Dissolved oxygen. Bronte Tilbrook for Craig Neill, Kelly Brown and Erik van Ooijen

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1 Dissolved oxygen Bronte Tilbrook for Craig Neill, Kelly Brown and Erik van Ooijen

2 GOOS Biogeochemistry Panel: Defining Essential Ocean Variables for Biogeochemistry 1. The role of ocean biogeochemistry in climate How is the ocean carbon content changing? How does the ocean influence cycles of non-co 2 greenhouse gases? 2. Human impacts on ocean biogeochemistry How large are the ocean s dead zones and how fast are they changing? What are rates and impacts of ocean acidification? 3. Ocean ecosystem health Is the biomass/production of the ocean changing? How does eutrophication and pollution impact ocean productivity and water quality?

3 Deoxygenation Keeling et al., 2010 Detection O 2 Macronutrients (NO 3, PO 4, Si, NH 4, NO 2 ) Transient Tracers Material Export and/or O 2 /Ar Carbonate system

4 Impact Essential Ocean Variables, GOOS/IOC CO 2 system 10 Particulate Matter Export Oxygen 5 Trace Nutrients Suspended Particulates 14 C Tracers Macronutrients N 2 O, Dissolved Organics 13 C Bio-optics DMS Dissolution rates Pigments Methane Radium, Radon, Halocarbons Oxygen Isotopes Feasibility 10

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7 SBE 63

8 FLOATS SOTS NRS SOOP

9 Physical and Biological Controls on surface O 2 Total O 2 = O 2 (phys) + O 2 (bio) = O 2 (phys) + O 2 (O 2 /Ar) Shadwick et al 2014

10 WQM: SBE43 P. Jansen

11 North Stradbroke Island WQM dissolved oxygen SBE43 WQM dissolved oxygen data for NRS sites appear to be poor quality Sensor reliability, drift and calibration issues Accuracy about +- 10µmol/l, although the data returns are poor

12 Optode calibration Dissolved oxygen calibration system designed and built by Craig Neill (CSIRO) water jacketed chamber for temperature control to ± 0.01 C over T range = 1 to 35 C gas mixing and bubbling for O 2 saturation = 0 to 110% Reference: Winkler titrations ± 0.6 µmol/l compared to saturation up to 20 optodes per calibration run

13 Saturation (%) Optode Calibration Calibration of raw optode output (bphase) at range of temperatures and dissolved oxygen concentrations with winkler titrations used as reference. Polynomial fit: O 2 = f(bphase, T) Calibration makes a significant improvement on factory calibrations Optode 886 offset to initial calibration (mmol/l) Temperature ( C) Errors in calibration are not constant with temperature or O 2 saturation multiple point calibrations are needed

14 [O 2 ] error (umol l -1 ) Multipoint calibrations & accurate reference measurements Errors in [O 2 ] predicted by various calibration methods Red - Factory polynomial with linear correction (T min = 3.5) Orange - Polynomial fit to new winkler reference data (T min = 0.7) Green - Uchida fit to new winkler reference data (T min = 0.7) Temperature

15 % Oxygen saturation Offset (umol l -1 ) % Oxygen saturation % Oxygen saturation Careful use of reference sensors can save time Initial calibration of 5 reference optodes, winklers at every non-zero point. temperature Fit to Uchida or polynomial function for each optode [O 2 ] pi =f(p,t) Corrected (initial cal + offset) data from 5 reference sensors is averaged to generate a complete set of reference values for each batch. temperature Subsequent calibration batches, winklers on subset, automated sensor vs sensor data on all points. Fit calibration curves for all sensors temperature Fit correction offset for each of 5 reference optodes Off=[O 2 ] ref [O 2 ] pi [O 2 ] p

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17 Maria Island Maria Island

18 Conclusions optode technology = better data returns multipoint calibrations and high quality winkler titrations are essential for good quality data secondary reference sensors and some winker titrations can speed up calibrations pre and post deployment calibrations should be used where possible to correct drift and identify biofouling problems predeployment checks (e.g. measurement in air may provide a quick method to monitor drift).

19 Optodes Luminecsence quenching Sensing foil platinum porphyrine complex Blue-green light absorbed by sensing foil (lumiphore) Lumiphore emits light at red wavelengths. The delay in the emission of red light depends on the presence of oxygen