Lecture 17 Ricklefs Notes, Pages 516-521

(Headings taken from Ricklef Book)

In aquatic systems, nutrients are regenerated slowly in deep water sediments
Terrestrial and aquatic nutrient cycling systems are not very different in chemical and biological processes, but the main difference between the two is that organic matter tends to fall to the bottom of aquatic systems where it is:
1. Separated from aquatic plants and algae, and
2. Falls into an environment that lacks oxygen
Productivity is high in aquatic systems where bottom sediments are close to the photic zone (for example in shallow lakes), or where there is some mechanism that can bring nutrients back to the photic zone.
To some extent, excretion and decomposition of microbes in the photic zone can regenerate some nutrients.
Sedimentaion of nutrients is major process in aquatic systems.

Stratification hinders nutrient cycling in aquatic systems
Vertical mixing requires some energy, which is often wind.
A thermocline acts as a barrier for vertical mixing
However, salinity, ice formation, and large changes in temperature can encourage vertical mixing
Vertical mixing can have two effects:
1. Bring nutrients to the photic zone, which promotes production
2. Carry phytoplankton below photic zone, which decreases production
A typical pattern that occurs in temperate lakes is a thermocline is established in the summer, decreasing production towards the end of the season and then stratification breaks down in the fall, increasing nutrient mixing.
In high and low latitudes, a thermocline tends not to form, but stratification may develop in tropical areas
Similar concepts apply to marine environments, but the relationships are more complex

Oxygen depletion facilitates regeneration of nutrients in deep waters
If stratification occurs over long periods of time in lakes, bacteria may use up all the oxygen and begin to respire with sulfate, increasing concentrations of reduced sulfur.
Nutrients can still accumulate in this anaerobic environment because bacteria are not able to nitrify ammonium and elements have increased solubility as they shift to their oxidized forms

Nutrient inputs control production in freshwater and shallow-water marine systems
Aquatic environments can be classified in many ways based on nutrients and productivity, but there are two main classifications:
1. Oligotrophic (low amounts of nutrients)
2. Eutrophic (high amounts of nutrients)
Overproduction of organic matter within an aquatic system can negatively effect it, since it can lead to oxygen being used up at a faster rate than it can be produced by photosynthesis, therefore creating conditions of oxygen depletion.

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