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.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
2.1. Experimental design The experiment followed a nested hierarchical design with the species’ recent
feeding history i.e. recently-collected vs. short-term starvation for 361 days being nested within time of day i.e. daytime between 10:00 and 12:00 h vs. nighttime between
22:00 and 24:00 h which in turn was nested under species Table 1. Recently collected specimens were considered fed organisms since preliminary gut content analysis
indicated partly digested and recently eaten material. Each experimental run consisted of a control flask without an organism and three flasks each containing one organism of the
D .T. Dy, H.T. Yap J. Exp. Mar. Biol. Ecol. 251 2000 227 –238
229 Table 1
Nested hierarchical design outlining the various independent and dependent variables where D 5 daytime, N 5 nightime, F 5 recently collected presumed fed, S 5 starved for 361 days
Independent variables Echinoderm species
T . gratilla
O . incrassata
P . nodosus
sea urchin brittle star
starfish Time of day
D N
D N
D N
Recent feeding history F
S F
S F
S F
S F
S F
S Dependent variables
← ——————————
Oxygen consumption ——————————
→ ←
—————————— Ammonium excretion
—————————— →
← ——————————
Phosphate excretion ——————————
→
same species. The experimental run for each factor combination was conducted twice to give a total of six replicates.
2.2. Collection of the test organisms Echinoderms were collected from a coral reef flat in the eastern side of Mactan Island,
Central Philippines Fig. 1. Collection involved skin diving and careful handpicking of the organisms, and placing them in plastic containers. They were immediately brought to
the laboratory and acclimated for at least 6 h before subjecting them to the incubation experiments. The test animals were not fed during acclimation to minimize the release of
fecal matter during the incubation proper which would possibly affect the measurements.
2.3. Experimental procedures Before each experiment, seawater was collected at the same site as the organisms,
passed through a 10-mm filter bag and poured into 2.7-l reaction flasks. The salinity ranged between 32.3 and 34.0‰. After initial water samples for dissolved oxygen, and
ammonium and phosphate measurements were collected from each flask, the acclimated organisms, previously washed with filtered seawater, were placed in the reaction flasks.
The flasks were filled with filtered seawater and made to overflow for 1 min and then covered, after which they were placed in a water bath temperature between 23.2 and
25.08C for 1–2 h. At the end of the incubation period, each flask was placed on top of a magnetic stirrer with gentle stirring for 3 min to homogenize the medium, after which
final samples for dissolved oxygen, ammonium and phosphate concentration were collected.
After incubation, the volume of the organism was measured by seawater displace- ment. The organism was dried in an oven at 908C for at least 48 h or until constant
weight. 2.4. Seawater analysis
Samples for dissolved oxygen measurement before and after each incubation were
230 D
.T. Dy, H.T. Yap J. Exp. Mar. Biol. Ecol. 251 2000 227 –238
Fig. 1. Map of the collecting site Maribago, Mactan Is., Central Philippines.
collected in duplicate and analyzed using the classic Winkler method adapted by
21
Grasshoff et al. 1983 with a precision of60.05 mg l . All nutrient samples were
collected in triplicate and analyzed within 3 h of collection. Ammonium determination was done after the phenol–hypochlorite procedure of Strickland and Parsons 1972
while inorganic phosphate was analyzed using the ascorbic acid–molybdenum reaction and the procedure patterned after Grasshoff et al. 1983. Absorbance was measured
with a spectrophotometer at 640 nm for ammonium and 880 nm for phosphate using a 1-cm cell.
D .T. Dy, H.T. Yap J. Exp. Mar. Biol. Ecol. 251 2000 227 –238
231
2.5. Statistical analysis Tests for significant differences due to the recent feeding history of the organisms
which was nested under time of incubation which in turn was nested under species were carried out using nested hierarchical analysis of covariance ANCOVA at P 5 0.05
with temperature and salinity acting as covariates. Dependent variables oxygen consumption, ammonium and phosphate excretion were initially tested for normality
and homoscedasticity. Data were transformed i.e. oxygen consumption using log ,
10
ammonium excretion using square root and phosphate excretion using the sine function because the dependent variables were heteroscedastic. In cases where significant
differences were detected among the dependent variables after ANCOVA, Tukey’s H.S.D. test was used for post-hoc comparisons of the main effects. Planned comparisons
were carried out to compare effects of recent feeding history and time of incubation for each echinoderm species. To check whether there was a correlation between oxygen
consumption and nutrient excretion in each species, we used Pearson product-moment correlation Zar, 1984.
3. Results
