Assessing Sensitivity to Eutrophication of the Southern Puget Sound Basin Bos, J.K., Newton, J.A., Reynolds, R.A., Albertson, S.L. Washington State Dept. of Ecology, Olympia, WA Figure 1. Stratification intensity as estimated by the difference in sigma-t (Ds t ) between the surface and bottom of the water column. Higher numbers indicate stronger stratification, requiring more wind or tidal energy to mix the water column. Figure 2. Dissolved oxygen concentration (mg/L) in near bottom waters. Dissolved oxygen levels below 5 mg/L are considered stressful to marine biota health. Levels below 3 mg/L have deleterious effects on marine organisms.. Figure 3. Distribution of surface chlorophyll a concentration (mg m -3 ). Chl a is used as a measure of phytoplankton biomass.. Figure 4. Distribution of surface nitrate concentration (  M). Values below 5  M are possibly limiting to phytoplankton growth.. Figure 5. Measurements in Carr Inlet during September (Top panel): False color plot of the distribution of chlorophyll a fluorescence (relative units) versus distance from the northern head of the inlet, overlaid with contours of nitrate concentration (  M). (Bottom panel): Scatter plot of nitrate vs. phosphate concentrations from discrete water samples. The line represents best-fit regression of the relationship for phosphate concentrations greater than 1  M. Introduction and Approach Assessment of marine water quality data from the Washington State Dept. of Ecology’s long- term Marine Waters Monitoring Program (part of PSAMP) from shows that many sites in South Puget Sound would be sensitive to nutrient addition or eutrophication. This assessment is based on indicators, including: persistent density stratification; low dissolved oxygen concentrations; high levels of fecal coliform bacteria; high ammonium concentrations; and non- measurable levels of dissolved inorganic nitrogen during the phytoplankton growth season. Additionally, the South Puget Sound basin has physical characteristics that make it susceptible to eutrophication effects. These features include: shallow bathymetry; slow flushing times; physical stability; numerous inlets with poor circulation; and a large ratio of shoreline to basin. Along with these features, high projected human population growth and subsequent development in the region demand close observation of South Puget Sound water quality variables. Unfortunately, long-term monitoring data has been collected from 3-5 stations only in the Southern Puget Sound basin, yet a high degree of variation in water quality properties is evident. In 1998, the Marine Waters Monitoring Group began intensive studies as part of project SPASM (South Puget Sound Area Synthesis Model) to better characterize the spatial and temporal variation of water properties in the South Puget Sound basin. Objectives were: 1 ) Describe spatial and temporal patterns in water quality variables in South Puget Sound. 2 ) Identify sites within South Puget Sound that are sensitive to the effects of eutrophication. 3) Assess factors controlling plankton production in this basin. 4 ) Provide calibration data for the hydrodynamic and water quality models of this basin, currently in development at the Dept. of Ecology (see Albertson talk and Pelletier poster). Cruises occurring seasonally from , along with two cruises in 1994 and 1997, have provided a comprehensive set of data for analysis of nutrient and other water quality dynamics in this region. Figure 6. Seasonal view of primary productivity rates in natural (Ambient) and nutrient-enhanced (Spike) surface seawater samples. The nutrient spike was 30  M NH 4 and 3  M PO 4. Data shown are Apr. 99, Jul. 00, Sep. 99 and Dec. 99. The December data were multiplied by 10 in order to be visible. Figure 7. The percent increase in surface primary productivity due to an added nutrient spike for experiments conducted in Apr. 99, Jul. 00, Sep. 99 and Dec. 99. Spatial and Seasonal Perspectives Landsat Image of the Southern Puget Sound Region 07 July Density in Southern Puget Sound is largely controlled by salinity gradients. Increased rainfall and riverine input in the winter create areas of stronger stratification. Stratification decreases in the spring as freshwater inputs diminish, but many areas continue to exhibit strong density gradients throughout the year. Some areas of South Puget Sound already have low dissolved oxygen concentrations; stratification plays a role. Stratification prevents bottom-water oxygen levels from being replenished by gas exchange and mixing. Stratification also enhances phytoplankton production during the spring and summer, which sinks to the bottom resulting oxygen debt as the material decomposes. In the winter, oxygen levels are generally uniform throughout the water column from strong mixing and lack of photosynthesis. As the phytoplankton growing season progresses, bottom water oxygen levels decrease, nearing depletion or low values in some areas like Budd, Carr, and Case Inlets. Strong phytoplankton blooms occur, but not consistently in the same places. The blooms appear localized and short-lived, constrained by stratification and nutrient levels. Concentrations of chlorophyll a are uniformly low during the winter months due to lack of light. Very high chlorophyll a concentrations, indicating blooms, can be found during the spring and summer. Surface nitrate can be un- measurable in some areas of South Puget Sound, indicating possible nutrient limitation of phytoplankton production. Dissolved inorganic nitrogen is generally high in Puget Sound waters. Levels may be drawn down during the summer months, in a stratified water column, by phytoplankton population growth. These are the water columns that would be susceptible to effects of eutrophication from added nutrients from point and non-point sources. Evidence that nitrogen is controlling (i.e. can limit) phytoplankton growth: 1) Nitrogen control of phytoplankton growth is suggested by the strong overlap in chlorophyll with that of nitrate contours during the growing season (top). 2) Further, nitrogen, not phosphate, is indicated as the limiting nutrient in this system, as shown by a typical marine (Redfield) ratio between the elements (~16:1 N:P), with nitrate going to zero when excess phosphate is still found. Phytoplankton productivity is relatively high and, as predicted from results above, can be enhanced by nutrient addition. An annual integrated production estimate is about 1000 g C m -2 y -1. These rates are substantially higher (e.g., 3- 5x) than those from many other estuaries like San Francisco Bay or Chesapeake Bay. Nutrient addition experiments resulted in significant increases in productivity; up to 1.5 g C m -3 d -1 was produced in excess over ambient production from nutrient enhancement. Nutrient addition can enhance phytoplankton production by as much as 300%, indicating some regions in South Puget Sound are very sensitive to effects from eutrophication. Enhancement was found at all South Puget Sound stations to some extent, but the highest percentage increases occurred in Carr Inlet. Acknowledgements: WA State Dept. of Ecology: Kara Nakata, Carol Falkenhayn, John Summers University of Washington: A Plethora of Student Volunteers! Conclusions The following observations clearly indicate that inlets in the South Puget Sound basin are sensitive to effects from eutrophication.  Stratification of Southern Puget Sound inlets occurs variably throughout the year.  Dissolved oxygen levels in bottom waters are drawn down during the summer. Levels reach the biological stress level (5 mg/L) in Case and Carr Inlets, and drop to harmful levels (2 mg/L) in Budd Inlet.  Inlets are well-mixed and replete with nutrients during the winter, but show surface depletion of nitrogenous nutrients during the spring and summer, indicating considerable utilization by phytoplankton.  Discrete measurements of chlorophyll a show concentrations indicative of phytoplankton blooms (15-60 ug/L). These blooms appear concurrently with the depletion of surface nitrate, although the location is random and non-repetitive. The factors causing such transience in these particular blooms are not well-understood from the cruise data.  Phytoplankton production is limited by nitrogenous nutrients during the growing season. Nutrient addition experiments resulted in a substantially greater rate of primary production (up to 300%), especially in late summer. Water quality matters concerning eutrophication effects should be focused most strongly on Carr and Case Inlets. Smaller inlets exhibit nutrient sensitivity at various times, but are fairly well-mixed such that strong dissolved oxygen gradients do not appear.