Meredith Howard Karen McLaughlin Nikolay Nezlin

Transcription

1 Meredith Howard Karen McLaughlin Nikolay Nezlin

2 SCCWRP Objectives For Diversion Determine the fate of effluent nitrogen How quickly is effluent nitrogen consumed or transformed? Is effluent nitrogen being utilized by the biological biomass? Determine how DO concentrations are affected by pathways of N transformation and other processes

3 Approach Dissolved Nutrients: DIN (NO 3, NO 2, PO 4 ) and NH 4, DOC, TN, TDN, TP, TDP Particulate Nutrients: PN, POC Stable Isotope Analysis: Dissolved Natural Abundance (NH 4, NO 3 ) Particulate Natural Abundance Rate Estimates: Nitrification Experiments

4 Tracking the Fate of Effluent Nitrogen Measure ambient concentrations of nitrate and ammonia Assess rates of key process Nitrification Nitrogen uptake rates Productivity and respiration Measure natural abundance stable isotopes to trace distinct sources

5 Ambient Nitrogen Was Mostly Nitrate Pre- Diversion Outfall pipe on Nitrogen concentration (um) Nitrite+Nitrate Ammonia 25 9/6/212 Nitrite+Nitrate 9/2/212 2 Ammonia 15 5 Nitrogen concentration (um) Outfall pipe off 222S 223S 223D 225S Nitrogen concentration (um) 225D 227S 227D 228S 228D 222S 223S 223D 225S 225D 227S 227D 228S 228D 1 Day Post- Diversion Outfall pipe on Nitrite+Nitrate Ammonia 25 /3/212 Nitrite+Nitrate /17/212 2 Ammonia 15 5 Nitrogen concentration (um) Post- Diversion Outfall pipe on 222S 2183S 2183D 2223S 2223D 222S 223S 223D 225S 225D 227S 227D 228S 228D 225S 225D 236S 236D Low ambient ammonium throughout the study

6 Tracking Effluent Ammonia Sources have equal magnitudes but the form of nitrogen is different: Upwelled nitrogen is 9% nitrate Effluent nitrogen is 8-9% ammonia From Bight 8 Study Nearshore seawater concentrations indicate nitrate is primary form Examine nitrification as potential source of nitrate

7 Nitrification Rates are Higher Near Outfall Pre- Diversion Outfall pipe on Station S228 S227 S225 S223 9/6/212 9/2/212 S228 Station S227 S225 S223 Outfall pipe off S222 Surface Depth S222 Surface Depth Nitrification Rate (nmol L -1 d -1 ) Nitrification Rate (nmol L -1 d -1 ) 1 Day Post- Diversion Outfall pipe on Station S236 S225 S2223 S2183 /3/212 /17/212 S228 Station S227 S225 S223 Post- Diversion Outfall pipe on S222 Surface Depth S222 Surface Depth Nitrification Rate (nmol L -1 d -1 ) Nitrification Rate (nmol L -1 d -1 ) In situ rates of the pre-diversion magnitude would result in conversion of 1e 6 2e 6 kg of ammonia to nitrate per day per cubic km For Reference: OCSD discharges ~ 1.4 X 4 kg of ammonia per day

8 Difference In Nitrification Rates Seems to Track with Effluent Plume Before Diversion starts CDOM at 225 at depth and locations off shore During Diversion CDOM are reduced offshore

9 Nitrification May Contribute to Dissolved Nitrate Pool Near Outfall Rates could transform all effluent ammonia into nitrate Occurs at all time points Could explain low ambient ammonia concentrations

10 Nitrogen Source Tracking: Approach Stable isotope source tracking Principle: Different sources have unique isotopic signatures that can be tracked Application: Measure isotopic composition of sources and ambient samples Ambient samples should fall along mixing line(s) Tracers d 18 O and d 15 N of nitrate d 15 N of ammonia d 15 N of phytoplankton and zooplankton biomass

11 How Much Nitrate is from Effluent Relative to Other Sources? Approach: d 18 O and d 15 N of Nitrate and Ternary Mixing Models Purple triangle: Mixing between upwelled nitrate and effluent nitrate d 18 O ( O / OO ) Upwelled NO 3 Effluent NO 3 Effluent NH d 15 N ( O / OO ) Theoretical Nitrified NH 4 Nitrification incorporates 1 O atom from H 2 O (d 18 O = ~) and 2 from dissolved O (d 18 O = ~24); nitrified effluent should have d 18 O = ~16)

12 Ambient Nitrate Isotopic Signatures Atmospheric deposition? End-member values pending Pre-Diversion Cruise: Sept 6 th Effluent discharge location is 225 Purple triangle: Mixing between upwelled nitrate and effluent nitrate d 18 O ( O / OO ) 3 2-2S Upwelled NO 3 3S 7S 3D 8S 5S Effluent NO 3 7D8D Ambient NH 4 5D Effluent NH 4 Theoretical Nitrified NH d 15 N ( O / OO ) 18 to 97 percent of nitrate pool is from effluent in deep water samples pre-diversion

13 Nitrification is Contributing to the Dissolved Nitrate Pool Near Outfall Pre-Diversion Mid-Diversion d 18 O ( O / OO ) 3 2 9/6/212 7S Upwelled NO 3 3D 2S 5S 8S 7D 8D 5D 3S Ambient NH 4 Effluent NH 4 d 18 O ( O / OO ) 3 2 9/2/212 Upwelled NO 3 8S7S 3S2S 5S Ambient NH 4 3D 7D5D Effluent NH 4 - Effluent NO d 15 N ( O / OO ) - Effluent NO d 15 N ( O / OO )

14 Effluent Contributes Significantly to Dissolved Nitrogen Pool Overall Station Fraction from effluent % % % % % Use ternary mixing model to calculate the relative contribution of each source to the measured value (deep samples only)

15 How Much Effluent Nitrate is Incorporated into Biomass? Approach: d 15 N of Particulate N and Binary Mixing Models d 15 N ( o / oo ) Effluent NH 4 Under-estimate Effluent NO 3 Over-estimate Upwelling NO 3 S222 S223 S225 S227 S228 Real answer is somewhere in between

16 Evidence of Effluent Nitrogen in Food Web d 15 N ( o / d 15 N ( o oo ) / oo ) /6/212 S222 S223 S225 S227 S228 9/2/212 S222 S223 S225 S227 S228 Particulate 2uM net Tow 2uM Net Tow Effluent NH4 Effluent NO3 Upwelling NO3 d 15 N ( o / d 15 N ( o oo ) / oo ) /3/212 S236 S225 S2223 S222 S2183 /17/212 S222 S223 S225 S227 S228

17 Effluent Contributes Significantly to Phytoplankton Biomass Station Fraction of Particulate from effluent (using NH 4 end-member) % % % % % Use binary mixing model to calculate the relative contribution of each source to the measured value

18 Post Diversion Subsurface Bloom Post Diversion: Phytoplankton were comprised of an average of 43% effluent ammonia (13 87%) d 15 N ( o / oo ) /6/212 Surface DCM Effluent NH4 Effluent NO3 S23 S26 S223 S224 S226 S2224 S2225 S2226 S233 S235 S236 S2354 S243 S245 WQ1 Upwelling NO3

19 Can We Account for All Effluent Nitrogen? Simple Mass Balance Keep with the 1:3 dilution scheme Box is 3 km x 3 km x 3 m (1.4 x 11 Liters) Homogenously mixed Average mean daily effluent discharge for Sept: 7 MGD N load into the box = Effluent NO3 + Effluent NH4 Assume nutrients stay in the box for 1 or 2 days Nitrogen measured in situ should equal effluent nitrogen discharged In situ N = NO 3 + NH 4 + Particulate N

20 Can We Account for All Effluent Nitrogen? Effluent nitrogen is roughly accounted Effluent Discharge Into The Box Measured 1 day accumulated discharge 9.9 x 5 moles N 2 days accumulated discharge 2 x 5 moles N Size of the Box Dilution Measured In Situ 1.4 x 11 Liters 1:3 13 x 5 moles N 4 x 11 Liters 1: 38 x 5 moles N

21 9/8 9/13 9/18 9/23 9/28 /3 /8 /13 Average Chlorophyll Concentration (ug/l) C:N Ratios Were Low During Diversion Average C:N ratios during diversion about half compared with spring 2 Average extracted chl increases initially, then decreases 3.5 Fall 212 Avg C:N Ratio = 3. Spring 2 Avg C:N Ratio = (22 Transect line only) >9% Diatoms during both time points

22 Conclusions Effluent nitrogen is a big player in biogeochemical cycling during the Fall 212 Effluent contributes significantly to dissolved nitrogen pool Nitrification transforms effluent ammonia to nitrate Effluent is a large fraction of dissolved nitrate pool at depth Effluent contributes significantly to phytoplankton biomass Effluent nitrogen comprised up to half of the total nitrogen in phytoplankton pre-, during-, and post- diversion Influence of effluent nitrogen is evident up the food web Simple box model analysis suggests all effluent N is accounted for in the dissolved and particulate N pools