Define gross and net primary productivity, explain the abiotic factors that limit marine primary production, and describe how productivity is measured and where the most productive marine zones occur

What this dot point is asking

QCAA wants you to define primary productivity, distinguish gross from net, explain what limits it in the sea, and describe how it is measured and where it is highest. This is a heavily quantitative area, so expect IA1 stimulus with productivity figures, light and nutrient profiles, and trophic calculations.

Gross and net primary productivity

Producers such as phytoplankton, seagrass and algae fix energy by photosynthesis. The total amount they fix per unit area per unit time is the gross primary productivity (GPP). Producers use some of that energy for their own respiration. What is left is the net primary productivity (NPP):

NPP is the energy actually available to herbivores and the rest of the food web, so it is the figure that matters most for ecosystem productivity. Productivity is usually expressed as mass of carbon or oxygen fixed per square metre per year, or as grams of biomass.

What limits marine productivity

Two abiotic factors dominate.

• Light decreases rapidly with depth. Net photosynthesis is only possible in the upper sunlit layer (the photic zone). Below the depth where photosynthesis just balances respiration, the compensation depth, producers cannot grow.
• Nutrients, especially nitrogen, phosphorus and in places iron, are scarce in surface waters because sinking organic matter carries them down. Production stops when nutrients run out, even in bright light.

Productivity is therefore highest where light and nutrients overlap, which happens where deep, nutrient-rich water is brought up into the light.

Where productivity is highest

• Upwelling zones, where wind and currents draw cold, nutrient-rich deep water to the sunlit surface, are the most productive open-ocean regions and support major fisheries.
• Estuaries and coastal waters receive nutrients from rivers and land and are shallow enough for light to reach the bottom, so they are highly productive.
• Coral reefs are highly productive locally despite low surrounding nutrients, because tight internal nutrient recycling and the coral-zooxanthellae partnership retain and reuse what is there.
• The open ocean gyres are nutrient-poor and have low productivity despite covering huge areas, which is why they are sometimes called marine deserts.

How productivity is measured

Marine scientists estimate productivity several ways:

• Light and dark bottle method. Water samples are sealed in clear and opaque bottles. Oxygen change in the light bottle measures net production; the dark bottle measures respiration; adding them gives gross production.
• Chlorophyll concentration, measured directly or by satellite, indicates how much phytoplankton biomass is present and is used to map ocean productivity globally.
• Carbon uptake using tracer carbon measures the rate of fixation directly.

These methods generate the kind of paired data that IA1 questions ask you to process and interpret.

Australian context

Australia's tropical waters are generally low in nutrients, which is one reason the clear Coral Sea supports reefs rather than dense plankton blooms. Seasonal upwelling along parts of the southern coast, such as the Bonney Coast upwelling in South Australia, boosts productivity and supports feeding aggregations. Understanding what controls productivity is the foundation for the energy-flow and connectivity dot points, because all of that energy starts here.