7.2: Drivers of & Patterns in Primary Production

What do Primary Producers Need?

For terrestrial plants, many factors affect productivity, including light, temperature, nutrients, soil, and water. For phytoplankton, soil is obviously not needed, and water availability is not an issue. Temperature is generally more stable in the ocean than on land, so for phytoplankton, productivity comes down to the availability of light and nutrients.

 

Light

Since light is vital for photosynthesis, phytoplankton and other primary producers are limited to the uppermost layers of the ocean where light is abundant enough to sustain the reaction. As depth increases, light intensity decreases until there reaches a depth where photosynthesis can no longer occur (Figure \(\PageIndex{1}\)). The region through which sufficient light for photosynthesis can penetrate is called the photic or euphotic zone, which extends down to about 200 m.

 

In addition to undergoing photosynthesis, phytoplankton also respire, consuming some of the organic compounds they produce. Rates of respiration are not light dependent, and respiration occurs at all depths and light levels. Therefore, as depth increases the rate of photosynthesis declines as light is diminished, until a point is reached where the rate of photosynthesis equals the respiration rate (Figure \(\PageIndex{1}\)). This depth is the compensation depth, and it marks the lower level of the photic zone, and represents the depth where net primary production ends. Below this depth, there is net respiration.

 

Figure \(\PageIndex{1}\) As depth increases the rate of photosynthesis declines as light becomes limited. The rate of respiration remains consistent at all depths. The depth where photosynthesis equals respiration is the compensation depth (PW).

 

Nutrients

Nutrients are required by all of the marine primary producers. The major nutrients required by phytoplankton are nitrogen and phosphorus, in the forms of nitrate NO₃^–, nitrite NO₂^2-, ammonium NH₄⁺, and phosphate PO₄^3-. Many phytoplankton, particularly the diatoms, also require silica, SiO₂, for shell formation. All of these nutrients occur in very small amounts in seawater, so they are often the limiting factors for phytoplankton growth in most situations, particularly the nitrogen compounds. For example, agricultural soil contains 0.5% nitrogen in the upper meter of soil, while surface ocean water contains about 0.00005% nitrogen, 1/10,000 the amount in soil.

 

Nutrients are not distributed evenly throughout the water column (Figure \(\PageIndex{2}\)). Near the surface nutrients are quickly utilized by phytoplankton as they become available, so surface waters are nutrient-poor. But as the phytoplankton are consumed or die they are recycled into particles of organic matter, such as fecal pellets or carcasses, that sink into deeper water. Once in deep water, decomposition of these materials releases the nutrients back into the water column. Because there are no producers to utilize them at depth, nutrients are more abundant in deeper water.

 

Figure \(\PageIndex{2}\) Representative nutrient profile in the open ocean. While this profile shows nitrate abundance, the profile is similar for other nutrients such as phosphate and silica (PW).

 

These deep water nutrients are out of reach of the phytoplankton at the surface. The thermocline and density stratification of the water column generally prevents the nutrient-rich deep water from mixing with the surface water. However, under certain conditions this nutrient-rich deep water may be brought to the surface through the process of upwelling. Where upwelling occurs there is usually high productivity as the phytoplankton can take advantage of the input of nutrients.

 

Temporal and Spatial Patterns in Primary Production 

Primary productivity varies both geographically and seasonally. Geographically, phytoplankton abundance generally decreases as you move from coastal to oceanic waters (Figure \(\PageIndex{3}\)). Coastal waters are more productive than the central ocean for two main reasons. First, runoff from land often contains a high abundance of nutrients which get deposited in coastal waters and stimulate production. Second, the shallower bottom along the continental shelf can trap nutrients and prevent them from sinking to greater depths. It is easier for these nutrients to be brought back to the surface when they remain trapped in the shallows. Conversely, the central ocean generally has very low primary production, as these areas are far removed from any terrestrial sources of nutrients, and the great depth prevents the deep nutrients from returning to the surface.

 

Global averages for ocean surface primary production are about 75-150 g C/m²/yr. Some highly productive areas include the California coast (200-300 g C/m²/year), the Southern Ocean (200-400 g C/m²/year), and the coast of Peru (200-400 g C/m²/year), all regions with significant upwelling. The central ocean, by contrast, produces less than 50 g C/m²/year.

 

Figure \(\PageIndex{3}\) Global surface ocean primary productivity, as measured by chlorophyll concentration (Provided by the SeaWiFS Project, Goddard Space Flight Center and ORBIMAGE [Public domain], via Wikimedia Commons).

 

Regional and seasonal changes in primary production are due to a combination of the availability of light and the amount of nutrients provided by water mixing above the thermocline. In tropical regions sunlight is plentiful throughout the year, so light is not a limiting factor. The surface water is always warm and there is always a pronounced thermocline, leading to highly stratified water that prevents the nutrient-rich bottom water from reaching the surface. Thus productivity in tropical water is always nutrient-limited, and productivity is low throughout the year (Figure \(\PageIndex{4}\)). Because tropical water is nutrient-poor with little phytoplankton production, the water is very clear, as is the case with water in the central ocean.

 

At the poles, the water is uniformly cold at all depths, so there is not much of a thermocline and little stratification, allowing mixing to occur year-round (section 6.2). This mixing distributes nutrients throughout the water column, so that for much of the year productivity will not be nutrient-limited. However, the polar regions may experience several months with little or no light during the winter, and the fluctuation in light levels leads to variation is seasonal productivity. In the winter months, mixing is occurring and nutrients are abundant, but there is no light, so there is no productivity. By late spring the sunlight returns, and combined with the abundance of nutrients, a spring/summer bloom of phytoplankton occurs (Figure \(\PageIndex{4}\)). By late summer, the nutrients have been depleted and zooplankton have been grazing on the phytoplankton, so the bloom begins to decline. In the autumn, light levels decline and prevent further production throughout the winter. But during the winter the mixing is distributing nutrients throughout the water, ready for the sun to return and stimulate a bloom in the following summer.

 

In temperate regions there is much more seasonal variation in the depth and intensity of the thermocline. The thermocline is shallower and stronger in the summer, and is deeper and weaker in the winter, so mixing of deep and surface water is more pronounced in the winter months. As with the poles, this winter mixing creates nutrient-rich water during the winter, but the lack of light limits winter productivity. When light levels increase in the spring, the combination of abundant light and nutrients creates a spring bloom of productivity. By late summer there is still plenty of light, but the nutrients have been depleted by the phytoplankton bloom, and the summer thermocline has prevented further mixing, so productivity declines. In the autumn, cooler temperatures weaken the thermocline, and increasing storms cause a deeper mixed layer to form, bringing nutrients back to the surface. At the same time, there is still sufficient light available that a smaller autumn bloom occurs (Figure \(\PageIndex{4}\)). But this bloom is short-lived, as light declines throughout the autumn and into the winter. Again, there is little production during the winter due to light limitations, but the winter storms and deep thermocline recharge the water with nutrients for the next spring bloom.

 

Figure \(\PageIndex{4}\) Seasonal patterns of productivity in the Northern Hemisphere. Tropical regions are always nutrient-limited and show low productivity. Polar regions are light limited in the winter and only display production during the late spring and summer months when light is available. Northern temperate regions have a spring bloom, and a smaller autumn bloom (PW).

 

A Case Study of the Temperate Ocean

North Atlantic right whales are one of the most endangered whales in the world. They were traditionally hunted and remain endangered today, with only a few hundred individuals surviving. Scientists are interested in learning more about right whale migration patterns to help protect them and save them from extinction. Recently, scientists discovered that these whales spend significant time feeding along the coast south of Cape Cod, Massachusetts, during March and April while they are moving from the south, where they breed during the winter, to the north where they feed during the summer and fall. These whales are filter feeders and eat a type of zooplankton called copepods, which in turn eat phytoplankton. Here, you will investigate why the whales’ food is so abundant south of Cape Cod at this time of the year. You will answer this question by looking at the distribution of phytoplankton (chlorophyll), nutrients (nitrate), water temperature, and sunlight (irradiance) in the ocean where these whales feed.

Now that we know how important patterns in primary production are, and the what these patterns look like at a global scale, let's focus in on the The summary below (Figure \(\PageIndex{5}\)) is based on the classic textbook version of how primary productivity changes in temperate oceans. 

 

Figure \(\PageIndex{5}\) Typical changes in solar irradiance, water stratification, nutrients and phytoplankton biomass over the year in the temperate ocean.

 

Use the data in the graphs below to further explore this explanation. You can interact with the data by

• turning on and off different variables to compare to chlorophyll
• hovering over a data point to view the data values of the different variables on that day
• turning on and off the different seasons to highlight winter, spring, summer and fallchat

 

 

 

Exercise \(\PageIndex{1}\)

Why is spring the best time for whales to feed in this region? What would limit their food availability in the winter? What about in the summer?

 

Answer

In the spring, there is finally enough sunlight to grow - and phytoplankton haven't yet used up all of the nutrients that were mixed throughout the water column during the winter (when there is low stratification and a lot of mixing due to storms). By the time the summer hits, the water is highly stratified, and plankton have used up all of the nutrients at the surface - there is plenty of sunlight, but all of the nutrients are trapped below the photic zone, where phytoplankton can't benefit from them. In the winter, the nutrients are mixed back up into the surface ocean as surface waters cool and stratification breaks down - but, at that point, there isn't enough sun. So spring is really the goldilocks period for phytoplankton growth in temperate ecosystems - and many organisms time their breeding to coincide with this period.