Lecture 9: Ricklefs 516-525

Chapter 24 pgs 516-525 notes
-most cycling of elements take place in aqueous mediums
- in bodies of water, organic matter sinks to the bottom and accumulates, and the nutrients are returned to zones of productivity only very slowly
- primary productivity is highest in shallow seas or in areas of strong upwelling currents
- winds supply most of the energy to mix shallow water and currents
- a thermocline can be established do to temperature difference (due to heating of the sun at the surface) or when fresh water mixes with salt water and the water density creates stratification
- although vertical mixing brings nutrient-rich water to the photic zone, it can also carry organisms below the photic zone where they cannot survive
- stratification causes anoxic condition in the bottom of lakes, and elements switch from oxidized to reduced forms and the nutrients accumulate
- lakes are classified from oligotrophic (poorly nourished) to eutrophic (well-nourished)
- phosphorus is the most important element in determining the fertility of most lakes
- sewage and agricultural drainage add nutrients increasing primary production - but often decomposers consume oxygen faster than it is regenerated by photosynthesis and as a result aerobic organisms often suffocate
- some of the most productive ecosystems are shallow water marine ones
- hypoxia – the depletion of oxygen to the extent that aquatic organisms can no longer survive. This is caused by excess amounts of nutrients such as N and P in estuaries. Rises in primary producer biomass are followed by rises in the population of bacterial decomposers. This surge in heterotrophic respiration removes much of the oxygen from the system, and as such, the areas become inhospitable to many species of fish and invertibrates.
- hypoxic zones have formed in most of the world’s estuaries
- some places in the ocean have low productivity due not to limiting nitrogen or phosphorus, but other elements such as iron and silicon
- Redfield Ratio is a 106:16:1 C:N:P ratio

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