Eutrophication: Nitrogen Loading
and the Coastal Ecosystem
Melissa Blakely
Ocean Ecosystems
Summary
Eutrophication is ‘the enrichment of water by nutrients, especially nitrogen and/or
phosphorus, causing an accelerated growth of algae and higher forms of plant life to
produce an undesirable disturbance to the balance of organisms present in the water and
to the quality of water concerned’ (Andersen, et. al, 2006). In other words, it is used to
describe the inputs and effects of nutrients into aquatic systems. There are two types of
eutrophication: natural and anthropogenic. Over geologic timescales coastal systems
throughout the world have at one time or another experienced some eutrophication
(Burkholder, et. al., 2006). The addition of nitrogen into the coastal systems through a
number of anthropogenic sources has increased the eutrophication of the coasts
significantly. The increase of nitrogen causes an increase in primary production forcing a
bottom-up stimulus that is seen throughout the ecosystems of coastal environments. In
turn a cascade up the trophic level instigates vast alterations in the food webs, species
composition, and biodiversity. Furthermore, coastal eutrophication has also been linked
to a number of Harmful Algal Bloom events (Scavia and Bricker, 2006). Some
management practices are in place to discourage eutrophication, but a wide range of
practices worldwide need to be modified before eutrophication of the coast will decrease.
Background/Introduction
In 1907 C. Weber first used the term eutrophication to describe what he was
seeing in the wetlands (Schindler, 2006). In the mid-1970’s eutophication became a
common word to describe processes and interactions that happen when an overload of
nutrients are added to an aquatic system (Burkholder, et. al., 2006). Eutrophication was
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originally studied in lakes and wetland areas because the natural progression of an
enclosed system is to become eutrophied. Due to natural aging lake basins become less
deep and more productive over a geological timescale, thus becoming eutrophied (Smith,
2006). There is also evidence that anthropogenic sources will speed up this process
(Andersen, et. al., 2006; Smith, 2006). Coastal eutrophication does not happen gradually
over time the way it does in enclosed system. They do have natural sources as well as
anthropogenic sources, but have only been looked at in the last 20 years because of
advances in determining biomass through primary productivity (Burkholder, et. al.,
2006).
In the marine environment, coastal eutrophication can be caused by natural events
such as upwelling and storms (Smith, et. al., 2006). Storms cause a mixed environment
to form, bringing nutrients up to the surface where they had been depleted before, and
upwelling brings nutrient cold water to the surface. Seasonally, marine environments
will experience brief periods of eutrophication because of this, usually in the summer or
early fall when phytoplankton blooms happen (Yoshiyama and Sharp, 2006; Bishop, et.
al., 2006; Piehler, et. al., 2002). Unfortunately, eutrophication has been greatly
accelerated due to inputs of nutrients from anthropogenic sources (Burkholder, et. al.,
2006). The main nutrients input into the coastal system are forms of nitrogen and
inorganic phosphorus. These sources enter the ocean through point sources and nonpoint sources. Point sources are can be described as inputs from a known source where
the observer knows where the nutrients have come from and exactly where they enter the
coastal systems (Scavia and Bricker, 2006). These include rivers, sewage pipes, and
waste management plants. Non-point sources most often refer to run-off from land,
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usually from agricultural use. The macronutrients phosphate, carbon, and nitrogen enter
the coastal ocean through these sources. Nearly two-thirds of the coastal waters along the
United States are moderately or severely eutrophied from an increase of nitrogen addition
(Howarth and Marino, 2006).
Eutrophication has wide-ranging consequences for ecosystem level changes in the
coastal marine environment. This is true especially along the coast where biodiversity is
high and small changes in the primary producers can have a ripple effect and cause large
changes in the structure of higher trophic levels. It may cause an increase in the rates of
primary productivity, changes in algal plant biomass, shifts in species compositions,
destruction of corals and sea grasses, death of fish and shellfish populations, decreases in
biodiversity, oxygen depleted bottom waters, and an increase in Harmful Algal Blooms
(Smith 2006; Howarth and Marino, 2006; Bishop, et. al., 2006, Scavia and Bricker,
2006).
Discussion
As described above, eutrophication is more than just a definition of an event. It
actually illustrates the effects that happen when large amounts of nutrient addition take
place. The cause of eutrophication is an influx of nutrients into the coastal ocean system.
The response to this is an increase in primary production because of the input of nutrients
which causes a cascade of effects to higher trophic levels. These effects have long
reaching complications on delicate coastal environments.
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Sources of Nutrients
There is a debate in the oceanography community as to whether phosphate or
nitrogen is the limiting nutrient for the marine environment (Burkholder, et. al., 2006).
Whether it is one or the other, evidence suggests that when either is in abundance primary
production will increase. Freshwater eutrophication became a hot top in the 1970’s when
scientists began noticing the increase of eutrophicated waters due to anthropogenic
sources (Howarth and Marino, 2006). Freshwater such as lakes and estuaries are
commonly phosphate limited (Smith, et. al., 2006). Research has implied that in the
coastal environment nitrogen rather than phosphorus is the controller for eutrophication
in coastal environments (Fig. 1) (Howarth and Marino, 2006).
Fig. 1. Adapted from Howarth and Marino, 2006. Time trend of nutrient enrichment into Laholm Bay in
southwestern Sweden from 1950 to 1988. Phosphorus loads decreased in response to management for
eutrophication. Nitrogen loads, which were unregulated, continued to increase.
During natural cycles, nitrate and other nutrients are brought up from deep depths
to the surface, either through winter storm mixing or upwelling. Subsequently when
physical conditions are correct this will cause acceleration in the growth of primary
producers (Andersen, et. al., 2006). In coastal waters, due to anthropogenic sources,
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species of nitrogen are the influential cause for an increase in phytoplankton biomass.
The average flux of nitrogen to the coastal environment in the United States has increased
by four to ten times in the last 20 years (Fig. 2) (Howarth and Marino, 2006).
Fig. 2. Adapted from Howarth and Marino, 2006. The global rate of creation of reactive nitrogen from
fertilizer and fossil fuel combustion.
Nitrogen is flushed into the coastal ocean by a number of ways. Most fertilizers
have shifted from phosphate based to a nitrogen source (Glibert, et. al., 2006). Along
coastal areas there are increased populations of humans, expanded land development and
an increase in agriculture (Scavia and Bricker, 2006). Areas with agriculture show a
much greater increase in nitrogen loading into the coastal environment. Nitrogen enters
the system mainly thru runoff, or non-point sources, and waste management plants.
Furthermore, there has been a change in the species of nitrogen in fertilizer and other
nitrogen input sources (Glibert, et. al., 2006). Urea is often seen in the coastal
environment and sometimes preferentially assimilated by phytoplankton species (Glibert,
et. al.; Kudela, et. al., unpublished data). In fertilizers, urea has increased from making
up 5% of fertilizers to almost 90% and often is removed from the soil as runoff. It is
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responsible for vast increases in primary production along coastal ecosystems (Glibert, et.
al., 2006).
Influence on Primary Producers
There is a direct correlation between biomass increase of primary producers and
nitrogen input into coastal waters (Fig. 3) (Smith, 2006). Primary producers, when not
limited by nutrients or other physical sources will grow until they are limited. Reports of
Chl a amounts, a proxy for biomass of primary productivity, have been recorded as
greater than 300 ug/L during eutrophic events (Piehler, et. al., 2002). High levels of
primary producers cause large amounts of submerged aquatic vegetation die offs because
of the rain of organic matter that collects on the bottom of the eutrophied region. At only
20 ug/L of Chl a a shift in the community structure from a diverse mixture to a
monoculture can be seen (Scavia and Bricker, 2006). In systems that show
eutrophication they are usually bottom-up controlled and the nutrient addition will
determine how long the system stays that way.
Fig. 3. Adapted from Smith, 2006. Relationship between annual mean Chl a and annual mean total
nitrogen concentrations in estuarine and coastal marine ecosystems.
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Ecosystem Responses
There are many responses to coastal eutrophication that affect the ecosystem. The
increase of organic matter that is seen when nitrogen input is increased can cause benthic
hypoxia or anoxia (Bishop, et. al., 2006). Often this hypoxia is seasonal. Almost all
shallow coastal areas have experienced periodic seasonal hypoxia at one time (Smith, et.
al., 2006). Along the Danish coast and Finnish seas there has been hypoxic or anoxic
events recorded with inputs of nitrogen into the system (Cartesen, et. al. 2006). More
than thirty different coastal areas in the United States experience seasonal hypoxic events
(Scavia and Bricker, 2006). For example, the Neuse River Basin and its corresponding
coastal ocean areas have shown bottom water hypoxic events every summer since the
1970’s. The hypoxic event usually lasts longer than ten days and kills benthic
invertebrates as well as pelagic fish. It also has a negative effect on migrating ducks that
depend on the benthic organisms and fish for food (Bishop, et. al., 2006; Piehler, et. al.,
2002). These events cause widespread food web alterations.
Not only the top predator trophic levels have to adjust to changes due to
eutrophication. Nutrient loading causes changes to the food web that start with first level
zooplankton heterotrophs (Olsen, et. al., 2006). In a European study which used short
term (less than three weeks) scales to study the planktonic food web its response to
eutrophication was considered. Olsen, et. al. found that on a short term scale
environments that are copepod dominated cannot exert enough top-down restraint to
control a bloom. The phytoplankton increased and the system toward the end of the
study had a decrease in copepods and an increase in flagellates. Furthermore, in other
studies the initial increase in phytoplankton were the picoplankton and bacteria which
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was then followed by nanoflagellates which was a change from the diatom dominated
system that was in place before the addition of nutrients (Lagus, et. al., 2007).
Food web alteration also involves species composition changes and a loss of
biodiversity. While an increase in nutrients will multiply primary producers, there is a
sizeable loss of biodiversity throughout the trophic levels (Vidjak, et. al., 2006). First
level heterotrophs shift either from a larger zooplankton size to flagellates or become
dominated by one species of copepods (Olsen, et. al., 2006). Fish larvae are also severely
limited in biodiversity, and over time the adult fish will match the juvenile species.
Although primary producers tend to grow exponentially, they too are decreased in
biodiversity (Vidjak, et. al., 2006).
Biodiversity is decreased in eutrophic coastal areas. Besides the monocultures
that exist, there is an affinity for primary producers in coastal areas with large amounts of
nutrients to become Harmful Algal Blooms (Scavia and Bricker, 2006). A Harmful Algal
Bloom just needs to be detrimental to higher trophic levels such as invertebrates or fish.
When primary producers are recorded in such high amounts, they can cause any number
of events (such as light depletion, hypoxic environments, and toxins) which harm fish,
invertebrates, and other epiphytes (Scavia and Bricker, 2006, LaRoche, et. al., 1997).
There has also been some evidence that eutrophication of coastal waters influences
diatom dominated systems to a species composition of dinoflagellates (Smith, 2006).
Dinoflagellates are responsible for most of the toxin producing species of Harmful Algal
Blooms.
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Policy
There is no set worldwide or nationwide policies that deal with eutrophication.
Many are local only. Since the 1970’s lawmakers have been working with scientists to
create a set of guidelines which will alert the public to eutrophication events. In the last
forty years management policy has evolved from random studies of algal growth to trying
to control elements which cause eutrophication (Schindler, 2006). Government
assessments have come up with procedures to test for primary production and oxygen
depleted bottom water as signs that eutrophy is occurring (Scavia and Bricker, 2006).
Nitrogen needs to be reduced in agricultural and other input sources first before
eutrophication events will stop happening. In a study by Cartesen, et. al., they compared
data over the last two decades at a site that had recorded eutrophication. The Danish
government decided to reduce their nitrogen input by 50% into coastal systems, and the
trend is encouraging. Biodiversity increased, hypoxic events decreased, and an overall
reduction of dissolved nitrogen of 48% in the coastal system (Cartesen, et. al., 2006).
Conclusion
Nitrogen loading is forcing eutrophication of coastal marine environments
throughout the world. While some nitrogen is from natural sources almost all is from
anthropogenic influences. Eutrophication of these coastal systems creates havoc on
delicate ecosystems by depleting biodiversity, causing fish kills, and possibly Harmful
Algal Blooms. Well over thirty coastal systems in the United States have some
eutrophication or are in danger of becoming so. There is no set worldwide or nationwide
policies that deal with eutrophication. When nitrogen inputs are reduced, the adverse
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effects of eutrophication are also decreased and the ecological status of the coastal oceans
will improve. The goal for the 21st century is to reduce nitrogen loading into the coastal
ecosystems because only then will eutrophication begin to decline and the coastal marine
system begin to thrive again.
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