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

Nutrient InputsPrimary cause of global rise in hypoxia is agricultural nutrients

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