with the heading and pressure records of the platform, give information on the vertical shear of the water column. Regions of high vertical shear were mostly confined to pycnoclines and micro-pycnoclines. Sudden changes in the biological microstructure were more often associated with density gradients than regions of high vertical shear.
http://jaffeweb.ucsd.edu/
OS53B-04 INVITED 14:25h
The Formation and Maintenance of Finescale Planktonic Structure in the Upper Ocean: Evaluating the Balance Between Vertical and Horizontal Processes
Numerous finescale (1 m to 3 m) and microscale (0.01 m to 1 m) observations of the vertical structure of plankton have revealed local maxima of phytoplankton and zooplankton within narrow layers (0.5 to 2 m thick). These layers have been observed to persist over ecologically relevant time intervals (hrs to days), while extending many kilometers in the horizontal dimension. What are the limits on the physical and biological processes that permit planktonic layers to persist on these temporal and spatial scales? This presentation will examine the formation and maintenance of planktonic layers through an evaluation of the interplay between intermittent small-scale vertical mixing and vertical gradients in horizontal advection. The analysis will focus on observations made over the Oregon continental shelf, where planktonic thin layers have been documented with four different systems: 1) a free-fall profiling system that provided centimeter-scale resolution of hydrographic properties, bio-optical properties, and vertical shear; 2) a undulating towed system (MiniBat) that provided 0.25m resolution of hydrographic and bio-optical properties; 3) a Bluefin AUV that provided vertical shear, hydrographic and bio-optical properties along a 15-20km track while undulating across a thin planktonic layer; 4) vertical shear and acoustic backscatter from a moored 500 KHz ADP. Data from all systems suggest that steep concentration gradients at the boundaries of planktonic layers correspond to local maxima in vertical shear within vertical intervals of local stratification. Although intermittent vertical mixing processes may erase steep vertical gradients in planktonic structure, it appears that the formation and maintenance of vertical structure of phytoplanktonic distributions, and their horizontal extent, are strongly influenced by advection and velocity shear.
OS53B-05 14:40h
Small-scale turbulence measurements with a free-falling DPIV profiler
We have recently developed and built a novel free-falling platform with a stereoscopic Digital Particle Image Velocimeter (DPIV) to observe and quantify microscale physical and biological structures in the upper ocean. Scattered light from a vertical sheet of laser illumination is imaged from both sides by sensitive CCD cameras. Sequences of images allow two-dimensional maps of three-component velocity to be constructed from cross-correlations between images. The profiler has the potential to provide direct estimates of turbulent kinetic energy dissipation in the near-surface open ocean. Dissipation rates can be estimated using finite-difference approximations for 8 of the 12 velocity gradient terms. Using a filter wheel in front of the camera lenses, scattered light and two wavelengths of fluoresced light can be imaged sequentially, allowing mapping of fluorescent particles to the turbulent structures in the water column. The spatial and temporal resolution of the system is set by the size of the imaging area (about 20x20 cm with 200 micron resolution), the camera frame rate (8 Hz), and considerations related to the DPIV cross-correlation technique. We discuss some results of preliminary tests both in the lab and at sea.
OS53B-06 14:55h
Observing microscale fluorescence field in turbulence
We have developed a freefall microscale structure profiler (TurboMAP) that measures the high-resolution fluorescence field in the upper ocean. TurboMAP carries conventional shear probe and FP07 and a new LED microscale fluorescence probe. We present the performance of TurboMAP, the calibration method of the LED probe, and the spectral feature of the fluorescence field at high wavenumbers. The high wavenumber end of the spectrum resembles a high Prandtl number spectrum. Thus we propose an apparent diffusivity for the fluorescence signals. A new laser fluorescence probe that resolve at least a ten times smaller scale than the LED probe will also be introduced.
OS53B-07 15:10h
Along-Isopycnal Variability in Temperature and Chlorophyll Fluorescence in the North Pacific
Planktonic chlorophyll in the ocean may be regarded as a reactive tracer. Two mechanisms which could lead to small- and meso-scale structure in the horizontal chlorophyll field are: 1) cholophyll sources or sinks, e.g. spatially varying growth/death rates of phytoplankton; and 2) advection--stirring of large-scale chlorophyll gradients by ocean currents. In the latter case, chlorophyll behaves as a conservative tracer, and would be expected to display a distribution statistically similar to that of other passive conservative tracers. Further, simple stirring models demonstrate that the locations of enhanced gradients in all tracers would be expected to coincide where the strain has been greatest. However, the directions of enhanced gradients formed in this way may be either parallel or antiparallel, depending on initial conditions and on the direction of the strain. Chlorophyll fluorescence and temperature were measured with a horizontal resolution of 4 meters on the 1025.5 kg/m$^3$ isopycnal along a 1000 km meridional transect in the eastern North Pacific. Probability density functions (PDFs) of fluctuation magnitudes of the concentrations of these two tracers are compared at each of a large range of lengthscales. At the smallest scales, the PDFs of fluorscence and potential temperature differ significantly. They are nearly identical at larger scales. By comparing the phase of wavelet transforms of each of the tracers, the tendancy for temperature and fluorscence gradients to line up is investigated over a large range of spatial scales. At horizontal lengthscales of order 10 km and larger, the wavelet phase difference between temperature and fluorscence tends to be close to 0 or 180 degrees--that is, the gradients tend to align, either in phase or 180 degrees out of phase. At smaller scales, the distribution of phase difference is uniform--there is no tendancy for gradients to coincide. These analyses suggest that at small scales, the horizontal temperature and fluorescence distributions observed were generated by different dynamics, but at larger scales both were governed by advective processes.
OS53B-08 15:25h
Planktivorous Fish Recognize Temporal Motion Patterns of Suspended Particles
Small planktivorous fish feed by selective captures of individual zooplankters. We realize that: 1) the predator, as well as the prey is suspended in the water column, which does not provide either with stable reference points; 2) the ambient flow field acts differently on the larger predators than on the much smaller prey; and 3) within the water column there are many suspended particles of lower nutritional value than the zooplankters represent. We investigated in the laboratory whether or not fish can distinguish between small targets moving with different swimming patterns, e.g. particles entrained passively in the ambient water flow versus entrained but actively swimming particles. We created in an aquarium computer-animated stimuli with motion patterns ranging from random to actual swimming motions of live animals. The results show that planktivorous fish can recognize temporal patterns in a visually homogeneous environment. Therefore, blue-water fish must process visual information similar to terrestrial animals processing auditory information.