5.1. Nutrient Enrichment of the Northern CCS

[44] The Juan de Fuca Eddy has been described as an “upwelling center”, allowing water to be raised from deeper depths than in classical wind‐driven upwelling [Freeland and Denman, 1982]. Upwelling in the eddy enriches the deep waters that flow into Juan de Fuca Strait as part of the estuarine circulation return flow. The penetration into the strait of this nutrient‐rich water mass is evident in a vertical section of ambient nitrate concentration measured in September 2003 (Figure 13). At the mouth of the strait, nitrate concentrations below 100 m (the approximate depth of the division between inflow and outflow) are >34 μM. Similar concentrations are present in bottom water along the strait axis, reaching ∼150 km east of the Strait entrance where strong mixing in shallow regions of high tidal currents mixes them upwards.

[45] The nutrient‐rich waters observed in the euphotic zone of the southern Vancouver Island shelf are thought to derive primarily from this estuarine entrainment of deep water into the outflowing surface waters of the strait [Crawford and Dewey, 1989]. However, direct upwelling into the eddy may also be an important source of nutrients to the region. In this section, water mass analysis is used to distinguish these two sources and examine their relative importance. We also illustrate how variability in the circulation of the eddy and prevailing wind conditions can modify nutrient supply from Juan de Fuca Strait to the eddy region and the Washington coast.

[46] Surface nitrate distributions, measured over the three years of field studies, illustrate a reasonably comprehensive set of conditions: early season upwelling, and late‐season upwelling, downwelling, and relaxation (Figures 14a and 14b, upper panels). In both June and September 2003 surveys, relatively high ambient nitrate concentrations are present along the entire Washington coast, a result of coastal upwelling. In June 2003, surface nitrate concentrations >5μM extend almost 50 km west off Cape Flattery, whereas later in the season (September) elevated nitrate concentrations extend much further seaward (∼100 km offshore), consistent with the seasonal increase in offshore extent of the eddy. Similar nutrient distributions are evident in 2005 survey results, with nitrate concentrations >5 μM extending ∼60 km offshore in July and ∼100 km offshore in September. In September 2004, during downwelling‐favorable wind conditions, moderate levels of surface nitrate (∼10μM) are still found in the eddy region, in contrast to the Washington coast where ambient nitrate ranges from undetectable (<0.10 μM) in the south to a maximum of ∼4.5 μM in the north.

[47] To differentiate the source waters contributing to the elevated nutrient concentrations, the contours from the water mass composition analysis are plotted on nitrate at 30 m, the shallowest depth at which a good fit to the data is generally achieved (Figures 14a and 14b, lower panels). The contours delineate areas that are composed of ≥50% Juan de Fuca source water or ≥50% California Undercurrent source water (both Core and Deep).

[48] In all surveys, both Juan de Fuca and California Undercurrent water contribute to the broad regions of high nitrate observed. In both early and late‐season surveys, the Juan de Fuca Strait source water is advected cyclonically around the denser California Undercurrent water, resulting in the wider offshore extent of high ambient nitrate. Later in the season, when the eddy is more developed, this water is transported much further seaward. During downwelling conditions (September 2004), the nutrient‐rich Juan de Fuca effluent is largely confined to a narrow‐band nearshore off Vancouver Island.

[49] At this depth (30 m) and below, the core of the eddy is comprised mainly of California Undercurrent water (Figures 10, 14a and 14b). In the three September surveys, the highest concentrations of nitrate at 30 m are in the eddy core, aligned with the maximum composition of California Undercurrent water. In 2004, ambient nitrate concentrations are reduced relative to the other years (Figure 8). In June 2003, highest nitrate concentrations at this depth are on the inner Washington shelf, associated with wind‐driven coastal upwelling of California Undercurrent water (Figure 14a).

[50] We conclude that in later summer, when the eddy is fully developed, direct upwelling of nutrient‐rich water is the dominant nutrient supply mechanism to the eddy interior, to depths at least as shallow as the base of the mixed‐layer. Earlier in the season, when the eddy is less developed and Fraser river outflow is at its peak, the Juan de Fuca source water may dominate nutrient supply to the eddy region. The two sources together contribute to making the northern extent of the California Current extremely nutrient‐rich. In both cases, macronutrients originate from the California Undercurrent, but both the timescales of delivery and the pathways through which they reach the euphotic zone are very different.