Colvos/East Passages 5/15/26

Welcome to Colvos & East Passages!

*Photo taken aboard the RV Welander at Point Defiance Dock. Credit 2026 Estuary Class.

The Colvos and East Passages are estuarine channels that run on either side of Vashon Island (Colvos on the western side, East on the eastern side). This is the first time the Estuarine Studies class has sampled these passages, making this a very unique day. We picked out and plotted our own stations, looking at where King County has stations so that we could compare our data to the county's.

The deepest parts of the sites sampled were 176 meters in East Passage and 124 meters in Colvos.

The freshwater that comes into Colvos is fueled from water coming up from the narrows and commencement bay (which is in turn fueled by the Puyallup River), while the East Passage is fueled from water coming down from the main basin of Puget Sound. These two flows cause the tides around Vashon Island to flow clockwise around the island, with water pushing up through Colvos and down through the East Passage. Due to the push of water from the narrows, the water column in the Colvos Passage is much more well mixed than the column in the East Passage.

Figure 1: Photo showing flow of tides in Colvos and East passages. Credit: (Seim and Gregg 1997) .

A senior thesis done on the passages looked into the flow of water out of Colvos, and found that a majority of the water that comes out of Colvos does not flow directly back into the East passage, instead settling in the middle of the water column, and being pushed back seaward into the nothern part of the main basin. (Ovall 2019)

Sampling Stations

Team A took morning samples at Stations 11 and 12 in East Passage. Team B took afternoon samples at Stations 15, 16, and 17 in Colvos Passage.

Figure 2: Map of station locations.

Due to rough weather conditions, three of the eight planned stations had to be eliminated from the plan; two stations further north into East Passage from Team A, and a duplicate of Station 11 from Team B.

Figure 3: Station information table created by Kendall Burch.

Meteorological Conditions During Sampling Period

*photo above of Ronnie and Andy sampling east passage, credit: Estuary Class 2026

The weather started cloudy with light drizzles, however through the run of Team A became much more windy, and the water became much more choppy, causing Team A to cut their sampling short for safety and time. When Team B went out, the water had calmed enough to do all but one of their stations without issue. Team A was bailing water with a bucket.

Figure 4: Barometric Pressure from 05/14/2026 to 05/15/2026 in Tacoma, provided by NOAA

Figure 5: Meteorological conditions for 05/14/2026 from Weather Underground, Lisabeula Station.

Figure 6: Meteorological conditions for 05/15/2026 retrieved from Weather Underground, Lisabeula Station.

Tidal Patterns During Sampling Period

Figure 7: NOAA 24 hour tide chart for day of sampling.

Figure 8: Stations sampling time and tide correlation chart by Kendall Burch.

Station 11 was sampled at the end of an ebb tide. Station 12 was sampled during a slack tide. Stations 15, 16, and 17 were sampled during a flood tide.

CTD Water Property Sections

Temperature

Figure 9: CTD temperature readings. Station numbers listed across the top of the chart.

Temperature generally decreases as depth increases, as cold water is denser than hot water, and sinks.

Salinity

Figure 10: CTD salinity readings. Station numbers listed across the top of the chart.

Salinity generally increases as depth increases, this is due to the salt content increasing density, and the more dense water sinks to the bottom.

Density

Figure 11: CTD density readings. Station numbers listed across the top of the chart.

Density increases as depth increases, causing more dense water to sit at the bottom. The main reason for more dense water is salinity, but temperature also has an effect on density.

Dissolved Oxygen

Figure 12: CTD Dissolved oxygen readings. Station numbers listed across the top of the chart.

Dissolved oxygen decreases with depth as distance from the surface means less proximity to air and less photosynthetic organisms creating oxygen.

Fluorescence

Figure 13: CTD fluorescence readings. Station numbers listed across the top of the chart.

Fluorescence is generally higher at the surface, as it is also related to phytoplankton. On this section in particular, our lowest value was above 2.5, hence why there appears to only be fluorescence in the top corner. There is more, however the scale of the section does not allow it to be seen.

Transmissivity

Figure 14: CTD transmissivity readings. Station numbers listed across the top of the chart.

This is the percentage of light moving through the water. It does not specify what could be blocking the light, just what percentage the CTD picks up.

Due to the scale of the fluorescence CTD section, we have elected to include the CTD profile of fluorescence. Station 12 has a large spike around 10 meters, which is what is visible in the section. The rest of the stations show little fluorescence overall, however it is important to see that there is at least some at the other stations and depths.

Figure 15: CTD depth vs fluorescence readings.

Comparative CTD data was available for Station 12 via King County Puget Sound Marine Monitoring who's station NSEX01, is located in the same area of East Passage. The data was from May 5, 2026, ten days before our teams' sampling day. NSEX01 showed warmer surface temperatures by approximately 1℃ and colder bottom temperatures by approximately 1℃, as well as higher dissolved oxygen levels by roughly 3.3 mL/L at the surface. All other King County CTD readings were very close to those gathered by Team A at Station 12.

King County NSEX01 Data

Figure 16: King County CTD data from sample site NSEX01.

Comparison of CTD Data with Discrete Water Sample Data

Figure 17: Correlations between CTD and discrete dissolved oxygen.

The correlation was relatively high with R2=0.779, meaning the data we got with the CTD matches well with the data we got through the discrete samples.

Figure 18: Correlations between CTD fluorescence and discrete chlorophyll.

The correlation was relatively high with R2=0.8914, meaning the data we got with the CTD matches well with the data we got through the discrete samples.

Secchi Disk

Phytoplankton

Figure 20: Phytoplankton net abundance table showing species presence per station.

Phytoplankton Net...

Phytoplankton net samples showed a big bloom happening at Station 12. Multiple Chaetoceros species were present in abundant to blooming quantities at this station, along with Actinoptychus senaris, Pseudo-nitzschia spp, and Thalassionema spp. Station 15 also showed multiple blooming species of Chaetoceros, along with Actinoptychus senaris and Ditylum brightwellii. Thalassiosira spp was blooming at Stations 11, 16, and 17.

Surface Phytoplankton...

Figure 21: Surface phytoplankton concentrations. Please note that color scheme for the phytoplankton bar graph is not standardized between sections of the webpage.

Surface phytoplankton concentrations mirrored what was observed in net samples, showing a big bloom at station 12 and a dominant presence of multiple Chaetoceros species.

*Image of phytoplankton net sample from Station 12 viewed under a microscope. Multiple Chaetoceros species can be seen.

*Image of a phytoplankton net sample freshly collected in the field. A bloom is apparent.

Nutrient Data

Nutrient Concentrations by Station

Figure 22: Ammonia concentration at each station

This graph shows the concentration in μM of ammonia at the surface and bottom of each station. Nutrient samples were taken from East Passage (stations 11 and 12) and Colvos Passage (stations 15, 16, and 17). The samples were taken at the surface and the bottom of each station.

Figure 23: Silicate concentration at each station

The concentration of silicates Si(OH)₄ µm from nutrient samples taken from Colvos (stations 15, 16, and 17) and East Passage (stations 11 and 12). The samples were taken at the surface and the bottom of each station. The concentration of SiOH₄ increases with depth at most stations except station 11, where it is slightly lower than at the surface.

Figure 24: Nitrite concentration at each station

The concentration of nitrites (NO2) in μM from nutrient samples taken from East Passage (stations 11 and 12) and Colvos Passage (stations 15, 16, and 17). The samples were taken at the surface and the bottom of each station.

Figure 25: Nitrate concentration at each station

The concentration of nitrates (NO3) in μM from nutrient samples taken from East Passage (stations 11 and 12) and Colvos Passage (stations 15, 16, and 17). The samples were taken at the surface and the bottom of each station.

Figure 26: Phosphate concentration at each station

The concentration of phosphates (PO4) in μM from nutrient samples taken from East Passage (stations 11 and 12) and Colvos Passage (stations 15, 16, and 17). The samples were taken at the surface and the bottom of each station.

Nutrient Correlation Graphs

Figure 27: Correlation graph of nitrites and nitrates

Correlation of Nitrates and Nitrites

This graph shows the correlation of NO2 (nitrites) and NO3 (nitrates) to each other. It shows a moderate correlation, with R2=.6845

Correlation of Silicates and Nitrates

This graph shows the correlation of Silicates Si(OH)4 and Nitrates (NO3). It shows a lower correlation, with R2=.5564

Figure 28: Correlation graph of silicates and nitrates

Figure 29: Correlation graph of nitrates and phosphates

Correlation of Nitrates and Phosphates

This graph shows the correlation of Nitrates (NO3) and Phosphates (PO4). It shows a high correlation, with R2=.9601

Figure 30: Table of all nutrient concentrations (µM) taken from Colvos (stations 15, 16, and 17) and East Passages (11 and 12) at the surface and bottom

Discussion of Observations

This sampling day was unique in that it was the only foul weather sampling day of the quarter. This affected the teams' ability to follow through with the sampling plan. Three stations had to be eliminated and no sediment samples were taken. The sampling that was accomplished spanned a tidal transition from ebb to flood. This transition is vibrant at these locations because there tends to be strong tidal mixing coming down through the Main Basin as well as from the intersection of Colvos Passage and the Narrows Passage sill. Waters at stations 11 and 12 were very choppy as the ebb and flood actions converged.

Station 11 was a re-do of station 50 from the Commencement Bay 2 sampling day when the CTD failed to get any readings at this location. This station may have the most dynamic input as a result of its location at the intersections of East Passage, Colvos Passage, and the outflow of the Puyallup River exiting Commencement Bay. All of the CTD and discrete data readings at this location were consistently the lowest at the surface, with sharp and shallow pycno/haloclines leading to an otherwise low stratification at mid to bottom depths. As the first station sampled that day, it was caught near the end of the ebb tide which may likely have been pulling fresh, cold Puyallup River water out of Commencement Bay and into the southern Main Basin.

A notable phytoplankton bloom was observed at station 12 (King County Station NSEX01), correlating with high CTD and discrete dissolved oxygen, chlorophyll, and fluorescence readings at this location. Low nitrogen and phosphorus levels were seen at this station as well, suggesting active phytoplankton absorption of nutrients at the time of sampling. Past studies of this area (Mickett et al. 2004) suggested that deep nutrient rich waters coming in from the Strait of Juan de Fuca are vertically mixed into shallower areas of the water column. This can create an environment that is conducive to productivity when the vertical mixing aligns temporally with presence of sunshine. This station was sampled right in the middle of a slack tide which may have caused the phytoplankton populations to be concentrated spatially as the ebbing waters and flooding waters converged.

Station 15 was located directly at the intersection of Colvos Passage and the sill at Narrows Passage. This is a dynamic location where, on a flood tide, incoming waters from East Passage are vertically mixed at the sill and then split to head either northward into Colvos or southward into Narrows, and on an ebb tide, outgoing waters pull from the South Basin through Narrows Passage and into either Colvos or East Passage. Surface readings for salinity and density were highest at this station, correlating with lowest surface temperatures. The station was sampled at the very beginning of the flood tide, when ocean fed waters would have been moving south into this location.

Readings from samples taken deeper in Colvos Passage (Stations 16, and 17) showed generally lower stratification in all data gathered compared to the other stations sampled. Past studies (Paulson et al. 1985) have shown strong tidal mixing at either end of Colvos Passage which lends to both low stratification as well as low residency time of water in this area.

Photos from Sampling Day

A Moment of Silence

During the ride out to the second station, Andrew lost his hat to the sea. Ronnie made a valliant effort to save it, alas, it was for naught. Let us mourn this loss.

Data Repository

Phytoplankton Nets & Surface

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