Aquarium Oxygen Drop and Bicarbonate Confound
Two 20 L aquaria are prepared with aged tap water and 12 Elodea shoots each. Tank A receives 0.5 g/L sodium bicarbonate; Tank B receives none. Each tank holds six pond snails and is kept at 24°C under 16 h light/8 h dark. On day 1, dissolved oxygen at dawn is 6.2 mg/L in A and 6.0 mg/L in B. After 14 days, dawn oxygen is 8.9 mg/L in A and 4.1 mg/L in B; dusk oxygen is 11.2 mg/L in A and 6.7 mg/L in B. Snails in B are alive but clustered at the surface; Elodea in B is yellow-green and fragile. A student concludes that the snails are dying from low oxygen and that Elodea cannot photosynthesize without added bicarbonate. You have pH strips, a light meter, and one week.
1. Evaluate whether the student's conclusion is supported by the evidence and identify alternative explanations.
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The conclusion overreaches. The snails in B are alive, so the claim that they are dying is not supported by the observations. Clustering at the surface is consistent with a behavioral response to low oxygen, but it is not proof of mortality. The oxygen pattern does show a real difference: Tank B falls from 6.0 to 4.1 mg/L at dawn, while Tank A rises, which suggests that photosynthesis in B is not keeping up with respiration by plants and snails. Bicarbonate is a carbon source for photosynthesis, so its absence could limit Elodea, but yellow-green fragile plants could also reflect nutrient deficiency, low pH, or light limitation. Bicarbonate also affects pH and alkalinity, so the treatment changes water chemistry, not just carbon supply. The dusk reading in B is still higher than dawn, which shows some net photosynthesis is occurring. The student needs direct measures of plant growth, pH, light, and snail survival before making a causal claim.
2. Design a one-week follow-up that separates the effect of bicarbonate from the effect of snail respiration on oxygen.
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Use at least four replicated tanks in a 2 × 2 design: bicarbonate present or absent crossed with snails present or absent. Keep plant mass, water volume, temperature, and light schedule identical, and measure dissolved oxygen at dawn and dusk each day along with pH, plant mass, and snail behavior. Include dark bottles or a dark period measurement to estimate respiration separately from net photosynthesis. Randomize tank positions and rotate them to control for uneven light. If only two tanks are available, run a crossover: measure both tanks without bicarbonate, then add bicarbonate to one tank and swap treatments after a washout period, while tracking pH. The key comparison is whether oxygen declines when snails are present even with bicarbonate, and whether Elodea grows when bicarbonate is added without snails. That separates carbon limitation from respiration load.