1.8 - Primary Productivity

Learning Objectives

The main goal is explaining how solar energy is acquired and transformed by living organisms. The associated skill is describing an environmental concept or process.

Primary Productivity Basics

Primary productivity is defined as the rate at which solar or light energy is converted into organic compounds. Those compounds, like glucose or cellulose, are created through photosynthesis over a given time frame.

Measurement: Productivity is often measured in units of energy per area over time. The standard units are kilocalories per meter squared per year, $\text{kcal}/\text{m}^2/\text{year}$ .

Plant growth: Productivity can be understood simply as the rate of plant growth. Areas with higher productivity have higher plant growth.

Productivity and Biodiversity

There’s a significant trend linking productivity to biodiversity:

High productivity leads to high biodiversity. Ecosystems with high primary productivity have more plant material available, which means more food and shelter for consumers. That allows the ecosystem to support a greater variety and number of species, making it more biodiverse.

Calculating Productivity

To accurately measure the energy available in an ecosystem, we have to distinguish between the total energy captured and the energy actually stored. Plants use some of the energy they capture for their own internal processes, like transportation and tissue repair, similar to how humans use energy for digestion.

Term	Definition	Analogy
Respiration Loss	The energy plants consume for basic, vital internal processes needed to stay alive.	The “taxes” taken out of the plant’s paycheck.
Gross Primary Productivity (GPP)	The total amount of sunlight captured by plants and converted into energy. It represents the total photosynthetic rate.	The total paycheck amount earned.
Net Primary Productivity (NPP)	The energy that remains after respiration loss. This is the energy the plant can store as biomass (actual growth), and it’s what’s left over for consumers.	The take home pay the plant can save in the bank.

The formula for NPP:

\text{NPP} = \text{GPP} - \text{Respiration Loss}

Worked Example: Calculating NPP

A prairie has a GPP of 6,000

\text{kcal}/\text{m}^2/\text{yr}

, and its plants use 3,600

\text{kcal}/\text{m}^2/\text{yr}

for respiration. The prairie covers 3 hectares. How much energy do the plants store as new growth each year?

NPP per square meter: subtract respiration from GPP.

\text{NPP} = 6{,}000 - 3{,}600 = 2{,}400\ \text{kcal}/\text{m}^2/\text{yr}

Share of GPP that's stored: $\frac{2{,}400}{6{,}000} \times 100 = 40\%$ , which matches the 40% figure below.

Convert the area to square meters (1 hectare = 10,000 $\text{m}^2$ ):

3\ \text{ha} \times \frac{10{,}000\ \text{m}^2}{1\ \text{ha}} = 30{,}000\ \text{m}^2

Multiply by the area. The $\text{m}^2$ units cancel, leaving kcal per year:

2{,}400\ \frac{\text{kcal}}{\text{m}^2 \cdot \text{yr}} \times 30{,}000\ \text{m}^2 = 72{,}000{,}000\ \frac{\text{kcal}}{\text{yr}} = 7.2 \times 10^7\ \text{kcal/yr}

On the FRQ, write each step with its units like this. Points are often given for the setup, not just the final answer.

Measuring Productivity in Water: Light and Dark Bottles

In aquatic ecosystems, productivity is usually measured with dissolved oxygen (DO), since photosynthesis makes

\text{O}_2

and respiration uses it up.

Fill three bottles with water from the same spot and measure the DO in one right away (the starting DO).

Seal one clear bottle (light) and one bottle wrapped in foil (dark), and leave both in the water for 24 hours.

Light bottle: photosynthesis and respiration both happen, so its change in DO equals NPP.

Dark bottle: only respiration happens, so its drop in DO equals respiration.

GPP = NPP + respiration.

Example: the starting DO is 8.0 mg/L. After 24 hours, the light bottle has 9.2 mg/L and the dark bottle has 7.1 mg/L. NPP = 9.2 - 8.0 = 1.2 mg

\text{O}_2

/L/day. Respiration = 8.0 - 7.1 = 0.9 mg

\text{O}_2

/L/day. GPP = 1.2 + 0.9 = 2.1 mg

\text{O}_2

/L/day.

Photosynthetic Efficiency

Photosynthetic efficiency measures the fraction of incoming solar energy that plants capture and convert into usable biomass (NPP).

Sunlight capture: In most ecosystems, only about 1% of total incoming sunlight is captured by plants and used for GPP.

Lost energy: Roughly 99% of incoming sunlight is either reflected off the plant’s surface or passes right through the leaves without being captured.

Conversion to biomass: Of the small amount of energy captured, only about 40% (which works out to roughly 0.4% of the total incoming sunlight) is actually stored as biomass, or NPP.

Factors Driving Productivity Trends

Some ecosystems are naturally more productive because they get more sunlight or have more efficient plants, but the distribution of highly productive biomes comes down to three key factors:

Water availability: A high amount of available water supports growth.

Temperature: Warmer temperatures allow plant growth year round.

Nutrient availability: High nutrient levels in soil or sediment fuel growth.

Productivity Level	Biome Type	Defining Factors / Deficiency
High Productivity	Swamps, marshes, tropical rainforests, coral reefs	High access to water, high temperatures, and high nutrient availability, such as the vast organic matter in wetlands.
Low Productivity	Desert	Deficiency of water and nutrients, thanks to sandy soil.
Low Productivity	Tundra	Deficiency of temperature (too cold for year round growth) and low access to liquid water, since most is frozen.
Low Productivity	Open Ocean	Deficiency of nutrients (lack of organic matter), despite abundant water and warm temperatures.

Per Square Meter vs. Total Productivity

Productivity per square meter and total productivity can rank ecosystems differently. Here are classic estimates of NPP (as grams of dry plant matter, from Whittaker and Likens, 1975):

Ecosystem	NPP per area	Area covered	Total NPP
Tropical rainforest	about 2,200 $\text{g}/\text{m}^2/\text{yr}$	about 17 million $\text{km}^2$	about 37 billion metric tons/yr
Open ocean	about 125 $\text{g}/\text{m}^2/\text{yr}$	about 332 million $\text{km}^2$	about 41.5 billion metric tons/yr

Setting up the open ocean total (1

\text{km}^2

10^6

\text{m}^2

, and 1 metric ton =

10^6

g):

332 \times 10^{6}\ \text{km}^2 \times \frac{10^{6}\ \text{m}^2}{1\ \text{km}^2} \times 125\ \frac{\text{g}}{\text{m}^2 \cdot \text{yr}} \times \frac{1\ \text{t}}{10^{6}\ \text{g}} = 4.15 \times 10^{10}\ \text{t/yr}

Per square meter, the rainforest is about 18 times as productive. But the open ocean covers about 20 times as much area, so its total NPP is larger. Satellite estimates agree: the oceans make up nearly half of all NPP on Earth (Field and others, 1998).

Light and Photosynthesis in Water

Water absorbs sunlight quickly, and it absorbs some colors faster than others. This limits how deep photosynthesis can happen.

Red light is absorbed fastest. Most red light is gone within the top 1 m of water, which is why everything looks blue-green underwater.

Blue light goes deepest. Only in the clearest ocean water does it reach deeper than 100 m, and in cloudy (turbid) water, light may only reach a few meters down.

Why it matters: below the depth light reaches, there's no photosynthesis (the aphotic zone, see 1.3), so nearly all aquatic productivity happens in a thin top layer.

Adaptations: aquatic photosynthesizers have ways to use the little light that's left. Many algae have extra (accessory) pigments that absorb the blue and green light that reaches deeper water and pass that energy to chlorophyll. Red algae use a red pigment called phycoerythrin, which lets them grow deeper than most other algae; a red alga found 268 m deep off the Bahamas (reported in 1985) is among the deepest photosynthesizers known. Phytoplankton float near the surface, and kelp grows tall to reach the light.

Big picture: NPP works like the fuel gauge on a car. GPP is the total fuel you pumped in, respiration loss is what the engine needs just to stay running at idle, and NPP is the remaining fuel available to actually move the car down the road, or in the ecosystem’s case, to feed the consumers.

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