An Ever-Changing World

The environment faces various challenges like pollution, habitat destruction, and overfishing. On top of that, climate change often makes these challenges even worse [1]. In response, scientists and policymakers work together to identify problems in the environment and to find ways to keep it healthy. Importantly, this job needs long-term data, which is not always available because most serious environmental monitoring only started in the 1960s. When data are available, they are typically only for information collected after environmental problems were identified. This gap in information leads to major challenges for scientists trying to provide effective advice to help solve these issues.

Fortunately, many environmental challenges leave clues in the mud at the bottom of lakes, known as sediment. These clues are called proxies, and they can be analyzed to understand how the environment has changed [2]. In the rest of this article, we explain how a lake can track environmental changes, how some of the proxies can be studied, and how these clues are used to help fix environmental problems.

A Lake’s Time Machine

One of the most effective tools scientists use to understand past environmental changes is paleolimnology, a scientific field that studies lake sediment to understand changes in the environment. Lakes form through many processes, like glacier movement digging out holes in the Earth. Once water fills the hole, sediment begins to collect at the bottom. The sediment can come from the atmosphere, from the land, and from within the lake (Figure 1). Over time, sediment layers build up, tracking the physical, chemical, and biological history of the area where the atmospheric (air-based), terrestrial (land-based), and aquatic (water-based) inputs came from. Sediments can accumulate for thousands of years.

The process of studying the environment using paleolimnology follows a few common steps (Figure 2). Imagine, for example, that you were a paleolimnologist (a scientist who studies lake sediments) interested in understanding why fish populations are changing in certain lakes. You would first select a lake where the fish species are affected (Figure 2A). Then, you would use a tool that looks like a big plastic straw, called a corer, to collect the sediment sample. The corer is inserted into the sediment to collect the core.

Once a sediment profile is collected, you would take it to a lab and cut it into sections, like slicing a loaf of bread. Next, the sediment is dated so that you know how far back in time your “time machine” goes (Figure 2C). Various methods can be used for dating, like counting annual layers, similar to counting the rings of a tree to learn how old it is.

The most common way to age sediment is by using radioisotopes, which are elements that decay (or disappear) over time because they are unstable. You may have heard of one of the radioisotopes that paleolimnologists use: carbon-14. Carbon-14 or radiocarbon is often used by archaeologists who age bones that can be thousands of years old. For sediment that is not quite so old, a different radioisotope called lead-210 is most often used. Different radioisotopes decay at different speeds. If the element decays slowly, like carbon-14, it can be used to age things up to 50,000 years old—after that, there is too little carbon-14 left to measure. In comparison, lead-210 disappears after just about 150 years. Once a sediment core is dated, you will next choose which clues, called proxies, you will examine to address the environmental problem at hand (Figure 2D).

Putting Together the Evidence

A sediment core holds clues about the environment in the form of physical, chemical, and biological information called proxies. Paleolimnologists will select proxies that change because of the environmental problem that is being studied. Ultimately, the goal of the paleolimnologist is to unlock all the clues in the “time machine”, to understand what the environment used to look like. Below, we talk about a few essential proxies and how they can be used (Figure 3).

Environmental DNA

All organisms, from microbes to plants to animals, shed cells, leaves, and hair into the surrounding environment. These materials contain DNA, a molecule that holds the genetic information unique to each organism. When it rains, DNA is washed into nearby lakes and is preserved in the sediment, allowing scientists to measure changes in biological communities through time (Figure 3A). For example, scientists have used the presence and disappearance of mammoth DNA in sediment cores to help learn when these animals went extinct [3]!

Metals

The concentrations of metals in sediment change due to natural processes, like erosion, or from human activities, like mining. If, for example, a factory is built that releases toxic metals like cadmium or lead as a by-product, these metals can pollute the nearby environment as they leave the factory from smokestacks or through wastewater from factory processes. When these pollutants reach a lake, they can collect in sediment and the metal concentrations can be measured.

Sub-Fossils From Aquatic Organisms

There are lots of algae and tiny animals that get preserved in sediment profiles when they die. These tiny, preserved organisms are called sub-fossils. Given their small size, scientists use microscopes to identify the sub-fossils (Figure 3C). One example is the group of algae called diatoms, which have cell walls made of glass! Because these cell walls are so strong, they do not decompose, so they are preserved in sediment. Different diatom species can only survive in certain water conditions, so changes in the diatom communities found in sediment cores can be used to understand changes in lake conditions. For example, some diatom species need lots of light to survive, and others do not. If a lake’s water gets darker, the diatom community will shift from types that need lots of light to survive to those that need less light.

The larval stages of certain fly species, such as chironomids, are also common in lakes. Their mouth parts are preserved in sediment (Figure 3D), and different species can be used to understand changes in the lake that diatoms cannot tell us about, like changes in temperature or oxygen concentrations. Paleolimnologists look at how the diatom and chironomid communities have shifted throughout a sediment core to understand how the lake has changed over many years.

Pollen

Although paleolimnologists are usually most concerned about what is happening inside a lake, changes in the surrounding terrestrial environment (such as changes in vegetation) are also important to understand. For example, they might ask “When was this area deforested?” or “How did farming affect the environment?”. Pollen grains come in lots of different shapes depending on the plant that produced them, all of which can blow into lakes where they sink to the bottom and preserve in the sediment. Paleolimnologists study changes in vegetation by using a microscope to see when shifts in pollen percentages occurred (Figure 3E).

Pigments

All plants and algae produce a pigment called chlorophyll to convert sunlight into energy. Scientists can use specialized tools to measure how light bounces off sediments, which can tell them about the amount of chlorophyll that was in the environment (Figure 3F). If chlorophyll increases over time, that means the lake had more plants and algae growing in it, which might happen, for example, when fertilizers from farming are washed into the lake.

Building The Case

Once the sediment core is dated and the proxies are analyzed, paleolimnologists can weave the clues together to tell the story of environmental change through time. These results are then shared with policymakers and the public, so informed decisions can be made to fix environmental challenges. The long-term data from sediments are one of the most important things that policymakers use to make changes to protect the environment [4].

Paleolimnology in Action

One of the most famous early uses of paleolimnology was to understand the effects of acid rain on the environment. Over 40 years ago, in the 1970s and 1980s, people in certain parts of the world noticed that rain had become very acidic. Imagine rain with a pH almost like lemon juice! This unhealthy rain caused lakes to acidify and harmed animals and plants in those lakes. Scientists thought that the acidification was coming from air pollution from factories and mines. But, since no data had been collected before those industries were built, some people argued that the lakes were naturally acidic.

To pinpoint what caused the changes in lakes, paleolimnologists looked at changes in diatoms, invertebrate sub-fossils, and pigments in sediments to determine what the environment was like before industries producing acidic emissions began polluting the atmosphere. The results were conclusive—many lakes were far more acidic than they were before emissions began, indicating that pollution from industries were in fact affecting the acidity of rain [5].

After the culprits of environmental harm were identified, many governments established anti-pollution policies to minimize future acidification impacts and to start the recovery process. Many lakes have improved, but some are still on the road to recovery, showing how environmental impacts can have very long-term effects. Without the data from paleolimnology, many of the improvements would not have happened.

Putting it All Together

Lake sediment can be important for protecting the environment because sediment cores can serve as “time machines” that help paleolimnologists understand what the environment was like in the past. Learning how to interpret the clues found in sediment helped scientists understand and communicate about how the world has changed, due to both human and natural factors. This knowledge can help us to protect the environment, keeping it as healthy as possible for many years to come.

Glossary

Sediment: ↑ The material that settles at the bottom of lakes over many years. It is made up of material that comes from within the lake and the surrounding environment.

Proxies: ↑ The measurable clues, such as sub-fossils, studied in sediment cores to reconstruct past environmental conditions in lakes and the surrounding environment.

Paleolimnology: ↑ The study of the physical, chemical, and biological information stored in lake and river sediments. Someone who studies paleolimnology is called a paleolimnologist.

Radioisotopes: ↑ Unstable forms of elements present in the environment, typically used to determine the age of a sample. Typical radioisotopes used in paleolimnology are carbon-14 and lead-210.

Sub-fossils: ↑ The remains of organisms, like diatoms and chironomids, that are preserved in sediments. They are different from true fossils because sub-fossils do not mineralize into rocks.

Diatoms: ↑ A group of algae with glass (siliceous) cell walls that are well preserved in sediments. Different species thrive in different environments, so their species distributions can be used to reconstruct past conditions.

Chironomids: ↑ A group of insects whose larval stages typically live in lakes. Chironomid head capsules are specific to their species and so can be used to reconstruct past underwater oxygen levels and temperature.

Pigments: ↑ Natural colored compounds from various plants, algae, and bacteria. Changes in the amount and types of pigments can be used to reconstruct past microbial populations in and around the lake.

Conflict of Interest

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

Acknowledgments

The authors are grateful to Poorna Patange and Zoe Kane for illustrating the figures. Our work on paleolimnology is mostly funded by the Natural Sciences and Engineering Research Council of Canada.