CICEET Progress Report for the period 8/01/00 to 2/15/01

Project Title: Using d¹⁵N to monitor N loading at NERR sites along the western US coast
Principal Investigator(s): Brian Fry

Accomplishments
Scheduled Tasks:
The main task for the last six months were data analysis and synthesis.

Progress on Tasks
Mr. Arian Gace has completed a draft chapter for his master's thesis that synthesizes the main results of the NERR study. Data from the study has been compiled into 2 appendices.

Difficulties Encountered
The project is going smoothly, with Mr. Gace scheduled to hand in a final thesis April 12, 2001, so that we can also meet the final CICEET report deadline for this project, May 30, 2001.

Anticipated Success in Meeting Project Objectives in Scheduled Project Period
This project continues is on schedule.

Preliminary Data
We have been synthesizing data for each of the four west coast NERR sites, and present some of the major trends in terms of "bubblegraphs." These graphs show site-by-site trends for seasonally averaged data, with beach and coastal marine values providing the basic reference point for understanding values within the 4 west coast estuaries. Averages for salinity, nitrate, ammonium and d¹⁵N of four bioindicators (filter feeders, green macroalgae, particulate organic material and sediments) were assigned into categories, then plotted on site maps to show geographic variation within each estuary. The nitrate and ammonium data were normalized via dividing by average marine values measured at end-member beach stations, while d¹⁵N values of the bioindicator samples were normalized by subtracting marine end-member values. The appended graphs (Figures 1-4) show these normalized data in bubblegraph views of the four west coast estuaries.

Padilla Bay (Figure 1a and Figure 1b). Salinities at the 12 Padilla Bay stations were generally mesohaline, with freshwater in two sloughs draining local watersheds (Joe Leary Slough and No Name Slough, stations 5-7 and 8 respectively, Figure 1a). Ammonium and nitrate concentrations were near marine values in the open bay stations, with higher concentrations in the sloughs (Figure 1a). d¹⁵N values of bioindicators were generally near marine values, with higher d¹⁵N values in some nearshore stations. Lowest d¹⁵N values occurred in green macroalgae taken from the freshwater side of the control structure in Joe Leary Slough (station 5, Figure 1a). Low d¹⁵N values were correlated with high ammonium concentrations found in freshwaters at this last site, and also in other samples collected from the Tijuana estuary (see below). Our preliminary interpretation is that while watershed nitrate inputs usually increase ^d15N values in estuarine biota, strong ammonium inputs can lead to the opposite result, i.e., low ^d15N values.

South Slough (Figure 2a and Figure 2b). South Slough had a moderate salinity gradient from high values at beach stations to low values in freshwater creeks (Figure 2a). Ammonium and nitrate concentrations were generally close to marine values (Figure 2a). d¹⁵N values for bioindicators were also generally close to marine values (Figure 2b), a result that might be expected if watershed N loading is small in South Slough.

Elkhorn Slough (Figure 3a and Figure 3b). Salinities were generally high in most of this estuary (Figure 3a), consistent with strong marine flushing. Both ammonium and nitrate concentrations were strongly elevated in portions of the watershed draining into the estuary (e.g., stations 3-5, Figure 3a). High d¹⁵N values of macroalgae and consumers were also observed in this estuary where highest DIN levels were observed (Figure 3b).

Tijuana Estuary (Figure 4a and Figure 4b). As at Elkhorn Slough, high salinities and nutrient concentrations were evident for many landward stations in Tijuana Estuary (Figure 4a). Bioindicator samples collected along Oneonta Slough (towards station 3, Figure 1a) showed increasing ^d15N values associated with higher DIN levels (Figure 4a and Figure 4b). These patterns are consistent with the basic prediction of this research, that higher ^d15N functions as an indicator of anthropogenic N loading. Lower ^d15N values were observed along the main axis of the estuary that received sewage effluent (Figure 1a and Figure 1b, stations 7,8 and 9), even though d¹⁵N values of ammonium were high (ca. 11o/oo). As at Padilla Bay, results from Tijuana suggest that while watershed nitrate inputs usually increase ^d15N values in estuarine biota, strong ammonium inputs can lead to the opposite result, i.e., low ^dd15N values.

Tasks and activities for the next reporting period

Tasks for the next reporting period
Prepare final report.

Work plan to accomplish tasks
We are working with Mr. Gace to get a make sure he meets all deadlines to hand in his thesis on April 12. The first chapter of his thesis will be the basic final report summarizing this project.

Concerns or difficulties
No special concerns to report here.

Expenditures
Expenditures are within the expected range and allocated budget.