What causes underwater plant collapses?

Underwater plants play a keystone role in shallow aquatic ecosystems: they stabilize clear-water conditions with high biodiversity and their decline can cause a shift to a turbid state with phytoplankton dominance. Various mechanisms have been suggested triggering underwater plant collapse. We hypothesized that usually consumption by fish and waterfowl (herbivory) alone does not cause plant collapse, but that additional stress by periphyton shading is needed.

We elaborated this idea by fitting an underwater plant growth model with different herbivore grazing and periphyton shading scenarios. In addition, we performed a meta-analysis on existing experimental herbivore exclosure studies. The model supported our proposed hypothesis and the reviewed field studies appeared to point in the same direction. We suggest that a significant herbivore impact may be an early warning signal for an imminent plant collapse leading to a sudden shift of the system to turbid conditions.

Shading of underwater plants by periphyton increases their susceptibility to herbivory

Sabine Hilt and co-workers

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