Publication Summary

This study was conducted to assess whether primary production in Central Puget Sound and Possession Sound would be affected by the addition of nutrients (nitrogen, phosphorous), such as could come from wastewater treatment plant effluent. Measurements of phytoplankton production and its controlling factors over seasonal cycles were made from October 1998 through October 2001. Interannual variation was evident that appeared linked to differences in weather and ocean conditions. An increase in primary production due to the addition of nutrients was demonstrated within the study area during spring and especially during summer. Nutrient enhancement was most evident in Possession Sound.

Primary production was measured using the carbon-14 uptake method in order to assess production and nutrient dynamics in the Central Basin and Possession Sound regions of Puget Sound. Ambient and nutrient-spiked production rates for the entire euphotic zone were determined every 2-6 weeks at four stations from October 1998 through October 2001 (n=42). Nutrient (dissolved nitrogen and phosphate) concentrations, chlorophyll a, incident radiation, temperature, and salinity were also measured to examine factors affecting production rates. Seasonal variation in production is well-defined for all four stations, with values integrated over the euphotic zone during summertime (May-September) as high as 13,000 mg C m-2 d-1, which drop to less than 100 mg C m-2 d-1 during wintertime. During the middle of the growing season (April-June) a reduction in production was measured at all stations. Similar variation in biomass, as measured by chlorophyll a, was also seen. Considerable interannual variation in production was observed, potentially linked to differences in external physical forcings. Variations in water properties suggest strong oceanic input during 2001. A maximal primary productivity rate was not consistently found at any particular station. Increased primary production due to experimental addition of nutrients was seen at times at all stations during spring and, more often, summer months. Nutrient enhancement of productivity was most evident in Possession Sound. However, based on annual averages, all four stations exhibit substantial production increases in response to added nutrients. Possession Sound and Admiralty Inlet appear to be most susceptible to nutrient limitation towards the tail end of the summer snow melt, when the water column becomes increasingly stratified. Nutrient stimulation was seen at the Admiralty Inlet station during the winter, although light appears to be the primary determinant of production levels during the winter. Consideration of these results along with physical data and modeling will be required to assess regional sensitivity of this part of Puget Sound to nutrient addition.