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Alternative Stable States

        Because they compete for light and nutrients, aquatic plants and phytoplankton b) having clear water with abundant plant growth. These two conditions have often been referred to as “alternative stable states” (Scheffer et al., 1993; Moss et al., 1997; Scheffer, 1998). In general, dominance by rooted plants is preferable; they increase habitat for fish and invertebrates, lead to better water clarity, and typically enhance biodiversity. A diagram of the two “alternative stable states” is shown in Figure 41.

Fig. 41  (left) Pond with a diverse community of rooted aquatic plants, providing improved water quality, suppression of phytoplankton growth, and a greater variety of wildlife. (right) Pond dominated by phytoplankton, which shade out aquatic plant growth and lead to turbid green water and lower biodiversity (internet image, originally from Scheffer 1998).

        In general, high nutrient levels favor dominance by phytoplankton over rooted aquatic plants, and aquatic plants tend to predominate in ponds with lower nutrients. As the word “stable’ implies, however, ponds with abundant phytoplankton and few plants tend to remain that way even if nutrients are reduced (following the dotted line in Figure 42). Likewise, aquatic plants resist replacement by phytoplankton even with moderate increases in nutrients (solid line in Figure 42).
        Dramatic shifts from a plant-dominated state to one dominated by phytoplankton, or vice versa, within the intermediate zone of alternative stable states are nonetheless possible. For example, treatment with herbicides, or introduction of grass carp (see companion document on Pond Management), or even a high-water year providing less light at the pond bottom may suppress plant growth and promote dominance by phytoplankton (vertical line 1). By contrast, a die-off of fish during a severe winter can, by releasing predation pressure on zooplankton, increase grazer control of phytoplankton and thereby increase light penetration enough to promote dominance by aquatic plants (vertical line 2).

Fig. 42  Trajectories of change in plant abundance with increasing nutrients (solid line) or decreasing nutrients (dashed line).

        In our study 7 ponds supported healthy plant populations, while 4 ponds were dominated by phytoplankton and had few aquatic plants. The two ponds with curly-leaf pondweed showed intermediate characteristics in being dominated by plants in early spring but by phytoplankton later in summer (see Pond Management section for additional information about the biology of this invasive species).