Eutrophication in coastal environments

Eutrophication in coastal environments

Eutrophication involves the 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. Eutrophication is the result of an anthropogenically induced alteration of the global nitrogen cycle, and just like climate change, should be regarded as a "global change". Eutrophication is usually treated scientifically and in terms of management as a local and regional phenomenon. Coastal regions throughout the world and Europe are affected by eutrophication.

What is eutrophication about?

Fig. 1. Cyanobacteria bloom, Western Baltic, 1997

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

Fig. 2. Noctiluca milaris bloom, German Bight, 2000

But is “eutrophication” good?

• In general: NO … it is actually ”bad” …
• Too many nutrients in the 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”

Effects of Eutrophication

The different processes and effects of coastal eutrophication are well documented and it has been considered as one of the biggest threats to marine ecosystem health for decades .

Fig. 3. Eutrophication flow diagram. Source: HELCOM, 2006

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.

Fig. 4. Eutrophication schematic. Source: HELCOM, 2006

General effects

Major effects of eutrophication include structure and function changes in the entire marine ecosystem and a reduction in stability. The following are 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 ratio causes potential nitrogen limitation; while a higher ratio leads to phosphorus limitation of phytoplankton primary production.

Primary production is usually limited by availability of light and nutrients. Nutrient enrichment increase phytoplankton primary production, which increases biomass, which decreases light penetration through water column. Light penetration is measured by Secchi depth - a 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

Finally, eutrophication issues are often divided into three groups:

1. Causative factors: inputs, elevated nutrient concentrations, Redfield ratio changes
2. Direct effects: primary producers, namely phytoplankton and submerged aquatic vegetation
3. Indirect effects (secondary effects): related to zooplankton, fish and ínvertebrate benthic fauna (animals living on seafloor).

Primary and Secondary Effects

Some important primary and secondary effects are discussed in the sections below.

Phytoplankton

Phytoplankton are at base of pelagic food webs in aquatic systems, have generation times from less that a few days, respond rapidly to nutrient concentration changes, and are often quantified in terms of:

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

Submerged aquatic vegetation

Submerged aquatic venetation are affected by eutrophication through:

1. Reduced light penetration and shadowing effects from phytoplankton can reduce the depth distribution, biomass, composition and species diversity; and
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

Additionally,

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

Oxygen depletion

Oxygen depletion, or hypoxia, is a common effect of eutrophication in bottom waters. This effect may be episodic, occuring annually (most common in summer/autumn), persistent, or 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), which is lethal to organisms.
• Anoxic periods cause release of phophorus from sediments - dissolved inorganic phosphorus (DIP), and ammonium is released under hypoxic conditions. DIP and ammonium in water column can enhance algal blooms.
• The 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.
• An example of the effect: Eelgrass responds to low oxygen concentrations, and dies off under these conditions (often in combination with high temperatures)

Invertebrate benthic fauna

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 - there are increased catches of fish and crustaceans during these times. It is difficult to predict when animals will return after eutrophication events. The area affected 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

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.

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.

Within Europe, regional seas such as the Baltic and Mediterranean Seas are currently adversely affected by eutrophication, with climate change expected to intensify these adverse impacts. As well as monitoring fresh water impacts on coastal areas, it will be important to monitor impacts between seas such as the Mediterranean and Black Seas. For example, the Black Sea is strongly eutrophic, and enters the Mediterranean Sea at the North Aegean near the borders of Greece and Turkey.

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.

More global approaches were considered in meetings such as the International Symposium on Research and Management of Eutrophication in Coastal Ecosystems from June 20 to 23, 2006 in Nyborg, Denmark. This meeting included a keynote speaker, a working seminar, produced some outcomes,and led to the creation of an European group to address the issue of climate change and eutrophication.

Causes in Baltic Sea

Human-mediated nutrient enrichment in the Baltic Sea can be caused by input of nutrients in form of:

1. Direct inputs from point sources (sewage treatment plants, industries)
2. Atmospheric deposition
3. Riverine inputs (from activities in the catchment: eg point sources, agricultural losses, atmospheric deposition, natural background losses (natural erosion and leakage of nutrients from areas without much human activities) and stream, river and lake retention)

Waterborne: Agriculture forestry, scattered dwellings, municipanlities, industries, natural background losses.

Airborne: Nitrogen compounds emitted to atmosphere:

• Nitrogen oxides: road transportation, energy combustion, shipping
• Ammonia emissions: mostly from agriculture.
• Distant sources

The role of agriculture in nitrogen inputs: The main source of nitrogen inputs in Baltic Sea is agricultural discharge via rivers, deriving from:

1. Soil cultivation
2. Fertiliser use
3. Use of manure
4. Intensive and uncontrolled agriculture

Aspects of Eutrophication problem in the Baltic sea

• Excessive phytoplankton blooms are major problem – especially of blue-green algae. There are commonly summertime algal blooms in most parts of Gulf of Finland, Gulf of Riga, the Baltic Proper and south-western parts of Baltic Sea

Problems caused:

• bathing people can hardly see their feet
• blue-green algae potentially toxic to humans and animals
• large mats of drifting algae deposited along shores and decay

Baltic Sea Solutions

The following steps are suggested

1. Establish overall goals and target values
2. Implement relevant measures directly linked to fulfillment of these overall goals and targets
3. Carry out monitoring
4. Conduct assessments
5. Evaluate whether the goals and targets have been fulfilled or not

Main drivers:

• European Directives (see links below)
• Decisions and recommendations adopted by HELCOM
• National action plans

EU Directives:

EC Urban Waster Water Treatment Directive

EC Nitrates Directive

EU Water Framework Directive

Marine Strategy Directive

See also

Theme 4 - Pollution

Water quality/pollution

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

Wikipedia: Eutrophication article

WWF Baltic Ecoregion Programme

References

Nutrients and Eutrophication in the Baltic Sea - Effects, Causes, Solutions (HELCOM) - main reference for this article