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DEPTH DISTRIBUTION OF CALCAREOUS ENCRUSTING ASSOCIATIONS IN THE NORTHERN RED SEA (SAFAGA, EGYPT) AND THEIR GEOLOGICAL IMPLICATIONS

Michael Rasser and W. E. Piller

Published: Proc. 8th Int. Coral Reef Sym. 1: 743-748. 1997

INDEX:

ABSTRACT
INTRODUCTION
STUDY AREA
METHODS
RESULTS
     Taxonomy
     Depth distribution of species
     Encrusting Associations
DISCUSSION
     Depth distribution of taxa
     Depth distribution of Encrusting Associations
     Geological implications
REFERENCES

 


ABSTRACT

Encrusting biota were sampled at two fringing reefs in the northern Safaga Bay (Red Sea, Egypt) in vertical transects between 0 and 51 m. Crust forming calcareous algae and foraminifera are common in both reefs, growing on various hard substrates. With increasing depth crusts get thicker and more affected by borings and cementation. Between 0 and 35 m, the associations of encrusting organisms are clearly dominated by coralline algae. Below 35 m, acervulinid foraminifera and peyssonneliacean algae dominate. A clear depth zonation with four species associations is observable (Lithophyllum kotschyanum - Association, Hydrolithon onkodes - Association, Sporolithon ptychoides - Acervulina inhaerens - Association, and A. inhaerens - Peyssonnelia sp. - Association). These distinct species - associations support the idea of using calcareous encrusting associations as depth indicators in fossil environments.

 

INTRODUCTION

Encrusting calcareous algae and foraminifera are abundant and important stabilisers and secondary framebuilders in modern and fossil reefs (Ghose 1977; Wray 1977; Flügel 1982). Crustose coralline algae, in particular, are quoted as one of the best (palaeo-)bathymetric indicators owing to their general wide depth distribution (Dodd and Stanton 1990) on one hand but sensitive reaction to changing light conditions on the other. This general opinion is contradicted by the number and quality of papers dealing with this topic. Only rare case studies are available in modern examples dealing with depth distribution of calcareous encrusters (e.g., Adey et al. 1982; Bosence 1984; Minnery et al. 1985; Vadas and Steneck 1988). Considering the fossil record, examples are even less abundant (e.g., Martindale 1992) and provide only very rough estimations. Even in exceptional cases, as in the Late Miocene reefs of Mallorca which provide the possibility of absolute reconstructions of water depths, the accuracy of results is limited (Perrin et al. 1995).

The main reason for the small number of studies of fossil calcareous encrusting communities in respect to their depth distribution is the lack of a good data base of modern examples from different biogeographical regions. In this paper we want to provide first informations on the depth distribution of calcareous incrusting associations from the Northern Red Sea.

 

STUDY AREA

Northern Safaga Bay is a topographically highly structured, relatively low energy shallow-water area with a maximum depth of 55 m. It is separated from the Red Sea proper to the east by a submarine ridge and a steep slope down to more than 200 m (Fig. 1). Owing to the rugged sea-bottom topography a complex facial pattern is developed (Piller and Pervesler 1989). As usual in the Northern Red Sea, salinities are high (40-46 ‰) and water temperature ranges between 21°C in winter and 29°C in summer; tidal range is <1 m (Piller and Pervesler 1989).

Within an integrated study of the Northern Bay of Safaga (e.g., Piller and Pervesler 1989; Piller and Mansour 1990; Nebelsick 1992; Piller 1994), two fringing reefs have been examined along 3 depth transects in respect to their encrusting calcareous biota. One transect, at Ras Abu Soma, is located along the main coast at the northern margin of the bay with a steep, rocky slope down to more than 200 m. The other reef fringes the 2 islands Tubya al-Hamra and Tubya al-Bayda inside the bay. This reef also exhibits a steep slope but reaches the basin bottom of the bay in approx. 30 m with gravelly and sandy sediments at the base.

 

METHODS

The reef at Ras Abu Soma was sampled along one transect between 0 and 51 m, the other reef between 0 and 25 m in two transects. Every 5 depth meter, samples were taken by SCUBA diving. 25 SEM samples, 32 thin sections, several polished slabs and peels were prepared and studied. The collected material allowed only a semi-quantitative analysis of crust distribution. Therefore, associations were predominantly defined by presence or absence of species. Samples are stored at the Institute for Palaeontology, University of Vienna, Austria.

 

RESULTS

Taxonomy

Six dominant encrusting species were determined: four corallinacean algae, one peyssonneliacean alga, and one acervulinid foraminifer. Additionally, four rare coralline algal species were observed, identification to the generic level, however, was impossible. A detailed taxonomic documentation of red algae of the northern Bay of Safaga is currently in preparation by the authors.

Taxonomical and anatomical terms in the description of coralline algae are used according to Woelkerling (1988).

 

Identification Key of the refered coralline algal species:

A. Tetrasporangia in broad sori, partially calcified walls between adjacent sporangia: S. ptychoides

B. Tetrasporangia in uniporate conceptacles

1. cells of contiguous filaments not connected by cell fusion: L. kotschyanum

2. cells of contiguous filaments connected by cell fusion

a. trichocytes predominantly arranged in horizontal fields: H. onkodes

b. trichocytes predominantly arranged in vertical fields: N. brassica-florida

 

Class Rhodophyceae Rabenhorst 1863

Family Peyssonneliaceae Denizot 1968

 

Peyssonnelia sp.

The thin thallus (40-60 µm) consists of large, mostly quadratic, basal cells (13-16 µm). Cells decrease in size towards the thallus surface (Fig. 2). The outermost cells are quadratically as well and measure on an average 5 µm. Usually the thallus is overlain by a layer of calcite crystals interpreted as cement.

Depth distribution: 40-50 m. Samples: T3R1/1, 2, 3; T3R2/2; T3R3/1.

 

Family Corallinaceae Lamouroux 1812

 

Lithophyllum kotschyanum Unger 1858

Crusts are usually not more than 1 mm thick. Branching is dichotomous, branches are 2-4 mm thick; branch tops are wider than the base but flattend. Cells of the noncoaxial core filaments reach up to 10 µm in diameter and 10-25 µm in length. The peripheral filaments consist of mostly quadratic cells with dimensions of 5-10 µm. The uniporate conceptacles are 300-400 µm in diameter and 150-180 µm high. This species can easily be distinguished by the typically branched habit and the regular cells (Fig. 3).

Depth distribution: intertidal and shallow subtidal of the reef flat and the back reef. Sample: EC95-20.

References: Johnson 1957; Verheij 1994; Piller and Rasser 1996.

 

Hydrolithon onkodes (Heydrich) Penrose and Woelkerling 1992

The collected specimens consist of monomerous thalli, up to 2.3 mm thick, with a 50-100 µm thick noncoaxial core. Core cells are up to 10 µm in diameter and 18 µm in length. Cells of peripheral filaments measure up to 6 µm in diameter and up to 10 µm in length. Cells of contiguous filaments are joined by cell fusions and secondary pit connections, in the core as well as in the peripheral filaments. The characteristic horizontal trichocyte fields are up to 90 µm in diameter, trichocyte cells measure approximately 14 x 16 µm (diameter x length). The trichocytes are visible on the thallus surface. Conceptacles are uniporate measuring 270 x 70-90 µm (diameter x height) (Fig. 4).

Depth distribution: 0-45 m. Samples: T3R2/1, 2; T3R7/2, 3; T3R8/1; T3R10; T3R11/1, a , b; T3R12/1, 2, a; T3R14.

References: Johnson 1957 (as Porolithon onkodes); Adey et al. 1982 (as Porolithon onkodes); Verheij 1994.

 

Neogoniolithon brassica-florida (Harvey) Setchell and Mason 1943

Maximum crust thickness 5 mm. The noncoaxial core is 70-100 µm thick. Mean diameter of core cells is 10 µm, the length is 25-40 µm. Cells of the peripheral filaments are usually quadratical (10-13 µm). Trichocytes occur predominantly in vertical rows (Fig. 5) consisting of 10 cells on an average (25-30 µm in diameter, 10-12 µm in height). Cells of contiguous filaments are joined by cell fusions and secondary pit connections, in the core as well as in the peripheral filaments. The uniporate conceptacles are up to 700 µm in diameter and approximately 150 µm high.

Depth distribution: 0-10 m. Samples: T3R9/1; T3R10; T3R13a.

References: Gordon et al. 1976 (as Neogoniolithon fosliei); Penrose 1992 (as N. fosliei); Verheij 1994 (as N. brassica-floridum ecad fosliei).

 

Sporolithon ptychoides Heydrich 1897

Crust thickness up to 2.5 mm. The noncoaxial core is 50-120 µm thick. Core cells are mostly 5 µm in diameter and more than 25 µm long. Dimension of peripheral filament cells is 9 x 7 µm (diameter x length) on an average. Sori are up to 800 µm in diameter, rising above the thallus surface (Fig. 6). They consist of sporangia which are approximately 40 µm in diameter and 80 µm in height and are characterised by stalk cells and a basal cell layer. Sporangial pores are 10-12 µm in diameter and surrounded by 12 rosette cells.

Depth distribution: 20-40 m. Samples: T3R3/2, 3; T3R4/1, 4; T3R6; T3R7/4.

References: Woelkerling 1988; Keats and Chamberlain 1993; Verheij 1993.

 

Class Rhizopoda Dujardin 1841

Order Foraminiferida Eichwald 1830

Familiy Acervulinidae Schultze 1854

 

Acervulina inhaerens Schultze 1854

A. inhaerens forms crusts with a thickness of up to 5 mm. The test consists of large globular chambers with characteristic pores in the walls. Diameter of chambers: up to 100 µm (Fig. 7). No juvenile chambers were found in our material.

Depth distribution: 5-50 m. Samples: T3R1-R9.

References: The generic and specific assignment is in accordance with Moussavian and Höfling (1993) and Perrin (1994) and in contrast to Hottinger et al. (1993) who prefer "Gypsina plana".

 

Depth distribution of species

Six dominant calcareous crust forming species were determined, each with a distinct depth distribution (Fig. 8): L. kotschyanum is restricted to the intertidal reef flat and the back reef, where it is dominant besides H. onkodes and N. brassica-florida; the latter occurs from the reef crest down to 10 m. Besides N. brassica-florida, H. onkodes is the dominant encruster at the reef crest and down to 15 m, but was observed even at 45 m. Below 20 m, H. onkodes is replaced in dominance by S. ptychoides which occurs to 40 m, but dominates the depth range from 20 to 35 m. It occurs together with the foraminifer A. inhaerens, found from 5 to 50 m, being most dominant below 40 m. Consequently, the depth range of 40-50 m is characterised by A. inhaerens and the less abundant peyssonneliacean alga Peyssonnelia sp. which is restricted to depths below 40 m.

 

Encrusting Associations

The restricted species occurrences allow a differentiation of four associations showing general depth related features (Fig. 9): associations are predominantly monospecific and crusts are thinner in shallow water; with increasing depth crusts get thicker, predominantly multispecific, and more affected by borings and cementation; in contrast, coralline algal crusts are thicker in shallow water (up to 1 cm), getting thinner downslope together with an increasing thickness of acervulinid crusts.

 

(1) Lithophyllum kotschyanum - Association

This association is prominent on the intertidal reef flats and in shallow subtidal back reef areas. It is characterized by L. kotschyanum, which is restricted to this association. Additionally, H. onkodes and N. brassica-florida are prominent. They either grow directly on corals or/and on each other. Both L. kotschyanum and N. brassica-florida form nearly monospecific rhodoliths and may even form frameworks on the reef flats (Piller and Rasser 1996). In contrast, no H. onkodes-rhodoliths were found.

 

(2) Hydrolithon onkodes - Association

This association ranges from the reef crest to 15 m and is characterised by H. onkodes. This species is accompanied by N. brassica-florida which occurs only above 10 m. H. onkodes has a wide depth range and was recorded to 45 m. Both species grow directly on corals or on A. inhaerens which has its shallowest occurrence at 5 m.

 

(3) Sporolithon ptychoides - Acervulina inhaerens - Association

Depth range: 20 to 35 m. This association is dominated by S. ptychoides. It either grows directly on corals or on A. inhaerens. The latter, however, never overgrows S. ptychoides. A. inhaerens is abundant in the samples, but was never observed on the sample surfaces in this association.

 

(4) Acervulina inhaerens - Peyssonnelia sp. - Association

This association occurs only below 40  m. At 40 m, the deepest occurrence of S. ptychoides, a shift from coralline algal dominance to acervulinid foraminifera and peyssonneliacean algae occurs. Coralline algae become rare and only one occurrence of H. onkodes was observed in our samples. The growth successions are characterised by intergrowings of algae and foraminifera with corals.

 

DISCUSSION

Depth distribution of taxa

Peyssonneliaceae

Peyssonnelia sp. is restricted to a depth of more than 40 m in the study area. Generally, peyssonneliacean algae are reported to be common from the shallower subtidal (Maggs and Irvine 1983) to a depth of 50 m, occuring even down to 120 m (James et al. 1988). According to Minnery et al. (1985) and Vadas and Steneck (1988), however, their abundance generally increases below 30 m.

Coralline algae

In the studied transects, coralline algae are the dominant encrusters between 0 and 35 m, with most taxa showing a particular depth range. Each genus is represented by only one dominant species. Hence, depth distributions of genera coincide with that of species. A comparison of our results with other localities, however, seems to be inappropriate on the generic level, since the depth distributions of genera are generally much wider. One example is the genus Sporolithon with a clearly restricted distribution from 20-40 m in our transects. Earlier studies report this genus to be most abundant in the lower subtidal (Adey 1986; Manker and Carter 1987), but it may even occur in the intertidal (Fravega et al. 1989). On the contrary, the genus Lithophyllum, restricted to the lower intertidal and uppermost subtidal in the study area, is recorded in the Gulf of Mexico down to 80 m (Minnery et al. 1985). H. onkodes shows a widespread depth distribution as well. In the studied transects it occurs from the intertidal down to 45 m and is recorded even down to 85 m (Dullo et al. 1990).

Another problem comparing distributions on the generic level is caused by rather fast changing genus concepts in the last years. The genus Hydrolithon, for instance, is represented with H. onkodes in the study area. This species was formerly assigned to the genus Porolithon. More recently Penrose and Woelkerling (1988, 1992) considered Porolithon congeneric with Hydrolithon. These authors, however, stated that not all species of Porolithon can be refered to Hydrolithon without proofing. Consequently, a comparison of Porolithon and Hydrolithon occurrences, basing on old literature data, is currently not adequate on the generic level. The same problem arises with the genus Goniolithon which is congeneric with Neogoniolithon (Setchell and Mason 1943; Woelkerling 1988).

Taxonomically, comparisons at the species level are less problematic owing to the more solid status of this basic taxon. Ecologically, however, comparisons of different species occurrences also cause problems, since they have different depth distributions at various locations. The species S. ptychoides, for example, is restricted to a depth of 20-40 m in the studied transects and thus would be expected to represent a typical deeper water species. In Indonesia, however, it occurs between 1 and 30 m (Verheij 1993). N. brassica-florida is restricted to 0-10 m in the study area. In southern Australia it is also most dominant from the intertidal down to 3 m (Penrose 1992); in Guam, however, it occurs down to 37 m (Gordon et al. 1976) and in Hawaii it is reported from 0 to 30 m (Adey et al. 1982).

Acervulinidae

In our transects, the acervulinid foraminifer A. inhaerens occurs from 5-50 m, but is dominant below 40 m. Generally, acervulinid foraminifers are more dominant in deeper waters, where light conditions reduce the competition of coralline algae. This is reflected by the occurrences of acervulinid macroids being dominant in deeper water environments (Hottinger 1983; Reid and Macintyre 1988; Prager and Ginsburg 1989) which was also found in the study area (Piller and Pervesler 1989). Their shallowest occurrences were recorded around 25 m, getting more abundant below 45 m.

 

Depth distribution of Encrusting Associations

Four distinct encrusting associations were differentiated. They change characteristically with increasing depth from pure coralline algal associations, to coralline algal - acervulinid associations and finally to acervulinid - peyssonneliacean associations. Comparable correlations between depth and encrusting biotas are recorded from the Caribbean (Reid and Macintyre 1988; Littler et al. 1990), although these papers deal with rhodoliths and/or foraminiferal macroids only.

The decreasing thickness of coralline algal crusts with increasing depth might be caused by a lower abundance of herbivorous grazers in deeper waters (Steneck 1985).

 

Geological implications

Most studies dealing with (palaeo-)depth distributions of encrusting biota focus on coralline algae; equivalent studies on peyssonneliacean algae and acervulinid foraminifers are rare. Only in exceptional cases were absolute reconstructions of palaeodepth possible (Perrin et al. 1995). Since coralline algae occur down to 200 m (Littler et al. 1990) and even deeper (Dodd and Stanton 1990) their potential use as palaeobathymetric indicators requires detailed taxonomic studies. Their palaeoecological applications, however, are difficult both on the generic and specific level. These problems are caused by broad and regionally changing depth distributions of modern coralline algal taxa and by an insufficient taxonomic background of fossils. Braga et al. (1993) proved that modern coralline algal taxonomy (Woelkerling 1988) is applicable to fossils but demonstrated on Lithophyllum albanense, now refered to Spongites, that the status of many fossil genera is uncertain. Consequently, a comprehensive revision of fossil taxa is necessary before coralline algae can seriously be used for palaeoecological interpretations both on the generic and specific level. Nevertheless, an application on a higher taxonomic level may reveal positive results. Reid and Macintyre (1988), for example, interpreted the growth succession in deeper water macroids from coralline algae to acervulinid foraminifera as a response to a Holocene sea-level rise. Even in the Eocene, Rasser (1994) was able to interpret sections as deepening upward sequences owing to the vertical succession of rhodoliths and foraminiferal macroids: coralline algal rhodoliths prevail at the base, peyssonneliacean algae increase upsection and foraminiferal macroids dominate in the uppermost part.

Consequently, the most accurate method for palaeobathymetric interpretations of Cenozoic sequences in respect to relative sea level trends seems to be the analysis of growth successions integrating as many taxa as possible. The current study clearly points out, that interpretations of growth successions are not restricted to rhodoliths and/or macroids but are also detectable in attached calcareous crusts.

 

ACKNOWLEDGEMENTS

The authors are greatly indebted to A. M. Mansour (Dept. of Geology, South Valley University/Qena) and M. Zuschin (Institute of Palaeontology, University of Vienna) for collaboration during field work. We thank B. Riegl (Institute of Palaeontology, University of Vienna) for fruitful discussions and help with the English. This study was supported by projects P 8090-Geo of the Austrian "Fonds zur Förderung der wissenschaftlichen Forschung" and by the "Hochschuljubiläumsstiftung der Stadt Wien".

 

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