Discussion:

CTD Profiles:

The profiles obtained on May 16, 2003 indicate that the Puyallup River plume leaves Commencement Bay along the northern portion of the bay. Looking at the terminus, middle, and mouth transect profiles that were obtained on this survey, a warmer, fresher, less dense, and more oxygen enriched layer can be seen in the upper 20 meters from the Puyallup River delta to the mouth of the Hylebos waterway along the terminus transect, becoming less exaggerated between the middle station and Brown's Point station along the mouth transect. This finding is consistent with the results of Cannon and Grigsby (1982).

Comparing the data obtained at the mouth transect on May 9, 2003 and May 16, 2003, it can be concluded that a colder and more dense and saline water layer moves into the bay near the bay's bottom with a flood tide. The profiles obtained along the mouth transect on May 16, 2003, which was sampled on a flood tide, show more stratification than that which was observed along the mouth transect on May 9, 2003, which was sampled on an ebb tide. This conclusion is supported by the profiles obtained on May 16, 2003 along the middle transect. These profiles show a colder, denser, and saltier mass of water moving into the bay along the bottom, with a warmer, less denser, and fresher layer riding out into the bay along the upper water column. Please see Mitsuhiro Kawase's model for a display of surface salinity and temperature changes within Commencement Bay and the surrounding Puget Sound area.

The plume of higher salinity, reduced dissolved oxygen concentration, lower temperatures, and increased density extending out into the middle of the bay along the mouth transect on May 16, 2003 from the Asarco Station was most likely the result of having an insufficient number of stations between the Asarco Station and the Mid Mouth Station. Given the depth differences between the Asarco and Mid Mouth Stations, the CTD data may be insufficient to produce accurate contours between these two points, which would be expected to follow the bathymetric contours rather than extending out into the bay at mid water column depths. Obtaining CTD data at a station located in between the Asarco Station and the Mid Mouth Station may provide sufficient data to overcome this type of error. However, the data obtained on May 9, 2003 along the same transect did not produce this distinct plume observed on May 16, 2003. This may be due to the fact that the water column was generally less stratified on May 9, 2003 than that which was observed on May 16, 2003. Obtaining the additional CTD data along the mouth transect may also reveal that upwelling occurs along the western side of Commencement Bay.

Also, the profiles obtained along the mouth transect do not accurately represent the stratification that actually exist in the deepest sections, given that the CTD was only lowered to a maximum depth of 120 meters. This is also true for the data obtained along the middle transect, given that the CTD was only lowered to a maximum depth of 120 meters at the CMB 003 Station and the Mid Mouth Station. Therefore, profiles lower than 120 meters should be analyzed with knowledge of this limitation.

Water Clarity:

The Secchi disk depths obtained from each station on May 9, 2003 and May 16, 2003 suggests that the water is significantly clearer along the mouth of the bay, and within the middle of the bay as compared to the bay's terminus. Looking at the Secchi disk depths obtained from May 16, 2003, the depths were nearly 6 times greater along the mouth and within the center of the bay than they were at the terminus of the bay. The average Secchi Disk depths obtained at each station within the bay during the May 2000 surveys support this general characterization given that the average depths were 7.33 meters along the mouth and at the middle, and 5.08 meters along the terminus of the bay. However, these differences are not as significant as the differences observed in the May 2003 surveys.

There are several explanations for the observed differences in water clarity between the mouth and middle of the bay, and the bay's terminus. First, the terminus of the bay is shallow in comparison to the mouth of the bay and is frequented by boat traffic, given the use of this bay as a major sea port. Boat traffic may be responsible for increasing suspended sediments from propeller wash. Also, the shallow waters in the terminus of the bay increase the potential for sediment suspension given that the surface is closer to the bottom. Secondly, the Puyallup River delta is a source of sediment loading for the terminus of the bay. Given that the Puyallup River is known for its high sediment loading, and the very visible plume of sediments that protrudes into the bay. Looking at the Secchi disk depth graphs for the May 2003 and May 2000 surveys, the lowest Secchi disk depths were obtained at the Puyallup River delta, where the disk disappeared within 1.5 and 3.25 meters, respectively.

Plankton:

Plankton data was only available for the May 2002 and 2003 surveys. In general, the phytoplankton densities were considerably higher during the May 2002 surveys as compared to the May 2003 surveys. However, zooplankton densities were considerably lower during the May 2002 surveys as compared to the May 2003 surveys. Also, the observed zooplankton densities were lower than that which was observed for phytoplankton, which makes sense given that zooplankton are consumers of phytoplankton and zooplankton, and therefore usually occur in lower abundances than the food web's base.

Due to the method by which samples were extracted for analysis, it was noted during the May 2003 surveys that large zooplankton (primarily Cnidarians) were consistently excluded from the sample. However, observation of the sample itself indicates that these larger zooplankton were very abundant. Therefore, the zooplankton densities reported here may not reflect the actual densities. Also, a large number of Zoea were observed at the mouth of the Thea Foss waterway on May 16, 2003. These Zoea were large enough to be seen readily by the unaided eye, and were not consistently withdrawn into the pipette when the sample was being transferred for analysis. Therefore, the zooplankton data may not accurately reflect the large populations observed here.

Looking at the results of the chlorophyll a analysis for the May 16, 2003, the results indicate that chlorophyll a concentrations at each station were ranked as follows; 5>6>8>4>2>3>7>1. Given that chlorophyll a concentration is a measure of the amount of chlorophyll a in the water, and phytoplankton are the producers of chlorophyll a, it would be expected that chlorophyll a concentrations would be positively correlated with phytoplankton density. However, the May 16, 2003 phytoplankton densities for each station were ranked as follows; 7>8>2>6>4>1>3>5. This shows almost the opposite pattern one would expect if this relationship were in fact true, which we have to assume is the case given basic biological knowledge. This finding is counterintuitive and suggests that there might be a source of error associated with either measurement (e.g., phytoplankton surveys and chlorophyll a assessment). If a determination was to be made, it would be most probable that error was associated with the phytoplankton counts, given that all individuals who conducted these counts were inexperienced, and learning as the samples were processed. Also, a relatively large mess screen (153 microns) was used during the May 2003 surveys. Therefore, variations in small phytoplankton densities may be the cause of the observed discrepancies.

General Conclusions:

1. From the data obtained during the May 2003 surveys, salinities ranged from 25.6 to 29.6 PSU, densities ranged from 19.4 to 22.8 kg/m³, temperatures ranged from 9.1 to 10.7 C, and dissolved oxygen concentrations ranged from 4.4 to 6.3 mL/L within the bay. From the data obtained during all surveys, salinities ranged from 25.6 to 29.6 PSU, densities ranged from 19.4 to 22.8 kg/m³, temperatures ranged from 9.1 to 10.7 C, and dissolved oxygen concentrations ranged from 4.4 to 6.3 mL/L within the bay. Combining all data obtained from within Commencement Bay since 2000, salinities ranged from 19.96 to 30.07 PSU, densities ranged from 15.15 to 23.30 kg/m³, temperatures ranged from 8.59 to 10.92 C, and dissolved oxygen concentrations ranged from 3.75 to 9.51 mL/L.

2. The Puyallup River plume appears to leave Commencement Bay along the northern portion of the bay, moving along the terminus and up along the northern shore and leaving between the middle of the mouth and Brown's Point. This finding is consistent with the results of Cannon and Grigsby (1982).

3. During a flood tide, a denser, more saline, and colder water mass moves into the bay along the bottom, with a warmer, less dense, and fresher layer riding along the upper water column. The water column is generally more stratified during a flood tide, and less stratifed during an ebb tide.

4. The highest currents occur along the mouth of Commencement Bay, with the greatest occuring near Brown's Point., with the lowest currents occuring near the terminus of the bay. Outside of Commencement Bay, the highest currents were observed in the Narrows.

5. Nutrient concentrations in the bay range from 0.46 to 2.90 umols/L phosphate, 30.08 to 67.53 umols/L silicate, 3.89 to 27.23 umols/L nitrate, 0.004 to 1.22 umols/L nitrite, and 0.00 to 3.80 umols/L ammonium.

6. Zooplankton densities were lower than the observed phytoplankton densities. Biological productivity, as represented by chlorophyll a concentrations and fluorescence was greatest in the upper water column (0 - 20 meters).

7. Dissolved oxygen concentrations were generally higher at the mouth and lower along the terminus of Commencement Bay.

8. The upper water column at the mouth of Commencement Bay, and in the middle of Commencement Bay are generally clearer that the upper water column at the terminus of the bay.

9. Upwelling may be occuring along the western side of Commencement Bay, although the data obtained thus far does not conclusively support this hypothesis.

1. Orginization of all data, since 2000, into a similar format would facilitate the analysis of changes in conditions over time.

2. If time allows, sampling an additional station along the mouth transect would enable the development of more accurate profiles along this transect.

3. Also, obtaing CTD data from deeper depths (e.g., >120 meters) will also enable the developemnt of more accurate profiles along the mouth and middle transects.

4. Phytoplankton and nutrient data should be added to the data repository for all available surveys.

5. Surfer profiles of CTD data obtained along all transects sampled for each survey should be produced to facilitate comparisons among surveys, including similar blank files.

Cannon, G.A., and M.W. Grigsby. 1982. Observations of currents and water properties in Commencement Bay. NOAA Technical Memorandum OMPA-22. 37 pp.

Return to Top