Date of Award

Summer 2007

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Ocean & Earth Sciences

Committee Director

Thomas C. Royer

Committee Member

Chester Grosch

Committee Member

John A. Adam

Committee Member

John M. Klinck

Committee Member

Thomas J. Weingartner

Abstract

The northern Gulf of Alaska marine ecosystem is very productive with a "nutrient paradox". Primary producers require light and nutrients for photosynthesis. A primary source of nutrients is the deep ocean, while light is available in a relatively shallow layer in the upper ocean. In most productive parts of the world oceans, nutrients are brought to surface waters by upwelling. However, in the northern Gulf of Alaska, the winds are generally downwelling inducing and the mechanism(s) by which nutrients are brought to the euphotic zone are not known. One mechanism that might bring nutrients into the euphotic zone is the deepening of mixed layers. This dissertation is the first study of mixed layer depths (MLDs) across the continental shelf of the northern Gulf of Alaska. Hydrographic and nutrient data have been collected as part of the GLOBEC NEP (GLOBal ocean Ecosystem dynamics North East Pacific) project along the Seward Line in the northern Gulf of Alaska. The Seward Line of hydrographic stations extends from the coast, across the continental shelf and beyond the shelf break. It intersects two major circulation features—the Alaska Coastal Current (ACC) on the inner shelf and the Alaska Current offshore of the shelf-break.

This dissertation contains calculations and descriptions of the across-shelf and temporal (seasonal and interannual) variability in the MLDs and assessments of the role of MLDs in providing nutrients to the euphotic zone. The MLDs across the shelf are deepest in late winter/early spring and shallowest in summer. In general, MLDs on the shelf are deeper than those offshelf, with deepest MLDs near the shelf-break. This annual cycle is primarily in response to freshwater discharge, winds and solar insolation. On longer timescales, four forcing mechanisms have been identified: the direct interaction of freshwater discharge and winds; an estuarine-type circulation controlled by freshwater discharge and winds; upwelling related to the curl of the wind stress; and interactions with anticylonic eddies.

Interannually, deep winter MLDs show a deepening trend near the coast and a shoaling trend mid-shelf and at the shelf-break. This might lead to greater productivity near the coast and decreasing productivity offshore. A primary source of nutrients to the region is the deep ocean, but the coastal runoff might be a secondary source at the inner shelf. The nutrients correlate well with MLDs on the inner and mid-shelf, where they play a significant role in the supply of nutrients to the euphotic zone. However, at the shelf-break and beyond, other mechanisms might be more important for supplying nutrients. Further studies need to be done to include the effects of bathymetric interactions and horizontal advection and to resolve the episodic wind events that are possibly responsible for deep mixing.

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DOI

10.25777/zm8q-n531

ISBN

9780549232513

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Oceanography Commons

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