A nitrogen budget for the Strait of Georgia, British Columbia, with emphasis on particulate nitrogen and dissolved inorganic nitrogen

Abstract. Balanced budgets for dissolved inorganic N (DIN) and particulate N (PN) were constructed for the Strait of Georgia (SoG), a semi-enclosed coastal sea off the west coast of British Columbia, Canada. The dominant control on the N budget is the advection of DIN into and out of the SoG via Haro Strait. The annual influx of DIN by advection from the Pacific Ocean is 29 990 (±19 500) Mmol yr⁻¹. The DIN flux advected out of the SoG is 24 300 (±15 500) Mmol yr⁻¹. Most of the DIN that enters the SoG (~ 23 400 Mmol yr⁻¹) is converted to particulate N (PN) in situ by primary production. However, most of the PN produced by primary production is remineralized (~ 22 000 Mmol yr⁻¹) back into DIN within the top 50 m. The PN budget for the SoG was further constrained by nitrogen isotope composition (δ¹⁵N) that indicated regional differences in the source of PN. The southern strait receives a much higher proportion of terrigenous PN, relative to marine PN, than does the northern strait. The difference is due to the influence of the Fraser River, which discharges 1950 Mmol yr⁻¹ of PN and 1660 Mmol yr⁻¹ of DIN into the southern strait. The overall anthropogenic contribution of PN and DIN to the SoG is minimal relative to natural sources (> 30 000 Mmol yr⁻¹). It is unlikely that the strait will be affected by eutrophication in the near future, although anthropogenic N sources, such as wastewater outfalls, may have significant local effects.