Lec 19: Marine Biogeochemical Cycles II

• Nitrogen (N)


  Critical component of proteins, chlorophyll, and nucleic acids
• Dissolved gas (N2) is abundant in seawater

  
• Most organisms cannot use free N2 in the atmosphere and oceans directly
• Nitrogen fixation


  Binding nitrogen with oxygen and hydrogen to make it useable to most organisms

  
• Nitrogen fixation is accomplished by specialized bacteria / cyanobacteria

  
• Oceanic regions are often N limited - i.e. growth of primary producers is held back by a lack of N
• Nitrate runoff from soil helps make coastal ecosystems more productive than the open ocean
• Forms of nitrogen available for uptake by living organisms

  • Ammonium (NH4+)
  • Nitrate (NO3-)
  • Nitrite (NO2-)

  
• N2 gas is not directly useable by biological organisms
• Nitrogen sources to the surface ocean

  • Rivers
  • Upwelling from the deep ocean
  • Deposition
  • Nitrogen fixation

  
• In the deep ocean, most nitrogen is in the form of nitrate (NO3-)

  
• Upwelling regions bring up deep ocean NO3- which yields higher production in these regions
• Nitrogen is lost from the ocean in anoxic sediments and low-oxygen regions where denitrifying bacteria respire nitrate to N2, and when nitrogen containing organisms and debris are buried in ocean sediments

  
• The North Atlantic is a site of deep water formation, which brings oxygen-rich surface water to the deep ocean

  Atlantic: More dissolved O2, but lower nutrient concentrations than the Pacific

  
• As deep water spreads across ocean basins, oxygen is removed through respiration and nutrients are added via decomposition

  
• Phosphorus (P)


  Used by all organisms to link the parts of nucleic acids and in molecules that carry energy within the cell
• Calcium Phosphate


  Used in the formation of bones, teeth and some shells
• Silica (Silicon dioxide)


  Used by several marine organism groups (diatoms, coccolithophores radiolarians) to make their skeletons

  
• Phosphorus and Silica Cycles

  1. Enter ocean in rivers and precipitation (rock weathering)

  2. Used by organisms at ocean surface

  3. P is released in organic form when organisms die → decomposers convert it back to inorganic form (phosphate) → available to be re-used by phytoplankton and bacteria

  4. Silica in shells is released when organisms die and is available to be re-used as ionized, dissolved form, silicate

  
• Water column profiles for nitrate, phosphate, and silicate are similar

  
• Phosphorus and Silica Cycles operate in 3 loops

  • Rapid recycling occurs in daily feeding, death, and decay of surface organisms
  • Slower loop occurs as bodies fall below the pycnocline and P and S escape downward into deep-ocean circulation.

  -A few hundred years may pass before the P or S is upwelled and again available in the

  sunlit surface waters where primary producers can take it up

  • Longest loop begins with P or S locked into rocks or shells that become marine sediments → sediment is subducted → P and S re-enter ocean system through volcanos

  -Could take millions of years

  
• Iron (Fe)


  Used in very small quantities in reactions of photosynthesis, certain enzymes crucial to nitrogen fixation and in proteins
• Other essential trace metals like zinc, copper, manganese are also used by organisms in small quantities mainly in enzymes
• Organisms don't need much Fe, but its concentration in the ocean compared to N and P, can sometimes be so low that phytoplankton growth is limited by Fe availability
• Fe is among most abundant elements in Earth's crust but nearly insoluble in oxygenated seawater
• The little dissolved Fe that is present is highly reactive, sticking to particles and sinking to the bottom of the water column
• Biogeochemical cycling of trace metals

  • Uptake and recycling in the surface ocean and regeneration (sometimes over long time periods) at depth
  • Differences between ocean basins
• Redfield Ratio


  • Ratio of major elements observed in the tissues of algae when macro-nutrients are not limiting productivity
  • C:N:P = 106:16:1
  • Named for A.C. Redfield, who first tried to quantify these stoichiometric ratios
  • Ratio is observed in zooplankton that feed on diatoms and most ocean water samples worldwide
• If nutrients are not available at necessary concentration it will limit productivity
• Generally, nitrate is more limiting than phosphate

  
• Limiting factor


  A physical or biological necessity whose presence in inappropriate amounts (too large or too small) limits the normal action of the organism
• Typical limiting factors in the oceans

  • Light
  • Temperature
  • Dissolved Nutrients
  • Salinity
  • Dissolved Gases
  • Acid-Base Balance
  • Hydrostatic Pressure
• In some regions of the ocean, photosynthetic production is relatively low even though the concentration of major nutrients is high (i.e. close to Redfield)
• Micronutrient (e.g. iron) limitation and grazing pressures reduces primary production below its maximum potential in High Nutrient Low Chlorophyll (HNLC) regions
• Large regions of the ocean are limited in major nutrients, but exceptions are regions of upwelling that provide a flow of deep water to the surface
• Cooler, deeper seawater is nutrient-rich, and areas of coastal upwelling are sites of high productivity
• Productivity values range from 1 to 4000 g C/m2/year, with the lowest in open ocean and highest in estuaries and coral reef ecosystems
• 90% of biomass from the euphotic zone decomposes before descending
• Differences in regional productivity are due to uneven distribution of nutrients and changes in availability of sunlight
• In the tropics, the intense thermocline prevents nutrient-rich water from rising to the surface and productivity is low throughout the year