Lake Seminole: Nutrients, Diel Oxygen, and Three Bad Fixes
Lake Seminole has been monitored monthly since 2018. In 2018, mean Secchi depth was 3.1 m, chlorophyll a was 7 µg/L, and dissolved oxygen was 7.5 mg/L at dawn. After a housing development with septic systems is built, 2023 values are Secchi 0.5 m, chlorophyll a 88 µg/L, and nitrogen and phosphorus concentrations three to five times higher. Dissolved oxygen in 2023 is 1.8 mg/L at 5 a.m. and 13 mg/L at 3 p.m. A fish kill of mostly large adult bass occurs in August. Water hyacinth, an invasive floating plant, covers 30% of the lake surface. Managers propose three actions: harvest the hyacinth, apply an algicide, and stock tilapia that eat algae.
1. Explain the dissolved-oxygen pattern across a day and why the August fish kill hit large adult bass hardest.
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The pattern is a diel cycle driven by photosynthesis and respiration. By day, dense algae photosynthesize and release O2, pushing dissolved oxygen to 13 mg/L, above saturation. At night photosynthesis stops, but algae, bacteria, and everything else keep respiring, so oxygen falls to 1.8 mg/L by dawn. The swing is large because algal biomass is large.
In August, warm water holds less oxygen, and 1.8 mg/L at dawn is below the tolerance of large adult bass, which have high oxygen demand per unit mass and cannot escape a whole-lake low. Decomposition of dead algae consumes additional oxygen, reinforcing the crash. The underlying driver is nutrient loading, which produced the bloom in the first place.
2. Evaluate the three proposed actions. Which could make the fish-kill problem worse, and why?
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Harvesting water hyacinth is the least risky option, but only if the removed biomass is taken out of the lake; plants left to rot will decompose and consume oxygen. The algicide is likely to worsen the short-term problem: killing the bloom creates a large pulse of dead organic matter, bacteria decompose it, oxygen falls further, and a second fish kill can follow.
Stocking tilapia is also risky. Tilapia add waste, can resuspend sediment, and may escape and become invasive. Because the root cause is external nutrient loading, the effective action is reducing nitrogen and phosphorus inputs through septic upgrades and fertilizer rules, paired with continued oxygen monitoring. Treating symptoms without cutting the load will not restore the lake.
3. Describe a controlled experiment that would identify whether nitrogen or phosphorus limits the algae, including treatments, controls, and how results would be interpreted.
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Collect lake water, filter out large grazers, and dispense it into identical bottles. Treatments are control (no added nutrients), +N only, +P only, and +N+P, each in triplicate. Incubate at lake temperature under identical light for four to seven days and measure chlorophyll a or cell counts before and after.
The treatment with the greatest increase identifies the limiting nutrient. If +P exceeds the control but +N does not, phosphorus limits. If +N+P exceeds both single additions, co-limitation is indicated. Include a dark bottle to correct for respiration and a killed control to check for contamination. Repeat across seasons, because limitation can shift. This manipulative design with replication and controls is stronger evidence than correlating nutrient concentrations with bloom size.