54
Reef Environments
have ciliated epithelium, such as trochophores, echinocardiums, corals,
clams, serpulid and sabellid worms (Stephens 1962; Sorokin 1977b, 1984b;
Sorokin and Wyshkwartzev 1973). The suspended organic nutrients are
consumed by sedimentary and filter-feeders. Nutrients contained in the
biomass of zooplankton, in passing over the reef oceanic waters, enter the
nutrients pool of the reef ecosystem through their consumption by
planktonic and benthic predators - mainly by corals and planktonovorous
fish (Hamner et al. 1988). These fish and also larvae of many bottom fish
and various bottom fauna while grazing zooplankton ex rete feces,
containing a bulk of nutrients consumed, which then are mineralized via the
food-web by animals, which feed of feces, including the corals themselves
(Meyer et al. 1983).
2.2.3 Nitrogen Fixation
The assimilation of atmospheric nitrogen by nitrogen-fixing microbial and
cyanobacterial periphytonic and endolithic associations wide-spread in
bottom reef biotopes is the basic process in building up the positive balance
of nitrogen in reef ecosystems. The first observations on the dynamics of
nitrogen in waters passing over the actively functioning reefs, even with a
predomination of macrophytes that had to consume inorganic nitrogen from
them, nevertheless invariably recorded a permanent and intensive release of
nitrogen by them, mainly in the form of ammonia and in organic compounds
(Johannes et al. 1972; Webb et al. 1975; Wiebe et al. 1975; Wiebe 1976).
The conclusion that this enrichment of oceanic waters with nitrogen passing
over the reef is the sequel of an intensive Nz-fixation in its bottom biotopes
seemed to be quite natural, accounting for the abundance of nitrogen fixing
cyanobacteria in them (Hatcher and Hatcher 1981; Paerl et al. 1981). The
reef-bottom biotopes enriched with organic matter are also optimal sites for
the growth of azotobacter. Direct estimations of rates of N 2 fixation were
made with the aid of N I5 (Burris 1976) and with the acetylene method
(Hanson and Gundersen 1976; Capone 1977; Pehnhale and Capone 1981;
Wilkinson et al. 1984). They showed that the most active N2 fixation takes
place in periphyton, which covers the rubble, the dead corals and the flat
rocks, as well as in the upper layer of coarse Halimt::da sands. The
periphyton overgrowing these substrates contains a large quantity of various
Nz-fixing blue-green algae such as: Stychonema, Calotrix, Nostoc,
Hermothamnion, Hyella, Oscillatoria, Symploca, Lyngbia. The rates of
Nz-fixation in these sites varied within 1-9 (NI5-method) and 1030 Ilg N g -I h -I (acetylene method). Very high rates of Nrfixation were
recorded in zones where the macrophytes had been grazed out by fish and
were then replaced by periphyton (Wilkinson et al. 1984). Some sponges
harboring the cyanobacterial symbionts appeared to be able to fix
atmospheric N2 (Wilkinson and Fay 1979). In various reef biotopes the
Reef Environments
have ciliated epithelium, such as trochophores, echinocardiums, corals,
clams, serpulid and sabellid worms (Stephens 1962; Sorokin 1977b, 1984b;
Sorokin and Wyshkwartzev 1973). The suspended organic nutrients are
consumed by sedimentary and filter-feeders. Nutrients contained in the
biomass of zooplankton, in passing over the reef oceanic waters, enter the
nutrients pool of the reef ecosystem through their consumption by
planktonic and benthic predators - mainly by corals and planktonovorous
fish (Hamner et al. 1988). These fish and also larvae of many bottom fish
and various bottom fauna while grazing zooplankton ex rete feces,
containing a bulk of nutrients consumed, which then are mineralized via the
food-web by animals, which feed of feces, including the corals themselves
(Meyer et al. 1983).
2.2.3 Nitrogen Fixation
The assimilation of atmospheric nitrogen by nitrogen-fixing microbial and
cyanobacterial periphytonic and endolithic associations wide-spread in
bottom reef biotopes is the basic process in building up the positive balance
of nitrogen in reef ecosystems. The first observations on the dynamics of
nitrogen in waters passing over the actively functioning reefs, even with a
predomination of macrophytes that had to consume inorganic nitrogen from
them, nevertheless invariably recorded a permanent and intensive release of
nitrogen by them, mainly in the form of ammonia and in organic compounds
(Johannes et al. 1972; Webb et al. 1975; Wiebe et al. 1975; Wiebe 1976).
The conclusion that this enrichment of oceanic waters with nitrogen passing
over the reef is the sequel of an intensive Nz-fixation in its bottom biotopes
seemed to be quite natural, accounting for the abundance of nitrogen fixing
cyanobacteria in them (Hatcher and Hatcher 1981; Paerl et al. 1981). The
reef-bottom biotopes enriched with organic matter are also optimal sites for
the growth of azotobacter. Direct estimations of rates of N 2 fixation were
made with the aid of N I5 (Burris 1976) and with the acetylene method
(Hanson and Gundersen 1976; Capone 1977; Pehnhale and Capone 1981;
Wilkinson et al. 1984). They showed that the most active N2 fixation takes
place in periphyton, which covers the rubble, the dead corals and the flat
rocks, as well as in the upper layer of coarse Halimt::da sands. The
periphyton overgrowing these substrates contains a large quantity of various
Nz-fixing blue-green algae such as: Stychonema, Calotrix, Nostoc,
Hermothamnion, Hyella, Oscillatoria, Symploca, Lyngbia. The rates of
Nz-fixation in these sites varied within 1-9 (NI5-method) and 1030 Ilg N g -I h -I (acetylene method). Very high rates of Nrfixation were
recorded in zones where the macrophytes had been grazed out by fish and
were then replaced by periphyton (Wilkinson et al. 1984). Some sponges
harboring the cyanobacterial symbionts appeared to be able to fix
atmospheric N2 (Wilkinson and Fay 1979). In various reef biotopes the