Hypoxia: Causes and Consequences of Low-Oxygen Marine Environments

1. Hypoxia: Causes and Consequences of Low-Oxygen Marine Environments Outline • Key terms and introduction • Major factors controlling hypoxia • Regional case studies • Impacts and Review

2. 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

3. Global Importance of Hypoxia • 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 Hugo Ahlenius, UNEP/GRID-Arendal

4. Major Components of Hypoxia • 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 Longislandsoundstudy.net

5. 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

6. Nutrient Inputs Primary cause of global rise in hypoxia is agricultural nutrients

7. 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

8. Geological influences on water mixing Fjords.com • 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 effectif flushing is reduced Institute of Marine Science Norway Sverdrup et al 1942

9. Oxygen Solubility • 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 Kimberley Schulz

10. 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

11. Phytoplankton directly affect oxygen levels • Phytoplankton oxygenate waters through photosynthesis • Phytoplankton respiration (just like bacteria) also consumes oxygen • Daily cycle that matches light cycle Marinespecies.org http://ux.brookdalecc.edu/staff/ sandyhook/taxonomy Marinebio.net

12. … but indirect effect of phytoplankton is greater • 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) Ocean.si.edu NOAA

13. 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

14. 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. Balticseanow.fi Oz.coasts.au Balticseanow.fi

15. Major Components of Hypoxia • 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 Longislandsoundstudy.net

16. 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 http://oceantoday.noaa.gov/happnowdeadzone/ Serc.carleton.edu J. Bartlet, UVM

17. Scale of Mississippi River dead zone compared to Alabama 2011 Dead Zone (one of largest ever) Current Dead Zone area

18. 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

19. Black Sea – Largest hypoxic zone in the world • 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

20. Hypoxia on the Grand Strand • 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! McCoy et al. 2011

21. 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

22. 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

23. 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 • Maintaining natural filters • Wetlands • Dissipating river outputs • Agricultural nutrient reduction • Run off buffers on farms • Fertilization methods • Human Development • Impermeable surfaces • Waste water treatment Less of this Less of this More of this!