Interacting Organisms Filter Water

Biological Strategy

Wetland ecosystems

Mary Hoff

Image: Herbert Aust / Pixabay / Free non-commercial use

Control Erosion and Sediment

Ecosystems must maintain their soils on site and prevent it from being washed away. Organic matter, minerals, and other nutrients in soil support organisms from microorganisms to the largest mammals. But because of the pull of gravity, water flows unless impeded. Therefore, the presence and actions of many, diverse organisms hold soils in place and slow water to allow sediments to settle out rather than be washed downhill. For example, in forests, downed logs that fall crossways to the slope efficiently slow water flow, trapping soils carried by the water and causing water to soak into the soil. In areas lacking logs and other structures, water can flow so fast that it cuts into the soil, causing erosion that washes the soil away.

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Cycle Nutrients

In ecosystems, there is no such thing as waste. Instead, one organism’s waste can be considered as another organism’s resource, and the cycling of nutrients may be the most important form of recycling in living systems. But sometimes those nutrients are transformed by one organism into a form that isn’t readily used by other organisms. For example, lignin is a complex organic molecule found in the cell walls of plants. In a forest, it’s one of the hardest molecules to break down in woody vegetation. Therefore, ecosystems include organisms that are particularly well-adapted to break down different types of nutrients. In the case of lignin, some fungi and bacteria release lignin-modifying enzymes that can break down the lignin to form carbohydrates and other life-supporting chemicals.

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Detoxification/Purification of Air/Water/Waste

Ecosystems must be effective at cleaning air, water, and waste because so much of what is in the air, water, and waste is valuable to organisms as resources. In contrast to functions that detoxify or purify at the organism level, this is at a larger scale. To be effective at the ecosystem level, it isn’t the individual efforts of a few organisms that detoxifies and purifies, but the actions of a wide diversity of organisms each contributing to the whole. For example, as water flows through an ecosystem, either aboveground or in the soil, a diversity of organisms take different actions to slow it down, capture excess nutrients from it, and break down harmful chemicals into more benign ones.

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a mountain with snow is in the background of a landscape with trees and shrubbery

Ecosystems

Eco- ("household"), system ("organized whole")

An ecosystem consists of all the biological and nonbiological elements that make up a defined area. That definition might seem vague, but that means it’s flexible: We can apply the word across different scales and times. Ecosystems can be small, like a single tide pool, or large, like coastal wetlands. Ecosystems can include plants, animals, bacteria, fungi, rocks, temperature, light, soil, and climate—all linked through the flow of energy and cycle of nutrients. Each part of an ecosystem interacts with, and is influenced by, other members of the system. These interactions are dynamic, leading to changes to any ecosystem over time.

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Wetland plants, bacteria, and the physical environment work together to remove particles and pollutants

Introduction

Roiling and churning as it rushes downhill, water from a sudden cloudburst stirs up muck from the bottom of a river, carrying fine bits of soil, organic materials from decomposing organisms, and more toward a lake or ocean. But on the way, something amazing happens. As the slope lessens, the water slows and spreads out, creating a water-laden habitat rich in plants and microorganisms. No longer rushed along by fast flow, the tiny particles are beckoned by gravity to settle to the bottom or are trapped by webs of leaves, stems, and roots. The microorganisms and plants soak up nitrogen, phosphorus, and other chemicals. As the water meanders slowly through the wetland, it becomes cleaner and clearer along the way.

A schematic illustration showing water in a wetland and the processes that help clean water.

Multiple organisms and processes interact in a wetland to remove sediment, nutrients, and nitrates from the water flowing through them.

The Strategy

For instance, dense communities of wetland plants slow down water flow, which gives more time for solid particles to settle out and nutrients to be consumed by plants and bacteria. In addition, the leaves, stems, and roots of wetland plants provide a large surface area on which bacteria and other microbes can attach. Certain wetland bacteria consume nitrate (an ion containing nitrogen) in the water and convert it into nitrogen gas, which is released into the atmosphere. This process of denitrification tends to be the way that most nitrogen is removed from the water in wetlands. The plants take up some nutrients, but this is temporary storage as the nutrients are released again when the plants die and decompose. Nonetheless, the presence of plants and their interaction with other organisms in the ecosystem facilitate the wetland’s ability to clean water flowing through.

Image: Chesapeake Bay Program / CC BY NC - Creative Commons Attribution + Noncommercial

The Potential

Some communities and businesses already use natural and constructed wetlands to filter polluting chemicals and particles out of wastewater. A better understanding of how different organisms complement and synergize in this effort—for instance, when the output of one organism is the input for another—can lead to more efficient, effective, and economical removal of undesirable components, reducing stress on ecosystems downstream and improving water quality for human use and consumption.

Understanding the role and interactions of different participants in the system also opens the door to designing water-cleansing systems able to handle specific contaminants, such as microplastics, pharmaceuticals, and precious metals, as they are introduced into wastewater.

John Todd Ecological Design

The Eco-Machine from John Todd Ecological Design is a custom-built wastewater treatment system that purifies water without chemicals.

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Biological Strategy

Beaver Dams Cleanse Streams by Slowing Water

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Last Updated December 4, 2017

References

“As human activities continue to alter the global nitrogen cycle, the ability to predict the impact of increased nitrogen loading to freshwater systems is becoming more and more important. Nitrogen retention is of particular interest because it is through its combined processes (denitrification, nitrogen sedimentation and uptake by aquatic plants) that local and downstream nitrogen concentrations are reduced…We show that wetlands retain the highest proportion of total nitrogen loading, followed by lakes and then rivers. The differences in the proportion of N retained among systems is explained almost entirely by differences in water discharge. Denitrification is the primary mechanism of nitrogen retention, followed by nitrogen sedimentation and uptake by aquatic plants.” (Saunders and Kalff 2001:205)

Journal article

Nitrogen retention in wetlands, lakes and rivers

Hydrobiologia, 443: 205-212 | 01/01/2001 | Saunders DL; Kalff J

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Reference

“Through-flow wetlands, defined here as wetlands through which water flows on its way to streams or other water bodies, are often situated at points of ground water discharge, serially along a stream’s flow path, or parallel alongside streams and rivers. With their characteristically shallow depths and high rates of biological productivity, such wetlands typically function as sinks for excess nutrients in through-flowing water (Zedler 2003), which can be significant at the landscape level in watersheds where large quantities of water move through wetlands.” (O’Brien et al. 2012:221)

The wetland vegetation is always present in constructed wetlands. It is generally accepted that macrophytes play important role in removal of pollutants but the roles of macrophytes are mostly indirect, such as provision of oxygen to rhizosphere, provision of substrate (roots and rhizomes) for the growth of attached bacteria or insulation during cold periods (Brix, 1997; Brisson and Chazarenc, 2009; Vymazal, 2011b, 2013). However, there is growing evidence that selection of macrophytes may be important for the treatment efficiency of a constructed wetland and that the direct role can be underestimated. (Vymazal et al. 2021:4)

Journal article

Recent research challenges in constructed wetlands for wastewater treatment: A review

Ecological Engineering | 2021 | Jan Vymazal, Yaqian Zhao, Ülo Mander

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