- Phosphorous recycling feedback (local - well established): In shallow lakes and rivers, the sediments on the lake or river floor typically contain high levels of phosphorous that have accumulated from the settling out of decomposing algae and other organisms. Under eutrophic conditions, the loss of the rooted plants means that the sediments can easily become resuspended due to wave action or the activities of bottom-feeding fish. The resuspended nutrients then become available, promoting further growth of algae, and thereby reinforcing the eutrophic state (Scheffer et al. 1993, Scheffer 1997).
- Phosphate solubility feedback (local - well established): In deep lakes, the eutrophic state is maintained by a different mechanism. In deep lakes temperature gradients create different layers of water: the epilimnion or upper layer is warm and well oxygenated, while the hypolimnion is a lower and colder water layer (Carpenter 2003). When the hypolimnion is oxygenated, phosphorous is captured by iron molecules in an insoluble form. Thus, it is not available to primary producers such as algae. However, algal blooms lead to the depletion of oxygen levels in the lower water layers through the decay of organic matter. When oxygen levels become depleted, phosphate is released in a soluble form that can be used by algae. Algal blooms thereby trigger the recycling of phosphorous in a way that reinforces the eutrophic state (Carpenter 2003).
- Phosphorous recycling feedback (local - well established): In oligotrophic regime, the phosphorous recycling feedback is weak due to the dominance of macro algae in water bottoms. These plants help absorb excess phosphorous from the water column and also stabilize the sediments on the lake floor. Then, phosphorous is kept trapped unavailable for algae (Carpenter 2003).
Shift from Oligotrophic to Eutrophic lakes
Important shocks (eg droughts, floods) that contribute to the regime shift include:
- Floods (local, speculative): Floods are shocks for lake eutrophication since they bring unusual amount of sediments and make the lake turbid. Turbidity reinforce the phosphorous recycling feedback by blocking light to reach the bottom macro-algae vegetation.
The main external direct drivers that contribute to the shift include:
- Nutrients input (local, well established): Excess phosphorous inputs to freshwater systems typically derive from fertilizers applied to agricultural lands, urban storm water runoff, and untreated sewage disposal (Carpenter 2003). The excess of nutrients in water often cause algal blooms. The excessive rates of plant growth and decay that characterize algal blooms lead to depletion of oxygen levels in the water. When oxygen levels fall below the levels needed for respiration, it may lead to widespread kills of fish and shellfish. In addition, algal blooms prevent sunlight from penetrating to rooted plants (macrophytes) growing on the bottom of lakes or rivers.
The main external indirect drivers that contribute to the shift?
- Population growth (global, speculative): Population growth leads to higher demand of food.
- Food demand (local-regional, speculative): Higher food demands usually stimulate intense agriculture, both as expansion of agricultural frontier or increase of fertilizers use to increase yield.
- Agriculture (regional, well established): Agriculture often requires the use of fertilizers. When soils are eroded or washed, fertilizers run downstream increasing nutrients input into lakes and rivers.
- Urban growth (global, well established): Urban growth increase the production of sewage which is also rich in nutrients. It also increase the water runoff on the urban landscape.
- Deforestation (regional, well established): Deforestation and poor agricultural management can accelerate, in magnitude and frequency, the runoff of phosphorous from agricultural lands (Smith and Schindler 2009). Deforestation increase landscape fragmentation and facilitates landscape conversion to agriculture. Both reduce the capacity of the landscape to retain water in the soil, accelerating erosive processes and runoff of nutrients.
- Rainfall variability (regional, speculative): Rainfall variability is expected to change with climate change in some areas of the world. Although it is not clear where or to what extent, it is definitely likely to influence the frequency of flood events and exacerbate erosion in the watershed.
- Global warming (global, speculative): global warming is expected to decrease the resilience of lakes to eutrophication due to increased strong rainfall and higher temperatures.
Slow internal system changes that contribute to the regime shift include:
- Phosphorous in water (Local, well established): The accumulation of phosphorous in the water column usually triggers excessive production of phytoplankton (i.e., algal blooms). In faster-flowing rivers, phytoplankton tends to be washed downstream, and excessive growth of plants such as water hyacinth (Eichhornia), duckweed (Lemna) or water fern (Azolla) may be stimulated instead (Scheffer 1997). Algal blooms, in turn, trigger larger ecosystem changes. The excessive rates of plant growth and decay that characterize algal blooms lead to depletion of oxygen levels in the water. When oxygen levels fall below the levels needed for respiration, it may lead to widespread kills of fish and shellfish. In addition, algal blooms prevent sunlight from penetrating to rooted plants (macrophytes) growing on the bottom of lakes or rivers