Download Marine Primary Productivity: Measurements and Variability

Marine Primary Productivity:
Measurements and Variability
Why should we care about productivity?
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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.a Measurement of primary production in seawater
samples by using light and dark bottles
Fig. 1.b Measurement of photosynthesis with
light and dark bottles at various depths.
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:
( R − R D ) ×W
rate of production = L
R ×t
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where R is the total radioactivity added to a sample, t is the number of hours of
incubation, 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
Rate of production
Environment
(grams of carbon
fixed/m2/yr)
Pelagic Environments
Arctic Ocean
Southern Ocean (Antarctica)
Subpolar seas
Temperate seas (oceanic)
Temperate seas (coastal)
Central ocean gyres
Equatorial upwelling areas**
Coastal upwelling areas
Benthic Environments
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0.7-100
40-260
50-110
70-180
110-220
4-40
70-180
110-370
Salt marshes
Mangrove forests
Seagrass beds
Kelp beds
Coral reefs
2670-700
370-450
550-1,100
640-1,800
1,500-3,700
Terrestrial Environments
Extreme deserts
Temperate farmlands
Tropical rain forests
0-4
550-700
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.
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