Productivity in Aquatic Ecosystems
Primary productivity
Primary productivity is the rate of production of biomass using an external energy source and inorganic sources of carbon and other elements.
- Productivity refers to the rate of energy accumulation in an aquatic ecosystem through photosynthesis and the formation of organic biomass.
- Most productivity in aquatic systems is carried out by phytoplankton.
Productivity in aquatic systems depends on light, temperature, and nutrients, which together determine where and when photosynthesis can occur.
Light Availability and the Photic Zone
Photic zone
The photic zone is the upper layer of a body of water that receives enough sunlight to support photosynthesis.
- Sunlight is the main energy source for photosynthesis, but water absorbs and scatters light rapidly.
- The photic zone, the depth where sunlight penetrates sufficiently for photosynthesis, is typically the upper 80-150 m of the ocean.
- Below this depth, light levels are too low for photosynthesis, marking the start of the aphotic zone.
- Water turbidity, depth, and dissolved particles influence how far light penetrates.
In clear tropical waters, light may reach depths of 150 m, while in turbid coastal waters, photosynthesis may be limited to 10 m.
Thermal Stratification
- Thermal stratification is the layering of water in oceans and lakes based on temperature and density differences.
- Warm, less dense water floats above colder, denser layers, forming a vertical gradient.
- The boundary layer separating these two regions is called the thermocline.
Thermocline
A thermocline is a layer in a body of water where temperature decreases sharply with depth, restricting vertical movement and mixing.
Seasonal Stratification in Temperate Waters
- Summer: Warm surface water forms a stable layer over colder deep water, preventing nutrient mixing.
- Autumn: Cooling surface water sinks, breaking the stratification, a process is called turnover.
- Winter: Water becomes nearly uniform in temperature, allowing complete mixing.
- Spring: Increasing light and temperature lead to spring phytoplankton blooms, supported by nutrients mixed upward during winter.
The spring phytoplankton bloom in the North Atlantic occurs after winter mixing replenishes surface nutrients, followed by increasing sunlight in spring.
Analogy- Think of stratification like a layered cake.
- During summer, each layer stays separate, but in winter, it’s mixed into one uniform layer.
Nutrient Mixing and Upwelling
Nutrient mixing
Nutrient mixing refers to the vertical movement of water that redistributes dissolved nutrients (like nitrates and phosphates) throughout the water column.
- Nutrient mixing is the vertical movement of nutrients between deep and surface waters.
- It occurs through wind-driven turbulence, seasonal turnover, and upwelling processes.
- Mixing replenishes surface nutrients that are constantly depleted by phytoplankton uptake.
Upwelling
Upwelling
Upwelling refers to the vertical movement of deep, cold, nutrient-rich water to the surface due to the displacement of surface waters by wind.
- Upwelling occurs along coastlines and near the equator, where winds push surface waters away, allowing deeper water to rise.
- Upwelling zones are among the most productive ecosystems globally, supporting fisheries and dense plankton populations.
Mechanisms of Mixing
- Wind-driven circulation: Surface winds push water away, allowing deeper water to rise (common along coastlines).
- Thermohaline circulation: Global ocean currents transport nutrients and oxygen between surface and deep layers.
- Seasonal overturning: Occurs when surface temperatures cool, causing mixing between layers.
