Two Lakes, Two Food Webs
Two adjacent lakes receive runoff from the same watershed. Lake A is clear with abundant submerged macrophytes and a diverse invertebrate community; its fish are mainly visual predators. Lake B is turbid, with phytoplankton dominance, few macrophytes, and a fish community dominated by a benthic-feeding species. A fertilizer spill adds 50 kg of nitrogen and 5 kg of phosphorus to Lake B. Within one season, Lake B's chlorophyll a doubles, dissolved oxygen at dawn falls below 3 mg/L, and a fish kill occurs. Lake A shows no change. Both lakes have the same depth and mean temperature.
1. Explain why Lake B responded so strongly to the nutrient addition while Lake A did not, using the concepts of limiting nutrients and alternative stable states.
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Lake B was already phytoplankton-dominated and turbid, meaning light was the limiting factor for submerged plants but nutrients limited phytoplankton. Adding nitrogen and phosphorus relieved that limitation, so phytoplankton grew rapidly. Lake A was macrophyte-dominated and clear; its phytoplankton were light-limited because macrophytes stabilized the sediment, took up nutrients, and kept the water clear. The two lakes represent alternative stable states maintained by feedbacks: in Lake B, turbidity suppresses macrophytes, which would otherwise bind nutrients and clear the water; in Lake A, clear water favors macrophytes, which keep it clear. The same nutrient load therefore produces a large response in B and little in A. The N:P ratio of the spill (~10:1 by mass) is also close to the Redfield ratio favorable for algal growth, reinforcing the response.
2. The dawn oxygen drop and fish kill occurred together. Explain the mechanism linking the algal bloom to the oxygen crash, and why the measurement was taken at dawn.
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The bloom creates a large diurnal oxygen swing. During the day, photosynthesis by dense phytoplankton produces oxygen, sometimes supersaturating the water. At night, photosynthesis stops but respiration by algae, bacteria, and other organisms continues, consuming oxygen. Just before dawn - after the longest period without photosynthesis - dissolved oxygen reaches its minimum. The measured value below 3 mg/L is hypoxic for most fish. Bacterial decomposition of dead algae further consumes oxygen and can release nutrients, fueling more blooms. The fish kill follows because hypoxia impairs gill function and aerobic metabolism. Measuring at dawn captures the worst-case oxygen deficit; a midday measurement could misleadingly show adequate or high oxygen. This is why continuous monitoring or dawn sampling is standard in eutrophication assessment.