Regime Shifts DataBase

Large persistent changes in ecosystem services

Aquatic ecosystems, such as ponds, canals, ditches or tropical lakes can experience shifts between submerged and floating plant-dominated regimes when the concentration of nutrients (i.e. nitrogen and phosphorus) in the water column changes. The two possible regimes and their associated ecosystem services are:

Submerged plant-dominated regime

This regime is dominated by submerged plants such as Elodea nuttallii (better known as waterweed, a rapidly growing, long and stringy plant) that grow underwater in aquatic ecosystems such as ponds, canals, ditches or tropical lakes. They can be found in water bodies with a low water depth, clear water, low turbidity and weak water-level fluctuations. Typically, their roots are connected to the sediment pool. The regime of submerged dominated plants is also characterized by a low-nutrient concentration in the water column.

Floating plant-dominated regime

This regime is dominated by floating plants such as Eichhornia crassipes, Salvinia molesta, Lemna gibba or Pistia stratiotes found in aquatic ecosystems such as ponds, canals, ditches or tropical lakes. In many places these are invasive exotic species. These plants have their leaf surface exposed to the atmosphere and their roots are not connected to the sediment floor. Typically, a floating plant dominated regime is characterized by dark, anoxic and high nutrient conditions in the water column. 

Submerged to floating plant dominance

The main direct driver that leads to the shift from submerged to floating plant dominance is nutrient enrichment of the water body, typically associated with fertilizer and manure runoff from agricultural activities in the catchment. Strong water-level fluctuations associated with large rainfall events or water abstraction for irrigation and other uses can also enhance nutrient input from the shoreline.

This shift can also be caused by a change in water depth (associated with rainfall events and water abstraction) or turbidity (resulting from pollution, sediment runoff, and algal growth). These both directly affect the availability of light, which in turn affects photosynthetic activity and therefore the growth of submerged plants. The deeper the water body the darker it gets with increasing water depth. The more turbid the water body the less light available for photosynthetic activity in the deeper water layers. Floating plants are therefore in a better position to compete for in situ light than submerged plants. Hence, the deeper or more turbid the water body is, the more likely a shift to a floating plant dominated system. Moreover, the floating plant regime shift can be caused by the introduction of invasive species. Invasive floating plant species might grow very rapidly and aggressively outside their natural environment with the consequence that they might take over an entire water body by reducing the incidence of light, which in turn lead to a decrease of submerged plants. A famous invasive floating plant is Eichhornia crassipes which causes substantial problems in many parts of the world.

When nutrient concentrations in the water are realtively low, nutrients settle onto the sediment floor and are taken up by submerged plants. Submerged plants can grow at low nutrient concentrations in the sediment, and their growth removes nutrients from the system, keeping the nutrient concentration in the system low.

However, once the nutrient concentrations in the water exceed a certain threshold due to excessive nutrient input from eg fertilizer runoff, it becomes possible for floating plants to establish and grow in the water body. The roots of floating plants are not connected to the sediment pool, and are therefore dependent on the availability of nutrients in the water column in order to grow. 

The growth of floating plants in the water column decreases the amount of light that reaches the deeper water layers, and therefore reduces photosynthetic activity of submerged plants, eventually leading to their death. The loss of the submerged plants in turn destabilizes the nutrients trapped in the sediment floor, which become resuspended in the water column, further increasing the nutrient concentration in the water column and enhancing the growth of the floating plants. Once the system has shifted to a floating plant regime, the dark and anoxic conditions under floating plants leave little opportunity for submerged plant life, and it may be very difficult to restore the system to a submerged plant regime. 

Floating plant dominated systems decrease fisheries and plant life due to the dark and anoxic conditions under the leaf surface. They can also have a negative impact on navigation in lakes and canals, water purification and recreation opportunities. Moreover, floating plants can clog water supply pipes for households, ditches or even a river mouth. They can also complicate fishing. In addition, there is a greater risk of pathogens that can negatively affect human health in floating plant dominated systems. 

The main management option to enhance the resilience of the submerged plant regime is to control the nutrient levels in the system, and prevent the establishment of significant numbers of floating plants through a mechanical or chemical removal.

A drastic harvest of floating plants in a shallow water body that has some submerged plants and a not too high nutrient level can shift to a floating plant-dominated regime back to submerged plant-dominated regime (see also the Dutch Ditches Case Study). The amount of harvest needed for a shift is predicted to increase with the water nutrient level. Harvesting the floating plants allows light to penetrate to the deeper water layers and enables submerged plants to re-establish. Once re-established, the submerged plants can help reduce nutrient levels by absorbing nutrients for their growth and trapping suspended nutrients in the sediment.