Swan Lagoon: Internal Loading and a Single Funded Intervention
Swan Lagoon is a 42 ha shallow coastal lake (mean depth 1.4 m, hydraulic residence time 1.2 yr). Total phosphorus (TP) averages 85 µg/L, chlorophyll a 38 µg/L, Secchi depth 0.6 m, and summer phytoplankton is 80% cyanobacteria. External P load is 120 kg/yr; sediment release assays give 12 mg P m⁻² d⁻¹ under anoxia and 1 mg P m⁻² d⁻¹ when the sediment surface is oxygenated, and the hypolimnion is anoxic for 140 days each summer. A 3-year enclosure experiment (three 1 m³ enclosures per treatment, all in one bay) found that adding piscivorous perch halved chlorophyll a and tripled Daphnia density, but only in enclosures where TP was below 100 µg/L. A proposed dredging project would cost NZ$14 million and remove the top 30 cm of sediment. The regional council has funded only one intervention.
1. Estimate the internal P load and compare it with the external load. Explain why reducing external P alone would be slow to improve the lake.
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Area = 42 ha = 420,000 m².
Anoxic season: 12 mg m⁻² d⁻¹ × 420,000 m² = 5.04 kg d⁻¹; over 140 days = 706 kg P. Oxic season: 1 mg m⁻² d⁻¹ × 420,000 m² × 225 days = 94.5 kg P. Total internal load ≈ 800 kg P/yr, against an external load of 120 kg/yr - internal loading is roughly 6–7× external.
The lake's P budget is therefore dominated by recycling from sediment, so cleaning up the inflow would leave TP near 85 µg/L for years. Hysteresis explains why: high TP fuels phytoplankton production, whose decomposition drives oxygen demand, anoxia and further P release, which sustains high TP. The lake has a positive feedback and two stable states, so external load reduction must be large enough, and sustained long enough, to break the loop - otherwise the sediment keeps the lake in the turbid state.
2. How much weight should the enclosure experiment carry for a whole-lake decision? Identify the specific inferential problems.
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The result is encouraging in direction but weak in inference. (1) Pseudoreplication: three enclosures per treatment, all in one bay, so bay is confounded with treatment and there is no replication at the scale of interest. (2) Scale mismatch: 1 m³ enclosures exclude fish movement, wind mixing and the sediment anoxia that drives internal loading; the dominant feedback in the lake is removed by design, so the experiment tests a mechanism, not the lake. (3) Conditionality: the effect appeared only below TP 100 µg/L. Mean TP is 85 µg/L, so the lake sits inside the window - but marginally, and summer and anoxic-season values will exceed it, so the window may close exactly when biomanipulation is needed. (4) Response variables: chlorophyll halved and Daphnia tripled, but there is no evidence that cyanobacteria specifically declined, nor that the effect persisted beyond one season.
The experiment shows a mechanism can operate at small scale. It does not show it will operate at 42 ha. The appropriate next step is a whole-lake trial with before–after–control–impact monitoring across several bays and multiple years.
3. With one funded intervention, what would you recommend? Give quantitative justification and decision triggers.
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Recommend attacking the anoxic period first, because it controls the largest single term in the budget. Keeping the sediment surface oxygenated would drop release from 12 to about 1 mg m⁻² d⁻¹, removing roughly 700 kg P/yr - about six times the entire external load - at a fraction of NZ$14 million. Pair this with continued catchment load reduction so the sediment store is not simply recharged.
Add piscivore stocking only if TP is held below 100 µg/L, and monitor Daphnia density and Secchi as intermediate responses, since those are the variables the enclosure experiment actually measured.
Dredging removes the sediment store but is only justified once external and internal loads are controlled; otherwise the store re-accumulates. It also risks resuspending anoxic, nutrient-rich and possibly contaminated sediment. On cost-effectiveness per kg P removed, oxygenation plus catchment control is the better single investment, with dredging held as contingency.
Triggers: if summer TP remains above 100 µg/L after two years of oxygenation, reopen the dredging case. Monitor TP fortnightly, dissolved oxygen at the sediment interface, Daphnia density and cyanobacterial biovolume.