Hypoxia

Nutrient Enrichment has two main effects

1. Hypoxia 2. Change of community structure

Dissolved Oxygen and Hypoxia

• ~8 mg/L = saturation (note that warmer water can hold less dissolved gas)

• <5 mg/L is a violation of water quality laws (depends on the classification of the body of water)

• <4 mg/L some species will avoid a place• 2 to 0 mg/L = hypoxia to anoxia: 50%

mortality in 1-4 days

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)

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)

Large increases in N loads can result in anoxia and fish and invertebrate kills.

These get people’s attention!

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?

Loss of cover of seagrass meadows in Waquoit Bay through the end of the 20th century (Modified from

Valiela et al. 1992).

Altered structure of estuarine producers under increased nitrogen loads, for estuaries with different residence times.

From Valiela et al. (2001)

0

40

80

120

160

200

Nitrogen load (10 3 kg N y -1)

1965

1971

1980

19851990

15 18 21 24 27

Sca

llops

har

vest

ed (

103 L

y-1)

Exodus 7:20-21

…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…”