Assessment of Nutrient Supply by a Tidal Jet in the Northern Strait of Georgia Based on a Biogeochemical Model

We present a coupled three-dimensional biological-physical model for the Salish Sea and evaluate it by comparison to nitrate, silicate, and chlorophyll observations. It accurately reproduces nitrate concentrations with Willmott skill scores, root-mean-square error, and bias ranging from 0.84-0.95, 4.02-6.5 mu M, and -2.33-1.84 mu M, respectively, compared to three independent discrete sample data sets. A prominent feature of the model output is a tidal jet emanating from Discovery Passage producing a downstream plume of elevated surface nitrate. The signal is present from April to September, when surface nitrate is otherwise drawn down. It has a weak but statistically significant correlation to Discovery Passage tidal velocity (R=0.37, p<0.01). Within the turbulent jet and associated plume, the average rate of vertical nitrate supply due to mixing and advection across a depth of roughly 6m is 0.46 mu molm(-2)s(-1) between 15 May and 20 August 2015, compared to 0.10 mu molm(-2)s(-1) for the northern Strait of Georgia as a whole. Close to Discovery Passage, where velocities and shear are strongest, the majority of the vertical nitrate flux is due to mixing. As velocities weaken downstream, vertical advection becomes more important relative to mixing, but vertical velocities also decrease. The tidal pulses out of Discovery Passage drive waves that contribute net upward nitrate flux as far south as Cape Lazo, 40km away. The nitrate supply drives new production, consistent with existing observations. Similar dynamics have been described in many other tidally influenced coastal systems. Plain Language Summary A three-dimensional coupled physical-biological model has been developed that simulates currents, nutrients, and phytoplankton in the Salish Sea. Comparisons with observations show that the model successfully represents key aspects of the nitrogen cycle in the Strait of Georgia, including the seasonal cycle and regional differences. The model is used to investigate mechanisms through which tidal currents lead to nutrient supply to the surface layer, where phytoplankton have enough light to grow, in the northern Strait of Georgia. Strong tidal flow through the narrow Discovery Passage leads to turbulence that mixes deep nutrients to the surface. Tidal currents then advect the surface nutrient plume further into the Strait of Georgia. Additional mixing and vertical motion also occur downstream, increasing the amount of nutrients supplied to the surface.