4.3 Phytoplankton and primary production in the oceans

Phytoplankton are tiny marine plants that play a huge role in ocean ecosystems. They form the base of marine food webs and produce half of Earth's oxygen. These microscopic organisms come in various groups, each with unique features and impacts on ocean chemistry.

Phytoplankton growth depends on light, nutrients, and temperature. Their distribution varies across oceans and seasons, influencing global carbon cycles. Scientists use satellites and water samples to measure phytoplankton, helping us understand their critical role in marine ecosystems and climate regulation.

Phytoplankton Groups and Importance

Major groups of marine phytoplankton

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• Diatoms
  • Unicellular algae with siliceous cell walls made of silica (glass-like material)
  • Dominant in nutrient-rich areas such as coastal upwelling zones (California Current, Humboldt Current)
  • Major contributors to global primary production and carbon fixation, accounting for ~20% of total photosynthesis on Earth
• Dinoflagellates
  • Unicellular, often motile algae with two flagella for locomotion
  • Some species form harmful algal blooms (HABs) that can produce toxins (red tides, paralytic shellfish poisoning)
  • Important in marine food webs as primary producers and as symbionts in coral reefs (zooxanthellae)
• Coccolithophores
  • Unicellular algae with calcium carbonate plates called coccoliths covering their cell surface
  • Significant role in the global carbon cycle through calcification and carbon fixation, contributing to the biological pump
  • Blooms can affect ocean albedo (reflectivity) and contribute to the biological pump by sinking and transporting carbon to the deep ocean
• Cyanobacteria
  • Prokaryotic, photosynthetic bacteria such as Prochlorococcus and Synechococcus
  • Numerically dominant in oligotrophic (nutrient-poor) waters, particularly in the open ocean
  • Essential for nitrogen fixation in the ocean, converting atmospheric nitrogen (N₂) into bioavailable forms (ammonia, nitrate)

Factors Influencing Primary Production

Factors influencing ocean primary production

• Light
  • Phytoplankton require light for photosynthesis to convert CO₂ and water into organic compounds
  • Light availability decreases with depth due to attenuation (absorption and scattering)
  • Seasonal variations in light influence phytoplankton growth, with higher production during summer months
• Nutrients
  • Essential nutrients for phytoplankton growth include nitrogen (nitrate, ammonium), phosphorus (phosphate), silica (for diatoms), and iron (micronutrient)
  • Nutrient availability varies spatially and temporally, with higher concentrations in coastal and upwelling regions
  • Upwelling and mixing bring nutrients from deep waters to the surface, supporting primary production
• Temperature
  • Affects phytoplankton metabolic rates and growth, with higher temperatures generally increasing growth rates up to an optimal range
  • Optimal temperature ranges vary among phytoplankton species, with some adapted to cold polar waters and others to warm tropical waters
  • Stratification and mixing of water columns influence temperature and nutrient distribution, affecting phytoplankton growth and community structure

Patterns of phytoplankton distribution

• Spatial patterns
  • Higher biomass and production in coastal and upwelling regions due to nutrient input from rivers, coastal runoff, and upwelling of deep, nutrient-rich waters
  • Lower biomass and production in oligotrophic gyres (North Pacific Gyre, Sargasso Sea) where nutrients are depleted
  • Latitudinal gradients: higher production in temperate and polar regions during summer due to increased light availability and nutrient input from deep winter mixing
• Temporal patterns
  • Seasonal variations due to changes in light, temperature, and nutrient availability, with spring and fall phytoplankton blooms in temperate regions
  • Spring blooms occur when light increases and winter mixing provides nutrients, while fall blooms are triggered by cooling and mixing of surface waters
  • Interannual variability influenced by climate patterns such as El Niño-Southern Oscillation (ENSO), affecting upwelling and nutrient supply

Measuring Phytoplankton Biomass and Primary Production

Measurement of phytoplankton biomass

• Satellite remote sensing
  • Measures ocean color (chlorophyll-a concentration) to estimate phytoplankton biomass on a global scale
  • Provides global coverage and long-term data sets, allowing for the study of large-scale patterns and trends
  • Limitations include surface measurements (unable to detect deep chlorophyll maxima), cloud cover, and coastal interference (sediments, dissolved organic matter)
• In situ techniques
  • Chlorophyll-a extraction: measures phytoplankton biomass by extracting chlorophyll-a pigments from water samples using solvents (acetone) and measuring fluorescence or absorbance
  • $^{14}$C uptake: measures primary production rates by incubating water samples with radioactive carbon-14 and measuring the incorporation of $^{14}$C into phytoplankton cells
  • Oxygen evolution: estimates net community production by measuring changes in dissolved oxygen concentrations in light and dark incubations
  • Fluorescence: assesses phytoplankton physiological state and productivity using fluorometers to measure chlorophyll-a fluorescence, an indicator of photosynthetic efficiency

Role in the Global Carbon Cycle

Phytoplankton in global carbon cycle

• Carbon fixation
  • Phytoplankton convert dissolved inorganic carbon (DIC) such as CO₂ to organic carbon through photosynthesis, forming the base of marine food webs
  • Estimated to contribute ~50% of global primary production, making them key players in the global carbon cycle
• Biological pump
  • Process by which organic carbon is exported from the surface to the deep ocean via sinking particles and vertical migration of organisms
  • Phytoplankton-derived organic matter sinks as particulate organic carbon (POC) in the form of dead cells, fecal pellets, and aggregates (marine snow)
  • Sinking POC is remineralized by bacteria or buried in sediments, sequestering carbon for long time scales (hundreds to thousands of years)
• Carbon sink
  • Phytoplankton remove CO₂ from the atmosphere and surface ocean through photosynthesis, acting as a biological carbon pump
  • Long-term carbon storage occurs in the deep ocean and sediments, helping to regulate atmospheric CO₂ levels and climate
  • Changes in phytoplankton productivity and community structure can affect the efficiency of the biological pump and the ocean's capacity to absorb atmospheric CO₂