Hypoxia

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Hypoxia
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Nutrient Enrichment has two main effects

• Hypoxia
• Change of community structure

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Dissolved Oxygen and Hypoxia

• 8 mg/L saturation (note that warmer water can
  hold less dissolved gas)
• lt5 mg/L is a violation of water quality laws
  (depends on the classification of the body of
  water)
• lt4 mg/L some species will avoid a place
• 2 to 0 mg/L hypoxia to anoxia 50 mortality in
  1-4 days

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Effects of Hypoxia

• Under hypoxic/anoxic conditions organisms will
• First increase their breathing rate
• Then they decrease their activity or leave
• Then die
• Some use anaerobic metabolic pathways (oysters)

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Factors influencing Hypoxia

• Severity
• Duration
• Suddenness of Onset (e.g. wind sloshing and the
  Crab Jubilee in the Chesapeake)
• Species type (shellfish, finfish, eggs, etc.)
• Temperature (warm is worse)

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Large increases in N loads can result in anoxia
and fish and invertebrate kills. These get
peoples attention!
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Change of community structure

• Eutrophication can cause greater plankton and
  nuisance algae growth
• Increased phytoplankton growth causes greater
  shade on the bottom
• This can kill off important species of Submerged
  Aquatic Vegetation such as Eel Grass
• Why is SAV important?

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Loss of cover of seagrass meadows in Waquoit Bay
through the end of the 20th century (Modified
from Valiela et al. 1992).
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Altered structure of estuarine producers under
increased nitrogen loads, for estuaries with
different residence times.
From Valiela et al. (2001)
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Exodus 720-21

• and all the waters that were in the river were
  turned to blood. And the fish that was in the
  river died and the river stank, and the
  Egyptians could not drink of the water of the
  river