BIO3082 Lecture Notes - Lecture 20: Trophic State Index, Eutrophication, Species Richness
Lecture 20 – Eutrophication
Causes of Eutrophication
• Caused by excess nutrient additions
• Eutrophication: ecological effects of addition of excess nutrients to
environments
o Anthropogenic additions of excess nitrogen and phosphorous
o Affects grassland, freshwater and coastal ecosystems
• Nitrogen and phosphorous co-limit most ecosystems
o N and P – key limiting elements that determine phototroph
productivity, diversity and composition
▪ Both: increased spike in productivity
• Humans have increased agricultural productivity by fourfold by fixing N2 via
Haber-Bosch process
o 120 million tonnes of N2 into fixed nitrogen each year
o Humans mined large amounts of phosphorous and converted into
phosphate fertilisers → increased biological phosphorous cycling
How Eutrophication Affects Grassland Productivity and Richness
• Increased productivity of agricultural systems – in short-term
• Reduction of grassland plant species richness
• Change in physiochemical parameters and microbial biodiversity of native
soils
• Runoff into lakes, rivers and coastlines → algal
blooms and dead zones
• Major emissions source of greenhouse gas nitrous
oxide
• Eutrophication decreases species richness of
grassland
o Inverse relationship between grassland
species richness and total inorganic nitrogen
deposition (of 68 British Grasslands)
o Grasslands with more inorganic nitrogen
have fewer number of species
Multifaceted Reasons for Richness Decline *EXAM
Increased Competition
Increased Synchrony
Increased Toxicity
• Eutrophic soils favour growth of
fast growers (r strategists) at
slow growers (k strategists)
o Increases competition for
light: canopy layer
restricting light
transmitted to understory
• Species in natural grasslands are
adapted to low nutrient regimes
– perform better under high
amounts of nitrogen (fast
growers) → take away available
resources from k strategists
• In undisturbed
ecosystems,
asynchronous growth of
different species
maintains productivity
and diversity in eutrophic
ecosystems
o More temporal
variation in
productivity
o Greater synchrony
of species
• Species die off → another
one will take over
• Overall productivity of
ecosystem is stable →
increase biodiversity
• Eutrophication causes
long term
physiochemical
changes in soils
• Ammonium fertilisers
promote soil
acidification and
phosphorous fertilisers
harbour toxic cadmium
contaminants
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Document Summary
Haber-bosch process: 120 million tonnes of n2 into fixed nitrogen each year, humans mined large amounts of phosphorous and converted into phosphate fertilisers increased biological phosphorous cycling. Increased toxicity: eutrophic soils favour growth of fast growers (r strategists) at slow growers (k strategists, increases competition for light: canopy layer restricting light transmitted to understory, species in natural grasslands are adapted to low nutrient regimes. Perform better under high amounts of nitrogen (fast growers) take away available resources from k strategists. Eutrophication can facilitate invasions: weed species are competitive in eutrophic soils. Invasive exotic species nassella trichotoma outcompetes multiple native grasses at high nitrogen loads. Exam long answer: eutrophication has been shown to decrease species richness in a range of ecosystems. Describe the main ways that eutrophication can cause species declines. Support your answer with examples from both terrestrial and aquatic ecosystems. (10 marks)
