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Biogeochemical Mineral Cycles
  • Mineralization: transforming organic substances back into inorganic forms
    • Occurs in the water column, on sediments and in sediments
    • Different elements have different time scales of mineralization
  • Oxic mineralization: heterotrophic bacteria decompose organic material and produce CO2 and mineral nutrients
  • Anoxic mineralization: occurs in anoxic water (Black, Baltic Sea) and sediments; anaerobic bacteria utilize oxygen in SO4 and NO3 ions and produce highly reduced products: methane (CH4), hydrogen (H2), H2S, NH4
  • Chemosynthesis: anaerobic, chemoautotrophic bacteria can utilize these reduced but energy-rich substrates to reduce CO2 and form organic biomass
  • Importance: Rate at which essential, limiting nutrients are recycled
  • Limiting nutrients: can be nitrate (nitrogen), phosphate, iron (bioavailable iron!), silicate (for diatoms, silicoflagellates, radiolaria)
  • Si-Cycle: simplest, only inorganic forms; organism include Si in their skeletons, which is dissolved after their death; silica frustle of diatoms is covered by organic matrix, which first has to be degraded by bacteria
The Phosphorus Cycle:
  • Relatively simple and fast cycle: at the usual alkaline pH of seawater, organic phosphates are easily hydrolyzed to inorganic phosphate
  • Alkaline phosphatase: enzyme of algae and bacteria that helps converting organic phosphate into inorganic phosphate, which is subsequently taken up; can occur intra- or extracellularly 
  • Limitation: most marine systems are less P, more N limited; freshwater and coastal areas can be severely P-limited, though.
The Nitrogen Cycle
  • Complex because nitrogen occurs in different inorganic (NH4, NO3, NO2, N2) and organic (PON, DON) forms that are mostly biologically transformed
  • Dominant forms: nitrate, taken up by phytoplankton; ammonia, produced by bacteria, protozoa, zooplankton and consumed by bacteria and phytoplankton
  • Limiting nutrient in most marine systems for phytoplankton production
  • Nitrification: conversion of NH4 to NO2 to NO3 by nitrifying bacteria; uses oxygen!
  • Denitrification: conversion of NO3 to NO2 to N2 by bacteria; releases oxygen but net loss of nitrogen to water column!
  • Nitrogen fixation: conversion of atmospheric N2 into organic nitrogen by bacteria and cyanobacteria

The concept of „new“ and „regenerated“ primary production 

  • Concept: Refers to the origin of nitrogen used by phytoplankton, basically NO3 vs. NH4; some nitrogen originates from recycling by bacteria, the microbial food web or zooplankton (NH4), other nitrogen originates from inputs from deeper water layers, from land/river, or the atmosphere
  • „New Production“: relies on nitrate from upwelling, river/land, atmospheric input
  • „Regenerated Production“: relies on NH4 recycled within the euphotic zone
  • Nitrogen fixation: fixation of atmospheric N2 is new production!
  • f-ratio: ratio of new production to total production:

  • f = NP / (NP + RP) = NP / P
    f-ratio ranges from 0.1 in oligotrophic, open ocean waters to 0.8 in upwelling areas
  • Sedimentation of PON ~ f-ratio !

The Marine Carbonate System

  • Carbon dioxide: enters the ocean from atmosphere; steady exchange at surface
  • Solubility: in water is low! But CO2 reacts with water to form ions: carbonate (CO32-) and bicarbonate (HCO3-)
  • Most of marine CO2 is stored as bicarbonate Therefore, CO2 is never limiting photosynthesis in seawater, but it is in freshwater, because the lower pH of freshwater prevents transformation into bicarbonate 
  • Respiration: CO2 is added, reacts with water:

  • H2O + CO2 = HCO3- + H+; [H+] rises, which means pH sinks!
  • Primary production: CO2 is removed, HCO3- converts to free CO2 to keep the chemical balance; [H+] falls, which means pH rises!
    The Carbon Cycle

  • Carbon: The basic currency unit in ecology; it is tran ferred from inorganic to organic forms and back by biological processes
  • Photosynthesis and Respiration are the major pathways to transform forms of C
  • Calcification: Some marine organisms combine calcium with bicarbonate ions to make calcareous shells or skeletons
  • CO2 balance of calcification: Calcification produces CO2 !!!
    Ca2+ + 2 HCO3-  =  CaCO3 + H2O + CO2 
    Oceanic blooms of coccolithophorids and production of coral reefs  DO NOT help decreasing the atmospheric increase in CO2