recap

57.2 recap

Fixed carbon, be it from photosynthesis or chemosynthesis, is used to measure primary production. Net primary production is the amount of biomass incorporated into the tissues of primary producers after respiration, and it varies with latitude and ecosystem type. On land, NPP is highest in the tropics where temperatures are high year-round and rainfall is abundant. In the oceans, NPP peaks along continental margins and in areas of upwelling, where the main primary producers—phytoplankton and algae—have access to light and nutrients. Nutrient runoff from agriculture and wastewater can cause eutrophication in aquatic systems, resulting in algal blooms and deadly low oxygen conditions (hypoxia).

learning outcomes

You should be able to:

  • Analyze, with examples, the variation in NPP by latitude and biome in terrestrial and ocean ecosystems.

  • Describe the FACE experiment and evaluate, with examples, how results of such experiments might shed light on the relationship between atmospheric CO2 concentrations and plant production.

  • Describe the process, causes, and effects of eutrophication in aquatic ecosystems, and compare the importance of various nutrients in this process.

Question 1

Consider Figure 57.4. How does the average net primary productivity differ between the open ocean and algal beds and coral reefs? Why, then, is the percent of Earth’s NPP so much higher for the open ocean?

The open ocean has very low average net primary productivity (–100 g/m2/yr), compared with algal beds and coral reefs (2,500 g/m2/yr). However, because there is so much more open ocean (nearly 70% of Earth’s surface) than there are algal beds and coral reefs (only 0.1% of Earth’s surface), open ocean accounts for a much higher percent of Earth’s NPP.

Question 2

Atmospheric CO2 concentrations are rising at unprecedented rates. Is it reasonable to assume that primary producers on land and in the oceans will be able to appreciably lower global CO2 concentrations? What evidence is there to support your answer?

No, primary producers will not be able to lower global CO2 appreciably. CO2-enrichment experiments show that primary producers can increase their NPP under increased CO2 concentration, but there is a limit. That limit is set by other factors, such as nutrients, light, and water. In addition, the effects may be somewhat counteracted by respiration of CO2 by consumers feeding on primary producers.

Question 3

Describe the process of eutrophication and name the nutrient most responsible for this process in lake systems.

Eutrophication can, but does not always, result in the explosive growth of algae, which can severely deplete oxygen (hypoxia) when they decompose. The level of hypoxia is driven by the amount of algae subject to decomposition and the level of oxygen available in the system.

Once the energy captured by primary producers is fixed as NPP, consumption by heterotrophs transfers a fraction of that energy to higher trophic levels. Next we will consider in more detail the factors important to the movement and loss of energy between trophic levels.