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Hypoxia: Causes and Consequences of Low-Oxygen Marine Environments
I. Key terms and introduction
II. Major factors controlling hypoxia
III. Regional case studies
IV. Impacts and Review
Outline
Key Terms and Definitions• Hypoxia = low oxygen, in this
case low dissolved oxygen in aquatic environments
• Anoxia = Absence of oxygen
• How little is “low”• Depends on location• Good levels in most marine
water columns = 5-8 mg/L • Long-term exposure to <5
mg/L is harmful to larvae of many animals• Acute effects vary along
range VIMS
VIMS
Global Importance of Hypoxia
Hugo Ahlenius, UNEP/GRID-Arendal
• A natural long-term or temporary condition in many marine environments
• Number of marine systems experiencing hypoxia and severity has increased greatly due to human impacts.
• Consequences = sporadic fish kills, degradation or loss of benthic invertebrate communities, decreas in overall fisheries production (finfish and shellfish)
Major ecological and
economic losses
Major Components of Hypoxia
Longislandsoundstudy.net
• Nutrient inputs • Phytoplankton growth• Bacterial growth (direct and
indirect)• Increase is main cause of
global rise in hypoxic zones
• Physical Processes• Isolation of bottom waters• Change in oxygen solubility
with temperature
• Organic Carbon• Phytoplankton production• River or runoff inputs
• Oxygen Consumption• Biological oxygen demand• Redox chemistry
Nutrient Inputs• Nutrients (nitrogen, phosphorus, silica, iron)
required for phytoplankton growth
• Phytoplankton-made carbon is main source for oxygen-consuming bacteria
• Most hypoxic zones are nutrient-rich estuaries
• Global nutrient inputs to ocean have increased due to human activity (eutrophication)• Agricultural fertilizer• Sewage• Industrial waste• Impermeable surfaces• Loss of natural filters like forests and
wetlands
Physical Processes• Bottom waters must be isolated from mixing with atmosphere or from
inflows of oxygen-rich water to become hypoxic
• Stratification is the main internal process• Water column is divided vertically into layers of different density with
limited mixing between each• Thermal Stratification – occurs seasonally in many waters, solar
heating• Haline Stratification – Occurs in many esturaies, river inputs form
layer of less dense freshwater over more dense seawater
Geological influences on water mixing
Sverdrup et al 1942
Institute of Marine Science Norway
• Geomorphology also greatly affects hypoxia
• Formations like sills in fjords restrict flushing from oxygenated waters, enhance stratification
• Many fjords have anoxic bottom waters
• Alterations of river flows can produce similar effect if flushing is reduced
Fjords.com
Oxygen Solubility
Kimberley Schulz
• Inherent properties of gas in liquid – solubility decreases with temperature
• Seasonal cycle in dissolved oxygen due to temperature alone
• This and thermal stratification is why most hypoxia occurs in the summer
Physics also relieves hypoxia
• Lateral mixing with oxygenated ocean water or river flow can relieve hypoxia
• Complicated because river flow also causes hypoxia in some systems
• Wind-driven mixing is a major that reduces hypoxia
• Sometimes, only extreme wind events (ex: hurricanes) are sufficient
• Thermal inversion relieves hypoxia seasonally
Bioap.wikispaces.com
Phytoplankton directly affect oxygen levels
http://ux.brookdalecc.edu/staff/sandyhook/taxonomy
Marinespecies.org
Marinebio.net
• Phytoplankton oxygenate waters through photosynthesis
• Phytoplankton respiration (just like bacteria) also consumes oxygen
• Daily cycle that matches light cycle
… but indirect effect of phytoplankton is greater
Ocean.si.eduNOAA
• Aerobic bacteria consume oxygen during respiration, utilize organic carbon
• Several important sources of organic carbon• Dead/sinking phytoplankton, other detritus• Particulate and dissolved organics from river/wetland discharge• Dissolved organic carbon from groundwater
• Respiration increases with temperature (same for phytoplankton)
Chemical consumption of oxygen• Hypoxia is usually attributed to biological oxygen consumption
• Reduce compounds (rich in electrons) easily react with dissolved oxygen (greedy for electrons)
• Reduced minerals from runoff or anoxic sediments will consume oxygen when mixed with oxygenated water. Can cause rapid/temporary hypoxia
Positive feedbacks with hypoxia• Chemical demand for oxygen and reduced minerals can cause positive
feedbacks (vicious cycle where a problem makes itself worse)
• Reduced iron and sulfide from anoxic sediments will consume oxygen
• Anoxic sediments may release H2S at toxic levels to fish, invertebrates
• Oxidized iron and sulfur bind up phosphorus and other nutrients, but release them when anoxic
• Nutrients released from anoxic sediments can lead to more phytoplankton, more organic carbon, more hypoxia.
Oz.coasts.auBalticseanow.fiBalticseanow.fi
Major Components of Hypoxia
Longislandsoundstudy.net
• Nutrient inputs • Phytoplankton growth• Bacterial growth (direct and
indirect)• Increase is main cause of
global rise in hypoxic zones
• Physical Processes• Isolation of bottom waters• Change in oxygen solubility
with temperature
• Organic Carbon• Phytoplankton production• River or runoff inputs
• Oxygen Consumption• Biological oxygen demand• Redox chemistry
http://oceantoday.noaa.gov/happnowdeadzone/
Mississippi River Dead Zone• World’s 4th largest drainage basin
• In addition to agriculture, channelization has increased eutrophication, loss of coastal wetlands (positive feedbacks again)
• Seafood industry worth $2.8 billion
• 470 million pounds of seafood lost annually
Serc.carleton.edu
J. Bartlet, UVM
Scale of Mississippi River dead zone compared to Alabama
Current Dead Zone area
2011 Dead Zone (one of largest ever)
Black Sea – Largest hypoxic zone in the world
• Naturally anoxic bottom layer ~2000m thick
• Origin debated – once a freshwater lake, then flooded by Caspian and Mediterranean Seas after last ice age
• Also has expanding hypoxia problems near estuaries due to development
• Bottom waters fed by saline water from Mediterranean
• Surface waters fed by Danube and Dnieper Rivers
• Inflow from Bosphorus strait, wind mixing to weak to break stratification
Black Sea – Largest hypoxic zone in the world
Hypoxia on the Grand Strand
McCoy et al. 2011
• Hypoxia in Long Bay, SC was a mystery• No big rivers• No land masses to restrict water
flow• No massive blooms to fuel
hypoxia
• Problem is more geology and chemistry, less biology
• Organic carbon/nutrient-rich groundwater feeds a hypoxic water mass from below
• Dense upwelled water traps hypoxic water against shoreline
• Lesson: Hypoxia is complicated!
Impacts of Hypoxia
• Hypoxia is a complex process the relies mainly on high nutrient inputs and physical processes such as stratification
• Phytoplankton and bacteria dynamics, sediment chemistry regulate hypoxia
• Hypoxia may cause decline to already-dwindling fisheries, but true effect is debated (eutrophication may enhance some fisheries)
• Loss of benthic ecosystems in hypoxic zones is a major global ecological impact
• Hypoxia is expanding due to human activity
Coping with hypoxia
• Reminder: Hypoxic zones have occurred naturally, inherent aspect of many ecosystems
• Most of the hypoxia formations are highly complex• A wide variety of nutrient sources for eutrophication• Variation in wind, tides, river flows• Phytoplankton communities are incredibly diverse and
unstable
• Can’t directly alter hypoxia on a large scale, prevention is the only real solution
Hypoxia Prevention?• “An ounce of prevention is worth a pound of
cure” – especially true for HABs
• Addressing the key causes• Eutrophication• Climate Change
• Reducing eutrophication seems most likely to happen and most effective
1. Maintaining natural filters• Wetlands• Dissipating river outputs
2. Agricultural nutrient reduction • Run off buffers on farms• Fertilization methods
3. Human Development• Impermeable surfaces• Waste water treatment
Less of this
More of this!
Less of this