Nitrogenous Nutrient Sources and Sinks in the Juan de Fuca Strait/Strait of Georgia/Puget Sound Estuarine System: Assessing the Potential for Eutrophication

This paper estimates the overall nitrogenous nutrient budget for the Puget Sound/Strait of Georgia/Juan de Fuca Strait estuarine ecosystem, and the potential for eutrophication of this system by anthropogenic nutrient inputs. Large-scale eutrophication is unlikely for two reasons. First, ambient nitrate+ammonia concentrations are high (2–20 μM N) over much of the total area, so that total primary productivity is relatively insensitive to moderate increases or decreases. Second, exchange of water by estuarine and tidal currents is rapid (c. 1 year turnover time), and entering water carries naturally high nutrient concentrations. Natural nitrogen inputs by the estuarine circulation are very much larger than all other sources combined: 2600–2900 tonnes N day−1for the entire system and 1400–1500 tonnes N day−1for the inner basins (Strait of Georgia and Puget Sound)vs. <100 tonnes N day−1for sewage inputs, <160 tonnes N day1for river inputs plus sewage, <15 tonnes N day−1for coastal groundwater discharge exclusive of sewage, and <10 tonnes N day−1for atmospheric inputs. The largest loss terms for nutrients are also due to estuarine exchange. Surface-layer advective export of dissolved inorganic nitrogen is 2100–2400 tonnes N day−1through Juan de Fuca Strait and about 1000 tonnes N day−1from the inner basins. Advective export of organic nitrogen can be estimated only roughly, but is probably between 100 and 300 tonnes N day−1as dissolved organics, and 100–200 tonnes N day−1as living and detrital particulates. Due to the dominant role of estuarine exchange, the overall nutrient budget is likely to be strongly affected by variations in river discharge (affecting total flow) and offshore oceanographic conditions (affecting nutrient content of incoming deep water). Sensitivity to nutrient addition varies with location. The least sensitive sub-regions are the Strait of Juan de Fuca and the tidally-mixed passages linking it to Puget Sound and the Strait of Georgia. The most sensitive sub-regions are some tributary inlets and fjords that have low flushing rates and that adjoin urbanized shorelines