Pollution-Tolerant Macroinvertebrates: Clean Water Survivors?

Benthic macroinvertebrates are small aquatic animals and the aquatic larval stages of insects. They include dragonflies, stoneflies, snails, worms, and beetles. They are commonly used as indicators of the biological condition of waterbodies because they are relatively easy to collect and differ in their tolerance to pollution. Some macroinvertebrates are highly intolerant of pollution and require clean, well-oxygenated waters to survive. Other macroinvertebrates exhibit high tolerance to pollution and can survive in degraded or polluted waters with low oxygen levels. This raises the question: can pollution-tolerant macroinvertebrates live in clean water?

What are macroinvertebrates?

Macroinvertebrates are organisms that are large enough to be seen without a microscope and lack a backbone. They are also known as 'macros'. They inhabit all types of running waters, from fast-flowing mountain streams to slow-moving muddy rivers. They can also be found in still waters like lakes, ponds and tinajas.

They include many different types of insects in their larval or nymph form, as well as crayfish, clams, snails, and worms. Most macroinvertebrates live part or most of their life cycle attached to submerged rocks, logs, and vegetation. Some live within the soft sediments at the bottom of lakes and ponds, while others capture food that is drifting along in the current.

They are ecologically significant because they have the ability to integrate changes in both aquatic and associated terrestrial environments. They are sensitive to environmental change, which helps scientists detect physical, chemical, and biological changes in the ecosystems where they live. They are also a critical part of the food web in these environments.

Macroinvertebrates are also used to indicate water quality. They are affected by the physical, chemical, and biological conditions of the water and are unable to escape pollution, so they are good indicators of stream health. They are relatively easy to sample and identify, and their presence or absence is used to indicate clean or polluted water. Some are very intolerant of pollution, while others can tolerate it.

Why are they used as water quality indicators?

Macroinvertebrates are used as water quality indicators because they are good indicators of stream quality. They are affected by the physical, chemical, and biological conditions of the stream and can be classified into three groups based on their tolerance to pollution. Some macroinvertebrates are completely intolerant of pollution and require the cleanest water for survival, while others are moderately tolerant or highly tolerant. By evaluating the abundance and variety of macroinvertebrates in a waterbody, we can determine the biological condition of that waterbody. Waterbodies in healthy biological condition will support a wide variety and high number of macroinvertebrate taxa, including those that are intolerant of pollution.

They are also relatively easy to collect and identify, and they respond to human disturbances in predictable ways. They are often found attached to rocks, vegetation, logs, or burrowed into the sediment, and they are a critical part of the stream's food web. Additionally, they cannot escape pollution and can show the cumulative impacts of short-term and long-term pollution events, as well as the impacts of habitat loss that may not be detected by traditional water quality assessments.

Biological monitoring, which involves studying biological organisms and their responses, is used to determine environmental conditions and assess the health of a stream. Macroinvertebrates are commonly studied in wadable streams and are good indicators of stream quality because they are affected by various factors and cannot escape the effects of pollution.

In summary, macroinvertebrates are used as water quality indicators because they are sensitive to changes in their environment, easy to collect and identify, and provide valuable information about the health of a stream and the presence of pollutants.

How do they indicate water quality?

Macroinvertebrates are good indicators of water quality because they are affected by the physical, chemical, and biological conditions of the water body. They are relatively easy to sample and identify, and they respond to human disturbance in fairly predictable ways.

The presence or absence of certain types of macroinvertebrates can indicate the level of human disturbance and pollution in a water body. This is because each species has unique characteristics regarding habitat requirements, life history, behaviour, and pollution tolerance. For example, stonefly nymphs are very sensitive to most pollutants and cannot survive if a stream's dissolved oxygen falls below a certain level. If a biosurvey shows that no stoneflies are present in a stream that used to support them, it may indicate that pollution levels have increased or that there has been habitat loss.

The abundance and variety of macroinvertebrates in a water body can also indicate its biological condition. Generally, water bodies in healthy biological condition support a wide variety and high number of macroinvertebrate taxa, including many that are intolerant of pollution. Samples yielding only pollution-tolerant species or very little diversity or abundance may indicate a less healthy water body.

The basic principle behind the study of macroinvertebrates is that some are more sensitive to pollution than others. Therefore, if a stream site is inhabited by pollution-tolerant organisms and the pollution-sensitive organisms are missing, a pollution problem is likely.

How can we categorise them based on pollution tolerance?

Macroinvertebrates can be categorised into three groups based on their pollution tolerance:

Group 1: Pollution-Intolerant Organisms

These macroinvertebrates are highly intolerant of pollution and require clean, well-oxygenated water to survive. They include mayfly nymphs, hellgrammites (dobsonfly larvae), and adult riffle beetles. Stoneflies, caddisflies, and mayflies are also sensitive to changes in water quality. The presence of these species indicates good water quality, and their absence may suggest a problem.

Group 2: Moderately Pollution-Tolerant Organisms

This group of macroinvertebrates can tolerate a moderate level of pollution and are often found in slightly degraded environments. Examples include crustaceans such as crayfish and scuds, and some caddisflies, like the Hydrophsyche. While their presence suggests some disturbance, a large number of these organisms may indicate a decline in water quality.

Group 3: Pollution-Tolerant Organisms

These macroinvertebrates have a high tolerance for pollution and can thrive in degraded or polluted waters with low oxygen levels. Examples include leeches and worms. If Group 3 species dominate a habitat, it typically indicates poor water quality with elevated pollution levels. Their prevalence may signify ongoing pollution sources or environmental stress, requiring immediate remediation.

The categorisation of macroinvertebrates into these three groups is essential for assessing water quality and stream health. The Biotic Index (BI) of water quality is calculated based on the pollution tolerance of each type of macroinvertebrate found in a sample, with diversity being more important than abundance. This index helps determine if water quality is poor, fair, good, or excellent.

How can we monitor their populations?

Monitoring the populations of pollution-tolerant macroinvertebrates is an important task, as these organisms are key indicators of water quality and aquatic ecosystem health. Here are some methods and approaches to monitor their populations:

• Biological Surveys (Biosurveys): Biosurveys are a type of biological monitoring that involves collecting, processing, and analyzing aquatic macroinvertebrates to assess the health of a stream or water body. This method is useful because macroinvertebrates are affected by physical, chemical, and biological conditions and can indicate the cumulative impacts of pollution and habitat loss. They are relatively easy to sample and identify, and their presence or absence can signal the health of the ecosystem.
• Habitat Surveys: Habitat surveys are conducted alongside biosurveys to examine the quality of the habitat for macroinvertebrates. This includes assessing the rocks and sediments of the stream bottom, plants in and around the stream, leaf litter, and submerged debris. By evaluating the shelter and food sources provided by these habitats, we can identify areas that support higher populations of macroinvertebrates.
• Water Quality Monitoring: In addition to biosurveys, regular water quality monitoring is essential. This includes measuring parameters such as dissolved oxygen levels, temperature, nutrient levels, and pH. By identifying specific pollutants and their sources, we can better understand their impact on macroinvertebrate populations and implement targeted mitigation measures.
• Long-term Monitoring: Long-term monitoring of macroinvertebrate populations over several years can help identify trends and changes in their abundance and diversity. This data can reveal the cumulative effects of pollution, climate change, or habitat degradation, providing a more comprehensive understanding of their population dynamics.
• Sampling and Collection: Macroinvertebrates can be collected using various methods, such as dip nets and kick nets, especially in wadable streams. These samples are then processed and analysed in laboratories to determine their species, abundance, and diversity.
• Macroinvertebrate Indices: Indices are used to analyse the health of freshwater streams by scoring the presence and diversity of macroinvertebrate taxa. These indices provide a criterion, such as Excellent, Good, Fair, or Poor, to describe the biological integrity of the stream.
• Tracking Changes in Composition and Abundance: Monitoring programs aim to track changes in the composition and abundance of macroinvertebrate assemblages. This information helps identify any shifts in population dynamics that may be caused by environmental factors or human influences.
• Investigate Relationships with Other Factors: By studying the relationships between macroinvertebrate indices and other factors, such as fish communities, water quality and quantity, and stream channel morphology, we can gain a more comprehensive understanding of the ecosystem and potential impacts on macroinvertebrate populations.

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