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Chapter II: Relationships with Man, Freshwater Benthic Ecology

Soil & Water Conservation Society of Metro Halifax (SWCSMH)

Updated: October 09, 2013           Freshwater Benthic Ecology and Aquatic Entomology Homepage

Chemical vs Biological monitoring


Chapter II: Freshwater Benthic Ecology  click for info on PDF documents
"Pollution is a semi-nebulous term used to describe changes in the physical, chemical or biological characteristics of water, air or soil, that can affect the health, survival, or activities of living entities. Organisms respond to pollution usually in one of two ways, acutely or chronically. Acute effects result in serious injury to, or death of, the organism shortly after exposure to high concentrations of a pollutant. Chronic effects are realized following exposure to low concentrations of a pollutant, the results of which appear over time, often as serious diseases (e.g. cancers)" ................... Williams, D. D., and Feltmate, B.W. 1992.

Important links:

Relationships with Man

Urban and Highway Runoff

(also see Soils and Erosion, and Typical pollutants in stormwater runoff)

(Mackie, 1998)

Aquatic communities downstream of many municipalities change due to the effects of urban stormwater runoff and solid waste disposal. Stormwater runoff has similar constituents as highway runoff, with road salt, tars, oils, gasoline, metals and rubber tire derivatives entering streams as a broth of contaminants. Runoff from asphalt also has a significantly higher water temperature, often resulting in greater than a 10C increase in stream temperature immediately below the outfall. Silt loads are also high so that the runoff is hot and turbid. Much of the effluent results in increases in sediment concentrations of total hydrocarbons, aromatic hydrocarbons (both in lubricating oils and fuels), and heavy metals (e.g. lead in fuel, copper in brake linings, zinc and cadmium in tires, and chromium and copper in de-icing salts).

The toxicity of this "broth" of contaminants in the water and sediments is complex, with synergistic and antagonistic effects. Benthic and fish communities respond accordingly, but over the long term, fish may be eliminated and the benthos are dominated by pollution tolerant forms like tubificid worms and chironomid larvae.

In general, the type and size of the receiving water, the potential for dispersion, the size of the surrounding catchment area, and the biological diversity of the ecosystem are some of the factors determining the importance of runoff effects.

(Maltby et al., 1995)

Road-vehicle-related activities produce a number of potentially toxic substances including oil and tar products, dioxins, oxygenated compounds, halogenated phenols, metals, deicing salts, and asbestos. These contaminants are derived from a wide range of sources. For example, hydrocarbons are present in lubricating oils and fuels, whereas metals are present in fuel (e.g., lead), brake linings (e.g., copper),vehicle tires (e.g., zinc and cadmium), and road deicing salts (e.g., copper and chromium).

Contaminants from roads can enter river systems via runoff or atmospheric deposition, the relative importance of these two routes being dependent on the particular contaminant in question. Whereas low-molecular-weight polycyclic aromatic hydrocarbons (PAHs) are emitted mainly in the gas phase and are therefore dispersed in the atmosphere, higher-molecular-weight compounds are emitted in particulate form and are deposited on or near the road. Other contaminants may be associated with crankcase oil and leak directly onto the road surface where they become associated with particulate material.

Road runoff therefore contains a complex mixture of potential toxicants that are discharged, untreated, into receiving waters. The potential impact of road runoff on receiving water quality will be dependent on several factors, including volume of traffic, rainfall, and size of receiving water.

Many of the contaminants in road runoff are associated with particulate material and accumulate in the sediments of receiving waters where they may reach concentrations orders of magnitude greater than those present in the overlying water. The organisms most at risk, therefore, will be members of the benthic community as they are exposed to both dissolved and deposited contaminants. Although several different groups of benthic organisms may be used to assess water and sediment quality, most studies have concentrated on macroinvertebrates. These play an important role in energy flow and nutrient processing in freshwaters, as well as providing prey for vertebrates such as fish and birds.

The major energy sources in small streams are benthic algae and detritus, coarse particulate organic matter (CPOM) (e.g., leaf litter) being the dominant energy input in wooded streams. The breakdown of CPOM is brought about by a combination of microbial decomposition, macroinvertebrate feeding, chemical leaching, and physical abrasion. Studies have shown that conditioning of leaf material by fungi increases its palatability to macroinvertebrate shredders and that aquatic hyphomycetes (Fungi Imperfecti), in particular, play an important role in the microbial decomposition of leaf material. The processing of CPOM by microorganisms and shredders produces fine particulate organic matter (FPOM), which is consumed by filter feeders and collector-gatherers. The latter are in turn consumed by invertebrate and vertebrate predators. Hence, efficient decomposition is key to the energy budget (and therefore the integrity) of many stream ecosystems. A major rate-limiting step in the incorporation of CPOM into the freshwater food web is the conversion of detrital material into fungal and macroinvertebrate biomass.

1. Field Study- Abstract:The effects of motorway runoff on the water quality, sediment quality, and biota of small streams were investigated over a 12-month period. Downstream of motorway runoff discharges there was an increase in the sediment concentrations of total hydrocarbons, aromatic hydrocarbons, and heavy metals and an increase in the water concentrations of heavy metals and selected anions. Hydrocarbon contamination of sediments was positively correlated with potential contaminant loading (i.e., length of road drained/stream size). The greatest effect was observed at Pigeon Bridge Brook, a small stream receiving drainage from a 1,500-m stretch of the M1 motorway. The dominant PAHs in contaminated sediment at this site were phenanthrene, pyrene, and fluoranthene, whereas the dominant metals were zinc, cadmium, chromium, and lead. Differences between the station upstream and downstream of discharges in the diversity and composition of the macroinvertebrate assemblages were detected in four out of the seven streams surveyed. However, there was no evidence of an effect on either the diversity or abundance of epilithic algae. The diversity of the aquatic hyphomycete assemblage was only affected at the most impacted site. Reductions in macroinvertebrate diversity were associated with reductions in the processing of leaf litter and a change from an assemblage based on benthic algae and coarse particulate organic matter to one dependent upon fine particulate organic matter.

2. Identifying major toxicants- Abstract: Previous studies have provided prima facie evidence that runoff from the M1 motorway, UK, affects both the quality of the receiving water and the biota living there, in sites short distances from point sources- i.e., possible worst-case situations. Because discharges contain a wide variety of contaminants, both the identification of toxicants and the establishment of causal relationships between observed changes in water/sediment quality and biology are often difficult. In this particular case, the problem was addressed by conducting a series of toxicity tests using the benthic amphipod Gammarus pulex. The abundance of this species was greatly reduced downstream of the point where motorway runoff entered the stream. Stream water contaminated with motorway runoff was not toxic to G. pulex. However, exposure to contaminated sediments resulted in a slight reduction in survival over 14 d, and sediment manipulation experiments identified hydrocarbons, copper, and zinc as potential toxicants. Spiking experiments confirmed the importance of hydrocarbons, and fractionation studies indicated that most of the observed toxicity was due to the fraction containing polycyclic aromatic hydrocarbons. Animals exposed to contaminated sediments and water spiked with sediment extract accumulated aromatic hydrocarbons in direct proportion to exposure concentrations.

Physical disturbances- Urban land development

(Williams & Feltmate, 1992; Also see Soils and Erosion, and Typical pollutants in stormwater runoff)

Two major effects of urbanization on aquatic systems and insects are, (a) increases in sediment loads during construction phases, and (b) post-storm increases in the discharge of streams and rivers downstream from developments.

Motor vehicles- Urban areas

(Williams & Feltmate, 1992)

Motor vehicles impose an additional urban-related stress on aquatic insects. Higher values of COD in urban surface waters result due to hydrocarbons leaked from automobiles (i.e. oil and gas). Urban runoff also contains significant quantities of lead related to the proportion of catchment area allotted to motor vehicles and to the density of traffic.

Chemical disturbances- Road salt

(Williams & Feltmate, 1992; Mackie, 1998)

In many cold weather countries, road salts (NaCl, CaCl2, and KCl) are regularly applied in an attempt to de-ice motorways. The fate of these salts is to enter surface water and groundwater supplies, where they have both a proximal impact on stream- and lake-dwelling insects and a distal impact on those species found in groundwater outflows.


(Mackie, 1998)

In some watersheds, runoff from agricultural lands accounts for almost all the discharge into major tributaries.

Chemical disturbances- Pesticides

(Williams & Feltmate, 1992)

The ideal pest control agent would:
(1) kill only target species
(2) have no long- or short-term effects on non-target species
(3) break down into harmless chemicals in a short period of time
(4) not select for genetic resistance in the target organisms
(5) not affect predator/prey relationships or competitive interactions
(6) be more economical than not using pest control

Unfortunately, no such pesticide exists. A well known example of a control agent initially thought to be ideal, and later discovered not to be so, is DDT. By means of a circuitous path, aquatic insects can link DDT to the death of higher vertebrates. Further there is a marked paucity of studies that have examined how changes in biotic interactions (predator/prey, competition, etc.) might affect aquatic insect assemblages following exposure to insecticidal sprays.

Physical disturbances- Forestry practices

(Williams & Feltmate, 1992)

The impact of logging on aquatic insects is related primarily to two activities, road construction and tree felling.

Habitat Loss

(Mackie, 1998)

Total destruction of many wetlands has resulted in the direct loss of many species of birds and fish, merely by removal of their "homes". By removing the wetlands, one indirectly affects the quality and, therefore, the diversity of the receiving waters. Only the more resistant of the species survive, the weak are annihilated.

Habitat Fragmentation

(Mackie, 1998)

In some cases, only a part of the wetland or aquatic ecosystem is destroyed, resulting in their fragmentation. One example of habitat fragmentation (in some cases, destruction) is the construction of numerous dirt roads and concrete or asphalt highways across rivulets, creeks, streams and rivers within the same watershed.

Artificial Impoundments- Dams and reservoirs

(Williams & Feltmate, 1992; Mackie, 1998)

Chemical disturbances- Industrial pollutants

(Williams & Feltmate, 1992)

Industrial pollutants are generally point source in origin as they are usually discharged through pipes, ditches and sewers into bodies of water, at specific locations. Upon entering water, the chemical nature and concentration of pollutants will usually change as a result of four natural processes: dilution, biodegradation, biological amplification, and sedimentation. The rates at which these processes occur (particularly dilution and the oxygen-consuming process of biodegradation) vary directly as a function of the turn-over time of water in a system. When streams or lakes are overloaded with contaminants, and sediments become anaerobic and/or laden with heavy metals, the impact on aquatic insect communities can be severe.

Chemical disturbances- Oil spills

(Williams & Feltmate, 1992)

Although concerted efforts have been focused on examining the ecological consequences of oil spills in marine systems, only limited consideration has been given to the accidental release of oil in freshwater habitats.

Chemical disturbances- Mine waste

(Williams & Feltmate, 1992)

The harmful effects of mine waste on aquatic insects vary according to the type of mineral extracted, the size of the operation, the type of mine (surface or subsurface, hard or soft rock), and the local topography and climate. Generally, subsurface mining is less damaging to aquatic systems as, for each unit of mineral extracted, only one-tenth as much land is disturbed as would occur by extracting the equivalent unit from a surface mine. The factors that affect aquatic insects most severely are the release of toxic (mostly heavy metals) substances, increased silt loads, and higher levels of acidity.

Chemical disturbances- Acid deposition

(Williams & Feltmate, 1992)

In western and central Europe, Scandinavia, the northeastern United States, south-eastern Canada, and south-eastern China, acid deposition is a serious problem affecting aquatic and terrestrial systems. Although acid deposition is more commonly referred to as acid rain, this is a misnomer, as acids and acid forming substances are also deposited in snow, sleet, fog, dew, and as dry particles and gas.

[Img-acid-rain] Precipitation has a natural pH value of approximately 5.1 (5.0-5.6, depending on location), which forms when carbon dioxide, trace amounts of sulphur and nitrogen compounds, and atmospheric organic acids dissolve in atmospheric water. However, elevated levels of acidity result when the primary air pollutants, sulphur dioxide and nitrous oxide, enter the atmosphere at disproportionately high rates (due primarily to the burning of fossil fuels), and react to form secondary air pollutants.

The potential impact of global warming on the ecology of aquatic insects

(Williams & Feltmate, 1992)

Although a few studies have examined whether the microdistribution of aquatic insects within habitats is influenced by temperature, the collective evidence thus far indicates that such habitat selection does occur.

Mechanisms that aid certain species in withstanding the effects of pollution

(Williams & Feltmate, 1992)

(Wetzel, 1983)

Benthic community structure in lakes usually consists of a rich fauna with high oxygen demands in the littoral zone above the metalimnion. Heterogeneity of substrata is great in the littoral; benthic-animal species diversity is greater in the littoral than in the more homogeneous profundal zone. As lakes become more productive, the number of benthic animals adapted to hypolimnetic conditions of reduced oxygen and increased decompositional end-products declines.

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