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.
- 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.
- River inflow: Freshwater inflows can disturb stratification and add nutrient-rich sediments to coastal zones.
The Peruvian (Humboldt) Current upwelling zone off South America supports some of the world’s richest fisheries, due to nutrient-rich waters rising to the surface.
Without mixing or upwelling, nutrients remain trapped in deep water, leading to low productivity in mid-ocean gyres.
Nutrient Loading
Nutrient loading
Nutrient loading refers to the addition of nutrients (natural or human-induced) into a water body, affecting productivity.
- Nutrient loading is the addition of nutrients (mainly nitrates and phosphates) to aquatic ecosystems from natural or anthropogenic sources.
- Natural sources include weathering of rocks, river runoff, and organic decomposition.
- Human sources include agricultural fertilizers, wastewater, and industrial discharge.
Effects on Productivity
- Moderate nutrient input boosts phytoplankton growth and supports higher trophic levels.
- Excessive input causes algal blooms, oxygen depletion, and fish kills, a process known as eutrophication.
The Gulf of Mexico Dead Zone forms annually due to fertilizer runoff from the Mississippi River, causing oxygen depletion over an area larger than 15,000 km².
Interconnection Between Stratification, Mixing, and Productivity
- Light Availability: Productivity depends on how deeply sunlight penetrates; the photic zone limits photosynthesis to upper layers.
- Nutrient Availability: Stratification traps nutrients in deeper layers; mixing or upwelling is essential to replenish surface nutrients.
- Temperature Gradients: Warmer surface waters intensify stratification, while cooling promotes nutrient mixing.
- Biological Recycling: When phytoplankton die, nutrients sink and accumulate in the hypolimnion, creating a cycle dependent on physical mixing for renewal.
Geographic Patterns of Productivity
1. Coastal and Shallow Waters
- Highest productivity due to river runoff, upwelling, and sunlight penetration to the seabed.
- Coastal zones act as nurseries for marine species (e.g., estuaries, mangroves).
- Nutrient recycling is faster in shallow areas.
Chesapeake Bay and Gulf of Mexico support high productivity but also suffer from nutrient pollution due to excessive agricultural runoff.
2. Upwelling Zones
- Among the most productive regions in the world.
- Continuous nutrient supply supports dense phytoplankton populations.
- Fisheries (e.g., anchovies off Peru) depend on this nutrient enrichment.
3. Open Oceans (Oligotrophic Waters)
- Low productivity despite high sunlight due to nutrient limitation.
- Clear tropical waters are biologically poor compared to coastal or upwelling regions.
4. Polar Regions
- High seasonal variation.
- Productivity peaks in spring and summer when sunlight returns, and mixing brings nutrients to the surface.
- Supports short but intense food chains (e.g., phytoplankton → krill → whales).
- How can we balance economic interests with ecological sustainability?
- Should governments prioritize short-term economic gains or long-term environmental health?
- Define thermal stratification and explain how it affects nutrient availability in aquatic systems.
- Describe how upwelling contributes to high productivity in coastal waters.
- Discuss why open ocean waters are often less productive despite abundant sunlight.
- Outline how nutrient loading from human activities can alter aquatic ecosystems.
- Explain the interconnection between light penetration, thermal layering, and nutrient cycling in determining productivity.
- Describe the seasonal pattern of phytoplankton productivity in temperate waters and link it to temperature and nutrient changes.
