Eutrophication

Eutrophication is an important process involving enrichment of water by excess nutrients. It can cause serious problems in the coastal zone through disturbance of ecological balances and fisheries, and through interference with recreational activities and quality of life.

What is eutrophication about?

Cyanobacteria bloom, Western Baltic, 1997

• It’s about increased productivity (conversion of light and carbon dioxide into living organic matter – a process being limited by nitrogen and/or phosphorus) and unacceptable ecological effects as algal blooms, oxygen depletion, kills of benthic animals and fish

• It’s caused by increased inputs of nutrients from point sources, activities in the upstream catchment (e.g. losses from agriculture) and atmospheric deposition.

What are we really talking about?

Eutrophication 

“eu” = “well” or “good”

“trope” = “nourishment”

But is “eutrophication” good?

• In general: NO … it is actually ”bad” …
• Too many nutrients in wrong places may cause problems and result in changes in structure, function and stability of the marine ecosystems

• Eutrophication is ”too much of a good thing”

Some definitions:

Noctiluca milaris bloom, German Bight, 2000

Eutrophication 

An increase in the supply of organic matter

A condition in an aquatic ecosystem where high nutrient concentrations stimulate growth of algae which leads to imbalanced functioning of the system.

Alternative proposal 

The enrichment of water by nutrients, especially nitrogen and/or phosphorus and organic matter, causing an increased growth of algae and higher forms of plant life to produce an unacceptable deviation in structure, function and stability of organisms present in the water and to the quality of water concerned, compared to reference conditions

Effects of Eutrophication

Effects of eutrophication on marine ecosystems are well known:

• algal blooms resulting in green water
• reduced depth distribution of submerged aquatic vegetation
• increased growth of nuisance macroalgae
• increased sedimentation, increased oxygen consumption
• oxygen depletion in bottom water, and
• sometimes dead benthic animals and fish.

Eutrophication schematic. Source:HELCOM, 2006

Major effects include structure and function changes in the entire marine ecosystem and a reduction in stability. Responses to increased nutrient inputs:

1. Corresponding increase in nutrient concentrations
2. Change in ratio between dissolved nitrogen and phosphorus in the water: DIN:DIP ratio. Optimal is 16:1 – called the Redfield ratio.

• significantly lower: potential nitrogen limitation;
• higher N/P ratio: phosphorus limitation of phytoplankton primary production

Primary production:

• usually limited by availability of light and nutrients
  • nutrient enrichment increase phytoplankton primary production
  • which increases biomass -- decrease light penetration through water column
  • (light penetration measured by Secchi depth)
• Decreased Secchi depth can reduce colonization depth of macroalgae and seagrasses

Responses to nutrient enrichment (pelagic ecosystems) involve a gradual change towards:

1. Increased planktonic primary production compared to benthic production
2. Dominance of microbial food webs over linear planktonic food chains
3. Dominance of non-siliceous phytoplankton species over diatom species
4. Dominance of gelatinous zooplankton (jellyfish) over crustacean zooplankton

Eutrophication issues:

1. Causative factors

• inputs, elevated nutrient concentrations, Redfield ratio changes

1. Direct effects

• primary producers, namely:
  • Phytoplankton
  • Submerged aquatic vegetation

1. Indirect effects (secondary effects)

• related to:
  • Zooplankton
  • Fish
  • Invertebrate benthic fauna (animals living on seafloor)

Phytoplankton

• are at base of pelagic food webs in aquatic systems
• generation times: from less that a few days
• respond rapidly to nutrient concentration changes

Often quantified in terms of:

1. Primary production
2. Biomass (chlorophyll-a concentration, or carbon biomass)
3. Bloom frequency

Submerged aquatic vegetation:

Are affected by eutrophication through:

1. Reduced light penetration and shadowing effects from phytoplankton can reduce the depth distribution, biomass, composition and species diversity
2. increased growth of filamentous and short lived nuisance macroalgae at cost of long lived species lead to a change in structure of macroalgae community with reduced diversity

• seagrass meadows and perennial macroalgae are important nursery areas for coastal fish populations
• short-lived (annual) nuisance macroalgae favoured by large nutrient inputs

Oxygen depletion:

• “hypoxia”: a common effect of eutrophication in bottom waters

May be:

1. Episodic
2. Occur annually (most common in summer/autumn)
3. Persistent
4. Periodic in the coastal zone

• Lethally low oxygen concentrations depend on the species.
• Fish and crustaceans have higher oxygen requirements; other speices lower.
• Hypoxic and anoxic (no oxygen) conditions may results in formation and releast of hydrogen sulphide (H₂S) – lethal to organisms
• Anoxic periods cause release of phophorus from sediments. Dissolved inorganic phosphorus (DIP)
• Ammonium released under hypoxic conditions
• DIP and ammonium in water column can enhance algal blooms
• Predicted effect of global warming is to increase hypoxia with increased temperature. A 4 degree temperature increase is projected to results in a doubling of hypoxia in some parts of North Sea.
• eg eelgrass responds to low oxygen concentrations, and dies off (often in combination with high temperatures)

Invertebrate benthic fauna:

• can cope with oxygen depletion to varying degrees (days – month)
• if O₂ drops below zero and H₂S is released all organisms killed immediately
• mobile benthic invertebrates in sediment move to surface when O₂ decreases
  • increased catches of fish and crustaceans
• Difficult to predict when animals will return after eutrophication events
• Size of area plays a factor: small areas are recolonised and re-established more quickly

Climate change

• seas are important in element cycling – carbon and nitrogen cycle; phosphorus and silicate cycle
• ocean still takes up more carbon than it releases – depositing some in sediments

European Coastal Areas

The main source of nitrogen to European coastal waters is agricultural runoff discharged into the sea via rivers, identified as originating from sources of ammonia evaporation in animal husbandry and partly from fossil fuel combustion in traffic, industry and households. For phosphorus the major sources are treated and untreated discharges to the sea from households and industry as well as soil erosion.

Baltic Sea in focus

Eutrophication seriously affects the Baltic sea marine environment, resulting in algal blooms, reduced water clarity, oxygen reduction and death of bottom animals. The causes behind this are well known: discharges, losses and emissions of nitrogen and phosphorus to the aquatic environment. Reductions of discharges from municipal wastewater treatment plants and industries have been in focus for many years as have losses and emissions of nitrogen compounds from agriculture and traffic.

Solutions

Nutrient inputs must be reduced to levels that do not put at risk target values for mitigation of eutrophication. Integrated management strategies should enable characterization of all pressures on water bodies in order to develop a coherent approach to deal with the pressures in a cost effective manner.

EU Directives:

• EC Urban Waster Water Treatment Directive
• EC Nitrates Directive
• EU Water Framework Directive
• Marine Strategy Directive

See also

Wikipedia: Eutrophication article

External links

• Baltic Sea Parlimentary Conference
• BERNET: Baltic Eutrophication Regional Network
• BONUS for the future of the Baltic Sea
• European Environment Agency
• HELCOM
• HELCOM Indicator fact sheets: water exchange, winter nutrient concentrations, water clarity, algal blooms, chlorophyll-a concentrations, hydrography and oxygen in the deep basins
• MARE Research program on Baltic Sea environmental issues
• National Environment Research Institute (DK) Aquatic page
• Nutrients and Eutrophication in Danish Marine Waters
• OSPAR For the protection of the marine environment of the north-east Atlantic
• The Water Forecast
• WWF Baltic Ecoregion Programme

References

Authorship 02/01/2007, Jesper Andersen (jha@dhigroup.dk), DHI Water Environment Health.

(Caitlin 09:31, 18 January 2007 (Romance Standard Time))