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Midwater research

Physics and biology of oceanographic fronts in Monterey Bay
Lead Scientist/Project Manager: William Hamner (UCLA)

Oceanographic fronts are narrow, shallow regions of the epipelagic ocean where horizontal variations in temperature, salinity, density, and enhanced biological activity are large, and which are bounded by large areas of the ocean where horizontal gradients are small.  Intense biological activity is usually associated with fronts because fronts are areas where water masses of different density converge.  This convergence concentrates a narrow band at the surface of phytoplankton and zooplankton in the subducting water mass, leading to aggregations of predators and to increased secondary production.  Vertical diurnal migration of midwater micronekton occurs at night, with most of the biomass accumulating above the thermocline, creating intense vertical patchiness in addition to the horizontal patchiness at the surface convergence.

Most work on fronts has focused on physical oceanography, and without an appreciation of the physics it is not possible to design a sampling program to elucidate the chemistry or biology.  However, no investigation of a front has been conducted at night, nor has the importance of downwelling of particulate flocs, or marine snow, at fronts been investigated.  The role of midwater migrating micronekton and zooplankton on fronts at night has never been assessed, and pelagic-benthic linkages below fronts has been poorly studied.

We will examine the physics and biology of fronts in Monterey Bay during the day and night over a continuous 3-day period, using a two-ship sampling program along a convergence zone marked by an array of large surface drogues.  The ROV video system developed at MBARI provides the perfect tool for recording the fine-scale distributions and behavior of micronekton.  The study on fronts in Monterey Bay will be undertaken in collaboration with a team of MBARI biologists: Bruce Robison, Francisco Chavez, and George Matsumoto as primary collaborators and with Jim Berry and Ed DeLong as secondary collaborators.  Our primary collaborator on the physics of fronts at UCLA is James McWilliams, and, at UC Berkeley, Tom Powell.