Eddy-wind interactions stimulate extraordinary mid-ocean plankton bloomsOne-sentence summary: Mid-ocean eddies, together with wind-forced motions, cause episodic bursts of nutrient supply to the upper ocean, changes in plankton community structure, and export of organic material to the deep sea. Understanding the controls on primary production in the upper ocean is of fundamental importance for two main reasons. First, primary productivity sets a firstorder constraint on the energy available to sustain oceanic ecosystems. Second, fixation and subsequent sinking of organic particles removes carbon from the surface ocean (the so-called "biological pump"), which plays a key role in partitioning of carbon dioxide between the ocean and atmosphere. Geochemical estimates of new production (1) surpass the apparent rate of nutrient supply by vertical mixing by a factor of two or more in subtropical oceans (2-6), which constitute some of the largest biomes on earth. Two possible mechanisms to supply the "missing" nutrient locally include nitrogen fixation by cyanobacteria (7-10), and intermittent upwelling by mesoscale eddies and submesoscale processes (11-21).

Colonial diazotrophic cyanobacteria of the genus Trichodesmium are thought to play a significant role in the input of new nitrogen to upper layers of the tropical and subtropical oceanic ecosystems that cover nearly half of Earth's surface. Here we describe results of a transatlantic survey in which a noninvasive underwater digital microscope (the video plankton recorder), was towed across the North Atlantic at 6 meters per second while undulating between the surface and 130 meters. Colony abundance had a basin-scale trend, a clear association with anticyclonic eddies, and was not affected by hurricane-forced mixing. Subsurface abundance was higher than previously reported, which has important implications for the global ocean nitrogen cycle.

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A new Video Plankton Recorder (VPRII) has been developed for rapid surveys of plankton and seston in the size range of 100µm-1cm. The VPRII system includes: 1) a high-resolution digital camera (1Mpixel, 10-bits, 30Hz frame-rate), 2) a fast towfish capable of tow speeds up to 12 knots and 3-axis motion for automatic undulation and ship-wake avoidance, small diameter tow cable and winch for deployment on coastal vessels, and 3) new interface software (Visual Plankton) for automatic identification of plankton to major taxa and visualization of these taxa together with hydrographic data in real time. Camera and strobe optics are laboratory-adjusted to select the field-of-view (5-20mm), and depth-of-field is objectively calibrated using a tethered organism (e.g., copepod) and automatic focus-detection software. The VPRII towfish comprises a fuselage, a fixed main wing, and three servo-controlled tail fins: port and starboard for dive, climb, and roll control and rudder for lateral movement. Placement of the strobe (starboard wing-tip), camera (fuselage nose), and cantilevered tow-bridle minimize disturbance of the imaged volume. Compared with typical net surveys in shelf areas, the VPRII counts more plankton per station, quantifies ubiquitous fragile forms, automatically identifies plankton to major taxa and measures their size, quantifies scales of patchiness down to a few cm, and displays high-resolution distributions of plankton taxa and hydrography while underway. The VPRII is available to researchers via the Woods Hole Oceanographic Institution ship instrumentation pool. AcknowledgmentsWe thank the officers and crew of the R/V Oceanus for their support during field tests of the new VPR system. Engineers Pierre Tillier, Ken Peal, Nick Witzel, Ed Hobart, and Steve Faluotico made valuable contributions to the design and construction of the new VPR. Others involved in this project include

The roles of plankton behavior, stratification, and microstructure in the formation of fine-scale plankton layers were examined using a 3-dimensional video plankton recorder mounted on a remotely operated vehicle. Vertically compressed plankton patches were observed in association with a cold pool over the Southern Flank of Georges Bank, extending from the tidal mixing front to the shelf-slope break during the months of May and June, 1994, 1995, 1997. In June 1995, 3 major plankton layers were present: a 10 m thick layer above the thermocline, a 1 m thick layer within the thermocline, and a third, 2 to 5 m thick layer immediately below the thermocline. Energy dissipation rate was lowest in the central layer and increased in both top and bottom layers. Some passive organisms and particles, e.g. the colonial diatom Chaetoceros socialis and rod-shaped diatoms, were concentrated in all 3 layers, while marine snow particles were found only in transitional regions. All stages of Calanus spp. were present in high numbers on the fringes of all 3 layers, while Oithona sp. was found only in the thin, central layer. Plankton were significantly aggregated only when the motility number, Mn (i.e. ratio of plankton swimming speed/rms turbulent velocity) was greater than 3, suggesting dominance of plankton behavior over physical structure. Under both quiescent and turbulent conditions, the Lagrangian frequency spectra (f ) for swimming plankton and passive particles decreased with a slope of f -2 . However, in quiescent conditions, the magnitude of the spectrum for swimming plankton was 10-fold greater than for passive particles, illustrating a decoupling of plankton swimming from turbulent eddies. The air/water interface, the pycnocline, and multiple shear interfaces at density discontinuities act as boundaries to vertical zones where plankton behavior may succumb to or dominate background microstructure, thus providing a mechanism for formation of plankton and particulate layers.KEY WORDS: Fine-scale vertical structure · Thin layers · Plankton behavior · Turbulence Resale or republication not permitted without written consent of the publisherMar Ecol Prog Ser 267: [27][28][29][30][31][32][33][34][35][36][37][38][39][40][41][42][43] 2004 communities, species-specific patterns in abundance can form as a function of fine-scale physical structure (Owen 1989, Davis et al. 1992, Gallager et al. 1996b and may persist for many days (Donaghay et al. 1992, Cowles & Desiderio 1993, reviewed in Cowles et al. 1998. If this scale is undersampled or, worse, ignored, the result of persistent fine-scale structure will be gross underestimates of production (Cowles et al. 1998).Vertical fine-structure has been observed since the study of Eckart (1948), and is usually described in terms of mixing, such as the interaction between density stratification and horizontal shear (Gargett et al. 1984). One of the net results of shear is to redistribute horizontal variance onto vertical variance, producing the typical multiple-layer effect,...