Freshwater systems One response to added amounts of nutrients in
aquatic ecosystems is the rapid growth of microscopic algae, creating an
algal bloom. In
freshwater ecosystems, the formation of floating algal blooms are commonly nitrogen-fixing
cyanobacteria (blue-green algae). This outcome is favored when soluble nitrogen becomes limiting and phosphorus inputs remain significant.
Nutrient pollution is a major cause of algal blooms and excess growth of other aquatic plants leading to overcrowding competition for sunlight, space, and oxygen. Increased competition for the added nutrients can cause potential disruption to entire ecosystems and food webs, as well as a loss of habitat, and biodiversity of species. When overproduced
macrophytes and algae die in eutrophic water, their decompose further consumes dissolved oxygen. The depleted oxygen levels in turn may lead to
fish kills and a range of other effects reducing biodiversity. Nutrients may become concentrated in an anoxic zone, often in deeper waters cut off by stratification of the water column and may only be made available again during autumn turn-over in temperate areas or in conditions of turbulent flow. The dead algae and organic load carried by the water inflows into a lake settle to the bottom and undergo
anaerobic digestion releasing
greenhouse gases such as methane and CO2. Some of the methane gas may be oxidised by anaerobic
methane oxidation bacteria such as
Methylococcus capsulatus, which in turn may provide a food source for
zooplankton. Thus a self-sustaining biological process can take place to generate
primary food source for the
phytoplankton and zooplankton depending on the availability of adequate dissolved oxygen in the water body. Enhanced growth of aquatic vegetation, phytoplankton and algal blooms disrupts normal functioning of the ecosystem, causing a variety of problems such as a lack of
oxygen which is needed for fish and
shellfish to survive. The growth of dense algae in surface waters can shade the deeper water and reduce the viability of benthic shelter plants with resultant impacts on the wider ecosystem. Eutrophication also decreases the value of rivers, lakes and aesthetic enjoyment. Health problems can occur where
eutrophic conditions interfere with drinking
water treatment.
Phosphorus is often regarded as the main culprit in cases of eutrophication in lakes subjected to "point source" pollution from sewage pipes. The concentration of algae and the
trophic state of lakes correspond well to phosphorus levels in water. Studies conducted in the Experimental Lakes Area in Ontario have shown a relationship between the addition of phosphorus and the rate of eutrophication. Later stages of eutrophication lead to blooms of nitrogen-fixing cyanobacteria limited solely by the phosphorus concentration. Phosphorus-base eutrophication in fresh water lakes has been addressed in several cases.
Coastal waters IMAGE-Map of measured Gulf hypoxia zone, July 25-31, 2021-LUMCON-NOAA.png|Map of measured Gulf hypoxia zone, July 25–31, 2021, LUMCON-NOAA UNESCO global ocean deoxygenation map.png|Oxygen minimum zones (OMZs) (blue) and areas with coastal hypoxia (red) in the world's ocean
Estuaries, as the interface between freshwater and saltwater, can be both phosphorus and nitrogen limited and commonly exhibit symptoms of eutrophication. Eutrophication in estuaries often results in bottom water hypoxia or anoxia, leading to fish kills and habitat degradation. Upwelling in coastal systems also promotes increased productivity by conveying deep, nutrient-rich waters to the surface, where the nutrients can be assimilated by
algae. Examples of anthropogenic sources of nitrogen-rich pollution to coastal waters include sea cage
fish farming and discharges of
ammonia from the production of
coke from coal. In addition to runoff from land, wastes from fish farming and industrial ammonia discharges, atmospheric
fixed nitrogen can be an important nutrient source in the open ocean. This could account for around one third of the ocean's external (non-recycled) nitrogen supply, and up to 3% of the annual new marine biological production. Coastal waters embrace a wide range of
marine habitats from enclosed
estuaries to the
open waters of the continental shelf. Phytoplankton productivity in coastal waters depends on both nutrient and light supply, with the latter an important limiting factor in waters near to shore where sediment resuspension often limits light penetration. Nutrients are supplied to coastal waters from land via river and groundwater and also via the atmosphere. There is also an important source from the open ocean, via mixing of relatively nutrient rich deep ocean waters. Nutrient inputs from the ocean are little changed by human activity, although
climate change may alter the water flows across the shelf break. By contrast, inputs from land to coastal zones of the nutrients nitrogen and phosphorus have been increased by human activity globally. The extent of increases varies greatly from place to place depending on human activities in the catchments. A third key nutrient, dissolved
silicon, is derived primarily from sediment
weathering to rivers and from offshore and is therefore much less affected by human activity.
Effects of coastal eutrophication These increasing nitrogen and phosphorus nutrient inputs exert eutrophication pressures on coastal zones. These pressures vary geographically depending on the catchment activities and associated nutrient load. The geographical setting of the coastal zone is another important factor as it controls dilution of the nutrient load and oxygen exchange with the atmosphere. The effects of these eutrophication pressures can be seen in several different ways: • There is evidence from
satellite monitoring that the amounts of
chlorophyll as a measure of overall
phytoplankton activity are increasing in many coastal areas worldwide due to increased nutrient inputs. • The phytoplankton
species composition may change due to increased nutrient loadings and changes in the proportions of key nutrients. In particular the increases in nitrogen and phosphorus inputs, along with much smaller changes in silicon inputs, create changes in the ratio of nitrogen and phosphorus to silicon. These changing nutrient ratios drive changes in phytoplankton species composition, particularly disadvantaging silica rich phytoplankton species like diatoms compared to other species. (see also
OSPAR Convention) and the
Black Sea. In some cases nutrient enrichment can lead to
harmful algal blooms (HABs). Such blooms can occur naturally, but there is good evidence that these are increasing as a result of nutrient enrichment, although the causal linkage between nutrient enrichment and HABs is not straightforward. •
Oxygen depletion has existed in some coastal seas such as the
Baltic for thousands of years. In such areas the density structure of the water column severely restricts water column mixing and associated oxygenation of deep water. However, increases in the inputs of bacterially degradable organic matter to such isolated deep waters can exacerbate such
oxygen depletion in oceans. These areas of lower dissolved oxygen have increased globally in recent decades. They are usually connected with nutrient enrichment and resulting algal blooms. Climate change will generally tend to increase water column stratification and so exacerbate this oxygen depletion problem. An example of such coastal oxygen depletion is in the
Gulf of Mexico where an area of seasonal anoxia more than 5000 square miles in area has developed since the 1950s. The increased primary production driving this anoxia is fueled by nutrients supplied by the
Mississippi river. A similar process has been documented in the Black Sea. == Extent of the problem ==