Abstract
2012: Simulation of annual biogeochemical cycles of nutrient balance, phytoplankton bloom(s), and DO in Puget Sound using an unstructured grid model
Tarang Khangaonkar, Brandon Sackmann,Wen Long, Teizeen Mohamedali, Mindy Roberts
Nutrient pollution from rivers, nonpoint sourcerunoff, and nearly 100 wastewater discharges is a potentialthreat to the ecological health of Puget Sound with evidenceof hypoxia in some basins. However, the relative contributionsof loads entering Puget Sound from natural and anthropogenicsources, and the effects of exchange flow fromthe Pacific Ocean are not well understood. Development ofa quantitative model of Puget Sound is thus presented tohelp improve our understanding of the annual biogeochemicalcycles in this system using the unstructured grid Finite-Volume Coastal Ocean Model framework and the IntegratedCompartment Model (CE-QUAL-ICM) water quality kinetics.Results based on 2006 data show that phytoplanktongrowth and die-off, succession between two species of algae,nutrient dynamics, and dissolved oxygen in PugetSound are strongly tied to seasonal variation of temperature,solar radiation, and the annual exchange and flushing inducedby upwelled Pacific Ocean waters. Concentrations inthe mixed outflow surface layer occupying approximately5–20 m of the upper water column show strong effects ofeutrophication from natural and anthropogenic sources,spring and summer algae blooms, accompanied by depletednutrients but high dissolved oxygen levels. The bottom layerreflects dissolved oxygen and nutrient concentrations ofupwelled Pacific Ocean water modulated by mixing withbiologically active surface outflow in the Strait of Juan deFuca prior to entering Puget Sound over the Admiralty Inlet.The effect of reflux mixing at the Admiralty Inlet sill resultingin lower nutrient and higher dissolved oxygen levels inbottom waters of Puget Sound than the incoming upwelledPacific Ocean water is reproduced. By late winter, with thereduction in algal activity, water column constituents ofinterest, were renewed and the system appeared to reset withcooler temperature, higher nutrient, and higher dissolvedoxygen waters from the Pacific Ocean.