Productivity & Nutrient Dynamics in Oceans (HL) (DP IB Environmental Systems & Societies (ESS))

Productivity & Nutrient Dynamics in Oceans

Thermal stratification and nutrient mixing

• Thermal stratification:

  • In many parts of the ocean, the water forms layers based on temperature, with warmer water on the surface and colder water below

    • This is known as thermal stratification

  • When the water layers are strongly separated, nutrients from the deeper, colder water cannot mix into the surface layers

    • This limits productivity

  • In temperate regions, thermal stratification can vary with the seasons

    • In summer, surface waters warm up and form a clear thermal layer, but in winter, cooling of surface waters allows vertical mixing and nutrient exchange

• Nutrient mixing:

  • Nutrients that are essential for phytoplankton growth often sink to deeper waters

  • Mixing processes, such as wind-driven currents and seasonal overturning, bring these nutrients back to the surface

    • Vertical mixing occurs most often in the spring and autumn in temperate regions

    • During these seasons the water column is more uniform in temperature, allowing for the upward movement of nutrients

Limiting factors for aquatic productivity

• Light:

  • Light is crucial for photosynthesis

  • Its availability decreases with depth

  • This limits productivity to the photic zone (the upper layer of the ocean where sunlight penetrates)

  • Turbidity can also reduce the amount of light reaching phytoplankton

    • This can limit productivity even in shallow areas

• Temperature:

  • Warmer waters generally increase metabolic rates and the speed of nutrient cycling

    • This can boost productivity

  • However, if waters are too warm, it can lead to thermal stratification

    • This can prevent nutrient mixing and reduce overall productivity

• Nutrients:

  • Nutrients like nitrogen, phosphorus, and iron are critical for phytoplankton growth

  • In many parts of the ocean, these nutrients are a limiting factor

Patterns of aquatic productivity

Geographic patterns

• Coastal areas:

  • Coastal waters, especially in shallow seas, are highly productive due to nutrient inputs from rivers and upwelling

  • These areas, such as estuaries, often serve as nurseries for marine species

• Tropical regions:

  • Tropical oceans, especially in shallow areas, tend to be more productive due to constant sunlight

  • Coral reefs, like those found in the Great Barrier Reef, are examples of highly productive tropical ecosystems supported by nutrient recycling and symbiosis between species

• Upwelling zones:

  • Upwelling zones are among the most productive regions in the ocean

  • Winds and ocean currents along some coastlines drive surface water away, allowing cold, nutrient-rich water to rise

  • This nutrient enrichment of surface waters provides abundant resources for phytoplankton

Seasonal changes

• Temperate regions:

  • In temperate oceans, productivity follows a seasonal pattern

  • In spring, increased light and nutrient mixing lead to spring blooms of phytoplankton

  • In summer, productivity slows due to thermal stratification

  • In autumn, cooling surface waters and stronger winds allow nutrient mixing again, causing a smaller autumn bloom before winter sets in

Awaiting image: Northern hemisphere temperate waters

Image caption: Seasonal changes in light, temperature, nutrients, stratification and productivity in temperate waters in the Northern Hemisphere