Trophic Cascade and Nutrient Loading in a Shallow Lake
A shallow lake receives runoff from agricultural fields. Summer monitoring shows total phosphorus (TP) 120 µg/L, chlorophyll a 45 µg/L, Secchi depth 0.6 m, and hypolimnetic dissolved oxygen 2 mg/L. The lake historically supported submerged macrophytes but now has dense phytoplankton blooms. Managers remove 80% of planktivorous fish (bluegill) and stock piscivorous largemouth bass. One year later, chlorophyll a is 25 µg/L and Secchi depth 1.2 m, but TP remains 110 µg/L. A nearby reference lake has TP 30 µg/L, chlorophyll a 8 µg/L, and Secchi depth 2.5 m. Zooplankton grazing pressure increased after fish removal, especially by large Daphnia. The lake is still fed by the same agricultural drain, and sediment phosphorus release is high in late summer. The lake has no submerged plants remaining.
1. Explain how removing planktivorous fish reduced chlorophyll a without reducing total phosphorus. Include the trophic cascade and nutrient recycling pathways.
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Removing planktivorous fish releases zooplankton from predation. Large Daphnia increase and graze down phytoplankton, so chlorophyll a falls and Secchi depth improves. This is a top-down trophic cascade: fish → zooplankton → phytoplankton. TP remains high because external agricultural runoff and sediment phosphorus release continue. Fish removal may also reduce nutrient recycling by fish, but the dominant reason chlorophyll fell is increased grazing, not lower phosphorus.
2. Predict whether the chlorophyll reduction will persist if external phosphorus loading continues unchanged. Justify using trophic cascade strength and nutrient stoichiometry.
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The reduction is unlikely to persist. High TP keeps the lake's carrying capacity for phytoplankton high; grazing can be overwhelmed, especially when cyanobacteria or inedible algae dominate. Trophic cascades are weaker in eutrophic lakes, and sediment phosphorus release can sustain blooms. Stoichiometry matters: high P relative to N can favor cyanobacteria, which are poorer food for Daphnia. Without reducing external and internal P loading, the clear-water state is likely temporary.
3. Design a monitoring plan to distinguish top-down from bottom-up control. Include at least three variables and a comparison.
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Monitor TP, chlorophyll a, Secchi depth, zooplankton biomass and community composition (especially Daphnia), and fish abundance. Compare the manipulated lake with the reference lake before and after treatment. Also run nutrient-addition and zooplankton-exclusion mesocosm experiments. If chlorophyll tracks TP across lakes and years, bottom-up control dominates; if chlorophyll falls while TP is unchanged and zooplankton rise, top-down control is supported.