Remarks on the Distribution of LargerForaminifera (Protozoa) from Belau (WesternCarolines)

著者 "HOHENEGGER Johann"journal orpublication title

南太平洋海域調査研究報告=Occasional papers

volume 30page range 85-90URL http://hdl.handle.net/10232/16891

Kagoshima Univ. Res. Center S. Pac, Occasional Papers, No. 30, 85-90, 1996 35Survey Team II, Report 6. The Progress Report of the 1995 Survey of the Research Project,

"Man and the Environment in Micronesia"

REMARKS ON THE DISTRIBUTION OF LARGER FORAMINIFERA

(PROTOZOA)

FROM BELAU (WESTERN CAROLINES)

Johann Hohenegger

Introduction

Larger Foraminifera are marine single-celled organisms attaining the greatest size within

protozoans. They hide their protoplasm in calcium carbonate tests that measure more than 2

mm in diameter, sometimes reaching more than 100 mm. All living larger Foraminifera house

symbiotic microalgae and are thus restricted to the photic zone.

Two main factors regulate the distributions of larger foraminifers within coral reef com

plexes. Differences in water turbulence, mostly correlated with substrate type, and light availa

bility are managed in various ways. Test constructions in combination with attachment mechan

isms of the propoplasm combat strong water currents, while light penetration is handled by test

ultrastructure. Larger foraminifers inhabiting intertidal and extremely shallow subtidal environ

ments block high irradiation by thicker tests or porcelaneous structures, making the walls im

penetrable. In contrast, species living near the base of the photic zone facilitate light penetration

by thin test walls and by developing light-collecting mechanisms such as nodes and pillars

(Hohenegger 1994, Hottinger 1977, 1983).

In consideration of these complexities in test form, coral reef environments are characte

rized by different distributions of larger foraminifera, based on high correlations between ecolo

gical gradients and foraminifers. Therefore, changes in ecological factors, like global warming,

rising CO2 levels, and acidification of sea-water, are reflected in the distribution of larger

forams (Pecheux 1995). They can become important indicators of minor and major environmental disturbances, and will provide better information in respect to bleaching effects than cor

als and other macro-invertebrates (Glynn 1993, Hallock & Talge 1993).

The distribution of larger foraminifers in the Western Pacific was studied in detail on the

Ryukyu Islands (Hohenegger 1994). A comparison with faunas from Belau described by Hal-lock (1984) is useful in order to confirm the results gained in Okinawa, since the WesternCarolines, where larger foraminifers show the highest diversification, can be regarded as the

radiation center of Pacific species. Hallock's description of foraminiferan distribution from Be

lau dealt with depth dependency but neglected differences in environments of similar depths.

Furthermore, sampling was performed down to 30 m only, and ecological demands of species

living in deeper regions of the archipelago were not described. Therefore, a detailed study of

environmental dependencies of larger foraminifers from Belau is necessary.

Results

The marine environment of the Belau archipelago is characterized by differentiation into

four larger units. A broad barrier reef (unit 1) with steep slopes to the ocean (unit 2) surrounds the whole island group (Fig. 1) except the island Angaur in the South and the northern

86J. Hohenegger

atolls. Northeast of the largest island Babeldaob, this barrier transgresses into a fringing reef. Alot of smaller islands scatter the southeast of the lagoon, while its western part shows a densepattern of patch reefs. Lagoon channels (unit 3) between islands or patch reefs are less than 20mdeep near the barrier reef, but the central part of the lagoon and channels at the western sideof the small islands arch can reach 60 m depth (unit 4).

Sediments were investigated at sample locations only (Fig. 1). Highly diverse corals grow inthe upper part of steep reef slopes bordering the outside barrier. Sandy sediments are rare at alldepths in less steep region. The frontal crest of the barrier reef shows coral rubble cemented bycoralline algae, resulting in hard reef rocks. Small macroalgae, like Hypnaea and Halimeda, inhabit small grooves between exposed crest parts.

Coral carpets (Reiss &Hottinger 1984) with coral rubble as the dominant sediment typeare developed in beach parts of the lagoon islands close to channels. Larger macroalgae{Sargassum, Halimeda) settle in transition zones of carpets to sandy bottoms, which are frequent in shallower areas «1 m) behind the zones of coral growth. Sometimes large sea-grasses{Enhalus, Thalassia, Halophila) cover calm regions of these sand flats. Bottoms of shallowchannels «20 m) and the deeper lagoon show coarse-grained sand, while fine-grained sandsand silty sediments characterize deeper bottoms (>20 m) of restricted basins between islands(Arimizu Bay, Fig. 1).

All 34 samples were taken by either snorkeling, SCUBA diving, dredging, or grab sampling. These samples were investigated using the method described by Hohenegger (1994).Sampling in deeper parts of the outer slope was successful in 86 m(Western Passage, Fig. 1)and a rich fauna with the nummulitids Planostegina operculinoides and Cycloclypeus carpenteriwas detected; these species also characterize the deeper fore reef fauna in Okinawa.

Clustering of samples resulted in aclear assemblage differentiation, which can be separatedin two larger classes . The first contains extremely shallow-water species (Fig. 2), the secondcomprises samples from the upper subtidal down to 60 mdepth (Fig. 3).

Three distinct sample groups are found in the shallowest regions of the archipelago(Fig. 2). The star sand foraminifers Baculogypsina sphaerulata and Calcarina gaudichaudiidominate in the lower intertidal of the frontal reef crest. They cling with their spines to smallmacroalgae as Halimeda and Hypnaea and are thus able to withstand strong water turbulence.Beside these abundant forms, Amphistegina lobiferoc and the soritids Amphisorus hemprichiiand Marginopora vertebralis are common. Standing crop is high (150 indiv.-100 cm"2), but tentimes lower than in Okinawa reef crests. Characteristic species of the second sample group fromthe shallowest subtidal of the lagoon are Neorotalia calcar, Calcarina hispida, and Calcarina de-francii, all belonging to star sand foraminifers, but quite distinct from the high energetic formsof the first group. Calcarina gaudichaudii, a prominent member of the high energetic group, isan element of this second cluster too, but with smaller proportions (Fig. 2). Amphisorus hemprichii and Amphistegina lobifera are represented in both clusters as well, while the small-sizedMarginopora vertebralis is replaced by its large relative Marginopora kudakajimaensis. Star sandforams cling with their spines to macroalgal thalli; large soritids prefer attachment to coral rubble. Standing crop changes from 70 indiv.-lOO cm"2 on pure rubble to 450 indiv.-lOO cm"2 inSargassum-dom'maXed environments.

The third group of extremely shallow reef flat samples (Fig. 2) is characterized by sandybottoms bearing sea-grass and/or filamentous macroalgae. The foraminiferal fauna (100 to 350indiv.-lOO cm"2 standing crop) of these environments with calm water conditions is domin-

Remarks on the Distribution of Foraminifera

:i Pele Iiu %; ViM^MMBM

Fig. 1. Sample location map of the Belau archipelago.

ated by Peneroplis planatus, P. perlusus, and the archaisinid Laevipeneroplis proteus, which can

be easily identified by its green color based on chlorophycean endosymbionts. Beside these spe

cies, Amphistegina lessonii appears to be connected with deeper faunas of the shallow lagoon.

This species is abundant in environments of the shallow lagoon (20 indiv.-lOO cm ), accompanied by Amphisorus hemprichii, the nummulitid Heterostegina depressa, and its relative Amphistegina radiata (Fig. 3).

Sandy sediments of the deeper lagoon show characteristic assemblages of larger foraminifers

with standing crops four times higher than in the shallow lagoon (80 indiv.-lOO cm"2). Amphistegina radiata is the most frequent species here, together with the nummulitids Operculina ammo-noides, Heterostegina depressa, Nummulites venosus, and the rare archaiasinid Parasorites orbito-

litoides. Amphistegina bicirculala and A. papulosa supplement this association in the deepest

parts of the lagoon (Fig. 3). An Operculina-spe,c\es differing from Operculina ammonoides bytest form is the single representative in restricted areas below 20 m, where basin bottoms consist

of fine sediments. Standing crop in this environment reaches the highest value of all

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investigated samples (> 500 indiv.-lOO cm"2).Reef slope assemblages are similar in composition to deeper lagoon faunas, yet demonstrate

a significantly lower mean standing crop (20 indiv/100 cm"2). Amphistegina radiata is abundantin the upper 30m of the slope, accompanied by A. lessonii. The former loses its dominant rolein the deeper parts; A. bicirculata replaces A. lessonii there, while Apapillosa becomes a competitor of its relative A. radiaAa. Heterostegina depressa is an important element of all upperslope associations.

The fauna from Belau and Okinawa are similar in foraminiferal content and distribution.

Minor differences are caused by topographic conditions. The reef slope fauna of Okinawa ismuch richer than its equivalent from Belau because the slopes are generally flatter. Back-reefforaminifers, in contrast, show a much more differentiated distribution in Belau than in Okinawa, since the lagoon environments are more diverse in the southern archipelago. Nevertheless,the main trends in environmental response of larger forams are extremely similar in both island

groups.

References

Glynn, P.W. 1993. Coral Reefs, 12: 1-17.Hallock, P. 1984. J. Foramin. Res., 14: 250-261.

1985. Paleobiology, 11: 195-208.& Talge, H.K. 1993. Rosenstiel School Marine Atmosph. Sci., June 10-11, 1993:

8-13.

Hohenegger, J. 1994. P. S. Z. N. I.: Marine Ecology, 15: 291-334.Hottinger, L. 1977. Utrecht Micropaleont. Bull., 15: 35-110.

1983. Utrecht Micropaleont. Bull., 30: 239-253.

Pecheux, M. J.-F. 1995. Geobios, 28: Addendum, 565-566.Reiss, Z. & Hottinger, L. 1984. The Gulf of Aqaba. Ecological Studies, 50. 354 pp. Springer

Verlag, Berlin.