Physics and Biology of Ocean Fronts

Bruce Robison (MBARI), Bill Hamner (UCLA)

Oceanographic fronts are narrow, shallow regions of the epipelagic ocean where large horizontal variations in temperature, salinity, density, and biological activity exist and are bounded by large areas of the ocean where horizontal gradients are small. Intense biological activity is usually associated with fronts because they are areas where water masses of different densities converge. This results in phytoplankton and zooplankton at the surface of the subducting water mass being concentrated in a narrow band at the surface, leading to both the aggregation of predators and increased secondary production.

Intense patchiness on ocean fronts creates serious sampling problems related to the assessment of the population and behavior of zooplankton. Although converging water masses continually deliver zooplankton to the subduction zone,  it is the behavior of the plankton that results in patch formation in pelagic ecosystems, and this behavior has rarely been assessed. Patchiness on ocean fronts can be addressed only by a highly focused effort that uses the front itself as the frame of reference. This compares to the typical synoptic oceanographic program where the front is sampled at regular intervals along a predetermined grid, causing patches to be undersampled.

Because fronts are idiosyncratic and dynamic in space and time, it is critical to collect simultaneously as much information as possible on the physics and the biology of each front. Vertical migration of midwater micronekton occurs at night, with most of the biomass accumulating above the thermocline. This creates intense vertical patchiness in addition to the horizontal patchiness generated by the surface convergence. Patchiness of some micronekton species can be measured acoustically, however, many species are acoustically transparent. Therefore, the fine-scale distribution of these species must be determined visually, requiring the use of an ROV, manned submersible, or blue-water scuba techniques.

The best way to investigate ocean fronts, therefore, is to conduct short, intensive investigations that use multiple tools and multiple platforms, and which engage a variety of investigators with complementary talents. We examined the physics and biology of fronts in Monterey Bay during the spring and fall of 1999 during day and night over a continuous four-day period during each season. We used an ROV to assess the horizontal depth distributions and behavior of zooplankton and micronekton in Monterey Bay.

Data Index Aircraft AUV CODAR
Drifters Moorings Satellites Ships