CFP Water Quality & Management 2 — Questions and Answers
Question 1: Which form of nitrogen is most directly toxic to fish at elevated concentrations?
- Nitrate (NO3-)
- Ammonium (NH4+)
- Un-ionized ammonia (NH3) (Correct answer)
- Nitrite (NO2-)
Correct answer: Un-ionized ammonia (NH3)
Un-ionized ammonia (NH3) is the primary toxic form because it readily crosses gill membranes, unlike the ionized ammonium form.
Question 2: A fisheries manager measures a total alkalinity of 15 mg/L CaCO3 in a pond. This value indicates the water is:
- Well buffered against pH swings
- Moderately buffered
- Poorly buffered and susceptible to pH crashes (Correct answer)
- Oversaturated with calcium
Correct answer: Poorly buffered and susceptible to pH crashes
Total alkalinity below 20 mg/L CaCO3 is considered poorly buffered, making the system vulnerable to rapid pH fluctuations.
Question 3: During a fish kill investigation, you find DO = 0 mg/L at dawn and water temperature = 28°C. The most likely cause is:
- Ammonia toxicity spike
- Overnight algal and bacterial respiration depleting oxygen (Correct answer)
- Thermal stratification trapping cold water at surface
- Nitrite poisoning from runoff
Correct answer: Overnight algal and bacterial respiration depleting oxygen
Dense algal blooms and microbial respiration consume dissolved oxygen throughout the night, commonly causing pre-dawn DO crashes in warm, eutrophic systems.
Question 4: What is the primary function of a constructed wetland used as a fish farm effluent treatment system?
- Increasing salinity before discharge
- Removing suspended solids, nutrients, and BOD through biological and physical processes (Correct answer)
- Oxygenating effluent via mechanical aeration
- Lowering water temperature through evaporative cooling
Correct answer: Removing suspended solids, nutrients, and BOD through biological and physical processes
Constructed wetlands use substrate filtration, plant uptake, and microbial activity to reduce suspended solids, nitrogen, phosphorus, and oxygen demand in effluent.
Question 5: Which chemical is most commonly applied to raise pond pH and total alkalinity in acidic aquaculture ponds?
- Ammonium sulfate
- Agricultural limestone (CaCO3) (Correct answer)
- Potassium permanganate
- Alum (aluminum sulfate)
Correct answer: Agricultural limestone (CaCO3)
Agricultural limestone (calcium carbonate) dissolves slowly to neutralize acidity and raise both pH and total alkalinity in pond waters.
Question 6: The term 'chemical oxygen demand' (COD) in water quality analysis measures:
- The oxygen required by fish at a given temperature
- The oxygen consumed when all oxidizable organic and inorganic matter is chemically oxidized (Correct answer)
- The oxygen deficit relative to saturation
- The oxygen produced by phytoplankton per day
Correct answer: The oxygen consumed when all oxidizable organic and inorganic matter is chemically oxidized
COD measures the total oxygen demand of a water sample using a strong chemical oxidant, capturing both biodegradable and non-biodegradable organic compounds.
Question 7: Secchi disk depth is used in fisheries management primarily as a proxy for:
- Water temperature stratification depth
- Water clarity and phytoplankton biomass (turbidity) (Correct answer)
- Benthic habitat suitability
- Dissolved oxygen at depth
Correct answer: Water clarity and phytoplankton biomass (turbidity)
Secchi disk depth provides a simple field estimate of water transparency, which reflects phytoplankton density and suspended sediment loads.
CFP Water Quality & Management 2
Which form of nitrogen is most directly toxic to fish at elevated concentrations?