4 Primary production

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Marine Primary Productivity: 
Measurements and Variability 
 
Why should we care about productivity? 
 
• Photosynthetic activity in oceans created current O2-rich atmosphere 
• Plankton form ocean sediments & fossil fuels 
• Plankton are a critical part of “carbon pump” that influences atmospheric CO2 
• Phytoplankton form the base of food webs and associated biological diversity 
• Limits to productivity may limit the amount of harvestable biomass from ocean 
ecosystems. 
 
Productivity is the amount of living tissue produced per unit time. It is often estimated in 
terms of carbon contained in living material and expressed as grams of carbon (g C) 
produced per day, in a column of water intersecting one square meter of sea surface (g C 
m-2 d-1), from the surface to the seabed. 
 
Primary productivity is the amount of plant tissue build up by photosynthesis over time. 
It is so called because photosynthetic production is the basis of most of marine 
production. It is worth to mention here that there are other types of primary production 
that are carried out by bacteria capable of building organic materials through 
chemosynthetic mechanisms, but these are of minor importance in the oceans as a whole. 
 
Gross primary productivity is the total amount of organic carbon manufactured by 
primary producers. Primary producers however, immediately respire some of the organic 
matter they make to meet their own energy needs, so it is not available as food to other 
organisms. We are more interested, however, in Net primary production, which is the 
organic matter that is left over or the carbon that is available to higher trophic levels. Net 
primary production is also defined as the energy remaining after respiratory needs have 
been met. 
Net primary production = Gross Primary Production - Respiration 
 
Net primary production by photosynthesis can be estimated by measuring either the 
amount of raw material used up (CO2 ) or the amount of end products given off (O2) by 
photosynthetic organisms in the sunlight. Marine ecologists have attempted to develop 
accurate and relatively simple methods to estimate primary production. 
 
Secondary productivity refers to production by the organisms that consume the 
phytoplankton; and those that consume the organisms responsible for secondary 
production are said to be engaged in tertiary productivity. 
 
Standing stock or standing crop refers to the number of organisms per unit area or per 
unit volume of water at the moment of sampling. 
For phytoplankton, this can be measured by microscopic cell counts of preserved 
phytoplankton filtered from seawater samples, and the standing stock is given in number 
of cells per volume of water. However, because phytoplankton vary greatly in size, total 
numbers are not as ecologically meaningful as estimates of their biomass. 
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Biomass is defined as the total weight (total numbers × average weight) of all organisms 
in a given area or volume. 
It is possible to count numbers and measure volumes of phytoplankton electronically to 
provide an estimate of phytoplankton biomass, although cell volume may not always 
accurately reflect cell weight. Biomass is then expressed as the total volume (total 
numbers × volumes = mm3) of phytoplankton cells per unit volume of water. 
 
The distinction between standing stock and biomass is not always made evident, 
however, and often the terms are used synonymously. 
 
Measuring Primary Productivity 
 
¾ Oxygen Technique 
The oxygen technique relies upon the fact that oxygen is released in proportion to the 
amount of photosynthesis. Because oxygen is released during photosynthesis, changes in 
oxygen concentration can be used to estimate primary productivity. A water sample is 
first collected and the zooplankton are strained from it, using a 150-300 µm plankton net. 
The remaining water is divided into two biological oxygen demand (BOD) bottles and 
the dissolved oxygen is determined. One bottle is covered with dark paper while the other 
is left uncovered (Figure 1.a). Both bottles are incubated in the light for several hours. 
Dissolved oxygen is measured at the end of the experiment. 
 
 
 
 
 
Fig. 1.b Measurement of photosynthesis with 
light and dark bottles at various depths. 
 
Fig. 1.a Measurement of primary production in seawater 
samples by using light and dark bottles 
 
The clear bottle allows light to enter, so both photosynthesis and respiration take place, 
whereas the dark bottle provides data on respiration only. Thus, the amount of oxygen 
produced in clear bottles reflects the net primary production. 
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To calculate the total or gross primary production of organic matter by photosynthesis 
the amount of respiration must be known. This can be measured from the amount of 
oxygen used up in the dark bottle, where only respiration occurs because the light is 
blocked (gross primary production = net primary production + respiration). 
This light-dark bottle technique can be also applied by incubating in various levels of 
artificial light, or by actually dropping a string of bottles over the side and incubating 
them in natural light (Figure 1.b). With the artificial light technique, it is necessary to 
measure the light intensity as a function of depth, so that the measurements can be used 
to calculate the amount of photosynthesis in the natural environment. 
Practically speaking, respiration is often not actually measured, but is usually taken to be 
a value that corresponds to 10 percent of the total oxygen increase. 
 
¾ Chlorophyll extraction method 
A known volume of water is filtered, and plant pigments are extracted in acetone (or 
other organic solvent) from the organisms retained on the filter. The concentration of 
chlorophyll a is then estimated by placing the sample in a spectrophotometer to measures 
the extinction of different wavelengths in a beam of light shining through the sample. 
 
¾ Fluorometer technique 
Instead spectrophotometer an instrument called a fluorometer can be used for measuring 
chlorophyll concentration in acetone extract from its fluorescence. A fluorometer 
produces a certain wavelength of ultraviolet light which will cause chlorophyll to emit a 
red fluorescence, and this device can then estimate the amount of chlorophyll in a volume 
of water. Compared to spectrophotometer, fluorometer is more sensitive, it requires less 
sample, which may make it a better choice for chlorophyll analysis in less productive, 
ultra-oligotrophic systems. 
 
¾ The Pump and Probe Fluorometer 
PumpProbe Fluorometer is a submersible double-flash pulse fluorometer for continuous 
measurement of chlorophyll concentration and photosynthesis rate in situ. The method is 
very sensitive, and a fluorometer towed from a research vessel can rapidly record changes 
in chlorophyll concentration over large distances of sea surface 
 
¾ Radiocarbon Technique 
The most popular method of measuring productivity in the sea is the 14C method. In this 
method, bicarbonate ion is labeled with the radioactive isotope of carbon, 14C (The 
common nonradioactive isotope is 12C.) A small measured amount of radioactive 
bicarbonate (HCO3-) is added to two bottles of seawater containing phytoplankton. One 
bottle is exposed to light and permits photosynthesis and respiration; the other is shielded 
from all light so that only respiration takes place. The amount of radioactive carbon taken 
up per unit time is later measured on the phytoplankton when they are filtered out of the 
original samples. This radioactivity is measured in an instrument known as a scintillation 
counter, and primary productivity (in mg C m-3 h-1) is calculated from: 
( )rate of production = L DR R W
R t
− ×
× 
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where R is the total radioactivity added to a sample, t is the number of hours ofincubation, RL is the radioactive count in the 'light' bottle sample, and RD is the count of 
the 'dark' sample. W is the total weight of all forms of carbon dioxide in the sample (in 
mg C m-3), and this is determined independently, by, for example titration. The 
productivity is expressed as the amount (in mg) of carbon fixed in new organic material 
per volume of water (m-3) per unit time (h-1). 
The radiocarbon technique is preferable in waters of low productivity, because of the 
very low changes in oxygen concentration during photosynthesis. 
 
¾ Satellite Color Scanning 
Another method commonly used by scientists is satellite imagery which provides even 
broader spatial coverage of phytoplankton abundance and thus productivity. This 
technique is based on the fact that the radiance reflected from the sea surface in the 
visible spectrum (400-700 nm) is related to the concentration of chlorophyll. Satellites 
equipped with special cameras take color pictures of the sea surface and beam the images 
to earth. Using computers, scientists carefully analyze the photos, paying special attention 
to the characteristic green color of chlorophyll. Because water colour changes from blue 
to green as chlorophyll concentration increases, the relative colour differences can be 
used as a measure of chlorophyll concentration. 
FIG. 3 Color detectors in a satellite measure the radiance of the ocean, or the light that is reradiated after 
the sun’s light encounters and partially penetrates the ocean (the light that reaches the ocean is called the 
irradiance). 
 
Satellite measurements are not as sensitive as others and have restrictions of limited 
depth penetration, but they provide useful patterns of relative plant production on a global 
scale. Phytoplankton abundance can be estimated over vast areas of the sea surface by 
applying various correction factors, taking account of the weather at the time the photo 
was taken, and relating the results to actual chlorophyll measurements made from ships. 
Satellites are the only means for assessing the large-scale distribution of phytoplankton. 
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FIG. 4 Worldwide, year-round integrated estimate of chlorophyll concentration derived from data 
collected by the SeaWiFS satellite. False colors are used to represent the data, according to the 
accompanying scale. 
 
Regional Variation in Productivity 
In the ocean the amount of primary production varies dramatically from one region to 
other (the table below). Some marine environments are as productive as any on the earth. 
Others are biological deserts, with production as low as any desert on land. 
 
Table Typical rates of primary production in various marine environments 
 
Environment 
Rate of production 
(grams of carbon 
fixed/m2/yr) 
Pelagic Environments 
Arctic Ocean 0.7-100 
Southern Ocean (Antarctica) 40-260 
Subpolar seas 50-110 
Temperate seas (oceanic) 70-180 
Temperate seas (coastal) 110-220 
Central ocean gyres 4-40 
Equatorial upwelling areas** 70-180 
Coastal upwelling areas 110-370 
Benthic Environments 
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Salt marshes 2670-700 
Mangrove forests 370-450 
Seagrass beds 550-1,100 
Kelp beds 640-1,800 
Coral reefs 1,500-3,700 
Terrestrial Environments 
Extreme deserts 0-4 
Temperate farmlands 550-700 
Tropical rain forests 460-1,600 
 
Figure 5 shows the overall pattern of productivity in the ocean. Productivity in the ocean 
depends largely on the physical characteristics of the environment, in particular on the 
amount of light and nutrient that are available. Different oceans have different overall 
levels of primary productivity, which are determined by latitude, ocean basin shape, 
wind-driven surface currents, and the influence of surrounding continents. 
• average primary productivity in the oceans is ~50 g C/m2/yr 
• 300 g C/m2/yr considered relatively high rate of primary productivity 
• low rates of primary productivity typically 20 to 30 g C/m2/yr 
 
 
 
FIG. 5 Distribution of primary production in the oceans. (After Koblentz-Mishke et al., 1970.) 
 
 
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Productivity in the Eastern Mediterranean Sea 
The Eastern Mediterranean Sea is considered as one of the most oligotrophic regions in 
the world, both in terms of primary productivity and chlorophyll a concentrations. The 
primary productivity of the Eastern Mediterranean Sea is ranging from 50 to 150 mg 
C/m2/y, with a net decrease towards the eastern basin. The extremely low organic 
production, or oligotrophy, of the offshore waters of the Levantine Basin is manifested in 
the extreme transparency of the water, measured by the disappearance of a Secchi disc at 
a world-record depth of 53 meters. 
The construction of the High Dam had great impact on the fertility of the coastal waters. 
Before the construction of the dam, the Nile flood used to annually provide the Egyptian 
coast with large amounts of nutrients (see table 1). 
 
Table 1 Summary of pre-dam Conditions of Nile outflow. 
 
This nutrient-rich flood water, or Nile Stream, was detected off the Palestine coast (see 
the next satellite image). The fertilizing effect of the inflow of the nutrient-rich water 
during the flood season once resulted in exceptionally dense blooms of phytoplankton off 
the Nile Delta. This "Nile bloom" provided sustenance to sardines and other pelagic 
fishes. It also constituted a large source of detrital material, which forms a vital source of 
food for commercially valuable organisms such as shrimp. 
 
The decrease in fertility of the southeastern Mediterranean waters caused by the High 
Dam has had a catastrophic effect on marine fisheries. The average fish catch declined 
from nearly 35,000 tons in 1962 and 1963 to less than one-fourth of this catch in 1969. 
Hardest hit was the sardine fishery, primarily composed of sardine (Sardinella aurita), 
which is heavily dependent on increased phytoplankton during the flood season. Thus, 
from a total of 18,000 tons in 1962, a mere 460 and 600 tons of sardine were landed in 
1968 and 1969, respectively. The shrimp fishery also took a heavy toll as the catch 
decreased from 8,300 tons in 1963 to 1,128 tons in 1969.