Stream Health: Appearances Can Be Deceiving

Streams are often associated with cleanliness and purity, but they can be polluted too. Streams are susceptible to pollution from various sources, including dirt, bacteria, nutrients, and toxic chemicals. While some signs of stream pollution may be visible, such as dirt and debris, other pollutants like hormones, antibiotics, and heavy metals can be invisible to the naked eye. Streams with healthy ecosystems will have a lot of obstacles in their channels, such as rocks and fallen logs, which create pools of water and promote a slow, meandering movement. They will also have vegetation on their banks, which helps to stabilise the stream and prevent erosion. So, while a stream may appear polluted due to debris or murky water, it could still be healthy if it possesses these characteristics.

The impact of physical habitat on stream health

The physical habitat of a stream is a key factor in determining its health. Physical habitat attributes such as channel substrate size and type, habitat complexity, and vegetation cover and structure in the riparian zone all play a crucial role in supporting the organisms within the ecosystem.

One of the most important aspects of physical habitat is the channel substrate, which includes the size and type of materials that make up the stream bed. This can vary from large boulders and cobbles to sand and silt. The substrate size and type influence the stability of the stream bed and the amount of habitable surface area available for aquatic animals. For example, larger substrates like boulders and cobbles can provide shelter and spawning sites for fish, while fine sediments can smother their gills and affect their ability to respire.

Habitat complexity is another critical factor and refers to the presence of obstacles in the stream channel, such as rocks and fallen logs. These obstacles create pools of water and increase the oxygen content, benefiting stream invertebrates and fish. They also provide diverse habitats for different species, including riffles, which are shallow, turbulent areas with coarse sediments that many species prefer.

The riparian zone, or the area of land immediately adjacent to the stream, is also essential. It includes the stream banks and the floodplain, which can provide additional habitat for wildlife and help control flooding by absorbing excess water. Vegetation in the riparian zone, such as trees and shrubs, can stabilize the stream banks, provide shade to keep water temperatures cooler, and slow down runoff to control erosion.

Human activities can have a significant impact on the physical habitat of streams. For example, uncontrolled stormwater runoff, agricultural practices, and channel alterations can lead to excess sedimentation, erosion, and a loss of vegetation. This, in turn, can affect the health of the stream and the aquatic communities that depend on it.

Stream system connectivity

Longitudinal connectivity refers to the pathways along the entire length of a stream. As the physical gradient changes from source to mouth, chemical systems and biological communities shift and change in response. The River Continuum Concept (RCC) can be applied to this linear cycling of nutrients, continuum of habitats, influx of organic materials, and dissipation of energy.

Lateral connectivity allows the stream access to its floodplain during high water events. This access is critical for the healthy ecosystem function. Nutrients and organic matter are transported to the stream from the floodplain, plant and wildlife species flourish in the diverse successional stages of inundated areas, and aquatic species gain access to seasonal habitats essential to their life cycles.

Vertical connectivity is represented by the connection between the atmosphere and groundwater. The ability of water to cycle through soil, river, and air as liquid, vapour, or ice is important in storing and replenishing water. This exchange is usually visualised as unidirectional, with precipitation falling onto land and then flowing over land or percolating through the ground to the stream. An equally important transfer of water occurs from the streambed itself to surrounding aquifers.

Temporal connectivity refers to the continuous physical, chemical, and biological interactions over time, according to a rather predictable pattern. These patterns and continuity are important to the functioning of the ecosystem. Over time, sediment shifts, meanders form, bends erode, oxbows break off from the main channel, channels shift and braid. A stream rises and falls according to seasonal patterns, depending on rain and snowmelt.

Biotic interactions

Algae are a critical biotic factor in stream ecosystems. As autotrophs, they convert sunlight into chemical energy and biomass through photosynthesis, providing a vital source of energy for the entire ecosystem. The presence of algae supports the existence of other primary producers, such as trees along the banks, water lilies, duckweed, and cattails.

Invertebrates are another essential component of stream biotic interactions. Invertebrate species commonly found in freshwater ecosystems include segmented worms, arthropods, and mollusks, such as earthworms, leeches, water beetles, mayflies, dragonflies, and mussels. These invertebrates serve as a food source for fish and other organisms in the surrounding communities, including bears and foxes.

Fish play a significant role in stream biotic interactions by feeding on both algae and invertebrates. They also serve as a food source for larger fish and other predators. Other animal species commonly found in streams, such as crayfish, spiders, frogs, water snakes, and birds, contribute to the complex web of biotic interactions.

The health of a stream ecosystem is closely tied to the balance and interactions between these biotic components. A diverse community of specialist species, each with specialized needs for feeding, spawning, and shelter, is indicative of a healthy stream. On the other hand, a stream dominated by a few generalist or non-native species is considered degraded and unhealthy.

Human activities can significantly impact the biotic interactions in stream ecosystems. For example, urban development often involves covering surfaces with concrete, creating what ecologists call "impervious surfaces." This maximizes runoff, erodes stream channels, and can lead to streams becoming "prisoners" of their channels, unable to overflow and spread over their floodplains. This loss of connection to the floodplain can have cascading effects on the entire stream ecosystem, including the loss of sediment deposition and the disruption of denitrifying bacteria that play a crucial role in cleaning streamwaters.

The role of vegetation in stream health

The presence and health of vegetation in and around a stream is a key indicator of the overall health of the stream. Vegetation plays a critical role in maintaining the stability of a stream by reducing erosion. The more vegetation there is, the healthier and more stable the stream is likely to be. Plants help to stabilise the soil on the stream banks, preventing them from collapsing or eroding into the stream. This is important for the protection of the property and infrastructure surrounding the stream, as well as for the organisms that live in and around it.

Stream bank vegetation also provides shade and cover for fish, perches for birds, and habitats for insects and other wildlife. Insects are an essential food source for many of the organisms in the stream ecosystem, including fish and birds. Therefore, the health of the vegetation directly impacts the health of the wider ecosystem.

Additionally, vegetation in the floodplain of a stream is important. When a stream overflows its banks and spreads over the floodplain, the vegetation benefits from the sediment deposited during the flood. The floodplain also acts as a reservoir, holding floodwaters that could be problematic downstream.

The presence of vegetation in and around a stream is one of the five factors that affect stream health, according to the Missouri Department of Conservation's Resource Assessment and Monitoring Program (RAM). The other four factors are physical habitat (channel shape, rock/soil makeup), stream system connectivity (how the watershed interacts with surface and groundwater), and biotic interactions (how different species interact).

The effects of human activity on stream pollution

Human activities have a detrimental impact on stream health, with pollution from various sources being a primary concern. Streams are affected by physical, chemical, and biological pollutants, which have far-reaching consequences for the overall stream ecosystem.

Physical pollutants, such as fine sediments, are a major source of streambed pollution. These sediments can carry and facilitate the accumulation of other contaminants, like heavy metals and hydrophobic compounds, in the underlying streambeds. This can lead to secondary pollution, as the contaminants persist and continue to affect the stream even after they are no longer detectable in the surface water.

Chemical pollutants also play a significant role in degrading stream health. Toxic halogenated compounds, metals, and pesticides are commonly found in streambeds. Pesticides, particularly legacy chemicals like dichlorodiphenyltrichloroethane (DDT) and polychlorinated biphenyl (PCB), are among the most prevalent chemical streambed pollutants. Urban areas contribute heavy metals and oil-based contaminants, which are often transported through vehicular emissions, sewage, and industrial releases.

In recent years, a new class of chemical pollutants, known as emerging contaminants, has been detected in stream and riverbeds worldwide. These include microplastics, pharmaceutical waste, and personal care products, which can have detrimental effects on fluvial ecosystems.

Biological pollutants, such as pathogens like Escherichia coli and coliform bacteria, also impact stream health. These biological contaminants can persist near the sediment-water interface and have adverse effects on benthic organisms.

Human activities, such as farming, clearing forests, building roads, and mining, contribute to the presence of these pollutants in streams. The clearance of forests and conversion of natural landscapes can lead to increased soil erosion and particulate matter entering rivers and streams. This sediment can smother fish eggs and carry toxic chemicals, harming aquatic life.

Additionally, urban growth and development can result in streams becoming "prisoners of their channels." Concrete-lined channels in urban areas prevent streams from naturally overflowing their banks and spreading onto floodplains. This can lead to increased flooding and erosion and compromise the health of the stream ecosystem.

The effects of human activities on stream pollution are far-reaching and complex. The degradation of stream ecosystems has serious consequences for the plants, animals, and natural processes that depend on them. It is crucial to adopt a holistic ecosystem approach to stream management and work towards mitigating the impacts of human activities on stream pollution.

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