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ESS Topic 4: Water and aquatic food production systems and societies
Terms in this set (77)
the hydrological cycle
is used to describe the movement of water on the planet.
Major water storages include:
Surface waters, such as streams, rivers and lakes
Ice caps and glaciers
Water vapour and clouds within the atmosphere
Groundwater within aquifers
Organisms, such as plants and animals.
Water flow into the atmosphere
Solar radiation drives the hydrological cycle.
Water transforms from liquid to vapour as the sun causes evaporation from land and sea surfaces. Through the biological process of transpiration water moves from the root system in plants to the leaves, where it is lost as vapour to the atmosphere.
The processes of evaporation and transpiration are collectively called evapotranspiration and are affected by climatic factors such as temperature and wind speed.
There may also be some contribution to water vapour through the process of sublimation. This is when ice or snow turns directly into water vapour.
As water vapour rises, it cools and condenses onto particles such as dust in the air. The resultant liquid water appears as clouds. These clouds are transported within the atmosphere by wind in a process known as advection.
Water flow out of the atmosphere
The water droplets within clouds grow and join together until they become too heavy. They then fall as precipitation returning to the surface of the earth, usually in the form of rain.
If temperatures are low enough, the process of deposition may occur. This is where water vapour in the clouds forms snow.
About 80% of precipitation falls directly into the sea and the rest falls on land.
In addition, when temperatures fall overnight, condensation can occur and the water vapour in the atmosphere is deposited on the ground as dew.
Water flow on land
The flow of water over land is referred to as surface run-off.
Vegetation can slow the movement of water. The leaves capture the first raindrops and if the rainfall continues the water will reach the ground where some may move into the soil by a process called infiltration.Plant stems also intercept the runoff, reducing the water flow rate which provides more time for infiltration to occur.
The plant root system can help to provide channels for the water to move through the soil more easily. Decayed plant matter can act as sponge absorbing the water.
Within the soil, the water can contribute to the soil moisture and by the process of absorption be taken up through the root system into plants.
Water flow into groundwater
If the subsurface is permeable, water will flow from the soil further down. This movement of water occurs under the influence of gravity and is referred to as percolation. The water moves into underground zones called aquifers and contributes to the groundwater storage.
The presence of vegetation which encourages infiltration can potentially increase the amount of stored groundwater.
Water flow out of groundwater
Groundwater may flow directly into the sea or into surface streams and rivers. Therefore, during drought periods, a reduction in this groundwater flow may have a severe effect on river levels and in turn the river ecology.
Water flow into surface waters
Precipitation contributing to surface run off may feed into streams, rivers and lakes. The movement of water within the stream is referred to as streamflow.
Water flow out of surface water
Water may flow from streams, rivers and lakes into the sea. Some water may evaporate and enter the atmosphere. In addition, there will be some uptake of water by plants and animals. This includes humans who commonly abstract water from rivers and lakes.
The global conveyor belt
The global conveyor belt, also called "thermohaline circulation" is driven by differences in water density, dependent on:
Temperature (thermo), the colder the water the more dense it is
Salinity (haline), the greater the salinity the more dense the water becomes.
How does the global conveyor belt influence weather and climate?
oceans warm and cool very slowly compared to the atmosphere. As warm water moves towards the poles, it transfers heat to the environment increasing the temperature.
Conversely, as cold water moves towards the equator, the water absorbs heat from its surroundings, lowering the temperature and affecting the weather.
Global warming could have a detrimental effect on the global conveyor belt. For example, higher global temperatures will lead to melting of glaciers and ice in the Polar Regions, the resultant increase in warm water input could prevent formation of the water density gradient leading to collapse of the global conveyor belt.
Positive feedback may exacerbate global warming. For instance, if the oceans become warmer, they will hold less carbon dioxide. Therefore, with less absorption of carbon dioxide from the atmosphere, global temperatures will increase further.
Impact of deforestation on the hydrological cycle
Deforestation: Forests are often harvested as a source of timber or cleared to make way for urban growth, industrial development or for agriculture.
Forest vegetation affects the hydrological cycle by:
Intercepting rainfall which protects the soil from the impact of the raindrops reducing soil erosion.
Impeding the movement of water which allows time for infiltration.
Absorbing water through the root system and reducing flow.
The forest acts like a sponge, absorbing water and reducing the amount of run-off and therefore the risk of flooding.
Through the process of transpiration, the forest increases air moisture which creates a wet micro-climate.
- increased soil erosion
- flooding downstream, reduce water quality + biodiversity
- In the absence of the forest, there is less water vapour in the atmosphere, therefore less rainfall and a drier climate. Through this process of positive feedback, the situation can become further exacerbated and potentially lead to a desert environment.
Impact of urbanisation on the hydrological cycle
Large parts of urban areas are covered with impervious concrete and tarmac, meaning water cannot pass through. This prevents water from soaking into the ground and percolating into aquifers.
As water moves through the catchment, flowing from roofs and along streets it can become contaminated with waste, oil and toxic metals.
This run-off is often diverted into drainage systems which discharge into nearby rivers and streams.
The increased volume of water entering surface waters can increase the risk of flooding. Aquatic habitats can also become degraded by the polluted run-off.
Sustainable urban drainage systems (SuDS) can be used to improve water quality and slow down the movement of water through the catchment and thereby reduce flood risk. Components of a SuDS strategy include:
Reducing quantity of run-off by encouraging water to pass into the ground and aquifer. For instance, use of porous material (such as gravel, porous concrete blocks and porous asphalt) on driveways and car parks that allow water to pass through into soil and groundwater stores.
Slowing the velocity of the run-off by diverting water from roofs and roads into soak-away or infiltration trenches. This reduces the surface discharge and provides protection from polluted water.
Ponds and wetland systems can also be used to intercept run-off and act as retention areas reducing flood peaks and the risk of flash floods. Additional benefits include creation of green spaces and areas for wildlife within urban environments.
Impact of agriculture on the hydrological cycle
The agricultural sector is the largest user of water. This is expected to continue to rise due to an increase in population requiring production of more food, and a change to a more meat-based diet.
Agricultural activity can encourage excessive abstraction of water, diminishing available water resources for other users. The water supply will not be sustainable if abstraction rates exceed the rate of replenishment.
Excessive irrigation coupled with poor drainage can increase soil salinity. When plants are watered, evaporation occurs and water moves up through the soil drawing salts to the surface. The soil can become sterile and unsuitable to grow crops.
As water flows through agriculture land, it can leach pollutants such as pesticides and fertilizers. Some pesticides are toxic to aquatic organisms. Highly soluble fertilizers such as nitrates can be problematic contributing to eutrophication of aquatic ecosystems.
Livestock produce animal waste, such as manure and slurry that can be washed by surface run-off into nearby streams and rivers. This animal waste contains disease causing pathogens, organic material and suspended solids.
The organic material can lower the oxygen levels within the aquatic ecosystems. Suspended solids within the water column can reduce light penetration and therefore reduce photosynthesis and overall primary production. If the particulates precipitate out, they could smother and kill organisms on the riverbed.
Availability of water for agriculture could be improved through use of rainwater collection schemes and reservoirs. The amount of water used in agriculture could be reduced by:
Changing to crop varieties that require less water.
Changing watering methods to be more efficient and effective and reduce water lost e.g. through drip irrigation systems.
Increase soil moisture retention and reduction in soil erosion through practices such as use of bunds, terracing, contour ploughing and use of winter cover crops.
Strategies to reduce water pollution from farming activity include:
Avoid application of fertilizers, pesticides or manure near watercourses or near groundwater abstraction points.
Avoid application of fertilizers, pesticides or manure during or just prior to rainfall periods.
Limit application of nitrogen fertilizers to match rates of uptake by the crops.
Collect and manage animal waste (slurry and manure) to prevent pollution run-off.
Reduce the use of pesticides by using alternative methods such as integrated pest control (IPM) which includes the use of natural pest predators, such as spiders, ladybirds and parasitic wasps and changing farming techniques, such as introducing rotation.
Inequalities of access to safe water occur:
Between urban and rural areas: People living in urban areas are more likely to be connected to piped water. Some rural areas are remote and can be difficult to access. In 2014, 82% of people without access to water lived in rural areas.
Between the rich and poor: Those with wealth are most likely to have a reliable water supply. The poor in urban areas may live in slums and shanty towns that often have poor water access.
Between social groups: Some groups within society may be marginalised e.g. based on ethnicity, language or religion.
Distribution of water: climate change
Climate change is contributing to changes in regional precipitation patterns, which directly affect water availability.
Climate scientists have made the following predictions:
Some already water stressed areas in the mid latitudes and dry tropics will receive less precipitation.
High latitudes and equatorial Pacific may experience more precipitation.
Weather patterns are likely to be more extreme with:
- Greater periods of dry spells resulting in drought conditions.
- Increases in intense precipitation potentially leading to flooding.
An increase in the melting rates of glaciers and snow will contribute to the risk of flooding.
Along coastal areas, a rise in sea level could lead to seawater contaminating surface and aquifer water supplies.
Overall more regions around the world are likely to experience water stress.
Water demand is expected to continue to rise due to:
Growth in population: The population is expected to increase to 9 billion by 2050. More water will be required for domestic use and for agriculture to produce more food.
Increase in affluence and standard of living: This results in higher water consumption. More water is used for washing, cleaning, gardening and recreational purposes.
Change to a more meat-based diet: The production of meat consumes more water than producing only fruit and vegetables.
Growth of industry: UNESCO estimates that global industrial use of water will increase to 24% by 2025.
Increase in urbanisation: The world's urban population is expected to grow to 6.3 billion by 2050. This will require further investment into the development of infrastructure to provide water resources.
is when demand exceeds the available supply over a certain time period or when the quality of water restricts its use.
Issues contributing to water stress include:
Over abstraction of groundwater, in which water is being used at a faster rate than it is being replenished. In coastal areas this is leading to contamination of aquifers by seawater referred to as saline intrusion.
Excessive abstraction of surface waters lowering water levels and the area covered by water. The Aral Sea in Asia was the fourth largest freshwater lake in the world but following high levels of abstraction has shrunk to less than 10% of its original size.
Pollution of surface and groundwater resources, such as sewage effluent, toxic industrial waste and agricultural run-off. Contamination of water resources increases the cost of clean-up.
Inefficient use of water resulting from:
Poor irrigation e.g. resulting in high levels of run-off and evaporation. Drip irrigation systems are used to release water directly to the roots of the plant.
Leakages within the water distribution system.
Repairing leakages within the distribution system is an on-going battle. In some countries the water distribution is deliberately broken to "steal" the water.
Inefficient use of water by industry. Changes in industrial processes can reduce the amount of water used.
Inefficient use of water by individuals.
Climate change which will alter rainfall patterns
Rivers without borders
Many river systems are shared by countries. This can result in some nations being dependent on sources of water that originate outside their borders. Countries upstream can control the flow to downstream neighbours. Throughout history disputes have arisen over the ownership of water resources. With increasing pressure on limited water sources, conflicts continue to arise throughout the world today.
Conflict in countries over water
over the Ethiopian Grand Renaissance Dam - egypt relies on it
Downstream neighbours were concerned over its potential to threaten their supply of water. The River Nile provides Egypt with its main source of water. Egypt initially perceived the construction of the dam as a threat to its national security and prior to the agreement had threatened military action. Negotiations are aimed at reducing disruption to the flow downstream. For example, Ethiopia has agreed to fill the dam gradually which will lessen the impact on flow.
over the spanish drought
- farmers vs. urban people
- heat from the sahara
Managing water resources I : Reservoirs
Reservoirs can be either natural or artificially created lakes, used to collect and store water. Reservoirs are built by damming rivers and flooding suitable valleys. The aim of the reservoir is to store water during periods of high rainfall to provide a plentiful supply throughout the year.
What additional benefits are there to a reservoir?
Generation of hydropower: Some reservoirs also incorporate hydropower schemes to generate electricity.
Flood control: In some regions, reservoirs are also used to capture floodwater and reduce the risk of flooding in downstream areas.
Navigation: The reservoir can provide transport route from one site along the shore to another.
Fisheries: Commercial fisheries have been developed in some reservoirs.
Recreational, aesthetic and scenic value: Reservoirs can be used for many recreational activities, such as water sports such as canoeing and water skiing. Picnic spots are often located along the shores of the lake for their aesthetic and scenic value.
Control of water quality: Sediment load of the water can be reduced in standing water. The particles in the water precipitate out, improving the water quality.
What are the potential impacts of building reservoirs?
Change of habitat: When an area is flooded to become a reservoir, there is a change from a terrestrial to an aquatic ecosystem. The establishment of a lake ecosystems introduces new freshwater habitats to the area. However, scarce terrestrial habitats and species may be lost.
Relocation of people: People may need to be moved out of an area that is to be flooded and relocated elsewhere. Whole towns and villages may be affected. It has been estimated that China's Three Gorges Dam on the Yangtze River led to the displacement of about 1.3 million people.
Change to the flow of the water: Much of the water from the reservoir is diverted elsewhere e.g. to urban areas for industrial and domestic use. Some of the water may re-join the river further downstream as waste water, potentially polluting the river.
Loss of fish and mammal migratory routes: Dam walls can block the migratory route of some fish and dolphins. In an effort to alleviate the loss of fish, fish ladders (concrete steps filled with water) are incorporated into the dam wall. Some fish are able to find and use the ladder allowing them to travel through the wall and continue along their way.
Sedimentation in the reservoir and loss of capacity: The sedimentation of particles from the water behind the dam wall reduces the holding capacity of the reservoir. In addition, this may not always be desirable for farmers downstream who rely on the nutrients in the sediments to fertilise their fields.
Managing water resources I : Artificial recharge
Artificial recharge is used to increase the amount of water stored in aquifers. It is a widely used in some countries to enhance water supplied, such as in the Netherlands, Germany and USA.
Methods can include:
Building a ditch or trench above an aquifer zone to intercept and collect run-off. The water collected gradually seeps into the ground and percolates through permeable strata into the aquifer.
Although a simple method, the acquisition of sufficient land can be expensive.
Alternatively water can be pumped directly from rivers or reservoirs into the aquifer via a borehole (a hole drilled into the ground). Pumping directly from a river with high sediment loads can cause clogging of boreholes.
Using reservoirs to store the water prior to pumping into the aquifer has the advantage of allowing the sediments to settle out reducing the sediment load.
Managing water resources I : Rainfall harvesting schemes
Rainwater harvesting involves collection of precipitation which falls on the roof of buildings. The rainwater is stored in tanks and can be used for domestic purposes, such as cleaning and gardening.
Rainwater collection reduces the risk of flooding and soil erosion. It is also a relatively cheap method, and easy to maintain. The water is relatively clean but should be filtered and disinfected if used for drinking.
Managing water resources II : Desalination
Reverse osmosis is the most common method used. Reverse osmosis occurs when external pressure applied is greater than the osmotic pressure and water molecules move from a high concentrated solution into a low concentrated solution.
In desalination, seawater is placed under pressure, which forces water molecules to move through a semi-permeable membrane leaving behind the salt molecules.
Managing water resources II : Water redistribution: water transfer schemes
These schemes often transport water from one river basin to another using pipes or canals. Water is taken from where it may be considered as surplus to where there is a water deficit. They are often grand, expensive projects.
This redistribution may address water demand in one area but can have adverse effects on the region of the donor river. For instance:
Abstraction of water may lower water levels affecting habitats, such as wetlands and associated species.
The disruption in the flow can affect fish and other biota living in the river.
The reduced amount of water may not be sufficient to meet the needs of local people.
Managing water resources II : Use of greywater
Greywater is used water that is clean enough to be used again. It includes water from baths, showers, wash basins and washing machines.
Greywater can be collected and used for toilet flushing and gardening. It is not suitable for drinking due to presence of some pathogens and contaminants.
Greywater reduces the amount of wastewater produced and requiring treatment, as well as reducing the amount of water that needs to be abstracted.
Communal systems for the collection and treatment of greywater are often more cost-effective than greywater use by individual households. These systems can incorporate:
Physical treatment: filtration to remove large particles and disinfection to kill pathogens.
Biological treatment: involving either bacteria or wetland systems to utilize nutrients and filter particles from the water.
Reducing demand of water
Increasing water efficiency through improved technology and processes
Public awareness campaigns
Education in schools to change long term behaviour that encourages water conservation.
Economic incentives (fines, increase cost)
Introduction of legislation and policies that incorporates efficient use of water
Changing to crops that require less water to produce
Reducing meat based diet that utilises relatively large amounts of water during production
Detecting and repairing leaks in the water distribution system.
The ocean can be divided into zones:
Epipelagic zone (depth of 0 to 200m): into which light penetrates allowing primary produces to grow. It is the most productive zone, with little photosynthesis occurring outside this area.
Mesopelagic zone (depth of 200m to 1,000m): where there is insufficient light penetration to allow for plant growth. This zone contains a diverse range of organisms.
Bathypelagic zone (depth of 1,000m to 4,000m): is also known as the dark zone due to the absence of light apart from that produced by any bioluminescent organisms present.
Abyssalpelagic zone (depth of 4,000m to 6,000m): is dark and the water temperature is just above freezing. Few organisms can withstand the high pressure in this zone.
Hadalpelagic zone (depth of more than 6,000m): usually includes trenches and canyons. The water again is very cold and life here needs to be adapted to extremely high pressure.
Marine trophic levels
Primary producers form the base of the food web and include phytoplankton and seaweeds.
Primary consumers include the zooplankton, small floating animals in the sea that graze on the phytoplankton. They consist of a diverse range of animals including ciliates, copepods and animal larvae.
Secondary consumers are small predators such as some fish (e.g. sardines, menhaden and herring) and the young stages of larger varieties of fish and jellyfish.
Tertiary consumers include top predators such as large fish (eg sharks, tuna and mackerel), marine mammals (e.g dolphins, seals and walruses) and birds (e.g penguins and albatross).
Thermal stratification occurs when:
Sunlight heats the upper layer of water and surface movements create a layer with a fairly consistent temperature.
Water movement below the surface mixed layer is reduced due to the calm weather and the sun continues to warm the water. Sunlight penetration declines with depth resulting in a temperature variation from the top to the bottom of the thermocline.
This thermal stratification prevents mixing occurring.
Strong winds and coastal currents can cause mixing of the water and break up the thermocline and redistribute nutrients back into the water column.
This process is referred to as upwelling, and contributes to an increase in primary production. El Nino events reduce surface current and wind driven upwellings, which has a negative effect on productivity in the area.
Coastal waters and shallow seas tend to be productive, because:
In shallow water any nutrients that precipitate out are re-suspended by wind and currents.
River input brings in more nutrients.
Sunlight may penetrate down to the sea floor resulting in relatively high levels of light that drives photosynthesis.
Lake ecosystems include the following zones:
Littoral zone: the shallow area of the lake that goes up to the shore area. This is where large freshwater plants called macrophytes occur.
Limnetic zone: covers the open water in the lake where there is enough light for phytoplankton to photosynthesise.
Euphotic zone: includes both the littoral and limnetic zone where there is sufficient light for photosynthesis to occur.
Profundal zone: the deep water where there is no light penetration.
Benthic zone: the lake bottom, where organisms live within the sediments or on the surface of the lake sediments.
Trophic levels in freshwater ecosystem:
Primary producers: phytoplankton and macrophytes. Phytoplankton includes freshwater varieties of diatoms, dinoflagellates, and cyanobacteria.
Primary consumers: zooplankton (e.g. waterfleas, copepods and rotifers) and water snails.
Secondary consumers: fish (e.g. perch, smelt, minnows), birds (e.g. ducks) and frogs.
Tertiary consumers: large fish (e.g. trout, charr and piranhas), large birds (e.g. kingfisher) and mammals (e.g. otters and humans).
Energy efficiency of aquatic food systems
Aquatic food systems are often considered to be less efficient than terrestrial food systems:
Primary producers in aquatic systems receive less light than terrestrial plants because some of the incoming light is absorbed or reflected by the water.
Compared to terrestrial foods, humans generally tend to eat organisms from higher up in the aquatic food chain. Not all the energy is transferred from one trophic level to the next, hence the longer the food chain and the more transfers, the greater the energy loss.
However, in aquatic systems less of the biomass may be lost as indigestible skeletal material (e.g. jellyfish have no skeleton) resulting in more efficient energy transfer.
In order to meet demand the intensity of fishing effort increased. Many countries moved from small scale local fishing to large scale commercial fishing involving:
Growth in number and size of fishing fleets.
Improvements in shipping vessels which allowed fishing to occur further from the shore and in deeper waters.
Larger ships allowing for longer periods at sea resulting in greater harvest of fish.
Technological developments have also increased the efficiency of harvesting fish. These include:
Development of sonar, radar and satellite technology to detect and track schools of fish.
The ability to process, preserve and freeze aquatic produce on the ship whilst still out at sea.
Changes to fishing gear, allowing for larger catches.
Types of nets used to fish include:
Trawler, Purse-seine, and Drift nets
Why is overfishing common?
Property rights: no one owns the fish. Fish swim through large areas and do not respect national boundaries. People often do not want to spend money on conserving the fish, if other competing countries will harvest them.
Zero sum game: this is the idea that in many situations someone gains at the expense of others. In order to conserve fish stocks you need to convince people to sacrifice short term gain to benefit in the future. For this to be a favourable option, the long term gain needs to be sufficiently lucrative and low risk. If you take action to conserve fish stocks, can you ensure your competitors will do the same and not harvest the fish? If not, then the group that choose conservation lose out completely.
Managing fish stocks
Use of quotas: Fish biologists estimate the maximum sustainable yield based on current stock levels and rates of replenishment. - TAC (total allowable catch) but some non-target species can also be caught
Reduction in fishing effort:
This is achieved through:
Reducing the number of boats fishing.
Restricting boat size.
-Restricting type of fishing gear used, including limits on size of nets and mesh size.
Large mesh sizes are used to reduce catch of juvenile fish.
Setting limits on the minimum size of fish that are allowed to be caught, which prevents fishing of small and young fish.
Restricting fishing times, such as having a closed season when fishing is not allowed.
Use of exclusion zones including Marine Protective Areas (MPAs)
Newfoundland case study
Newfoundland, had the largest cod stocks in the world. However, in the 1950s, with the adoption of modern technology, the level of fishing effort increased considerably. This involved use of:
Large shipping fleets with more efficient engines that allowed boats to stay out longer and cover more fishing grounds.
Factory fishing boats with the capability of processing and freezing fish on board.
Huge trawl nets that covered a larger area but also damaged the seabed.
More efficient detection methods to find fish.
Iceland case study
Following a decline in cod fish stocks, the Iceland government took action to enable them to continue fishing but at a rate that did not lead to collapse of stocks as had occurred in Newfoundland. This included:
Protecting territorial waters from fishermen from other countries (action also undertaken by Newfoundland to protect fish stocks).
Restrictions on fishing gear and fleet sizes.
Strict quotas that can be traded between fishermen.
Banning the disposal of any bycatch including undersize cod.
High level of enforcement by inspectors that police and monitor the seas.
Use of exclusion zones (where fishing is banned), for instance:
- Permanent closure of nursery areas.
- Seasonal closure of some areas during spawning times.
- Temporary closure of fishing areas if fish caught are too small in order to conserve juvenile fish.
Aquaculture is the farming of aquatic organisms: fish, molluscs, crustaceans, aquatic plants, crocodiles, alligators, turtles, and amphibians. Farming implies some form of intervention in the rearing process to enhance production, such as regular stocking, feeding, protection from predators, etc...
open based aquaculture system
are the most popular and involve farming the organisms within a natural aquatic ecosystem such as the sea or a lake.
This includes fish cages, clam beds and oyster rafts that are submerged in the water. Often juvenile fish are transferred from hatcheries (where fish eggs are incubated and hatched) into the fish cages to develop and grow. The fish farmer has little control over the environmental factors such as temperature which may affect growth rates of a species. Other potential issues include predation and poachers.
semi-closed aquaculture system
involve the abstraction and use of water from the sea or lakes within tanks or ponds situated on land.
This allows for greater control over environmental conditions such as temperature and water velocity. If required the water can be filtered to remove any predators or pathogens. Semi-closed systems tend to be more expensive than open-closed systems.
Environmental impacts of aquaculture
Increase in organic sediments - fish waste, no circulation, leads to the formation of toxic gases
Increase in available nutrients - increase food waste and faeces of fishes increases primary productivity - algal blooms if no water circulation
Use of medicines and hormones - antibiotics and hormones used to treat farmed fish - water contamination - affect other aquatic life.
Use of antifouling agents - Antifouling agents are also used to prevent growth of algae and other organisms on the cage - lead to mollusc deformity
Spread of disease - Within intensely stocked cages, disease can easily spread from one fish to another and even potentially to other fish outside the cages.
Escaped fish - escaped fish from cages may threaten wild stocks by competing for habitat and food, transmitting diseases, interbreeding
Attracted predators - Predators attracted by the farmed fish can become entangled and caught within the nets of the fish cages.
Managing environmental impacts
Action to reduce some of the environmental impacts of fish farming includes:
Reducing the waste from uneaten feed by careful selection of appropriate feed and not overfeeding (e.g. timing feeding sessions with care).
More effective application of any medicines to reduce losses to the environment.
Regular removal of any dead fish from the cages.
Moving the cages at regular intervals to prevent build-up of organic sediments and give the area time to recover.
Aerate the water to prevent anoxic conditions.
Removing the deposited waste from below the fish cages.
Locate fish farms where there is sufficient movement and exchange of water to:
- Reduce nutrient levels in the water.
- Reduce phytoplankton levels in the water and disperse any blooms.
- Reduce build-up of waste by dispersing it.
Inland and coastal pollution sources
Domestic sewage - organic - source of pathogens
Industrial discharge - organic matter, toxic metals & synthetic non-biodegradable compounds
Agricultural run-off - pesticides, fertilizers, manure, slurry or silage that may contain pathogens
Urban run-off - high levels of organic waste, suspended solids, oil and toxic metals.
Land development - suspended soils and sediments from deforestation & building works
Landfill sites - Disposal of waste on land can lead to leachates entering groundwater or surface waters - harmful to aquatic organisms
Accidental discharges - toxic metals or non-biodegradable synthetic compounds
Acid mine drainage - result of water percolating through either disused or active mines
Atmospheric input - toxic metals (e.g. lead and cadmium) and synthetic compounds (e.g. polychlorinated biphenyls (PCBs) and dichlorodiphenyl trichloroethane (DDT)).
Marine based pollution sources
Outfall pipes - Outfall pipes can be used to discharge material directly from the land to the sea e.g. sewage effluent (either treated or non treated sewage) and cooling waters from power stations.
Materials directly dumped at sea -
Sewage sludge from sewage treatment plants.
Disused platforms e.g. oil platforms.
Fly ash from power stations.
Dredging spoils from widening shipping channels.
Shipping activities -
Disposal of litter and other waste at sea.
Accidental discharges, such as oil spills.
Discharge of ballast waters - contain oil residues.
Exploitation of resources - Extraction of materials such as oil or gravel beneath the sea bed may cause marine pollution.
Effects of organic pollution
If organic waste is discharged from an outfall pipe, it creates turbulence in the water, maintaining oxygen levels.
As the material flows downstream and is degraded, oxygen levels fall. Some aquatic organisms may be deprived of sufficient oxygen levels and die.
If conditions become anoxic, anaerobic bacteria breakdown the organic matter into methane, ammonia and hydrogen sulphide. The later that has a distinct smell of rotten eggs.
These gases are toxic and result in fish kills. Once the organic material is broken down oxygen levels begin to recover.
Determining factors of oxygen depletion
Concentration of the organic discharge, rate of dilution and rate of aeration (including turbulence).
Temperature: water holds more oxygen at lower temperatures.
Pressure: pressure decreases with altitude and at lower pressure the water holds less oxygen.
Turbulence: the movement and agitation of the water, the greater the aeration increasing dissolved oxygen levels in the water.
Photosynthesis: during the day, the process of photosynthesis produces oxygen which enhances dissolved oxygen levels.
Respiration: this process utilizes oxygen and can lower oxygen levels.
The greater the concentration of the discharge the more oxygen is required. If the receiving waters are large and fast flowing, it will reduce the impact of oxygen utilization.
Effects of inorganic plant nutrients
leads to eutrophication, decrease in oxygen and cyanobacteria and algal blooms - increased rate of photosynthesis that can result in the blocking of other plants - loss of overall biodiversity
Problems associated with eutrophication include:
Water can become unsuitable for drinking unless expensive treatment methods are used. The excessive plants can clog filters and decomposing algae can have a detrimental effect on taste and odour.
Reduced recreation use of the water (e.g. boating, swimming and fishing) due to excessive plant growth or cyanobacterial blooms.
Reduced commercial value of the aquatic ecosystem e.g. due to loss of fisheries or loss of navigation routes used for trade.
Increase in water related diseases. The excessive plants can provide a habitat for insects and other organisms that spread disease.
Effects of toxic metals
Some metals such as zinc and copper are required as micro-nutrients by many organisms.
However, at higher levels these metals can be toxic by interfering with essential cellular processes.
Bioaccumulation can occur, where on continual exposure the levels of the metals build up within the organism over time.
Bio-magnification can also occur in which the levels of the metal then build up though the food chain.
Effects of synthetic compounds
They cover a wide variety of different compounds but of particular concern are the organochlorine compounds. They are non-biodegradable and can bioaccumulate e.g. polychlorinated biphenyls (PCBs) & DDT.
PCBs are able to bioaccumulate within organisms and through the process of biomagnification, levels increase further up the food chain.
Biological effects include:
Inhibition of phytoplankton growth.
Inhibition of oyster shell growth.
Adverse effect on fish reproduction.
Suppression of the immune system in birds resulting in death.
Adverse effect on the immune system and endocrine system of mammals linked to reproductive failure.
Yusho illness in humans which includes acne, darkening of the skin and respiratory problems.
PCBs have also been found to cause birth defects and cancer.
Effects of inert suspended solids
Suspended solids enter aquatic ecosystems from domestic and industrial effluent and as run-off.
The suspended solids suppress plant life by preventing light penetration.
They can clog feeding and respiratory structures and smother benthic organisms living on the river, lake or seabed.
Effects of hot water
A major source of hot water is the cooling water discharged from electricity generating power stations.
The warmer water discharged elevates the local water temperature. If subtropical species have been introduced to the aquatic ecosystem, they may find the conditions favourable and out-compete native species.
A higher water temperature will lead to lower concentrations of oxygen in the water which may result in an increase in the level of stress experienced by the aquatic organisms.
Effects of oil
Oil covers the water forming a surface film that prevents gaseous exchange and therefore can result in oxygen depletion within the water. In addition the oil film blocks out light and prevents photosynthesis.
Effects of pathogens
A variety of pathogens that include bacteria and viruses are contained in sewage effluent discharged into inland and coastal waters.
These waters are a potential hazard to health and are of particular concern if they are used for recreation (e.g. swimming) or are shellfish collection sites.
Shellfish grown in contaminated waters accumulate pathogens posing a significant health hazard if consumed.
Effects of plastic debris
Effects on aquatic animals:
Becoming entangled in the plastics and drown.
Ingesting the plastics which block their digestive system, which reduces feeding and can also cause internal injury and death.
Plastics also release polychlorinated biphenyls which can alter hormone levels, lead to reproductive problems, increase risk of disease and cause death.
Effects of light pollution
Artificial lights along coastal areas can have devastating effects on sea turtle populations.
Artificial lights disorientates hatchlings as they try to find their way to the ocean and may wander further inland instead increasing risk of death from predators, from dehydration or accidental death on roads.
Effects of noise pollution
Noise such as underwater sonar is considered to be a contributing factor to the beaching of whales and dolphins.
Effects of Invasive species
Invasive species are categorized by some scientists as a biological pollution. Some species may migrate via ocean currents and with the effects of global warming may acclimatize well to its new environment.
Assessing water quality: physical and chemical parameters
pH: often reflects the local geology and soil. Changes in water pH can affect reproduction and overall population growth rates. pH is commonly measured using a calibrated pH probe, although a rough guide can be provided using litmus paper.
Temperature: Temperature normally reflects changes in ambient temperature (the surrounding environmental temperature). It affects the amount of dissolved gases present in the water. Temperature can be measured in-situ using a thermometer.
Suspended solids: Suspended solids are small particles that can block sunlight penetrating through the water reducing photosynthesis. These small particles can also block the feeding and respiratory systems of some organisms.
The amount of suspended material in the water is determined by:
Filtering a known volume of the water sample using pre-weighed filter paper (A).
Drying out the filter paper and collected residue.
Weighing the dried filter paper and residue (B).
Calculating the weight of the dried residue = B-A in micrograms/litre (mg/l).
Alternatively an indirect measurement can be taken using either a turbidity meter or secchi disk:
A turbidity meter determines the amount of light scattered by the particles in the water. The greater the amount of suspended solids present, the higher the turbidity readings.
A secchi disk, as previously discussed is used to measure water transparency. The greater the amount of suspended solids the lower the light transparency in the water.
Assessing water quality: physical and chemical parameters 2
Total dissolved solids and conductivity: Measurement of the total dissolved solids (TDS) provides an indication of the amount of salts present. The TDS can be an indication of the geology or the type of effluent discharged into the water. It is measured indirectly using a conductivity meter.
Dissolved oxygen: (DO2) is often used as indication of the quality of the water. DO2 can be measured using an oxygen meter on site.
Biochemical oxygen demand (BOD): is the measure of the amount of oxygen used by organisms present in the water sample. It provides an indirect measure of the amount organic material that can be oxidised.
Process to measure BOD:
The initial dissolved oxygen reading of the sample is taken in mg/l.
One litre bottle is filled with the sample and sealed.
The bottle is incubated in the dark at 20°C for five days.
The dissolved oxygen levels are measured again.
The difference between the initial and final oxygen readings is the BOD5.
As a general guide for BOD5, waters with a value of 2mg/l or less are considered to be pristine. Whereas a BOD5 value of 20mg/l would indicate a badly polluted site.
Nutrients: The water can be analysed for nutrient either on site using test kits or taken back to the laboratory and measured using chemical analytical methods
Metals: Metals are not usually measured on site and samples are collected and taken back to the laboratory to be analyzed using Inductively Coupled Plasma by Optical Emissions Spectrometry.
Limitations to testing for physical and chemical parameters
They provide information for that specific sample at that particular time and chemical pollution can be quickly washed away.
If toxic material is discharged into an aquatic ecosystem it may be dispersed before sampling has occurred.
However, the ecosystem may have been severely damaged by the pollution resulting in death of species including fish and loss of biodiversity.
Assessing water quality: Biological monitoring
Biological organisms can indicate whether the water quality has declined and whether there have been episodes of pollution between periods of sampling. More commonly used are communities of species. Use of macro-invertebrates is very popular. (kick sampling)
Assessing water quality: Biotic index
Biotic indices are used to determine water quality using aquatic organism. Different biotic indices are used around the world and are determined by species that are specific to the region.
Assessing water quality: Microbial test
Additionally if the water is to be used for recreation or drinking purposes, it will be tested for pathogens of faecal origin. Indicator species such as Escherichia coli and Faecal Streptococci are used.
Water pollution management: Domestic sewage effluent
Sewage effluent can be routed away from sensitive areas e.g. groundwater and waters that are sensitive to eutrophication.
The effluent can be treated to breakdown the organic material and therefore reduce the BOD and also the amount of suspended solids. During treatment the amount of a nutrients and pathogens are reduced.
Levels of sewage treatment can involve various processes.
Preliminary treatment which involves: (i) screens to remove large objects that may otherwise damage the mechanical equipment or cause blockages and (ii) grit removal to prevent abrasion and wear of equipment and deposition in pipes and channels
Primary treatment in which the piped sewage is allowed to settle within primary sedimentation tanks during which time any settlable solids are removed. This reduces the suspended solids and BOD levels.
Primary treatment in which the piped sewage is allowed to settle within primary sedimentation tanks during which time any settlable solids are removed. This reduces the suspended solids and BOD levels.
Tertiary treatment is less common than primary and secondary treatment. It can involve a variety of different processes. For example:
Nitrate removal involves biological processes in which ammonium ions are oxidised to nitrates and then using denitrifying bacteria the nitrates are converted to nitrogen gas which can be lost to the atmosphere.
Ammonium → nitrites → nitrates (nitrosomonas bacteria and nitrobacter).
Nitrates → nitrogen gas (denitrifying bacteria).
Phosphate removal involves use of chemicals such as iron and aluminium salts which react with the phosphates and precipitate it out.
Macrophyte beds can be used to treat effluent from primary of secondary treatment. The effluent is passed through the beds of growing macrophytes e.g. Phragmites australis to remove suspended solids, nitrates, phosphates, metals and pathogens.
Once effluent is discharged, DO2 levels within the receiving waters can be increased by use of weirs, steps or waterfalls to aerate the water as it flows.
Water pollution management: Industrial discharge
Pollution from industrial discharge can involve replacing the chemical causing pollution with an alternative. For example, PCBs within electrical transformers have now been replaced with silicone and mineral oils.
The amount of pollutant discharged into the environment can be controlled through legislation.
Consent licenses providing permission to release effluent into surface or coastal water are usually required. These often have specific requirements in terms of quantity and quality of the discharge including maximum levels of potential pollutants.
What is an effective way of reducing phosphates in wastewater effluent?
The addition of iron salts.
Water pollution management: reducing pesticides
The amount of pesticide entering aquatic systems used can be reduced by:
Using alternative approaches to reduce pest such as biological control i.e. the use of natural predators.
Only applying pesticides when and where required rather than blanket spraying on a regular basis.
Using biodegradable pesticides which do not bioaccumulate or biomagnify through the food chain.
Using pesticides that are target specific and do not harm other species (non-target species). Some pesticides have been banned due to their toxicity on non-target organisms such as phytoplankton and fish.
Storing pesticides in impermeable containers and within areas that can contain any accidental spill e.g. with bunds.
Water pollution management: reducing fertilisers
The amount of nutrients from fertilizers entering aquatic systems can be reduced by:
Replacing soluble nitrate fertilizers with ammonium fertilizers.
Using organic fertilizers that release nitrates more slowly than most artificial fertilisers.
Only applying fertilizers where and when required by plant growth.
Only applying fertilisers at the rate necessary for plant growth.
Only applying fertilizers during dry weather to avoid it being washed away into nearby water systems.
Not applying fertilizers near any aquatic systems.
Water pollution management: organic waste
The amount of slurry, manure and silage effluent entering aquatic systems can be reduced by:
Avoiding spreading this organic and nutrient rich matter (when used as fertilizers) near water courses.
Only applying this matter as a fertilizer to land during dry weather.
Ensuring the slurry, manure and silage is contained and run-off collected and treated prior to discharge into the water body.
Water pollution management: run off
Reducing the amount of run-off can decrease the amount of pollution entering water systems from agriculture land.
Some of these techniques include:
Reducing the amount of water used by employing more efficient irrigation systems.
Use of contours and terraces that impede the flow of the water and potential pollutants.
Planting cover crops to intercept the rain and reduce run-off.
Using buffer zones to remove pollutants from agricultural run-off before it enters nearby aquatic ecosystems. Buffer zones are areas of vegetation that intercepts the run-off. It helps to improve water quality by trapping sediments, organic matter, nutrients, pathogen and pesticides. In addition buffer zones contribute to preventing soil erosion and provide a habitat for wildlife.
Water pollution management: managing eutrophication
Eutrophication of waters is often caused by domestic or industrial effluent discharges and run-off from farms.
Action to reduce nutrients which cause eutrophication entering aquatic ecosystems includes:
Substituting phosphates in detergents with an alternative such as Zeolite A.
Removing nitrates and phosphates from sewage effluent (as discussed above this is typically part of tertiary treatment of sewage effluent).
Diverting sewage effluent away from water systems that are vulnerable to eutrophication (e.g. may have low degree of dilution and dispersal properties).
More efficient use of fertilizers and appropriate methods of dealing with animal manure, slurry and silage effluent (as discussed above).
Using buffer zones to intercept runoff and absorb the nutrients.
Restricting access of livestock to aquatic ecosystems
Once the nutrients have entered the waters, the following approaches can be taken:
Use macrophyte channels to absorb the nutrients from the water. The macrophytes would need to be harvested to prevent nutrients re-entering the water. In some regions, macrophyte growth is seasonal and therefore its use is limited to certain times of the year.
Mix the water to aerate it and prevent anoxic conditions that will kill many aquatic organisms.
Dredge the bottom to remove sediments that contain nutrients and enhance eutrophication.
Use herbicides to control algal blooms, although this could be problematic if the aquatic system is a source of drinking water.
Mechanically remove the macrophytes and use e.g. as a fertilizer on land.
Use biological control e.g. fish such as Tilapia that feed off the algal bloom. However, the introduction of non-native species may cause changes in the community composition and threaten other species.
Following action to remove nutrients and algal blooms reintroduce native species back into the aquatic ecosystem.
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