| | Aquatic ecosystems are dynamic and complex natural systems. Erosion processes primarily control change in flowing water systems. Whereas internal recycling of inorganic and organic materials and inputs from the watershed primarily drive changes in lake systems. Lake productivity is mainly controlled by aspects of inorganic and organic biogeochemical cycling and links the activities of decomposers and primary producers within aquatic ecosystems (Wetzel 1983). Even slight imbalances in the biogeochemical cycle may result in drastic changes within an aquatic ecosystem. For instance, higher loading rates of phosphorus and nitrogen increases photosynthetic productivity and makes the cyclic interactions involving regeneration of inorganic nutrients and organic compounds increase in intensity (Wetzel 1983).
The process of sedimentation, the movement and deposition of soil particles and other sediments into lakes, can alter or imbalance aquatic ecosystems. Further, pollutants that are adsorbed onto soil particles and other sediments or dissolved pollutants entering a lake or river ecosystem can alter the natural biogeochemical cycling processes and facilitate progressive change of lake and river productivity. Productivity, in terms of aquatic ecosystems, is the rate of formation of organic matter (i.e. algae, aquatic plants, organisms) over a period of time including any losses that occurred during that period (Wetzel 1983). High productivity rates are associated with fertile or 'green coloured' lakes that experience large algal blooms during the summer months. This generally results from high external and internal loading rates of phosphorus and nitrogen. Phosphorus is the principal nutrient that limits primary production in fresh water as some species of algae can use atmospheric nitrogen (nitrogen gas). Phosphorus and nitrogen are essential macronutrients which are taken up by aquatic plants and algae and incorporated into plant organic matter. Therefore, excessive quantities of phosphorus and nitrogen can stimulate primary productivity in an aquatic ecosystem.
In addition to external loading, internal loading from lake sediments is also a significant source of phosphorus to lakes. Dissolved nutrients may be released from lake bottom sediments when there is a lack of dissolved oxygen at the sediment-water interface. When water in the lake recirculates or 'turns over' in the spring and fall, the nutrients which were released at the sediment-water interface and isolated in the bottom waters by a temperature-density gradient in the lake profile are recirculated to the surface waters and made available to algae. Pollutants that are attached to eroded soil particles or absorbed onto lake bottom sediments can be a long-term source of contaminants to aquatic systems.
As inorganic phosphorus and nitrogen are incorporated into plant organic material (i.e. algae and aquatic plants), they become part of the food web within aquatic ecosystems. Zooplankton, small aquatic organisms (e.g. insects), and other animals in the food web may graze on algae as a food source while these organisms may be a food source for other species (e.g. fish). Nutrients are recycled to the water through direct release by algae, breakdown of organic forms (decomposition) or wastes excreted by organisms.
Ammonia nitrogen, usually present as the ammonium ion (NH4+) in fresh waters, is the main product produced from the decomposition of plant and animal material. In the presence of oxygen, ammonium is transformed by nitrifying bacteria, such as Nitrosomonas and Nitrobacter, first into nitrite (NO2-) and then into nitrate (NO3-). Nitrate is transformed by denitrifying bacteria to elemental nitrogen or nitrogen gas (N2). In addition, some organisms are able to fix or use atmospheric nitrogen (N2) (e.g. bacteria and blue-green algae) as a nitrogen source.
Nutrients are continually recycled between inorganic and organic forms. Nutrient transformations are similar in lakes and rivers; however, the nutrient cycle in a lake becomes a nutrient spiral in a river as the flow moves nutrients downstream. Nutrient cycles or spirals are affected by inputs and outputs. Phosphorus and nitrogen can enter a lake in surface runoff, ground water, streams and by atmospheric deposition as well as be recycled from bottom lake sediments. If nutrients are in short supply, they can limit the growth rates of aquatic algae and plants. Excessive quantities of nutrients, specifically phosphorus, stimulate aquatic plant and algal growth resulting in the eutrophication of surface waters. Some lakes can be exceptionally sensitive to phosphorus inputs. Critical levels of phosphorus which can lead to the eutrophication of surface waters can be as low as 10 ug/L in some lakes (Wetzel 1983). In Alberta, the Alberta Ambient Surface Water Quality Interim Guidelines suggest 50 ug/L of total phosphorus to be a maximum concentration to prevent impairment of water quality. Highly productive lakes with excessive algal and weed growth are referred to as eutrophic whereas oligotrophic lakes have low rates of productivity and low inputs of inorganic nutrients (Wetzel 1983). Water bodies with little flushing, such as lakes and reservoirs, are susceptible to eutrophication through nutrient enrichment over geological time; however, the eutrophication of surface waters may be greatly accelerated by human activities (CCME 1987).
Eutrophic conditions are aesthetically unpleasant and prevent recreational activities such as boating and swimming. In a river or canal, an increase in aquatic weeds can decrease channel capacity. Conversely, lakes with little aquatic plant or algal growth and low concentrations of phosphorus and nitrogen are referred to as meso- or oligo-trophic. If lakes become limited by nitrogen, green algae and diatoms are displaced by blue-green algae that are capable of fixing atmospheric nitrogen. Noxious blue-green algae tend to be unpalatable by zooplankton and therefore tend to form large scums and algal mats on the surface of the water causing severe odour and aesthetic problems. Blue-green algae can also produce deadly neuro- and hepato-toxins that are released into the water when the algal bloom dies. These toxins are a potential threat to livestock and human health (Kotak et al. 1993). Despite all the problems associated with eutrophication, the abundant growth of plants has some positive side-effects for nature lovers. Water birds and other wildlife are attracted by the abundance of food organisms that live among rooted weeds.
In rivers, rooted plant growth is more often regulated by physical factors including light limitation due to bank shading, velocity and timing of flow, and available substrate rather than by nutrients. However, increased concentrations of nutrients, such as phosphorus, can increase aquatic plant and algal growth in slow-flowing prairie rivers.
When plants become too abundant, they can change the chemistry of the water. Aquatic plants, like terrestrial plants, photosynthesize to produce oxygen in the presence of sunlight whereas they use oxygen (respire) at night or in the absence of sunlight. During the day they produce more oxygen than they use; at night they consume more than they produce. When aquatic plants and algae die off, by exhausting the nutrient supplies or through natural processes, they begin to decompose. Decomposition of organic matter, such as dead plant material and livestock wastes, consumes oxygen from the water column resulting in anoxic (no oxygen) conditions. Since fish and other aquatic organisms cannot survive without oxygen, large fish kills can result. Decaying plant material also produces highly offensive visual and odour problems. The decomposition of plants can also result in taste and odour problems in the water, particularly when certain types of bacteria are present. Furthermore, the eutrophication of lakes can cause changes in the fish community. Sport fish (e.g. trout) which are high quality food fish are replaced by low quality fish (e.g. carp). Although the collapse and decomposition of an algal bloom in a lake is more dramatic than in a river, rivers can also be severely degraded by decaying plant material. However, decomposing material is progressively moved downstream and oxygen can be mixed into the water through turbulent flow.
Concentrations of phosphorus, ammonia, iron and manganese are greatly influenced by the presence of oxygen in lake water and sediments. Ammonia is a breakdown product of proteins. When little or no oxygen is present at the sediment-water interface, concentrations of ammonia can be quite high. Ammonia is toxic and represents a further threat to aquatic life. Sediments in lakes can contain a lot of iron, manganese and phosphorus. These can be released in large quantities from the bottom of the lake when oxygen levels are very low. Iron and manganese can cause treatment problems in water treatment plants. Phosphorus released in the water fuels yet more plant growth. Bottom sediments can provide a significant source of phosphorus to prairie lakes. Phosphorus bound to the bottom sediments is released to the overlying waters when the water overlying the bottom sediments in a lake become anoxic. During the summer, most deep lakes experience a temperature-density gradient in the lake profile from warmer surface (epilimnion) waters and cooler bottom (hypolimnion) waters. The process of establishing a density gradient in a lake, either in winter or summer, is termed stratification. Phosphorus-rich hypolimnetic water is isolated at the bottom of the lake if the water is anoxic and a temperature-density gradient exists. However, when the temperature gradient disappears during the spring and fall, the entire water column in the lake mixes or 'turns over'. This allows the phosphorus-rich bottom water to circulate to the lake surface where the combination of available nutrients and sunlight facilitates algal growth. An additional source of phosphorus to lake water is the resuspension of sediments. Sediments can release phosphorus when resuspended into the water column of a lake by wind action.
Herbicides, insecticides and other chemical pollutants which enter lakes and rivers will continue to degrade through chemical processes such as photodegradation and biological degradation by microbes. Some toxic substances, however, can also be absorbed and retained by aquatic organisms (bioaccumulation) either directly from the water or consumed with food. Contaminants that are not degraded can be passed on in the food web with each predator accumulating the pollutant of the organisms it had eaten (biomagnification) (Figure 5) (Bioaccumulation and biomagnification of pollutants in the food web (source: Environment Canada. 1991. A primer on water quality: questions and answers. Environment Canada, Conservation and Protection. 67 p.)). Biomagnified toxins or pollutants can result in major problems further up the food web. For example, double-crested cormorants in the Great Lakes basin have had bill deformities from the bioaccumulation of the insecticide DDT (Environment Canada 1995). Further, the mortality of waterfowl and other wild birds has increased because of decreased egg shell strength and the consumption of sport fish has been limited or banned in certain areas due to the bioaccumulation of pollutants. |
|