Chemical and biological structure and transport of a cool filament associated with a jet‐eddy system off northern California in July 1986 (OPTOMA21)

Abstract

During the OPTOMA21 cruise, from July 7 to 19, 1986, the distributions of nutrient, pigment, bio‐optical, and physical variables were mapped in a jet‐eddy system located off Point Reyes and Point Arena, California. The goals of this mapping were to describe the three‐dimensional variability of the filament and its relation to the nutrient and phytoplankton distributions offshore, to examine the interaction between the filament and coastal water, and to estimate the transport of nutrients and phytoplankton by the jet system. Several cool filaments were distinguishable at distances of more than 35–50 km from the coast in satellite imagery during this period. The juxtaposition of these features as well as the presence of an offshore anticyclone and a cyclone south of the filament anchored to the coast at Point Arena led to complex patterns in all variables, aided by the apparent alongshore variability in the source of upwelled water. The seaward jet associated with the filament entrained coastal upwelled water, with low temperatures and high nutrient and pigment concentrations, so that the filament maintained its characteristic low temperature and high chlorophyll in the offshore zone. The filament extended ∼250 km offshore, where it made a cyclonic bend southeastward toward the coast. Salinity and nutrient concentrations generally increased across the filament into the cyclonic eddy circumscribed by the filament, although their fronts were not always coincident with temperature fronts. Chlorophyll fluorescence maxima were associated with the cool filament core, but elevated levels were also associated with the low‐salinity, slightly warmer water on the outer edge of the filament along its entire track. Subduction within the filament resulted in elevated chlorophyll concentrations extending to ∼100 m depth, nearly twice the estimated euphotic zone depth. Phytoplankton taxa within the filament were characteristic of those found within coastal upwelling regions. As was expected, the cyclonic eddy contained high salinity and high nutrient concentrations, but temperature at the surface was higher than was expected for the salinity, suggesting that this water may have been at the surface for several days to weeks. Chlorophyll concentration was low at the center of the eddy, in contrast to the water in the coastal upwelling region and within the filament, suggesting that there was little horizontal exchange between the cyclone and the cool filament and that the high‐salinity water may have originated from local (open ocean) upwelling or it may have been displaced seaward from a near‐coast origin several weeks earlier. The seaward jet associated with the filament resulted in a large transport of nutrients and phytoplankton biomass offshore, but the shoreward flow, downstream from the cyclonic bend, reduced the net transport to the offshore region. Shoreward nitrate transport was 85 to 90% of seaward transport, similar in proportion to the shoreward volume transport (∼90%) relative to the seaward volume transport. In contrast, shoreward chlorophyll transport was about 40% of seaward transport, indicating that the meandering jet may supply significant phytoplankton biomass to the offshore region. The meandering jet and filament system had a clear imprint on the chemical and biological structure of the region. This structure has implications to the fluxes of organic material in the region and it is probably significant in organizing the interactions among different trophic levels within the system.