channels cause the tidal currents on the rising flood tide to prefer the 
East Channel route on their way to the Tacoma Narrows. The turbulent 
mixing in the Narrows during the flood mixes the surface water and water 
from depth in the East Channel together sending it into South Sound. On 
the ebb this well-mixed water is preferentially discharged into Colvos 
Passage which is narrow and shallow. This water exits this channel as a 
surface flow to rejoin the main basin opposite Alki Point. 
This constitutes tidal pumping that produces a net clockwise circulation 
about Vashon-Maury Islands and selectively forces deep water up from depth 
at the south end of East Channel converting it to a surface water seaward 
flow near Alki Point. Tidal pumping combined with recycling at the en¬ 
trance sill increases the two-layer net circulation in the main basin above 
what is required in and out of the Sound to maintain salt and water bud¬ 
gets. This circulation also assures that waters of the main basin in 
contact with the urban centers can move elsewhere in the Sound to exchange 
with any of the appended basins or small embayments. 
The rapid two-layer net circulation in the main basin plays a significant 
role in controlling primary production. In the spring and early summer 
the addition of snowmelt runoff and surface heating combine to produce 
stability in the top 100 ft. of the water column. This enhances bloom 
conditions for phytoplankton and dissolved oxygen levels increase as 
nutrients in the surface layer decrease. This surface layer of the main 
basin, however, is actively being advected towards the mixing sill at 
Admiralty Inlet. If 50 percent recycling occurrs, then the surface water 
with its high density of phytoplankton population, lowered nutrients and 
high oxygen values is being mixed downward and returned to the main basin. 
This distributes phytoplankton advectively to the 500 to 700 ft.-thick 
deep layer at about 1/2 the average concentration found in the TOO ft.- 
thick surface layer. This means that below the photic layer there can at 
times be about 3 times the total plant biomass found in the surface layer. 
Much of the phytoplankton is exported to the Strait. The tidal pumping 
action at the Tacoma Narrows sends the main basin deep water with its sus¬ 
pended phytoplankton and nutrients back to the surface layer and the 
photic zone. 
The strong net advective processes in the main basin, sill mixing, and 
tidal pumping, export of phytoplankton and its advection to depth, act to 
ventilate the deeper waters of the main basin and continually supply 
nutrients to the surface layer. These natural flux rates of dissolved 
oxygen and nutrients are so large that anthropogenic sources of nutrients 
and BOD in the main basin are insignificant if well dispersed. At 930 ft. 
in the main basin the percent saturation of dissolved oxygen rarely drops 
below 65 percent whereas the percent saturation at 200 - 300 ft. depth 
in the Strait and along the coast can be less than 30 percent when up- 
welled type oceanic water is present in the summertime. 
Particulates other than plankton are present in Puget Sound. They come 
from rivers, shore erosion, and urban sources. An annual average value 
7 
