Nitrogen from the Oceans

Ocean water contains nitrogen, which enters the Salish Sea from the Pacific Ocean through the Strait of Juan de Fuca. Water also leaves the Salish Sea across this same boundary. More nitrogen enters than leaves, resulting in a net source of nitrogen to the Salish Sea, and eventually, to Puget Sound. The main exchange of oceanic nitrogen to Puget Sound occurs across Admiralty Inlet.

The interaction of coastal upwelling, lunar tides, and estuarine circulation govern this exchange of water. First, upwelling along the Washington Coast brings nutrient-rich, dense water to near the surface. Second, tides bring water in and out. Finally, heavier salty ocean water enters near the bottom, while lighter fresh river water exits near the surface; this is called estuarine circulation. Each process varies seasonally or with phases of the moon.

Rough estimates of oceanic nitrogen entering the Strait of Juan de Fuca and Admiralty Inlet are available from Mackas and Harrison (1997). Using a salt-balance approach, this study estimated an annual average net nitrate flux of 400,000 – 600,000 kg/d entering the Strait of Juan de Fuca.

They also estimated an annual average nitrate flux of 117,000 kg/d to Puget Sound at Admiralty Inlet, with a monthly range of 0 – 244,000 kg/d.

The amount of nitrogen entering Puget Sound from the ocean is larger than local sources of nitrogen from rivers, wastewater treatment plants, and other sources. The magnitude and month-to-month variability of oceanic nitrogen sources are determined by larger oceanic processes, such as Pacific Ocean upwelling and local weather events, and cannot be controlled through management activities. Since most of the nitrogen in the ocean is not from human sources, oceanic nitrogen is part of the natural background nitrogen contributions to Puget Sound.