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Net physical transports, residence times, and new production for Rivers Inlet, British Columbia Hodal, Michal

Abstract

A hydrographic dataset from the 2008-2009 Rivers Inlet Ecosystem Study (RIES) field program was used (a) to provide a more complete oceanographic description of Rivers Inlet, British Columbia and (b) to develop the first quantitative estimates of estuarine circulation and new production for this system. Water column observations show a highly stratified two-layer estuarine structure, particularly in the spring and summer months when river discharge and atmospheric heat inputs were high. The net air-sea heat flux had a seasonal range of approximately 220 Wm⁻² and peaked almost a month earlier in 2008 than in 2009. The main source of river input comes from the Wannock River. As temperatures begin to rise in the spring, the river discharge can suddenly increase by an order of magnitude (from about 100 m³ s⁻¹ to almost 1000 m³ s⁻¹) in less than two weeks. Residence times (ie. first-order estimates of estuarine circulation) were estimated for every cruise using salinity and temperature budgets in a two-layer box model parameterization of the flow structure. The results show that upper box residence times vary seasonally with river discharge; dropping from about 14 days in the winter to as low as 4 days in the spring at the freshet onset. An earlier flushing event in 2009 caused residence times to drop earlier and could have caused higher advection losses for phytoplankton in the early spring. Overall, residence times averaged to about 7 days for the upper layer and about 165 days for the lower layer during periods of high river discharge, and about twice that during periods of low river discharge. Deep water in the lower layer below the sill was renewed almost once a year in summer and was affected only by vertical diffusion during the rest of the year. Finally, a spring/summer new production estimate of 0.6-1.7 gCm⁻²d⁻¹ (which implies about 110-300 gCm⁻²y⁻¹ assuming no production during the other months) was obtained by combining transport estimates with observations of nutrients to infer a surface nitrate sink. This range compares well with independent estimates made in nearby regions.

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