Adding a limiting nutrient can increase primary productivity, but excessive nutrient input can destabilize aquatic ecosystems. This process is eutrophication.

Suppose runoff introduces large amounts of nitrogen or phosphorus into a lake. If one of those nutrients was limiting, phytoplankton and algae can grow rapidly, producing an algal bloom.

The bloom does not simply mean that the ecosystem now contains more useful energy. Several downstream effects can occur:

1. dense algae reduce light penetration and can suppress submerged plants;
2. bloom organisms eventually die or release organic matter;
3. decomposers consume that organic matter;
4. decomposer respiration consumes dissolved oxygen;
5. oxygen concentrations can fall to hypoxic levels, too low for many animals, or anoxic levels, with essentially no oxygen.

The causal sequence is therefore

excess limiting nutrient
        ↓
increased primary production
        ↓
more organic matter to decompose
        ↓
increased microbial respiration
        ↓
dissolved O₂ depletion
        ↓
stress or death of aerobic organisms

Some blooms also involve species that produce toxins, but toxicity is not required for eutrophication to cause ecological damage.

Eutrophication shows why increasing a resource can have indirect negative effects. Nutrient cycles, primary productivity, decomposition and cellular respiration all interact to determine the final ecosystem response.