Alternative States Hypothesis

Despite their prevalence in the world landscape, shallow lakes and ponds had historically received little scientific attention than deep lakes until the early 1990s, when several seminal works proposed ideas to explain the functioning of these unique and complex ecosystems.

Shallow lakes do not always change smoothly from one condition to another in response to external pressures. Instead, changes are often sudden and may trigger important modifications in the complex network of stabilizing mechanisms that previously kept the lake in a stable condition. The 'alternative stable state

hypothesis' states that different environmental states can occur under similar conditions with relative stability against external perturbations. Particularly, over an intermediate range of nutrient concentrations, temperate shallow lakes may exhibit extensive coverage of submerged (or floating-leaved) plants, clear water, and high biodiversity or, conversely, have few or no plants and a turbid-water, often phytoplankton-dominated state (Figure 2). Even different basins of the same lake may show such alternative states. The range of nutrient concentrations (total phosphorus) is typically considered to be between 0.025 and 0.15 mg TP although the upper limit may be much higher (several mg l_1), particularly in small lakes and ponds, notably if fish abundance is low or fish are absent.

At low nutrient concentrations (usually below 0.025 mg TP L_1), the plant-dominated state is the most likely to occur, because phytoplankton is nutrient-limited while plants can use nutrients from the sediments. Lakes dominated by plants have a higher biodiversity of invertebrates, amphibians, fish, and waterfowl. By contrast, at high nutrient levels plants are rare and lakes are typically turbid. The shift to the turbid state does frequently not happen gradually together with the increase in nutrient level, but abruptly when a particular lake-specific nutrient threshold is reached. With nutrient enrichment, several changes in the structure of fish communities occur and small specimens (such as cyprinids in temperate European lakes) become dominant. This has a consequent strong predation pressure on zooplankton and grazer macroinvertebrates, such as snails. The increased biomass of bottom-feeding fish also facilitates the resuspension of sediments and higher inorganic turbidity. The biomass ratio of zooplankton to phytoplankton typically decreases from 0.5-0.8 in mesotrophic lakes to less than 0.2 with phosphorus concentrations above 0.10-0.15 mgTPl-1, implying that the zooplankton is no longer able to control phytoplankton biomass. The reduced grazing pressure on phytoplankton and epiphytes, leads, in turn, to adverse growing conditions for submerged plants. The transitions between states also result in changes in the habitat of dominant algae (from benthic to pelagic), but not necessarily in increased abundance of primary producers. Owing to the new conditions in the lake, the reduction of external nutrient loading does not always easily result in the return of the plants.

Many large shallow lakes lack vegetation despite having low or moderate nutrient concentrations. This likely happens because the effect of the winds on sediment destabilization and resuspension, particularly strong in large systems, prevents the successful establishment of rooted plants. These often turbid lakes present generally low productivity (as also phyto-plankton is limited, in this case for light), and lower biodiversity.

Another alternative state is represented by the dominance of free-floating plants (from small duckweeds to large-bodied water hyacinth). This may occur at high nutrient concentrations in the water and usually high water column stability (Figure 2). Since very low air temperatures can strongly damage these plants, their dominance is more likely to occur in warm regions of the globe, such as the subtropics and tropics. In contrast to rooted plants, free-floating plants do not have access to the nutrient pool in the sediments. Their leaves are in larger contact with the atmosphere than with the water, thus reducing the possibility of taking up nutrients other than carbon through their leaves. Free-floating plants are

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Higher probability of phytoplankton or free-floating plant dominance

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Higher probability of phytoplankton or free-floating plant dominance

Higher probability of submerged plant dominance

Figure 2 General model of the alternative states in shallow lakes, over the gradient of nutrients (phosphorus) where the three main alternative states: phytoplankton-dominated, submerged plant-dominated and free-floating plant-dominated, may occur. Modified (adding free-floating plants) with permission from the original model for temperate lakes published in Moss etal. (1996).

superior competitors for light, whereas submerged plants can grow at lower nutrient concentrations in the water and reduce these concentrations further. Nutrient enrichment therefore reduces the resilience of lakes against a shift to dominance of free-floating plants and phytoplankton. The probability for these three main states to occur changes along gradients of water transparency (and therefore light availability) and nutrients and is likely affected by morphological features of the lakes, such as size or shoreline development (Figure 3).

Each of these environmental states is stabilized by several physical, chemical, and biological processes that act as positive feedbacks. For instance, the presence of submerged plants may promote a local increase in water clarity of up to 90% compared to control conditions, and under this light-enhanced environment, plants, algae on the plant surfaces (periphyton) and benthic algae grow even better while reducing nutrient and the light availability for phytoplankton competitors. The following higher physical stability of the sediments (and consequent reduction of nonbiotic turbidity) helps maintain and expand a clear-water, submerged plant-dominated state. Submerged plants also reduce available nutrients for phytoplankton, epiphyton, and floating plants, either by direct luxurious uptake or enhanced denitri-fication. Besides, submerged plants can indirectly promote higher grazing pressure on the algae competitors by offering habitat and refuge to large-bodied zooplankton and several grazing macroinvertebrates (e.g., snails, mussels) against visual predators. According to laboratory studies, some plant species (e.g., Myriophyllum spicatum, Chara spp, Ceratophyllum demersum, Stratiotes aloides) may excrete allelopathic substances that can inhibit algal growth, although this has not yet been fully substantiated by field experiments.

Perturbations of different nature (e.g., climate-related or most likely anthropogenic) must occur to lose some of these buffer mechanisms and shift the lake from one condition to the other. An increase in nutrient loading will lead to a weaker stability of the system, and small perturbations may suffice to cause a switch to the turbid state. Factors other than turbidity, such as pronounced changes in water level, waterfowl herbivory, wind exposure, and a fish community dominated by benthivorous fish, may negatively affect the development of submerged plants and promote the shift to a less desirable state (turbid waters, or phytoplankton or free-floating plant dominance). The upper limit in nutrients to promote the loss of submerged macrophytes and a shift to these alternative states is not absolute and seems to depend on e.g., lake morphometry and size.

Alternative Stable StatesMeerhoff Alternative Stable States

Figure 3 Alternative states in shallow lakes. Effect of nutrient loading on the equilibrium biomass of phytoplankton (above) and free-floating plants (medium) with respect to the biomass of submerged plants. The arrows indicate the direction of change if the system is out of equilibrium (i.e., the dashed equilibrium is unstable). Catastrophic shifts to an alternative equilibrium occur as vertical transitions in the scheme. Lower panel: probability of occurrence and dominance of the three alternative communities along gradients of nutrients and turbidity. Redrawn with permission after Scheffer etal. (1993) TREE 8: 275-279, (Copyright Elsevier); Scheffer etal. (2003) PNAS 100: 4040-4045, (Copyright (2003) National Academy of Sciences, USA, and Ecosistemas 2004/2, Meerhoff and Mazzeo (2004).

Figure 3 Alternative states in shallow lakes. Effect of nutrient loading on the equilibrium biomass of phytoplankton (above) and free-floating plants (medium) with respect to the biomass of submerged plants. The arrows indicate the direction of change if the system is out of equilibrium (i.e., the dashed equilibrium is unstable). Catastrophic shifts to an alternative equilibrium occur as vertical transitions in the scheme. Lower panel: probability of occurrence and dominance of the three alternative communities along gradients of nutrients and turbidity. Redrawn with permission after Scheffer etal. (1993) TREE 8: 275-279, (Copyright Elsevier); Scheffer etal. (2003) PNAS 100: 4040-4045, (Copyright (2003) National Academy of Sciences, USA, and Ecosistemas 2004/2, Meerhoff and Mazzeo (2004).

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