Measuring Nitrate, Phosphate, Salinity, and Dissolved Oxygen
Why measure?
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Nitrate (NO₃⁻) and phosphate (PO₄³⁻) regulate basal productivity; enrichment signals anthropogenic inputs and can trigger eutrophication with cascading biotic effects.
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Salinity (via specific conductance/salinity) integrates dissolved ions from geology and land use; elevated levels affect osmoregulation and community composition.
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Dissolved oxygen (DO) reflects the balance of photosynthesis, respiration, reaeration, and temperature; low DO stresses or kills sensitive taxa. Diel DO reveals ecosystem metabolism.
Materials Needed
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Clean sampling bottles (rinsed with sample water)
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Cooler with ice packs
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Test kits for nitrate and phosphate
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Conductivity meter
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DO meter with temperature sensor
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Calibration standards/blank water; datasheets
Nitrite
Why measure?
Indicates dissolved inorganic nitrogen availability and anthropogenic inputs; key for diagnosing eutrophication risk and calculating nitrogen loads (with discharge).
Method
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Collection and prep: Collect water sample with sample bottle bottle facing upstream
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Kit procedure: Follow the manufacturer's instructions. Use provided reagents, timing, and mixing exactly; avoid contamination of cuvettes.
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Reading: Compare color to chart or read with a portable photometer at the specified wavelength. Run a kit blank with deionized water and, if possible, a check standard.
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Record the reading on your datasheet.
Phosphate
Why measure?
Often the limiting nutrient; increases can drive algal blooms and DO depletion, degrading habitat.
Method
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Collection and prep: Collect water sample
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Kit procedure: Use molybdate blue (ascorbic acid) test kit per instructions; adhere to reaction time window.
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Reading: Compare to color chart or use a portable photometer. Include a blank and, if possible, a low level check standard.
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Record the reading on your datasheet.
Salinity
Why measure?
Reflects cumulative dissolved ions affecting organism physiology and community composition; useful for tracking catchment stress and mixing.
Method
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Calibration: Calibrate with appropriate standard(s) following the meter guidance
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Field reading: Rinse probe with sample; measure mid column in flowing water; avoid bubbles; allow stabilization.
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Record the reading on your datasheet.
Dissolved Oxygen (DO)
Why measure?
Central to aerobic metabolism; low DO indicates metabolic imbalance or high oxygen demand and can cause acute or chronic stress.
Method
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Calibration: Perform 100% air saturation calibration (and zero check if applicable) before sampling; verify after fieldwork.
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Field reading: Submerge probe gently at mid depth in flowing water; avoid bubbles/sediment. Allow stabilization.
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Reporting: Record DO (mg/L) and precentage saturation.
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Record the reading on your datasheet.
Simon F.S. Li Marine Science Laboratory
The Chinese University of Hong Kong
Hong Kong SAR, China