Substratum / Flow and hydraulics / Seasonal variations

Light and Shadell
Light is necessary for photosynthesis, and is often a defining feature when describing habitat. For example, fish or invertebrates may avoid sunny spots within a stream because they are more visible to predators. The largest influence that light has on aquatic ecosystems is its influence on plant growth.

The availability of light for photosynthesis varies seasonally in temperate regions, and is affected by factors that prevent it from reaching the stream surface and stream bed. Small streams in forested landscapes are shaded by forest vegetation or sometimes by valley walls. Larger streams tend to be less shaded by forest and valley walls and more so by increased particulate matter and water depth. As rivers increase in size, it is easier for unobstructed light to reach their surface, but the water itself shades its depths and so light reaching the streambed may be as limited as in a densely forested headwater stream.

Water temperatures and dissolved oxygen levels of streams are also influenced by the relationship between light and shade and in particular the density and height of streamside vegetation. A developed canopy provides shading which helps reduce water temperature ranges and rates of fluctuation. This reduces stress on biological populations and improves overall habitat quality.

Substratum--
The substratum or substrate is defined as the organic and inorganic material that makes up the bed of a river or stream. The inorganic material (ranging in size from silt, sand and gravel to pebbles, cobbles, boulders and bedrock) is usually eroded from the river basin slopes, river channel and banks and modified by the current. Sediment usually refers to soil particles, but may include organic materials that enter the water column from eroding land. Sediment consists of particles of all sizes, including fine clay, silt and gravel. The term sedimentation is used to describe the deposition of sediment particles in waterbodies.

The impact of substrate on aquatic biota depends on substrate size, organic content and interaction with other environmental factors. Not only do different substrates harbour different assemblages of animals, but the density, diversity and biomass of aquatic biota are also impacted. As a general rule, diversity and abundance tend to increase with substrate stability (which itself increases with mean particle size) and with the presence of organic matter. Sandy substrates are thus thought to be poorest, due to instability. Sediment may clog and abrade fish gills, suffocate eggs and aquatic insect larvae on the bottom. Sediment may also carry other pollutants into waterbodies. Nutrients and toxic chemicals may attach to sediment particles on land and ride the particles into surface waters where pollutants can settle with the sediment or become soluble in the water column.

Substrate heterogeneity is also important in controlling biota abundance and diversity. As mean particle size increases, physical complexity increases up to substrates dominated by large pebbles and cobbles. Mixed substrates provide a greater range of surfaces to colonize and microflow patterns.

Because of these relationships between particle size and heterogeneity, the relationships among particle size and faunal diversity, biomass and abundance are not linear. All three parameters tend to increase from sand to cobble but decrease for large boulders and bedrock.

Flow and Hydraulics--
Water velocity and the associated physical forces collectively represent perhaps the most primary environmental factor affecting the biota of running waters. The speed of current influences the size of particles of the substrate. Current affects food resources via the delivery and removal of nutrients and food items. Current velocity also presents a direct physical force that organisms experience within the water column as well as at the substrate surface.

Organisms must shelter from the current, develop attachment devices or perform energy costly work to hold their positions. Alternatively, they may prefer to live in zones of the stream where there is less current. These areas are typically located in back eddies behind obstructions, near shore or near bottom, depending on the physical characteristics of the stream.

Stream dwelling biota exhibit a wide variety of adaptations to use various current situations to their benefit. Diverse attachment and food gathering devices are found among aquatic biota including silk and other sticky secretions, hooks, claws, suckers, sweepers and filters. Some fish exhibit a variety of adaptations to life in fast or slow moving waters. Body shape, swim bladder modifications and fin configuration can aid fish to hold themselves or to feed in stronger currents.

Seasonal Variations--
The physical factors discussed above can vary greatly according to the seasons. Temperature may limit activity during winter, light may be abundant in early spring but limiting after the canopy leafs out. In temperate or high altitude regions such as the headwaters of the Mekong, ice and snow can limit incoming light.Similarly, seasonal flow changes are able either to increase or reduce the input of inorganic or organic matter, nutrients and when present, man-made sources of pollution.

Streams in regions with low seasonal precipitation may dry up during the dry season. This flow intermittency is a characteristic feature strongly influencing the aquatic life and habitats of many streams in tropical regions. Flow levels and aquatic habitats in streams at higher latitudes and alpine areas in Tibet are more dependant on runoff from melting snows. The flow regime and associated aquatic habitats in these streams are dominated by ice, reduced winter discharge, and spring freshets such as high flows due to melting snow and ice.

A key consideration in looking at the seasonal variation of stream environments and its effects on biota is the annual hydrologic cycle of flood and drought. Many species of invertebrates and fish are adapted to changes in discharge and flooding. This includes migratory behaviours between dry- and wet- season habitats and involves longitudinal and lateral movements. Some of these have extraordinary capacities to cope with low oxygen concentrations, high temperatures and desiccation. These same types of adaptations are evident in many species of riparian vegetation which in turn can determine the abundance and diversity of aquatic animals.

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