Question

Freshwater lakes have been suggested to have two alternate stable states, one of clear water dominated by macrophytes and one of murky water with high phytoplankton levels. If this is correct, would you predict that measurement of the clarity of lake water for many lakes would be bimodal with a peak of lakes at the clear end of the spectrum and another peak of lakes at the murky end, with few lakes in between? Why might this prediction be incorrect? Peckham et al. (2006) did this analysis for Wisconsin lakes and discuss the results obtained from satellite measurement of lake transparency.

Short answer

If lakes have two stable states, clarity measurements might be bimodal. However, factors like frequent state transitions or measurement variability could blur this pattern.

Step-by-step solution

Understanding the Hypothesis

The given hypothesis suggests that freshwater lakes have two alternate stable states: one characterized by clear water dominated by macrophytes, and the other by murky water with high levels of phytoplankton. This implies that lakes don't transition easily between the two states, as they are each stable on their own.

Predicting the Bimodal Distribution

If the hypothesis is correct, measuring water clarity across many lakes should show a bimodal distribution. This means that if we plot the frequency of lakes against water clarity, we would expect one peak in the frequency plot at the clear end (representing the clear water state) and another peak at the murky end (representing the murky water state), with relatively few lakes possessing intermediate clarity values.

Considering Reasons for Incorrect Prediction

A bimodal distribution may not be observed if the transition between states is not as stable as assumed. Factors such as nutrient input, temperature, or human activity might cause frequent transitions between states, leading to more lakes with intermediate clarity. Additionally, natural variability in lake ecology or measurement imprecision may also blur the distinction between stable states, reducing the bimodality.

Key concepts

Stable States

In lake ecology, the idea of **stable states** refers to different conditions under which a lake can naturally settle. For example, a lake might be stable with very clear water dominated by aquatic plants called macrophytes. Alternatively, it might stabilize in a murkier setting where tiny organisms like phytoplankton are more widespread. The key point about stable states is that these conditions are resistant to change. This means that even if small disturbances happen—like slight changes in temperature or nutrients—the lake tends to return to its original condition. These stable states are like 'resting places' for the lake's ecosystem.

Understanding these different states is crucial because it helps us predict how a lake might react to environmental pressures or human activities. A lake in a stable clear-water state is likely to stay clear until a significant change occurs, like an intense nutrient runoff that boosts phytoplankton growth.

Water Clarity

**Water clarity** in lakes refers to how clear or murky the water is. This aspect is visually evident; clearer water means you can see deeper into the lake, while murkier water limits visibility. Clarity is mainly influenced by the presence of suspended particles like soil, organic matter, or organisms, including phytoplankton. Clear water usually indicates a balance of nutrients that sustain a healthy ecosystem dominated by macrophytes. This balance allows sunlight to penetrate deeper, supporting plant life. Conversely, murky water could mean there are either more nutrients or disturbances causing more phytoplankton to flourish.

Lake managers often use clarity as a key indicator of the lake's health and ecological state. If water clarity is measured over time across multiple lakes and is found to be consistently clear or murky, it supports the idea of lakes having alternate stable states.

Phytoplankton

Phytoplankton are tiny, floating organisms that utilize sunlight to produce energy through photosynthesis, very much like plants. They play a crucial role in **lake ecosystems** as they form the base of the food web. In high numbers, they can give water a green tint, decreasing clarity. Their population in a lake is influenced by nutrient levels, water temperature, and light availability. When nutrients, such as nitrates or phosphates, are abundant, phytoplankton can rapidly multiply, leading to blooms. These blooms can cause the water to become murky, shifting the lake from a clear state to a more turbid one.

While an essential part of the ecosystem, too many phytoplankton can lead to problems like clogging fish gills or producing toxins that affect animals and humans. A murky water state with abundant phytoplankton can indicate nutrient imbalance in the lake.

Macrophytes

**Macrophytes** are large aquatic plants that grow in or near water and play a significant role in maintaining the clear water state of a lake. These plants include species like pondweeds and lilies and can be seen around the edges or beneath the water's surface. Macrophytes offer various ecological benefits:

• They provide habitat and food for fish and other aquatic species.
• Macrophytes help stabilize sediments, reducing lakebed disturbance which can keep the water clearer.
• They also compete with phytoplankton for nutrients, which helps control their abundance.

Their presence is generally associated with clear water conditions because they allow more sunlight into the lake, which further supports their growth.

A dominance of macrophytes often signals a well-balanced ecosystem where aquatic life is thriving, and water quality is high.

Bimodal Distribution

The term **bimodal distribution** in the context of lake ecology refers to having two distinct groups or peaks when measuring water clarity across many lakes. When we measure and plot lake clarity, we expect to see two peaks: one for lakes that have very clear water and another for lakes with murky water, with fewer lakes showing average clarity. This distribution suggests that lakes are often found in either of the stable states—with clear or murky waters. However, some factors could prevent this distinct bimodal pattern from being evident. These include:

• Frequent shifts due to external factors like weather changes or human activities.
• Natural variability in lake conditions that cause gradual transitions between states.
• Accuracy in measuring tools could blur the results.

Understanding bimodal distribution and potential reasons for its absence can provide insights into lake management and conservation strategies.