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  Effects and consequences  
  Nutrient concentrations
  Changes in N:P:Si ratio
  Phytoplankton primary production and biomass
  Microbial loop and the pelagic system
  Light and sedimentation
  Oxygen concentrations
  Seasonal signals
  Submerged aquatic vegetation
  Benthic fauna
  Social consequences
  Conceptual understanding of eutrophication
Figures 1.2
Seasonal variation at the station “Gniben” located in south-western Kattegat. Monthly averages during the period 1998-2001.

Figure 1.2 A
Runoff, salinity and temperature

Figure 1.2 B
N- and P-concentrations

Figure 1.2 C
Primary production

Figure 1.2 D
Chlorophyll a + Carbon-biomass

Figure 1.2 E
Oxygen concentration in bottom water

Figure 1.3
Conceptual model of marine eutrophication with lines indicating interactions between the different ecological compartments. A balanced system in Danish marine waters is supposedly characterised by:
1) A short pelagic food chain (phytoplankton - zooplankton - fish)
2) Natural species compositions of planktonic and benthic organisms
3) A natural distribution of submerged aquatic vegetation
Nutrient enrichment results in changes in the structure and function of marine ecosystems as indicated with bold lines. Dashed lines indicate release of hydrogen sulphide (H2S) and phosphorus, which is positively linked to oxygen depletion. Based on OSPAR 2001 and Rönnberg 2001.

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Effects and consequenses

Overloading with nitrogen, phosphorus and organic matter can result in a series of undesirable effects. The major impacts of eutrophication include changes in the structure and functioning of marine ecosystems and reduction of biodiversity.

Eutrophication as nutrient enrichment means elevated and/or increased trends in inputs of nutrients from land, atmosphere or adjacent seas. And consequently elevated dissolved inorganic nitrogen (DIN) and dissolved inorganic phosphorus (DIP) concentrations during winter.

The optimal DIN:DIP ratio (N/P-ratio) for phytoplankton growth is 16:1 (based on molar concentrations) and called the Redfield ratio. Significant lower deviations of the N/P-ratios indicate potential nitrogen limitation and higher N/P-ratios potential phosphorus limitation of phytoplankton primary production. Deviations from the Redfield ratio limit phytoplankton primary production, and affect phytoplankton biomass, species composition and consequently food web dynamics. Redfield ratios for diatoms for Si:N and Si:P ratios are 1:1 and 16:1, respectively (based on molar concentrations), and the abundance of silicate relative to nitrogen and phosphorus effect the growth of diatoms.

Primary production is most often limited by the availability of light and nutrients. Nutrient enrichment will therefore increase phytoplankton primary production. Consequently there will be an increase in phytoplankton biomass. Elevated phytoplankton production and biomass will increase sedimentation of organic material. Changes in the pelagic ecosystem could enhance the sedimentation. See microbial loop below.

The microbial loop may be enhanced by changes in the species composition and functioning of the pelagic food web when growth of small flagellates rather than diatoms is stimulated. The shift in phytoplankton cell size leads to lower grazing by copepods and possibly an increased sedimentation. The smaller cells will on the other hand increase the relative importance of grazing by ciliates and heterotrophic dinoflagellates. A larger fraction of the primary production is consequently channelled through the protozooplankton before it is available to the copepods. The general responses of pelagic ecosystems to nutrient enrichment can in principle be a gradual change towards:

  1. Increased planktonic primary production compared to benthic primary production.
  2. Microbial food webs dominate versus linear food chains.
  3. Non-siliceous phytoplankton species dominate versus diatom species.
  4. Gelatinous zooplankton (jellyfish) dominate versus crustacean zooplankton.

The Secchi depth, a measure of the turbidity and light penetration in the water column, is negatively affected by chlorophyll. Increased trends in inputs of nutrients increase phytoplankton biomass and reduce the Secchi depth. This decreases the colonisation depths of seagrasses and macroalgae.

Increased animal and bacterial activity at the bottom due to increased amounts of organic matter settling to the bottom increases the total oxygen demand. The increase can lead to oxygen depletion and release of H2S from the sediment. This will induce changes in community structure or death of the benthic fauna. Bottom dwelling fish may either escape or die.

Many of the eutrophication effects as well as some of the driving forces have a pronounced seasonal variation. Freshwater run-off, temperature and salinity have a strong seasonal signal. The same is the case for inorganic nutrient concentrations, phytoplankton primary production, chlorophyll a concentrations, phytoplankton biomass and oxygen concentration in bottom water. The seasonal variations are illustrated in Figure 1.2. This assessment report will, as mentioned in the preface, neither analyse the role of eutrophication on the seasonal succession in these parameters nor the changes in turnover or fluxes of nutrients during spring, summer and autumn.

In the Danish marine areas a significant portion of the primary production during the spring sediments to the sea bottom. An increase in primary production means that the sediment will experience elevated inputs of organic material. This leads to increased bacterial activity, hence an increase in oxygen demand.

Eutrophication in general affects submerged vegetation in two different ways. Reduced light penetration and shadowing effect from phytoplankton can reduce the depth distribution, biomass, composition and species diversity of the plant community. Increased nutrient levels favour opportunistic macroalgae species. The stimulated growth of filamentous and annual nuisance species at the expense of perennials will result in a change in macroalgae community structure with reduced species diversity and reduced nurseries for fish. The dominance of filamentous macroalgae in shallow sheltered areas will increase the risk of local oxygen depletion.

The increased load of organic material to the bottom affects the macrozoobenthic community. The enrichment will enhance growth and increase species diversity and biomass. A change in community structure will follow favouring suspension and burrowing detritus feeders. Reduction in species diversity and biomass will follow at progressively higher levels of organic load, and opportunistic species will be favoured. Oxygen depletion will lead to a further reduction in species diversity, and mass mortality of most organisms, especially due to production of H2S in sediments.

Reductions in dermersal fish and shellfish due to oxygen depletion and harmful algal blooms will reduce harvests. In the case of commercial fisheries these changes have large economic implications. The increased risk of toxic and harmful blooms will also affect mariculture, which can also be influenced by oxygen depletion. Another consequence of toxic algal blooms is the risk of shellfish poisoning of humans by algal toxins. The recreational value of beaches especially for swimming is reduced due to reduced water quality induced by discoloration and foam formation by algal blooms or decaying rotting macroalgae. This could particularly impact tourism at beaches.

Figure 1.3 illustrates in a simplified way the effects and consequences of nutrient enrichment and eutrophication in the marine environment.

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Danish Environmental Protection Agency & National Environmental Research Institute • updated: