Seasonal and interannual variability of pigment concentrations across a California Current frontal zone

Abstract

Previously published physical and biological data document a zonally oriented frontal region within the California Current system separating colder and more eutrophic water north of ≈ 33° N from warmer, more stratified, and oligotrophic water farther to the south. Satellite images of phytoplankton pigment from the coastal zone color scanner from 1979–1983 and 1986 are used to examine the seasonal and interannual variability of both the latitudinal position of this front and the pigment concentrations associated with it. Many temporal and spatial characteristics of the pigment structure are repeated in different years, and a general seasonal cycle is described. Variations in the frontal structure are controlled primarily by changes in pigment concentration north of the front. Seasonality is minimum south of the front where concentrations remain low (< 0.5 mg m−3) throughout the spring, summer, and fall. The frontal gradient is typically strongest from late March until early June when higher concentrations (> 2.0 mg m−3) are present north of the front. Lower pigment concentrations within the sampled region (0.5–1.0 mg m−3) north of the front in mid‐late summer (June–August), resulting from a seasonal shift in the cross‐shelf distribution of pigment, reduce and often eliminate the pigment gradient forming the front. Concentrations greater than 1.0 mg m−3 typically extend 150–250 km farther offshore in spring (April) than in summer (June–July). Superimposed on this general seasonality is strong interannual variability in the magnitude of the frontal gradient, its latitudinal position, and the seasonal development of higher biomass in regions north of the front. Pigment concentrations during the El Niño year of 1983 are distinctly lower than those of other years. The patterns evident in the satellite data are compared with available in situ measured hydrographic data and nutrient and phytoplankton concentrations. A comparison of the seasonal and interannual variability of these patterns to surface wind shows little direct relation between frontal strength or position and wind forcing.