7 – Primary Production, Nutrient Recycling and Energy Flow through Coral Reef Ecosystems
65
damage is an important step in ‘coral bleaching’ (see
below).
The characteristics of photosynthetic versus irradiance curves, while being driven by light, can change to
some extent through the process of ‘photosynthetic
acclimation’ (often mistakenly referred to as ‘photoadaptation’, which implies wrongly that evolutionary
‘adaptation’ is involved). To optimise light capture in
low light while reducing the risk of photoinhibition,
photosynthetic organisms actively manipulate the efficiency of light capture (A) by adding or subtracting
chlorophyll and other pigments to or from the photosynthetic components responsible for light capture.
This ability to process captured light (that reduces the
relative risk of photoinhibition) is high in organisms
that grow in high light habitats, and low in organisms
that grow in low light. Organisms can change their
ability to process captured light by increasing the
capacity of their dark reactions (Calvin-Benson cycle)
to increase P N max . These changes define many of the differences between shade and light-acclimated photosynthetic organisms (Fig. 7.2B).
Over day-night cycles, the photosynthetic activity of
reef communities fluctuates between periods of net consumption (i.e. at night, Fig. 7.2C) and periods of net
production of organic molecules (i.e. during the day,
Fig. 7.2C). On the way between these two extremes, photosynthetic reef organisms pass through periods (usually a few hours after sunrise or before sunset) in which
the fluxes of carbon dioxide, oxygen and energy are
zero (I c , as described above). As the sun rises, the rate of
photosynthetic activity becomes positive, increasing toward maximum values in the middle of the day. At this
point, P G approaches P G max that may be several times the
absolute value of respiration. The daily photosynthesis
to respiration ratio (P:R ratio) can be derived from the
integrated photosynthetic activity over a day (P G 24 h ) divided by the respiration that occurred over the same
period (R 24 h ) and is used by many physiological ecologists to examine how dependent an organism is on
energy derived directly from light. P:R ratios can be calculated for short (hourly) or long (daily, yearly) periods.
Organisms that have P:R ratios that are greater than 1.0
are referred to as autotrophic (‘self feeding’; also referred
to as phototrophic or ‘light feeding’) while organisms
that have ratios of less than 1.0 are referred to as heterotrophic (‘feeding on others’). Organisms on coral reefs
have a wide range of P:R ratios. Photosynthesis to respiration ratios can be applied to communities as well, giving important insight into how much organic carbon or
energy is entering a particular patch of land or seascape.
In the latter case, these measurements are easily integrated into measurements of the net accumulation of
organic carbon per unit time (primary production).
These types of measurements provide an important basis from which to answer key questions such as whether
coral reefs are a net sink or source CO 2 .
Productivity varies across the globe, with some of
the highest values being found in tropical rainforests
(700–800 g C m
2 y
1
. The highest values that occur in
the ocean range from 200 to 500 g C m
2 y
1 and are associated with nutrient rich coastal areas where upwelling occurs (e.g. off the west coasts of South America
and Africa). The oceans in which coral reefs occur generally have low primary production rates of around
0.1–1.0 g C m
2 y
1
. Coral reefs, on the other hand, show
average rates of primary production that range from
3–100 g C m
2 y
1 (averaged over all components including sand and rocky substrata) with some components (e.g. algal turfs, macroalgae, symbiotic corals)
having rates that match those of the highest productivities in terrestrial systems).
There is no single value for the primary production
of all coral reefs. Primary productivity varies with such
characteristics as whether a reef is inshore or offshore,
or whether it is a high or low latitude coral reef (Fig. 7.3).
Values for primary productivity also vary according to
location within a reef. The well flushed conditions of
the fore reef have the highest primary production values (up to 1500 g C m
2 y
1 ) while the poorly flushed
and more variable conditions (in terms of temperature
and light) of back-reef areas can have very low primary production values (up to 300 g C m
2 y
1 ).
Primary productivity also varies with the component
of reef ecosystem considered. Benthic microalgae
(see Chapter 15) and corals (and other symbiotic invertebrates) can have primary productivities of up to
2000 g C m
2 y
1 .
In addition to sunlight and CO 2 , photosynthetic
organisms require inorganic nutrients, particularly
65
damage is an important step in ‘coral bleaching’ (see
below).
The characteristics of photosynthetic versus irradiance curves, while being driven by light, can change to
some extent through the process of ‘photosynthetic
acclimation’ (often mistakenly referred to as ‘photoadaptation’, which implies wrongly that evolutionary
‘adaptation’ is involved). To optimise light capture in
low light while reducing the risk of photoinhibition,
photosynthetic organisms actively manipulate the efficiency of light capture (A) by adding or subtracting
chlorophyll and other pigments to or from the photosynthetic components responsible for light capture.
This ability to process captured light (that reduces the
relative risk of photoinhibition) is high in organisms
that grow in high light habitats, and low in organisms
that grow in low light. Organisms can change their
ability to process captured light by increasing the
capacity of their dark reactions (Calvin-Benson cycle)
to increase P N max . These changes define many of the differences between shade and light-acclimated photosynthetic organisms (Fig. 7.2B).
Over day-night cycles, the photosynthetic activity of
reef communities fluctuates between periods of net consumption (i.e. at night, Fig. 7.2C) and periods of net
production of organic molecules (i.e. during the day,
Fig. 7.2C). On the way between these two extremes, photosynthetic reef organisms pass through periods (usually a few hours after sunrise or before sunset) in which
the fluxes of carbon dioxide, oxygen and energy are
zero (I c , as described above). As the sun rises, the rate of
photosynthetic activity becomes positive, increasing toward maximum values in the middle of the day. At this
point, P G approaches P G max that may be several times the
absolute value of respiration. The daily photosynthesis
to respiration ratio (P:R ratio) can be derived from the
integrated photosynthetic activity over a day (P G 24 h ) divided by the respiration that occurred over the same
period (R 24 h ) and is used by many physiological ecologists to examine how dependent an organism is on
energy derived directly from light. P:R ratios can be calculated for short (hourly) or long (daily, yearly) periods.
Organisms that have P:R ratios that are greater than 1.0
are referred to as autotrophic (‘self feeding’; also referred
to as phototrophic or ‘light feeding’) while organisms
that have ratios of less than 1.0 are referred to as heterotrophic (‘feeding on others’). Organisms on coral reefs
have a wide range of P:R ratios. Photosynthesis to respiration ratios can be applied to communities as well, giving important insight into how much organic carbon or
energy is entering a particular patch of land or seascape.
In the latter case, these measurements are easily integrated into measurements of the net accumulation of
organic carbon per unit time (primary production).
These types of measurements provide an important basis from which to answer key questions such as whether
coral reefs are a net sink or source CO 2 .
Productivity varies across the globe, with some of
the highest values being found in tropical rainforests
(700–800 g C m
2 y
1
. The highest values that occur in
the ocean range from 200 to 500 g C m
2 y
1 and are associated with nutrient rich coastal areas where upwelling occurs (e.g. off the west coasts of South America
and Africa). The oceans in which coral reefs occur generally have low primary production rates of around
0.1–1.0 g C m
2 y
1
. Coral reefs, on the other hand, show
average rates of primary production that range from
3–100 g C m
2 y
1 (averaged over all components including sand and rocky substrata) with some components (e.g. algal turfs, macroalgae, symbiotic corals)
having rates that match those of the highest productivities in terrestrial systems).
There is no single value for the primary production
of all coral reefs. Primary productivity varies with such
characteristics as whether a reef is inshore or offshore,
or whether it is a high or low latitude coral reef (Fig. 7.3).
Values for primary productivity also vary according to
location within a reef. The well flushed conditions of
the fore reef have the highest primary production values (up to 1500 g C m
2 y
1 ) while the poorly flushed
and more variable conditions (in terms of temperature
and light) of back-reef areas can have very low primary production values (up to 300 g C m
2 y
1 ).
Primary productivity also varies with the component
of reef ecosystem considered. Benthic microalgae
(see Chapter 15) and corals (and other symbiotic invertebrates) can have primary productivities of up to
2000 g C m
2 y
1 .
In addition to sunlight and CO 2 , photosynthetic
organisms require inorganic nutrients, particularly