The oceans may be divided into large biomes, or living regions (Figure 1). These zones are based on the distribution of marine organisms. The two major environments are the pelagic, which consists of the water column, and the benthic, which comprises the ocean bottom. The organisms that live in these zones can be classified in terms of the habitat they occupy.

Figure 1. Classification of life zones in the oceans.

The factors that affect primary productivity are (1) the availability of light, (2) the availability of nutrients and (3) the rate of grazing by primary consumers (herbivores).

Figure 2. Seasonal variations in nutrient elements, plankton biomass and light for a mid-latitude oceanic region

Questions

Answer the following questions in terms of physical and/or chemical factors that might explain, or partially explain, various divisions in the Figure 1 classification.

1. The division between the epipelagic and mesopelagic zones.
2. The division between the mesopelagic and bathypelagic zones. This boundary is about 1000 meters deep, and it is a level at which many physical and chemical changes occur.
3. The division between the supralittoral (above high tide) and littoral zones.
4. The division below the sublittoral zone.
5. Why is there no boundary at about 1000 meters in the benthic environment but a significant one at about the same depth in the pelagic environment? Think of what the main control on the benthic organisms might be that the pelagic organisms would not have to contend with, and vice versa.

Answer the following questions using Figure 2.

1. Why do the dissolved nutrients drop in the spring?
2. Why does the spring phytoplankton bloom start in the spring and die out in the early summer?
3. Why is there a difference in the steepness of the zooplankton biomass curves during the spring bloom?
4. What are some possible reasons for a fall phytoplankton bloom?
5. Where on earth would the plankton show a different seasonal pattern of growth and why?