Date of Award

12-2002

Level of Access

Open-Access Thesis

Degree Name

Master of Science (MS)

Department

Oceanography

Advisor

Andrew Thomas

Second Committee Member

Mary Jane Perry

Third Committee Member

Lee Karp-Boss

Abstract

Heceta Bank, a unique shallow bank on the southern Oregon shelf, is located within the California Current upwelling system. Four years (1998-2001) of 1-km resolution SeaWiFS ocean color satellite data of the Oregon coast are used to provide the first systematic description, and quantification of seasonal and interannual surface chlorophyll variability in the Heceta Bank region of coastal Oregon. The variability over the bank is examined with respect to wind forcing and surface temperature, and compared to the variability observed in topographically simpler shelf regions north and south of the bank. A seasonal cycle with lowest concentrations in the fall and winter (October- April), an increase in the spring and early summer (May-July) and maxima in the summer (July-August) is evident along the entire Oregon coast. A spatial pattern of elevated chlorophyll values nearshore and lower values offshore dominates during the summer months. The pattern is enhanced in strength and cross-shelf extent over Heceta Bank and is appears to mimic shelf bathyrnetry. The chlorophyll pattern, as well as the seasonal pattern of SST, is consistent with latitudinal gradients of local wind forcing in the area. Interannual differences over the study period show the magnitude of this pattern to be strongest in 2001 and weakest in 1999. Cross-shelf maximum extension of chlorophyll occurred later in the summer season during the 1997-98 El Nino year on Heceta Bank and north of the bank. South of the bank, wind forcing may have been strong enough to overcome the warmer spring temperatures attributed to El Nino to begin the seasonal elevation of chlorophyll in April. Temporal variability on 8-day time scales of chlorophyll concentrations on Heceta Bank are positively correlated with areas immediately north and south of the bank, however, the correlation between locations becomes insignificant when the distance exceeds lo of latitude. Comparison of chlorophyll variability over Heceta Bank with those of adjacent shelf regions (north and south of the bank) at similar depths were contrasted with variability at similar cross-shelf distances to examine the effect of a wider shelf on chlorophyll concentrations. Although at similar depths specific differences between sites are evident on the shelf, these differences are not consistent from year to year and chlorophyll variability north and south of the bank is similar to that on Heceta Bank. At an equal distance from shore however, the northern and southern sites have consistently weaker chlorophyll concentrations than those over Heceta Bank suggesting that shelf width exerts a strong control on the cross-shelf extent of chlorophyll off the Oregon coast. Cross-shelf extensions of chlorophyll > 4 mg m-3 over Heceta Bank is confined to the shelf (55 km) in the summer months. North and south of the bank, where the shelf is relatively narrow, chlorophyll > 4 mg m'3 extend up to 35 km from the coast, 20 km beyond the shelf break. The wider shelf over Heceta Bank results in elevated chlorophyll concentrations farther offshore than adjacent shelf regions.

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