1. We piggybacked on Fleenor’s 2 August DWSC experiments and obtained water samples every 2-4 hours from the houseboat, and low/high tide samples from the traveling boats. Analyses: Nitrate d15N/d18O (done) POM d15N/d13C (in progress) DOC-d13C (done) DIC-d13C (done) DO-d18O (almost done) Water-d18O/d2H (not done yet) Ammonium (archived)

2. The  18 O of DO reflects the ratio of productivity to respiration in the water column Respiration (consumption) dilution Photosynthesis (addition) (from Len Wassenaar, 2001)

3. 48-Hour Diel Cycle of DO concentrations and  18 O Typical pattern found in small lakes and streams: 140 % range in O 2 saturation in 24 hours 17 ‰ range in  18 O of DO over 24 hours (from Len Wassenaar, 2001) (are currently giving this a try at a few USGS studies in CA, OR, WA)

4. DO  18 O  13 C DIC How to tease out the main controls on DO levels in the DWSC (and elsewhere): Photosynthesis (P) Respiration (R) Nitrification (N) N R P P N R These processes will probably also cause diagnostic changes in the  18 O/  15 N of nitrate and the  15 N/  13 C of POM

5. Treatment plant flow  15 N NO3 = 15 ‰  15 N NO3 = 12 ‰  18 O O2 = 25 ‰  18 O O2 = 27 ‰  18 O O2 = 29 ‰  18 O O2 = 23 ‰ O2 = 1.0 O2 = 0.9 O2 = 0.7 O2 = 0.6 Cartoon showing how isotopic compositions of O 2, NO 3, and POM can be used to separate the effects of nitrification of ammonium vs respiration of algae as BOD reactions  15 N POM = 11 ‰  15 N POM = 10 ‰ Water samples would be collected along a grid, and at several depths