A long‐term, high‐resolution record of surface water iron concentrations in the upwelling‐driven central California region

Abstract

Seven years of observations of surface water iron concentrations in the Monterey Bay region of central California reveal a consistent annual cycle dominated by the injection of high concentrations of particulate iron each spring. A companion study of the water column near the upwelling center at the north end of the bay clearly indicates a sedimentary source for the iron. Local river discharge, during winter storm events, results in the deposition of a fine‐grained sediment “fluff” layer along the shelf. The initial rapid shoaling of isotherms at the onset of upwelling in spring brings water from the fluff layer to the surface. Concentrations of iron in both surface and source waters are then quickly diminished although upwelling intensifies, bringing the highest concentration of nitrate to the surface ∼3 months later. The concentration of a number of constituents measured in bottom water samples collected during the companion study strongly suggests that the rapid decrease in surface water iron concentrations is due to the depletion of the fluff layer following initial isotherm shoaling. The decoupling of iron and nitrate supply sets up the potential for iron limitation. However, on the basis of average Fe/NO3 ratios, we conclude that the region is not often iron limited. Iron limitation was apparent only one summer at the offshore station. Summer chlorophyll concentrations are highly correlated to dissolvable (unfiltered) iron concentrations, evidence in support of the role of particulate iron in meeting the ecosystem's iron requirements.