4.3 Phytoplankton and primary production in the oceans

Phytoplankton are microscopic photosynthetic organisms that form the base of nearly every marine food web. They're responsible for roughly half of all photosynthesis on Earth, making them just as important as terrestrial forests for oxygen production and carbon cycling. Understanding what controls their growth, where they thrive, and how scientists measure them is central to understanding how oceans function.

Phytoplankton Groups and Importance

Major groups of marine phytoplankton

Four major groups dominate ocean phytoplankton communities, each with distinct structures and ecological roles.

• Diatoms
  • Unicellular algae encased in cell walls made of silica (essentially glass). These ornate, rigid shells are called frustules.
  • Dominant in nutrient-rich waters like coastal upwelling zones (California Current, Humboldt Current), where they can form massive blooms.
  • Account for roughly 20% of all photosynthesis on Earth, making them one of the single most productive groups of organisms on the planet.
• Dinoflagellates
  • Unicellular algae that use two whip-like flagella to swim. This motility lets them migrate vertically in the water column to access light or nutrients.
  • Some species produce potent toxins and form harmful algal blooms (HABs), including red tides that cause paralytic shellfish poisoning.
  • Play a dual ecological role: they're primary producers in open water and critical symbionts inside coral tissue, where they're known as zooxanthellae and provide corals with most of their energy.
• Coccolithophores
  • Unicellular algae covered in tiny calcium carbonate plates called coccoliths. When these organisms bloom, the white coccoliths make the water so reflective that blooms are visible from space.
  • Their calcification process and eventual sinking contribute to the biological pump, transporting carbon from surface waters to the deep ocean.
  • Because they increase ocean surface reflectivity (albedo), large blooms can actually influence local climate by reflecting more sunlight.
• Cyanobacteria
  • Prokaryotic (no nucleus) photosynthetic bacteria. The two most abundant genera are Prochlorococcus and Synechococcus, which together are the most numerous photosynthetic organisms on Earth.
  • Dominate in oligotrophic (nutrient-poor) open ocean waters where larger phytoplankton can't compete.
  • Some genera (like Trichodesmium) are capable of nitrogen fixation, converting atmospheric $N_2$ into biologically usable forms like ammonia. This introduces new nitrogen into nutrient-depleted surface waters.

Factors Influencing Primary Production

Factors influencing ocean primary production

Three main factors control how much phytoplankton can grow in a given area: light, nutrients, and temperature. These factors interact with each other constantly.

• Light
  • Phytoplankton need light to drive photosynthesis, converting $CO_2$ and water into organic compounds.
  • Light intensity drops rapidly with depth due to absorption and scattering by water and particles. The depth at which photosynthesis balances respiration is called the compensation depth; below it, phytoplankton can't sustain net growth.
  • Seasonal changes in day length and sun angle drive much of the variation in primary production, with longer summer days supporting higher growth at mid and high latitudes.
• Nutrients
  • The key nutrients are nitrogen (as nitrate or ammonium), phosphorus (as phosphate), iron (a critical micronutrient), and silica (specifically needed by diatoms for their frustules).
  • Nutrient concentrations are generally higher in coastal zones and upwelling regions, where deep, nutrient-rich water is brought to the surface. The open ocean gyres, by contrast, tend to be nutrient-depleted at the surface.
  • In some regions like the Southern Ocean and equatorial Pacific, iron is the limiting nutrient despite abundant nitrogen and phosphorus. These are called HNLC (High-Nutrient, Low-Chlorophyll) regions.
• Temperature
  • Warmer water generally speeds up phytoplankton metabolic rates and growth, but only up to each species' optimal range. Beyond that, enzymes lose efficiency.
  • Different species are adapted to different thermal ranges: polar diatoms thrive in near-freezing water, while tropical cyanobacteria prefer warm surface layers.
  • Temperature also affects production indirectly through stratification. Warm surface water sits on top of cold deep water, creating a stable layer that traps phytoplankton in the light but also cuts off the nutrient supply from below.

Patterns of phytoplankton distribution

• Spatial patterns
  • Coastal and upwelling regions show the highest biomass and production, fueled by nutrient input from rivers, runoff, and upwelled deep water.
  • Oligotrophic gyres (North Pacific Gyre, Sargasso Sea) have low surface nutrients and correspondingly low phytoplankton biomass, though cyanobacteria persist there.
  • A general latitudinal pattern exists: temperate and polar regions can be highly productive during summer, when long days combine with nutrients mixed up during winter storms.
• Temporal patterns
  • In temperate oceans, the classic pattern is a large spring bloom followed by a smaller fall bloom. The spring bloom kicks off when increasing daylight coincides with nutrients that were mixed to the surface during winter. The fall bloom is triggered by cooling and renewed mixing after summer stratification breaks down.
  • Tropical regions show less seasonal variation because light is relatively constant year-round, though nutrient pulses from upwelling can still cause blooms.
  • Interannual climate patterns like El Niño-Southern Oscillation (ENSO) shift wind patterns and upwelling intensity, causing dramatic year-to-year changes in phytoplankton production across entire ocean basins.

Measuring Phytoplankton Biomass and Primary Production

Scientists combine remote and hands-on methods to track phytoplankton across scales ranging from a single water sample to the entire global ocean.

Satellite remote sensing

Satellites measure ocean color to estimate surface chlorophyll-a concentration, which serves as a proxy for phytoplankton biomass. Greener water generally means more phytoplankton.

• Provides global coverage and decades of continuous data, making it invaluable for tracking large-scale trends and seasonal cycles.
• Limitations: satellites only "see" the surface layer, so they miss deep chlorophyll maxima (layers of concentrated phytoplankton below the surface). Cloud cover blocks measurements, and in coastal waters, sediments and dissolved organic matter can interfere with color readings.

In situ techniques

• Chlorophyll-a extraction: Water samples are filtered, and the pigment chlorophyll-a is extracted using a solvent (typically acetone). The extract is then measured with a fluorometer or spectrophotometer. This gives a direct measurement of phytoplankton biomass in that sample.
• $^{14}C$ uptake method: Water samples are spiked with radioactive carbon-14 ($^{14}C$) and incubated in sunlight. After a set period, scientists measure how much $^{14}C$ was incorporated into phytoplankton cells. This directly measures the rate of carbon fixation (primary production), not just standing biomass.
• Oxygen evolution (light-dark bottle method): Paired water samples are incubated in transparent ("light") and opaque ("dark") bottles. The light bottle measures net production (photosynthesis minus respiration), while the dark bottle measures respiration alone. The difference gives you gross primary production.
• Fluorometry: Fluorometers measure chlorophyll-a fluorescence in real time, which reflects the photosynthetic efficiency and physiological health of phytoplankton. This can be done continuously from a ship or mooring, giving high-resolution data.

Role in the Global Carbon Cycle

Phytoplankton in the global carbon cycle

Phytoplankton are a critical link between the atmosphere and the deep ocean in the carbon cycle. They pull carbon out of surface waters through photosynthesis and, through a chain of processes, help store it in the deep ocean for centuries.

• Carbon fixation
  • Through photosynthesis, phytoplankton convert dissolved $CO_2$ into organic carbon compounds. This organic carbon then enters marine food webs as other organisms consume the phytoplankton.
  • Collectively, marine phytoplankton fix an estimated 50 gigatons of carbon per year, roughly equal to all terrestrial plant photosynthesis combined.
• The biological pump
  • The biological pump is the set of processes that transport organic carbon from the sunlit surface ocean to the deep ocean. It works in several ways:
    1. Dead phytoplankton cells, fecal pellets from zooplankton, and clumps of organic material (called marine snow) sink as particulate organic carbon (POC).
    2. As this material sinks, bacteria decompose (remineralize) most of it, releasing $CO_2$ and nutrients back into the water at depth.
    3. A fraction reaches the seafloor and gets buried in sediments, locking carbon away for hundreds to thousands of years.
  • The efficiency of the biological pump depends on what types of phytoplankton are present. Large, heavy cells like diatoms sink faster and export more carbon than tiny cyanobacteria.
• Carbon sink and climate regulation
  • By drawing $CO_2$ out of surface waters, phytoplankton photosynthesis lowers the partial pressure of $CO_2$ at the ocean surface, which allows more atmospheric $CO_2$ to dissolve into the ocean.
  • Long-term carbon storage in deep water and sediments helps regulate atmospheric $CO_2$ levels over geological timescales.
  • Changes in ocean temperature, stratification, and nutrient supply can shift phytoplankton community composition and productivity, potentially altering how effectively the ocean absorbs $CO_2$. This is a major concern under climate change scenarios.