Eutrophication

• eutrophication → it is the over enrichments of water by nutrients (mainly nitrogen and phosphorous), which causes excessive growth of algae, phytoplankton, and seaweeds.

the paper

• the paper talks about anthropogenically driven eutrophication (ADE)

• what causes eutrophication:

  • agriculture: fertilizers, manure, soil erosion, deforestration

  • livestock production

  • urban wastewater and sewage (many countries treat little or none)

  • urban stormwater runoff

  • industry effluents → waste water form industrial processes

  • aquaculture (uneaten feed and faeces)

  • fossil fuel combustion (NOx deposition)

  • climate change intensifying run off (storms, monsoons, flood)

• the chain reaction of eutrophication

  • increasing nutrients → increasing primary production → algal blooms (micro and macro) → algae die (due to reduced nutrients from over consumption) → bacterial decomposition consumes oxygen → decreasing oxygen → hypoxia → anoxia → dead zones → loss of seagrass, mangroves, corals, fish, benthic organisms → collapse of ecosystem function.

• HAB (harmful algal blooms)

  • ADE often leads to harmful algal blooms, which produce toxins:

    • neurotoxins

    • hepatoxins

    • cytotoxins

    • dermatoxins

  • examples of HABs

    • pseudonitzchia

    • ulva green tides

    • sargassum golden tides

• ecological impacts:

  • corals →

    • nutrients reduce coral calcification

    • this increases coral’s susceptibility to bleaching

    • increase in coral diseases

  • seagrasses →

    • light reduction from algal blooms → seagrass death (prevention of photosynthesis)

    • up to 60% of shoot loss with additional stressors

    • loss of habitat for higher trophic levels

  • fish and invertebrates →

    • hypoxia and HAB toxins → mass mortality

    • shifts in fish distribution

    • aquaculture collapse

  • ecosystem transformation →

    • marine forests are replaced by

      • opportunistic algae

      • free floating algae

      • anaerobic bacteria

• socioeconomic impacts:

  • reduced fisheries and aquaculture

  • tourism decline

  • toxic gases from decaying blooms → health impacts

  • loss of property value and recreational use

• solutions:

  • management/preventative:

    • wastewater treatment

    • improve land-use practives

    • reduce fertilizer use

    • restore mangroves, seagrasses, wetlands

    • marine protected areas

    • integrated land-sea management

  • corrective/remediation:

    • macroalgae cultivation (removes nutrients)

    • algal turf scrubbers

    • multitrophic aquaculture

    • restoration of affected ecosystems

    • adressing HABs directly

  • nature based solution (only applicable when the blooms are less frequent and eutrophication doesnt wipe out the whole area):

    • mangroves

    • seagrasses

    • salt marshes

    • polyculture → a method where multiple crop species are grown together in the same field at the same time

    • multitrophic aquaculture

the lecture slides

potential consequences 

• increased pelagic algal productivity

• changed composition of pelagic community

• increased sedimentation of organic material

• hypoxic or anoxic bottom water and sediment

• increased frequency of harmful algal blooms (HABs)

nutrients

• N → proteins, nucleic acids, energy carriers

• P → nucleic acids, energy carriers

• Si → structural (diatoms)

• Fe → co factor for many enzymes/proteins (including nitrogenase)

• C → all organic molecules (not a nutrient)

limiting nutrients

• stoichiometry, particularly N:P ratio

processes

• know the nitrogen fixation:

  • its when nitrogen from the air is combined with hydrogen to form NH4 so that terrestrial plants can uptake it more easily.

• know the phosphorous cycle

nutrient availability is changing

• increasing use fo fertiliser

• increased animal production

• increased aquaculture production

• sewage form a larger population

• more dams

• climate change

• increased global production of N (no new synthesis of P, but increased mining)

• large geographical differences, atmospheric transfer

N use

global fertiliser use seems to have increased exponentially from the 2010s. before that it was more of a gradual increase of the use of fertiliser

two critical processes

• denitrification:

  • microbial process, reducing nitrate (NO3^-) via nitrite (NO2^-) and nitric oxide to molecular nitrogen (N2)

• nitrogen fixation:

  • microbial conversion of moelcular nitrogen to ammonia (NH4⁺) through enxyme nitrogenase (containing Fe, Mn/V)

nutrient loading

• definition: the excessive input of nutrients into an ecosystem which can cause ecological harm. The nutrients usually are nitrogen and phosphorous

• oligotrophic → usually clear water, low nutrients, high oxygen, usually associated with healthy and balanced ecosystems

• mesotrophic → more nutrients than oligotrophic, but still balanced, moderate algal growth, can risk the formation of blooms if nutrient inputs increase, supports more biomass

• eutrophic → high nutrients, high productivity, frequent algal blooms, water becomes turbid, oxygen level may drop, this is the zone hwere anthropogenic eutrophication often starts

• hypertrophic → extremely high nutrients to a harmful level, severe nutrient pollution, dense algal blooms, possible toxin producing algae, massive oxygen depletion leading to hypoxia and dead zone, the worst case stage of eutrophication

recycling of P

• movement of phosphorous:

  • terrestrial → rivers → coastal → open ocean → sediments → back to surface (or buried)

• phosphorous originates from bedrock, which is weathered down physically or through chemical erosion

• weathering releases phosphate form rocks into soils

• plants take up P; decomposition recycles it; farming and fertilisers add more

• rivers trasnport dissolved and particulate p to the ocean

• in the ocean, algae use P; sinking bioamss carries P to deeper water and sediments

• some P is recycled upward (upwelling), but much is buried in sediments, completing the long term cycle