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
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.
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
- Increased planktonic primary production compared to benthic primary
- Microbial food webs dominate versus linear food chains.
- Non-siliceous phytoplankton species dominate versus diatom species.
- 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
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.