136
Benthic Microflora, Periphyton and Plant Associations
wet weight g-l (Sorokin 1978b). In the detrital ("regenerative") sediments
it could be ten times more (Sorokin 1973c). The gross primary production of
microphytobenthos varied from 0.1-0.3 gem -2 day-l in the coral sands of
the polluted or deteriorating reefs with a poor coral growth, up to 13 gem -2 in sands of the flourishing reefs. Its top value (11 gem -2) was
recorded in the greenish sand from the beach of the atoll Majuro (Sorokin
1978b). At its rates of 2-3 g C g-2 the gross photosynthesis in sand per 1 g
will be 100-150 Ilg C g-l day-I, which is not very far from the same values in
living corals calculated per weight of their colonies (150-300 Ilg C g-l day-I;
cf. Sect. 4.2). The PIB ratios in most coral sands investigated was most often
below 1, reflecting so the domination of heterotrophic processes in them.
Additional sources of organic matter are obviously needed to support them.
The latter could be the coral mucus and the detritus transported from the
autotrophic reef-flat biotopes. Thus the soft reef sediments, which cover
60-80% of the total reef-bottom area, are the sites of intensive promary and
heterotrophic production, which supports in them an abundant zoobenthos.
Microphytes and microheterotrophs of the soft bottom are responsible for
approximately 20-30% of the total primary production and for 40-% of the
total respiration of coral reef ecosystems as a whole.
The biomass of bacterial and algal microbenthic communities is a basic
food source for numerous sediment-eating benthic animals, like holothurians,
various worms and clams, as well as for many representatives of microbenthos and meiobenthos (Sorokin 1973c; Moriarty et al. 1985b; Hansen
et al. 1985; Hansen and Skilleter 1988). Experimental evaluations of the
grazing rates of bacteria demonstrated visible effects of decrease in the
density of microbial populations by holothurians, clams and microbenthic
copepods, and especially by the harpacticoids. Benthic bacteria are consumed also by nematods, protozoans, and turbellarians. Benthic grazers
consume the most part of their daily production. Their assimilation by
holothurians was proved in experiments with the use of radio labelled
bacteria (Sorokin 1978b).
4.2 Periphytonic Overgrowth
Vast surfaces of the porous lime reef rocks, as well as the rubble and the
heads of dead corals, are densely overgrown with periphytonic turfs. The
areas of these overgrown surfaces on the reef exceed 2-3 times the projective ones (Dahl 1973; Odum and Odum 1955). The periphytonic turfs are
2-5 mm thick. They look like brushes or like greenish or reddish mucous
covers depending on the kinds of dominating algae. The turfs are structured
like a microhabitats or microbioeoenoses, which unite the autotrophic and
the heterotrophic components in one mucoid structure. Among these components different bacteria and algae, ciliates and forams as well as other
Benthic Microflora, Periphyton and Plant Associations
wet weight g-l (Sorokin 1978b). In the detrital ("regenerative") sediments
it could be ten times more (Sorokin 1973c). The gross primary production of
microphytobenthos varied from 0.1-0.3 gem -2 day-l in the coral sands of
the polluted or deteriorating reefs with a poor coral growth, up to 13 gem -2 in sands of the flourishing reefs. Its top value (11 gem -2) was
recorded in the greenish sand from the beach of the atoll Majuro (Sorokin
1978b). At its rates of 2-3 g C g-2 the gross photosynthesis in sand per 1 g
will be 100-150 Ilg C g-l day-I, which is not very far from the same values in
living corals calculated per weight of their colonies (150-300 Ilg C g-l day-I;
cf. Sect. 4.2). The PIB ratios in most coral sands investigated was most often
below 1, reflecting so the domination of heterotrophic processes in them.
Additional sources of organic matter are obviously needed to support them.
The latter could be the coral mucus and the detritus transported from the
autotrophic reef-flat biotopes. Thus the soft reef sediments, which cover
60-80% of the total reef-bottom area, are the sites of intensive promary and
heterotrophic production, which supports in them an abundant zoobenthos.
Microphytes and microheterotrophs of the soft bottom are responsible for
approximately 20-30% of the total primary production and for 40-% of the
total respiration of coral reef ecosystems as a whole.
The biomass of bacterial and algal microbenthic communities is a basic
food source for numerous sediment-eating benthic animals, like holothurians,
various worms and clams, as well as for many representatives of microbenthos and meiobenthos (Sorokin 1973c; Moriarty et al. 1985b; Hansen
et al. 1985; Hansen and Skilleter 1988). Experimental evaluations of the
grazing rates of bacteria demonstrated visible effects of decrease in the
density of microbial populations by holothurians, clams and microbenthic
copepods, and especially by the harpacticoids. Benthic bacteria are consumed also by nematods, protozoans, and turbellarians. Benthic grazers
consume the most part of their daily production. Their assimilation by
holothurians was proved in experiments with the use of radio labelled
bacteria (Sorokin 1978b).
4.2 Periphytonic Overgrowth
Vast surfaces of the porous lime reef rocks, as well as the rubble and the
heads of dead corals, are densely overgrown with periphytonic turfs. The
areas of these overgrown surfaces on the reef exceed 2-3 times the projective ones (Dahl 1973; Odum and Odum 1955). The periphytonic turfs are
2-5 mm thick. They look like brushes or like greenish or reddish mucous
covers depending on the kinds of dominating algae. The turfs are structured
like a microhabitats or microbioeoenoses, which unite the autotrophic and
the heterotrophic components in one mucoid structure. Among these components different bacteria and algae, ciliates and forams as well as other