The Theory of Alternative Stable States in Shallow Lake Ecosystems
Dr Adrian E. Williams
APEM Ltd, Enterprise House, Manchester Science Park, Lloyd Street North,
Manchester, M15 6SE, UK. E-mail: ae.williams@virgin.net
Overview
The idea of alternative stable states was first proposed in the late 1960’s (1) and
within ecological communities was described mathematically in the 1970’s (2, 3).
The contemporary view of shallow lakes is that two alternative stable states can exist
over a range of nutrient concentrations: a clear water state dominated by submerged
macrophytes and a turbid phytoplankton-dominated state (4, 5, 6, 7, 8). Unique states
maybe found at either end of the nutrient continuum but over a wide range of nutrient
concentrations (<50 to several thousand µg l-1 TP) either of these two states can
exist, stabilised by a number of buffer mechanisms (5, 6, 9, 10, 11).
A ‘ball and cup’ diagram is a simple way to visualise this idea (Fig. 1). The position
of the ecological system (the ball) can change between the two states and settles at
the bottom of the cup (stable equilibrium). The slope of the cup determines both the
speed and direction of the system change to a stable state. At either low or high
nutrient levels one of the two stable states exists – the ball can only be in one
location due to the slope of the horizon. However between these points the system
can rest in either cup (state). The ease with which one stable state can be switched to
the other is indicated by the position of the two balls with respect to one another and
the slopes that separate them. Therefore if the starting position is a pristine, low
nutrient (oligotrophic), macrophyte dominated, clear water lake, the nutrient levels
can rise substantially before the only possible scenario is a turbid algal dominated
lake. However to reverse the situation takes more than just reducing the nutrients as
the ball can remain in the turbid state even at relatively low nutrient levels. To switch
the state prematurely requires that a force be applied to the ball to push it over the
edge of the algal cup and into the macrophyte state.
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Fig. 1 ‘Ball and cup’ schematic of the potential for two stable states (clear water
macrophyte dominated versus turbid algal dominated) to exist within a shallow lake
under a wide range of nutrient conditions
Therefore each stable state has a number of buffer systems that help maintain that
state and through top-down and bottom-up processes fish, plankton, invertebrates,
macrophytes and nutrients are all entwined in reciprocal feedback mechanisms that
not only determine the state that is present (12, 13) but can also be manipulated to
force a shift.
Macrophyte buffer mechanisms that help maintain and stabilise a clear water state in
shallow lake systems are numerous (4, 14, 15, 16). Macrophytes act as habitats and
refugia for macro-invertebrates and cladocerans that reduce the epiphyton and
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phytoplankton communities by grazing (6, 17). Davis (18) suggested that the plant
bed environment, often de-oxygenated, favours grazers by discouraging their
predators. Macrophytes may also release allelopathic chemicals (19), partake in the
“luxury uptake” of nutrients, removing available nutrients for phytoplankton (20, 21,
22) and reduce available nitrate by anaerobic decomposition processes such as denitrification (23). Moreover certain species of macrophytes have been shown to
oxidise the sediment and reduce the release of phosphorus. With less available
phosphorus in the water column phytoplankton populations are reduced (24, 25). In
addition stands of macrophytes reduce water movement within them. In consequence
suspended sediment re-settles and turbidity falls, aiding the growth of macrophytes
(26). Moss (10) suggests that even if light penetration is falling, submerged
macrophytes may switch from low growing forms to taller species and that plants
heavily encrusted with epiphyton may be able to shed their leaves and produce new
growth. These processes competitively disadvantage phytoplankton and can buffer
the macrophyte-dominated state.
Once established on the other hand phytoplankton can buffer the switch back to a
macrophyte state by growing much earlier in the season than macrophytes in
temperate regions. In doing so they can curtail the development of turions, rhizomes
or seeds by shading in spring or reduce the formation of propagules by shading and
competition for CO2 in late summer (10). Submerged plants require carbon for
growth but algae have shorter CO2 and HCO3 diffusion pathways owing to their
small size thus removing the carbon available to the bulkier macrophytes (27). Some
researchers have suggested that certain blue-green algae may even release chemicals
toxic to macrophytes (28). In addition, phytoplankton-dominated open water has few
refugia for grazing Cladocera, thus any Cladocera venturing forth are typically
removed by zooplanktivorous fish. This may be compounded by the fact that, with
reduced macrophytes, few large macro-invertebrates would be available to large fish,
favouring smaller-sized fish feeding largely on zooplankton (6). Finally, in poorly
flushed systems, phytoplankton species can consist of large filamentous or colonial
blue-green algal species that are inedible to zooplankton. This and the above
mechanisms all help to maintain phytoplankton-dominated water states over a wide
range of nutrient concentrations.
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By understanding the mechanisms that buffer and thus maintain the alternate stable
states it is possible to attempt to force a shift or switch from one state to another by
manipulating top-down and bottom-up processes. Such attempts are often made
during lake restoration projects when there is a desire to shift an algal dominated
turbid lake into a clear water macrophyte dominated state. However, the long-term
success of lake restoration projects, though they use knowledge of top-down and
bottom-up processes, is not guaranteed and unexpected results are common (29, 30,
31). The uncertainty that surrounds these outcomes often stems from a lack of
understanding about the buffer systems that stabilise the alternative states within the
specific lake under restoration and an over simplification of the top-down and
bottom-up processes involved (13).
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