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Submerged to Floating Plants

Feedback mechanisms

Submerged plant-dominated regime

The main feedback mechanisms that maintain the submerged plant-dominated regime are:

Sediment nutrient feedback (balancing, local scale, well-established):Submerged plants can grow at low-nutrient concentrations in the sediment. At the same time, they remove nutrients from the sediment which in turn reinforces their growth and keeps the nutrient concentration in the system low.

Nutrient recycling feedback (balancing, local scale, well-established):Nutrients in the water column settle and get trapped in the sediment, and held fast by the roots of the submerged plants. This mechanism removes nutrients from the water column and deposits them in the sediment, to be taken up by the submerged plants.


Floating plant-dominated regime

The main feedback mechanisms that maintain the floating plant dominated system are:

Shading feedback (reinforcing, local scale, well-established): The growth of floating plants in the water column decreases the amount of light that reaches the deeper water layers and therefore reduces photosynthesis activity and the growth of submerged plants.

Nutrient recycling feedback (reinforcing, local scale, well-established): When there are no submerged plants holding down the sediment on the floor of the water body, the nutrients trapped in the sediment become resuspended in the water column, further raising the nutrient levels in the water column.

Other important feedbacks in the floating plant-dominated regime are:

Water nutrient feedback (balancing, local-scale, well-established): High nutrient concentrations in the water column enhance the growth of the floating plants, while the growth of the floating plants removes nutrients from the water column.

Floating plant shading feedback (reinforcing, local scale, well-established): The growth of floating plants in the water column decreases the amount of light that reaches the deeper water layers and therefore reduces photosynthesis activity and the growth of submerged plants. 

Drivers

Shift from submerged to floating plants

Important Shocks that contribute to the regime shift

Rainfall events(local, well-establishd): Strong rainfall events can lead to fluctuations in the water-level which in turn can enhance nutrient input from the shoreline.

The main external direct drivers that contribute to the regime shift

Nutrient input (local, well-established): Floating plants have no direct access to the sediment pool of nutrients, and therefore rely on nutrients in the water column for their growth.

Sediment & pollutants input (local, well-established): The input of sediment or pollutants affects turbidity, which in turn affects photosynthesis acticivity and therefore the growth of submerged plants.

The main external indirect drivers that contribute to the regime shift

Agriculture fertilizer and manure runoff (local, well-established): Floating plants have no direct access to the sediment pool of nutrients, and therefore rely on nutrients in the water column for their growth.

Water abstraction (local, well-established): Water abstraction for the purpose of irrigation or other uses can lead to fluctuations in the water-level which in turn can enhance nutrient input from the shoreline.

Slow internal system changes that contribute to the regime shift

Water depth (local, well-established): The deeper a water body is, the darker it gets with increasing water depth, which in turn affects photosynthesis acticivity and therefore the growth of submerged plants.

Turbidity (local, well-established): The turbidity in the lake affects the availability of light, which in turn affects photosynthesis acticivity and therefore the growth of submerged plants. The more turbid a water body is, the less light is available for photosynthesis activity in the deeper water layers.


Shift from floating to submerged plants

The main external direct drivers that contribute to the regime shift

Harvest of floating plants (local, well-established): Harvesting the floating plants allows light to penetrate to the deeper water layers and enables submerged plants to re-establish.

Slow internal system changes that contribute to the regime shift

Nutrient concentration in the water column (local, well-established): Floating plants are dependent on nutrients in the water column. They cannot take up nutrients from the sediment pool, because their roots are not connected to it. Therefore, a large decrease of nutrients in the water column can shift a flaoting plant dominated regime to a submerged plant dominated regime.

Water depth (local, well-established): The deeper a water body is, the darker it gets with increasing water depth, which in turn affects photosynthesis acticivity and therefore the growth of submerged plants. 

Key thresholds

Submerged to floating plant-dominated regime

Water nutrient threshold: Threshold at which floating plants can become established and grow.

Shading threshold: Threshold at which submerged plants die.

Submerged to floating plant-dominated regime

Water nutrient threshold: Threshold at which floating plants can no longer grow in the water.

Leverage points

Harvest of floating plants (local, well-established): Harvesting the floating plants allows light to penetrate to the deeper water layers and enables submerged plants to re-establish. 

Ecosystem service impacts

Ecosystem services associated with the submerged plant dominated are freshwater, fisheries and biodiversity. Submerged plants are also able to improve water quality due to their water purification and pest/disease regulation capabilities. Submerged plants can contribute to recreation and aesthetic values.