Sediment Interactions with Aquatic Life
Sediment may have far reaching effects on the nature and biota of the lake or stream.
“Suspended solids” that stay in suspension can reduce light penetration and/or alters the rate of temperature change in different water levels.
“Settleable solids” and “silts” tend to settle out of suspension slowly on the lake or stream bottom.
Both forms of sediment can affect the photosynthetic efficiency in terms of productivity and biomass.
These solids coat and smother some bottoms: organisms disrupting the food chain. The stream bottom should not be blanketed to a depth of more than 1/4 inch by sedimentary deposits in order to prevent destruction of bottom fauna.
In streams, sediment has a tendency to “scour” algae populations from the scream bottom. Spring floods in areas where there are great amounts of meltwater with high sediment loads may reduce algal populations to almost nothing.
Higher aquatic plants also can be affected by sediment changing the stream bottom conditions by affecting the amount of sediment in the stratum available. Small, light and shifting sediments are particularly difficult substrata for benthic animals.
It is not possible for the animals to make tunnels in this type of substrate because it tends to envelope and overwhelm them. This may influence the number of organisms found in silted and unsilted stream bottoms.
In general, it is possible to conclude that sand is the poorest habitat, that bedrock, gravel and rubble on the one hand and clay and mud mixed with sand on the other support increasing biomasses.
Sediment Interactions with FishAdult fish usually can withstand a heavy suspended sediment load for several weeks, but by limiting the food supply sediment can have an indirect harmful effect on fish. In general, and disregarding any possible toxic effects attributable to substances leached out by water, suspended sediment may kill fish by causing abrasive injuries to the delicate external organs, such as gills, spiracles, and fins. The fine particles that settle out tend to destroy spawning beds and can kill eggs through suffocation.
The impact suspended sediment can have on fish reproduction is far reaching because of the influence on breeding habits. This happens because nearly all species of fish have fairly well defined breeding habits and requirements. These are usually more restrictive than those needed for ordinary life. Hence, they determine to a large extent the reproductive suitability of particular lakes, rivers and streams.
The great majority of freshwater fish spawn on a solid surface that they select fairly carefully. In still waters, rooted vegetation is the site favored by most species, but this is of much less importance than the bare substratum requirements among running water species. Many species select or prepare definite nest sites on or in stones, gravely substrates.
A common nest site among stream fish is a flat area under a large stone, to which the eggs are stuck in a cluster. This habitat is common to the sculpins, spotted and fantail darters, and several other small fish. These fish require the presence of suitably elevated stones. This restricts these fish to habitats where such conditions are available. Any change in sediment deposition can reduce the number of suitable spawning sites.
Other species of fish ‘Fan’ their tails to dig pits in stream gravel in which eggs are laid. For these fish to move the gravel, it must be of suitable size and reasonably free from sand and silt.
Similar to ‘Fanning’ is the vigorous cutting of pits called ‘Redds’ performed by salmonids. In this case, the female turns on her side and by vigorous flapping of the tail she lifts stones by negative pressure, and they are then carried doowstream by the currents. After the spawning act the female moves forward and digs again, thus burying the newly laid eggs. The quantities of gravel disturbed by these fish are large enough that the spawning site is clearly marked. An important point about these spawning constructions is that loose gravel allows the oxygenated water to pass through and hence to bring oxygen to the buried eggs.
The oxygen content of the interstitial water flowing through the gravel has been shown to relate to the percentage of successful hatch of pink and chum salmon eggs. In years with low stream discharge, the egg survival may be quite low because of death caused by oxygen shortage. These fish must find places in the gravel where movement of water through the gravel occurs before they begin to dig.
Brown trout, steelhead trout and Atlantic Salmon select places for spawning where there is a down-flow of water into the gravel. These places occur at the downstream ends of pools where the water flows into riffles. The brook trout selects places where there is an upward flow out of the gravel such as at the tail end of riffles. In either case, the buried eggs are constantly bathed in new water and oxygen. Any considerable introduction of silt that fills the intergravel spaces will block the passage of water and oxygen usually will prove fatal to the incubating eggs.
Mud and silts appear to be unpopular spawning sites used only by a few species that make shallow nests on muddy banks. Most fish which live in muddy reaches either move upstream to gravel, or like still water species, spawn on plants which are either aquatic or flood covered terrestrial plants.
In summary, species that construct nests or Redds are restricted not only in respect to size of material of the stream bottom (which they must be able to move), but by the need for the spawning site to be free of silt. Salmonids, and probably some other fish, are also restricted to places where there is an intergravel flow of water.