Publication Summary

A seasonal monitoring project was conducted from March through October 1992, with biweekly surveys conducted at up to 23 stations in Sinclair and Dyes Inlets. Measured parameters included temperature, salinity, depth, DO, dissolved nutrients (nitrate+nitrite-N, ammonium-N, and orthophosphate-P), Secchi disk depth, chlorophyll a (in situ fluorometry and discrete laboratory analyses), phaeopigment, light transmission, and phytoplankton taxonomy.

Sinclair and Dyes inlets are semi-enclosed, partially-mixed embayments located in central Puget Sound. Both inlets were designated as sensitive to eutrophication (i.e., nutrient enrichment) by an assessment of long-term water quality data conducted by Tetra Tech (1988a) and in the Urban Bay Action Plan for Sinclair and Dyes Inlets (Jacobson and Booth, 1990) for the Puget Sound Estuary Program (PSEP). An additional water quality problem, noted in the former report, was the presence of occasional low dissolved oxygen (DO) concentrations (e.g., 21 September 1987). The low DO concentrations are partially due to persistent (density) stratification and decay of organic matter.

In order to better quantify the timing and extent of these problems, a seasonal monitoring project was conducted from March through October 1992, with biweekly surveys conducted at up to 23 stations in Sinclair and Dyes Inlets. Measured parameters included temperature, salinity, depth, DO, dissolved nutrients (nitrate+nitrite-N, ammonium-N, and orthophosphate-P), Secchi disk depth, chlorophyll a (in situ fluorometry and discrete laboratory analyses), phaeopigment, light transmission, and phytoplankton taxonomy.

Precipitation in 1992 was notably less than normal (NOAA, 1993) which likely minimized the strength of salinity-induced stratification in both inlets. Stratification in the Sinclair Inlet during 1992 was strongest in the inner inlet, between the inlet head and Bremerton and was driven by changes in both salinity and temperature. As in the Tetra Tech (1988a) study, low DO concentrations (DO < 5.0 mg/L) were observed infrequently (2 of 14 surveys) and only in the inner portion of the inlet, near Port Orchard. The lowest DO concentrations were recorded at stations where phytoplankton concentrations were highest, though not necessarily simultaneously. Near-surface (1-m) nitrate+nitrite-N concentrations fell below reporting limits (0.01 mg/L) in inner and central Sinclair Inlet from early-May to mid-September, with a notable exception occurring on 8 July after a period of precipitation and when phytoplankton concentrations were unseasonably low. The highest nutrient concentrations (particularly ammonium-N) were recorded in the inner Sinclair Inlet, near Port Orchard and south of Bremerton. Phytoplankton blooms (chlorophyll a >10 mg/m3) occurred throughout the Sinclair Inlet during March through October, however, the highest chlorophyll a concentrations were consistently in the inner inlet, between the inlet head and Bremerton. Blooms of dinoflagellate species known to migrate vertically (Gymnodinium splendens) occurred during early-August at inner Sinclair stations. This migration may be linked to nutrient availability. Potentially harmful (e.g., to vertebrates including fish or humans) phytoplankton species (Pseudonitzschia pungens, Pseudonitzschia pseudodelicatissima, Alexandrium (Gonyaulax) catenella, and Heterosigma carterae) were present in varying quantities in both Sinclair and Dyes Inlets, though no reports of toxicity or fish kills occurred during this time.

Low DO conditions do not appear to be problematic in either Sinclair or Dyes Inlets; however, Sinclair Inlet does exhibit isolated events of low DO concentrations. Further water quality investigation of low DO conditions should focus on inner Sinclair Inlet, in the vicinity of Port Orchard. At times, this portion of the inlet showed elevated ammonium-N and phytoplankton concentrations as well as low near-bottom DO concentrations (e.g., 19 August 1992 data). More frequent sampling at that location in the late-summer to early-fall period would be necessary to assess how sporadic low DO concentrations are and, thus, whether these observations represent a significant water quality problem. Sampling of the benthic community in this area would be useful in determining impact of low DO concentration events on biota. Measured DO concentrations were different during low tide than during high tide in this study. A near-bottom moored sensor, capable of recording hourly data, could be used to determine the magnitude, temporal extent and variability of low DO concentrations. Monthly data available from the long-term monitoring stations SIN001 and DYE004 do not resolve short-lived changes in DO concentrations nor adequately reflect water quality conditions elsewhere in these inlets.