The Strait of Juan de Fuca influences tidal action and other physical properties within the San Juan Channel.  In addition, the degree to which these channels show a mixed water profile is related to the bathymetry of the basins and the presence of either flood or ebb tides.  Other factors also affect stratification in the vertical water column, such as seasonal fluctuations in freshwater inflow. 

Taken together, the incoming tide of seawater, the influx of freshwater, and the bathymetric formation of the channel floor provide excellent conditions for considerable mixing in the San Juan Channel.  Samples taken during the April 26, 2014, cruise show a distinctive, dense layer  deep in the Strait of Juan de Fuca, while the San Juan Channel exhibited a mixed water column throughout.  The map below reveals the rough bathymetry at opening of the San Juan Channel where it receives tidal flow from Strait of Juan de Fuca.  From this visual vantage point, we can see the passage between Lopez Island and San Juan Is land is relatively narrow, creating an opportunity for faster and more volatile water movement as currents surge past the configurations of the sea floor. 

The day’s samples were taken from aboard the R/V Centennial, captained by  Dennis Willows.  Field work was conducted by members of the Spring, 2014 Estuarine Field Studies course at the University of Washington- Tacoma under the direction of Cheryl Greengrove, Ph.D., Associate Professor, Interdisciplinary Arts and Sciences. 

The vessel embarked from Friday Harbor at roughly 9:00 a.m. (PDT) and low tide occurred at 9:48 a.m. shortly before sampling began.  The last sample was collected around 4:00 p.m. while a nearly two-meter high tide occurred at 5:00 p.m.  As a result, all samples were collected during flood (incoming) tide conditions.  Nutrient and microplastics samples were sent to other University of Washington laboratories for analysis.  All other samples were processed and analyzed by the field studies team at Friday Harbor Labs, a fully functioning marine research facility with the University of Washington, with the exception of phytoplankton counts.  Phytoplankton were fixed in Friday Harbor and later processed at the Tacoma campus laboratory.   

The table depicting samples collected shows dissolved oxygen, chlorophyll, nutrients, and phytoplankton were obtained at all stations while zooplankton was taken from two stations.  Sediments and microplastics were each sampled from one location.

The semi-diurnal tide (two high and two low tides each day) depicted in the NOAA chart below shows the sampling aboard the R/V Centennial took place within the six-hour period marking the second flood tide of the day.  Flood tides bring ocean water into the straits and channels.  The denser, salty water settles deep in these basins and creates a distinctive upper and lower layer within the water column.  This phenomenon was evident in the Strait of Juan de Fuca.  Alternatively, significant freshwater inflows, common during the spring season, are responsible for creating a well-mixed water column despite flood tide conditions.  This dynamic was evident in San Juan Channel.  More details and graphs about water column stratification can be found in the results/discussion section.   

Ebbesmeyer CC, Cannon GA.  2001.  Review of Puget Sound physical oceanography related to the triple junction region.  Seattle (WA):  Report for King County Department of Natural Resources.

Masson D, Cummins F.  2004.  Observations and modeling of seasonal variability in the Straits of Georgia and Juan de Fuca.  Journal of Marine Research Vol. 62(4): 491-516.

[NOAA] National Oceanic and Atmospheric Administration [Internet.] Silver Spring (MD): Center for Operational Oceanographic Products and Services; [cited 2014 Jun 6].  Available from http://tidesandcurrents.noaa.gov/waterlevels.html?id=9449880&units=metric&bdate=20140426&edate=20140426&timezone=LST/LDT&datum=MLLW&interval=6&action=

[PSAT] Puget Sound Action Team. 2007. 2007 Puget Sound Update: Ninth Report of the Puget Sound Assessment and Monitoring Program [Internet].  [cited 2014 May 23].  Available from http://www.psparchives.com/publications/puget_sound/update/07update/2007_PS_Update.pdf

Strickland R.  1983.  The fertile fjord.  Seattle (WA):  Washington Sea Grant Program, University of Washington.

Sverdrup K, Duxbury AC, Duxbury AB.  2005.  The world's oceans. 8^th edition. New York (NY):  McGraw Hill.  Ch 12.

[USGS] Warrick J, Stevens A, Miller I, Gelfenbaum G.  2011. Coastal processes of the Elwha River Delta:  Biological and physical patterns and processes prior to dam removal. [Internet.] United States Geological Survey [cited 2014 May 30].  Available from  http://pubs.usgs.gov/sir/2011/5120/pdf/sir20115120_ch5.pdf

Zamon J.  2002.  Tidal changes in copepod abundance and maintenance of a summer Coscinodiscus bloom in the southern San Juan channel, San Juan Islands, USA.  Marine Ecology Progress Series.  226:193-210.