62
Reef Environments
within 0-0.5mgm-2 (Table 2.6). The same flow rates were measured in
lagoonal coral sands containing a rich microphytobenthos. The in situ enclosure experiments in this biotope gave the values of flows Ae and Ae as
2 to 7mgPm- 2 day-1. A significant net release of P04-P in 24h balance
(+ At) was recorded in most sites of this biotope: 1-3 mg P m -2. In the same
kind of experiments by Harrison (1983) the At values were ±0.1-1.5,
changing sign in different parts of the lagoon. A low level of the balance
values At in coral reef bottom biotopes actually reflects the above-mentioned
semi-closed character of nutrients cycles in their main compartments - in
corals, in periphytonic communities, and in microphytobenthos of coral
sands. In bottom biotopes occupied by corals and periphytonic overgrowths
(Tables 2.3, 2.5; Fig. 2.10) at low ambient P04-P concentrations (0.10.5 mol) the balance At value is approaching zero. Thus, it seems that the
communities do not release any inorganic P04-P, while the reciprocal Ae
and Ae flows could be significant. But we leave unaccounted for in this
balance an intensive consumption flow of the organic P, which is consumed
by periphyton as its dissolved, and by corals as both dissolved and planktonic
fractions. It is just this unaccounted for part which covers a slight release of
P04-P by benthic communities and participates as well in the building up of
the stock of organic phosphate stocks and mineral phosphorite (apatite)
deposits in bottom sediments and in reef rocks (Entsch et al. 1983). The
bottom reef community then as a whole consumes phosphorus as well in its
inorganic as its the organic forms, thus providing a significant portion of the
necessary nutrients supply to its autotrophs.
The calculated ratios between the flows of P04-P and rates of metabolism
in bottom communities occupied by corals and rubble look like the following.
At average values of these lows (Ae, Ae) 5-20mgPm- 2 , and by rates of
photosynthesis and respiration of 2-4 g C m -2 per day the C: P ratio will be
100-400 by weight. A high degree of equilibrium in the processes of
nutrients dynamics between bottom and water seems to explain the fact of
the coral reef ecosystem's ability to control it via its biological mechanisms.
In fact, a low ambient level of the concentrations of inorganic nutrients is
preserved even in reef areas subjected to permanent pollution (Johannes
1972b; Kimmerer and Walsh 1981; Crossland 1983) or in the artificially
fertilized reef biotopes (Kinsey and Domm 1974), while drastic changes in
the structure of their benthic communities could happen (Wiebe 1985). This
ability to keep a low level of nutrients in water when building up their stock
in bottom biotopes and in rocks is probably related to the very existence of
coral reefs, because an elevated level of nutrients in reef waters increases
the competition from the side of algae, and also inhibits the growth and
undermines the health of corals. Elevated nutrients levels will cause a
decrease in their calcification and enhance their infestation with boring
animals (Kinsey and Davies 1979b; Highsmith 1979b).
Main pools of mobile nutrients exist mainly in the living biomass of
organisms, and (much less) in organic matter of detritus and sediments
Reef Environments
within 0-0.5mgm-2 (Table 2.6). The same flow rates were measured in
lagoonal coral sands containing a rich microphytobenthos. The in situ enclosure experiments in this biotope gave the values of flows Ae and Ae as
2 to 7mgPm- 2 day-1. A significant net release of P04-P in 24h balance
(+ At) was recorded in most sites of this biotope: 1-3 mg P m -2. In the same
kind of experiments by Harrison (1983) the At values were ±0.1-1.5,
changing sign in different parts of the lagoon. A low level of the balance
values At in coral reef bottom biotopes actually reflects the above-mentioned
semi-closed character of nutrients cycles in their main compartments - in
corals, in periphytonic communities, and in microphytobenthos of coral
sands. In bottom biotopes occupied by corals and periphytonic overgrowths
(Tables 2.3, 2.5; Fig. 2.10) at low ambient P04-P concentrations (0.10.5 mol) the balance At value is approaching zero. Thus, it seems that the
communities do not release any inorganic P04-P, while the reciprocal Ae
and Ae flows could be significant. But we leave unaccounted for in this
balance an intensive consumption flow of the organic P, which is consumed
by periphyton as its dissolved, and by corals as both dissolved and planktonic
fractions. It is just this unaccounted for part which covers a slight release of
P04-P by benthic communities and participates as well in the building up of
the stock of organic phosphate stocks and mineral phosphorite (apatite)
deposits in bottom sediments and in reef rocks (Entsch et al. 1983). The
bottom reef community then as a whole consumes phosphorus as well in its
inorganic as its the organic forms, thus providing a significant portion of the
necessary nutrients supply to its autotrophs.
The calculated ratios between the flows of P04-P and rates of metabolism
in bottom communities occupied by corals and rubble look like the following.
At average values of these lows (Ae, Ae) 5-20mgPm- 2 , and by rates of
photosynthesis and respiration of 2-4 g C m -2 per day the C: P ratio will be
100-400 by weight. A high degree of equilibrium in the processes of
nutrients dynamics between bottom and water seems to explain the fact of
the coral reef ecosystem's ability to control it via its biological mechanisms.
In fact, a low ambient level of the concentrations of inorganic nutrients is
preserved even in reef areas subjected to permanent pollution (Johannes
1972b; Kimmerer and Walsh 1981; Crossland 1983) or in the artificially
fertilized reef biotopes (Kinsey and Domm 1974), while drastic changes in
the structure of their benthic communities could happen (Wiebe 1985). This
ability to keep a low level of nutrients in water when building up their stock
in bottom biotopes and in rocks is probably related to the very existence of
coral reefs, because an elevated level of nutrients in reef waters increases
the competition from the side of algae, and also inhibits the growth and
undermines the health of corals. Elevated nutrients levels will cause a
decrease in their calcification and enhance their infestation with boring
animals (Kinsey and Davies 1979b; Highsmith 1979b).
Main pools of mobile nutrients exist mainly in the living biomass of
organisms, and (much less) in organic matter of detritus and sediments