The central California Current transition zone: A broad region exhibiting evidence for iron limitation

The transition zone (TZ) of the central California Current upwelling system (cCCS) is the boundary between the cold, saline, coastally upwelled water and the warm, less saline, oligotrophic waters of the offshore California Current (CC). The TZ is a broad region that regularly exhibits chlorophyll concentrations of 1-2 μg L−1 throughout the spring, summer, and fall seasons. Surface transect and vertical profile data from three cruises (May 2010, June 1999, and August 2011) between 34 and 42°N show residual nitrate concentrations (5-15 μM) and low Fe concentrations (most < 0.2 nmol kg−1) in the TZ. We therefore suggest that much of the TZ of the cCCS is an Fe-limited, high nutrient, lower than expected chlorophyll (HNLC) region. The main source of Fe to the cCCS is from upwelling through the benthic boundary layer (BBL) over the continental shelf sediments. Iron and NO3- in coastally upwelled water are transported via offshore moving filaments into the TZ. However, since some coastal upwelling regions with narrow continental shelves do not have much Fe to begin with, and since Fe is drawn down more rapidly relative to NO3- due to biological assimilation and scavenging, these filaments transport low concentrations of Fe relative to NO3- into the TZ. Weak wind curl-induced upwelling and vertical mixing in the TZ also deliver Fe and NO3- to the surface but at lower concentrations (and lower Fe:NO3-) than from strong coastal upwelling. Mesoscale cyclonic eddies in the TZ are important to consider with respect to offshore surface nutrient delivery because there is a marked shoaling of isopycnals and the nutricline within these eddies allowing higher nutrient concentrations to be closer to the surface. Since wind curl-induced upwelling and/or vertical mixing occurs seaward of the continental shelf, there is not enough Fe delivered to the surface to accompany the NO3-. By using Fe:NO3- ratios and calculated specific growth rates for diatoms, we demonstrate that the TZ of the cCCS shows evidence for Fe limitation of diatom blooms. The TZ also appears to progress further into Fe limitation as the upwelling season progresses from spring into late summer. This study provides some of the first field data to suggest that Fe is a critical bottom up control on the ecosystem in the TZ of the cCCS.