228 D
.T. Dy, H.T. Yap J. Exp. Mar. Biol. Ecol. 251 2000 227 –238
1. Introduction
Coral reefs, which are productive ecosystems that grow in low nutrient waters, are composed of communities of corals, other animals and plants, and produce moderately
sustainable fisheries with the input of few extra nutrients Wilkinson, 1994. In most tropical reefs remote from big land masses, the growth of reef macrophytes is limited
either by the availability of inorganic nutrients or the extent of grazing by fishes and sea urchins. Marine organisms excrete soluble inorganic nutrients to their surrounding
waters in the form of ammonia Wright, 1995 and phosphate Pomeroy and Bush, 1959. These soluble excretory products are available to nutrient-limited primary
producers and possibly contribute to the nutrient pool in coral reef waters. The role of macroinvertebrates in nutrient recycling has been studied by several authors. For
example, Qian et al. 1996 showed that the alga, Kappaphycus alvarezii, treated with wastes of the pearl oyster, Pinctada martensi, grew much faster than when it was not
exposed to oyster wastes. Taylor and Rees 1998 observed that 79 of the nitrogen required by a subtidal bed of fucalean algae came from ammonium excreted by mobile
epifauna. Ambler et al. 1988 showed that productivity in Gracilaria was higher when ammonium excreted by the scallop Argopecten purpuratus was introduced. Ammonium
excreted by sea urchins is another mechanism that maintains the high rates of primary productivity of algal turf on coral reefs Williams and Carpenter, 1988. Mukai et al.
1989 pointed out that nitrogen released from megabenthic animals in a tropical seagrass meadow did not satisfy the nitrogen required for seagrass production but was
enough to sustain planktonic and epiphytic algal production.
There are several factors affecting the regeneration of nutrients by macroinvertebrates in algal-dominated coral reefs. Aside from purely species differences and occupancy of
different trophic guilds, other factors may come into play Diehl and Lawrence, 1979; Stickle and Bayne, 1982; Davoult et al., 1991; Mingoa-Licuanan, 1993. We studied
nutrient excretion and oxygen consumption rates of three common species of ech- inoderms found in tropical coral reefs, namely: Protoreaster nodosus starfish,
Tripneustes gratilla sea urchin, and Ophiorachna incrassata brittle star. We hypoth- esize that aside from taxonomic affiliation, the recent feeding history of the organism as
well as time of day are factors that affect the above.
2. Materials and methods
