• 
• Japanese
• 
• IETC's Focal Area
• Waste Management
• Publications & Tools
• Meetings & Workshops
• Global Partnership on Waste Management
• Background
• Objectives
  & Expected Outcomes
• Framework
• Sub-focal Areas
• Information Platform
• Meetings
• Participation / Membership
• Water and Sanitation
• Publications & Tools
• Meetings & Workshops
• Resources
• Publications & Tools
• About IETC
• Background and History
• Director
• Activity Reports
• Supporting Foundations
• Internship
• Contact Us

Limnological Background

Physical processes determine the extent of stratification and mixing in lakes, which, in turn, determine ecosystem structure and function, and ecosystem responses to enrichment. Based on vertical density profiles, limnologists divide lakes and reservoirs into an epilimnion (upper mixing layer), a metalimnion (region with a strong gradient in density), and a hypolimnion (region below the metalimnion).

Flushing rate can have a significant influence on the responses of a lake to enrichment. Reservoirs and floodplain lakes can experience especially strong riverine flushing, at least in certain seasons. Shallow lakes with inflows and outflows can flush rapidly. Conversely, lakes that exchange water via seepage or those with large volumes have much longer residence times. While inflows often supply nutrients that enhance eutrophication, rapid flushing can reduce the time available for plant growth and result in less accumulation of biomass.

Biotic communities in lakes can be divided into those in the open water (pelagic region), those in deep-water sediments (profundal zone), and those in near-shore habitats (littoral zone). Responses to eutrophication vary among these areas, and physical processes and movements of organisms link the three regions. Pelagic organisms include phytoplankton, zooplankton, free-living and particle-attached bacteria, and fish. The biota inhabiting the profundal sediments includes a wide variety of invertebrates and microbes, and their abundance and species composition is influenced strongly by the extent to which the sediments are oxygenated or anoxic (oxygen-free). Emergent, submerged and floating vascular plants often are conspicuous in the littoral zone. These plants provide habitat for attached animals, algae and bacteria, and for free swimming fish and invertebrates.

Limiting Factors

Light and nutrients determine the growth of algae and aquatic vascular plants. Therefore, if these resources are in short supply, they can be considered limiting factors for plant growth. Although one factor seldom consistently limits plant growth under the varying conditions prevalent in aquatic ecosystems, dominant control, at a particular time and place, often can be attributed to a single factor.

Light availability plays a key role in the development of submerged aquatic vascular plants, which are usually rooted and can access sediments for nutrients. Hence, waters made turbid by suspended sediments or algal blooms, or shaded by floating aquatic plants are not conducive to the growth of submerged, aquatic vascular plants. In contrast, floating plants are well positioned to receive sunlight, and derive inorganic nitrogen and phosphorus from the water.

Phytoplankton abundance and species composition change as a function of the supply rate of nutrients and underwater light conditions. Some species of cyanobacteria, an algal group known to produce noxious conditions, can regulate their buoyancy and often become common as turbidity increases.

External Loading to Lakes

Rivers and streams are major routes of transfer of nitrogen and phosphorus to many lakes and reservoirs, and they integrate the various point and non-point sources of nitrogen and phosphorus within their watersheds. The mining of phosphate, the industrial fixation of nitrogen, and agricultural, industrial and domestic uses of nitrogen and phosphorus have increased during the last few decades. Other activities of modern societies, such as forest clearing, extensive cultivation and urban waste disposal, have enhanced the transport of nitrogen and phosphorus from terrestrial to aquatic environments. While point and non-point sources of nitrogen and phosphorus contribute to eutrophication, non-point sources often are dominant and present complex management challenges.

Atmospheric deposition via rain, snow and aerosols is an increasingly important external source of nutrients to lakes and reservoirs. Major sources of nitrogen to the atmosphere include burning of fossil fuels and forests, operation of internal combustion engines, and volatilization from feed lots and fertilized fields. Augmented phosphorus deposition can originate from phosphorus-rich soil particles on fertilized and cultivated agricultural fields.

Internal Recycling

Internal recycling of nitrogen and phosphorus from sediments of lakes and reservoirs can sustain eutrophic conditions for long periods even if external loading is reduced. Empirical studies and models that incorporate biogeochemical and physical processes are usually employed to evaluate the likelihood that internal recycling will compensate for lowered external inputs. Shallow, warm water lakes with a history of receiving nutrient-rich inflows are especially likely to maintain high rates of internal recycling.

Organic matter settles to the bottom and decomposes by aerobic or anaerobic processes. Decomposing organic matter reduces oxygen concentrations and can lead to or maintain anoxic conditions. Nitrogen and phosphorus release from the sediments to the overlying water is often increased under anoxic conditions.

Table of Contents

• Brochure
• 
• 



• International Year of Forests
• 
• 



• World Environment Day
• 
• 



• UNEP Campaign
• 
•