Simulation of annual biogeochemical cycles of nutrient balance, phytoplankton bloom(s), and DO in Puget Sound using an unstructured grid model

Abstract

Nutrient pollution from rivers, nonpoint source runoff, and nearly 100 wastewater discharges is a potential threat to the ecological health of Puget Sound with evidence of hypoxia in some basins. However, the relative contributions of loads entering Puget Sound from natural and anthropogenic sources, and the effects of exchange flow from the Pacific Ocean are not well understood. Development of a quantitative model of Puget Sound is thus presented to help improve our understanding of the annual biogeochemical cycles in this system using the unstructured grid FiniteVolume Coastal Ocean Model framework and the Integrated Compartment Model (CE-QUAL-ICM) water quality kinetics. Results based on 2006 data show that phytoplankton growth and die-off, succession between two species of algae, nutrient dynamics, and dissolved oxygen in Puget Sound are strongly tied to seasonal variation of temperature, solar radiation, and the annual exchange and flushing induced by upwelled Pacific Ocean waters. Concentrations in the mixed outflow surface layer occupying approximately 5-20 m of the upper water column show strong effects of eutrophication from natural and anthropogenic sources, spring and summer algae blooms, accompanied by depleted nutrients but high dissolved oxygen levels. The bottom layer reflects dissolved oxygen and nutrient concentrations of upwelled Pacific Ocean water modulated by mixing with biologically active surface outflow in the Strait of Juan de Fuca prior to entering Puget Sound over the Admiralty Inlet. The effect of reflux mixing at the Admiralty Inlet sill resulting in lower nutrient and higher dissolved oxygen levels in bottom waters of Puget Sound than the incoming upwelled Pacific Ocean water is reproduced. By late winter, with the reduction in algal activity, water column constituents of interest, were renewed and the system appeared to reset with cooler temperature, higher nutrient, and higher dissolved oxygen waters from the Pacific Ocean.

Sensitivity of Circulation in the Skagit River Estuary to Sea Level Rise and Future Flows

“…A portion of Skagit River freshwater is transported out (due south) from the estuary through the surface layers at C-C1, but the net volume transport is into the estuary (due north) and is dominated by the inflow through the lay- ers below the pycnocline at this location. This finding also is consistent with results published by Khangaonkar et al (2012) where tidally averaged flow at the C-C1 location showed an outflow through the shallow brackish surface layer and inflow into the estuary through the lower layers. In this study, we used a projected year 2070 hydrograph that included a 13% increase in the total inflow (Table 1) as reflected in mean flux through EE1.…”

“…The upgraded SSM was applied for the year 2008, and error statistics for water surface elevation, velocity, and salinity were regenerated to ensure that the overall quality of model performance was retained over the Puget Sound domain (see Table 1). The error ranges are of the same order of magnitude as those presented in Khangaonkar et al (2012) using 2006 data at the Puget Sound stations. An average bias of approximately -1 psu was noted in simulated salinity results with predicted salinities lower than observed data.…”

“…To compensate for this variability, and provide a representative future hydrograph, we decided to average 5 years of daily stream-flow projections from 2065 to 2069 and designated it as the year 2070 estimate. Estimates of hydrologic loads from all other smaller rivers and watershed and wastewater streams entering the Salish Sea then were developed through a combination of regression and scaling techniques based on watershed areas described in Roberts et al (2014) and Khangaonkar et al (2012).…”

“…The effect of future conditions on upstream salinity intrusion and net transport through the system is presented below. (Chen et al 2003) and has been discussed in detail previously , Khangaonkar et al 2012 (Flater 1996), freshwater inflows, wind, and heat flux at the water surface.…”

Multiscale modeling of coastal, shelf, and global ocean dynamics

“…Considering next physical-biogeochemical interactions, Khangaonkar et al (2012) combine the Finite Volume Coastal Ocean Model with the Integrated Compartment Model (CE-QUAL-ICM) water quality kinetics to characterize annual biogeochemical cycles and nutrient balances in Puget Sound. Using this combined system, the authors simulate phytoplankton biomass growth and characterize the relative contributions from natural and anthropogenic sources and from exchanges with the Pacific Ocean.…”