Lake Tipping Points Explained

posted Aug 22, 2017, 4:53 PM by Joseph Bickard
August 21, 2017 - LEA Lake News – by Staff Writer, Summer 2017: BRIDGTON: Do you remember ever playing on a see-saw that was balanced, but just ready to drop, given a little push? You were on a tipping point – the point where you shifted from being high in the air, to finding you derriere on the ground. Like see-saw, lakes have tipping points, only instead of high or low, they can alternate between clear versus turbid, pure versus algae-filled. And the push comes not from a shift in weight, but from shifting human and climate pressures from the surrounding landscape. Lakes Region Environmental Association (LEA) is working with dozens of partners statewide to uncover what governs how lakes change, and how close each lake is to that threshold. This information will be used to calibrate a protection strategy to insure that water quality is stabilized to prevent a lake from “tipping.”

There are three major factors that contribute to the deterioration of lakes: landscape features, lake basin shape, and water cycle dynamics. The building blocks of understanding and analysis begin with the physical characteristics of a lake’s watershed: the soils, slopes, drainage patterns, vegetation, and land uses. These factors dictate how much sediment and phosphorus are delivered to the lake by stormwater. Phosphorus is the trigger for algal growth, usually entering the lake attached to soil particles.

Next, comes bathymetry, or the shape of the basin. Water in deep lakes stratifies during summer in upper warm waters and colder bottom waters. Characteristics such as depth patterns, perimeter shape, coves and distribution of deep “holes” are unique for each lake and influence how the system reacts to sediment inputs and weather events.

Hydrology refers to the way water flows through the system: the tributaries, overland flows, how inputs mix with lake waters and the flushing rate (how long it takes a lake to fully exchange its water.” The rate at which water passes through the system has a big impact on chemical concentrations, stratification, and the nature of plants (algae) and animals (zooplankton) living in the lake.

The better we document watershed conditions, bathymetry, and hydrology, the better we can evaluate what we learn from water quality monitoring. LEA conducts a broad array of in-lake tests: clarity, temperature profiles (top to bottom), oxygen profiles, phosphorus profiles, pH, alkalinity, color, sediment chemistry, chlorophyll (to measure algal biomass), and algal diversity. To do this, we use a variety of equipment: electronic meters, water samplers, temperature sensors, oxygen and temperature meters, sediment core samplers, drag nets (to collect algae), and exotic equipment such as autonomous, instrumented (GLEON) buoys. Fluorometers can now measure chlorophyll, saving lab expenses and even differentiating between populations of different types of algae. Bottom-located sensors can measure stream depth, and flow meters compute volume passing through a stream channel. These are important tools for understanding hydrology. GPS-equipped depth meters allow us to collect bathymetric readings quickly to build depth maps.

The accumulated information about watersheds, basins, hydrology and in-lake conditions is allowing us to gauge the sensitivity of each lake. Each water body is a unique mix of conditions, but there are several universal factors.

-All Maine lakes are “phosphorus limited.” Algal growth is determined and limited by the amount of this nutrient in the water. If you increase phosphorus inputs, you increase algal growth. It is very important to control erosion, because soil particles are the source of most lake phosphorus. They are carried to the lake in stormwater.

-Except for very shallow lakes, most stratify during summer. Colder bottom waters can become devoid of oxygen when bacteria decompose dead algae that settle from the upper warm and sunny layer where they grow. This is why limiting phosphorus (and thereby algal growth) can help a lake maintain healthy oxygen levels. Oxygen is not an issue in the lake’s upper layer because winds continually mix and replenish it. Deep waters are thermally stagnant with a finite amount of oxygen accumulated while the lake is fully mixed and before stratification sets up in the spring.

-If the cold layer loses its oxygen (a condition called anoxia), phosphorus can be released from bottom sediments, potentially increasing algae growth. If sediments are rich in aluminum, this “recycling” of bottom sediment phosphorus does not occur because aluminum chemically binds phosphorus. This is why it is important to test sediment chemistry. Fortunately, most lakes we have tested in LEA’s service area have aluminum-rich sediments.

-Healthy lakes have diversity of microscopic algae and zooplankton. This is why it is important to study their populations. Some species of algae can even become toxic to humans if their populations flourish.

Weather can be the wild card for water quality. Heavy rains scour soils, bringing nutrients to the lake. High winds can break down stratification to mix phosphorus-rich bottom waters into the growing zone to trigger algal blooms. Early ice-out and late ice-in expand the growing season for algae and warm lake waters beyond that which is healthy. Weather stations located onboard LEA’s instrumented buoys are adding crucial information to the mix of knowledge. With buoys providing oxygen and temperature profiles every 15 minutes, we can track the impact wind is having on stratification and oxygenation.

As you can see, water quality and tipping points are complex topics. There are many factors interacting with each other. As data is gathered, a clearer and clearer picture merges of how fragile each unique lake is. Though the volume of data may sometimes seem overwhelming and confusing, a sort of “melody” emerges if you “listen” to each of the “rhythms and sounds” generated by the data. Drawing conclusions is an art and a science because the relationships and interactions of factors sometimes defy rigid scientific rules, requiring intuition to draw conclusions.