Abstract-Simulation of annual biogeochemical cycles


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

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 Finite-
Volume 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.