Question

1. study the map showing terrestrial and ocean productivity (2008): (a) in general, where is ocean productivity the lowest? (b) in general, where is ocean productivity the highest? 5. (a) are tropical open - ocean waters generally more or less productive than temperate and polar waters? (b) use what you have learned about ocean stratification in activity 12 to explain why. 6. study the light and nutrient profiles above left and explain why maximum productivity occurs at around 90 m depth. 7. (a) the graph (right) shows primary productivity in the oceans. describe and explain the shape of the curves: (b) about 90% of all marine life lives in the photic zone (the depth to which light penetrates). suggest why this is so.

Explanation:

1. (a) By observing the global ocean and terrestrial productivity map, ocean productivity is lowest in the central - gyre regions of the major ocean basins as they are oligotrophic areas with low nutrient availability.
2. (b) Ocean productivity is highest along the continental margins and upwelling zones. Continental margins have runoff of nutrients from land and upwelling zones bring nutrient - rich deep waters to the surface.
3. (a) Tropical open - ocean waters are generally less productive than temperate and polar waters.
4. (b) In tropical open - ocean waters, there is strong thermal stratification. The warm surface layer acts as a barrier, preventing the mixing of nutrient - rich deep waters with the surface waters. In temperate and polar regions, seasonal changes can break the stratification, allowing nutrient mixing and higher productivity.
5. At around 90 m depth, there is a balance between light availability and nutrient supply. Light is still sufficient for photosynthesis (since blue light can penetrate to this depth) and nutrients are available from deeper water mixing or from the breakdown of organic matter above, promoting maximum productivity.
6. (a) The curves show a peak in primary productivity at a certain depth (around 50 - 100 m) and then a decline with increasing depth. This is because at shallower depths, although light is abundant, nutrient supply might be limited. At deeper depths, light becomes limiting for photosynthesis.
7. (b) 90% of marine life lives in the photic zone because it is the zone where light is available for photosynthesis. Photosynthesis forms the base of the marine food web, providing energy and organic matter for other organisms. Also, oxygen production from photosynthesis supports aerobic life forms.

Answer:

1. (a) Central - gyre regions of major ocean basins.
2. (b) Along continental margins and upwelling zones.
3. (a) Less productive.
4. (b) Strong thermal stratification in tropical open - ocean waters prevents nutrient mixing.
5. There is a balance between light availability and nutrient supply at around 90 m depth.
6. (a) There is a peak at a certain depth (50 - 100 m) and then a decline. Shallow depths may lack nutrients and deeper depths lack light.
7. (b) Light is available for photosynthesis and oxygen is produced, forming the base of the food web and supporting aerobic life.