Lec 18: Marine Biogeochemical Cycles I

• Major components of living organisms

  • C
  • O
  • H
  • N
• Macronutrients


  • Nearly all of other 1% of mass of living things
• Atoms of major components


  Combine to make carbohydrates, fats, proteins, nucleic acids (DNA)
• Common to all living things
• Micronutrients


  Present in very small quantities but necessary for life
• Unity, not diversity, is the central message of biology
• Biogeochemical cycles


  Chemical parts of matter cycled through ecosystem
• Living organisms are sustained by huge, non-living (inorganic) chemical reserves
• Large-scale transport of elements between reserves and the organisms
• Sometimes the environment contains enough of a required element to sustain life; sometimes it is in short supply (limited)
• Detritus


  Dead remains and waste products
• The elements and the small molecules forming the tissues of an organism are always on the move
• They may cycle rapidly in and out of living things or they may be stored in the deep ocean or Earth for great spans of time
• The nature of the cycles dictate what will live where, which organisms will be successful and ultimately what the composition of the ocean and atmosphere itself will be

  
• Carbon Cycle


  Earth's largest cycle

  
• Carbon (C)

  Basic building block of life on Earth
• Carbon Cycle

  1. Centers the atmosphere as carbon dioxide (CO2) by the respiration of living organisms, volcanic eruptions, the burning of fossil fuels, etc.

  2. Photosynthesizes capture sunlight and use this energy to incorporate (or fix) CO2 into organic molecules → food or structural components

  
• When an animal eats a plant

  1. It can be incorporated into the animal's body for growth (~45% used for growth → e.g. CaCO3 skeletons)

  2. It can be respired by the animal (taken apart to harvest energy) (~45% used for respiration → CO2 to atmosphere)

  3. It can be wasted, excreted back into the seawater as dissolved organic carbon (DOC) (~10% loss as DOC → Microbial loop)
• Carbon Cycle

  1. Eventually the organisms– or their hard parts (CaCO3) – sink below the mixed layer in the ocean and fall to the seafloor

  2. Most of the carbon in CaCO3 is turned into CO2 by heterotrophic bacteria before it hits bottom

  3. Small amount (<1%) reaches the sediments where it is buried

  4. Over geological time the carbonate sediments can be uplifted and weathered so that C is eventually returned to the biologically active upper ocean

  
• Biological Pump


  The way in which material is removed from the euphotic zone to the seafloor – "pumps" carbon dioxide and nutrients from the upper ocean and concentrates them in the deep ocean and sea floor sediments

  
• Because of large amounts of CO2 available in the ocean and because atmospheric CO2 readily dissolves in seawater, marine organisms almost never suffer from deficient C
• Bottlenecks to marine life lie mainly in the nitrogen (N), phosphorus (P) and iron (Fe) cycles
• DIC

  Dissolved inorganic carbon
• DOC

  Dissolved organic carbon
• POC

  Particulate organic carbon
• Dissolved

  Pass through a filter
• Photosynthesis

  Produces oxygen

  
• Respiration


  Consumes oxygen
• Surface O2 concentrations are close to equilibrium with the atmosphere

  
• O2 concentrations in the water column decrease with depth as oxygen is consumed in respiration

  
• Most organic matter is consumed in upper 1000m, below which O2 concentrations increase again
• Deep ocean has higher O2 concentrations because rates of O2 consumption are low and there is a supply of cold, oxygen-rich waters from polar regions

  
• Eutrophication


  Artificial enrichment of waters by previously scarce nutrient

  
• Causes of eutrophication

  • Sewage
  • Fertilizer
  • Animal waste
• Can cause algal blooms
• Large areas of ocean eutrophication associated with extensive hypoxia (oxygen-poor water)

  
• Creates "dead zones" in the ocean that most higher order marine organisms cannot tolerate
• Associated with mouths of major rivers and spring runoffs
• Suffocates bottom dwellers
• Global dead zones – significant increase since 1960s