Tropical Zoology Coral reef community structure at ...

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Coral reef community structure at Ras Mohammed in the northern Red Sea a

b

M. M.A. Kotb , R. G. Hartnoll & A.-F. Ghobashy

c

a

Department of Marine Biology , Suez Canal University , Ismailia , Egypt b

Port Erin Marine Laboratory , University of Liverpool , Isle of Man , U.K. c

Department of Zoology , Suez Canal University , Ismailia , Egypt Published online: 01 Aug 2012.

To cite this article: M. M.A. Kotb , R. G. Hartnoll & A.-F. Ghobashy (1991) Coral reef community structure at Ras Mohammed in the northern Red Sea, Tropical Zoology, 4:2, 269-285, DOI: 10.1080/03946975.1991.10539495 To link to this article: http://dx.doi.org/10.1080/03946975.1991.10539495

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Tropical Zoology 4: 269-285, 1991


Coral reef community structure at Ras Mohammed in the northern Red Sea M.M.A. KoTB 1 , R.G. HARTNOLL 2 and A.-F. GHOBASHY 3 1 Department of Marine Biology, Suez Canal University, Ismailia, Egypt 2 Port Erin Marine Laboratory, University of Liverpool, Isle of Man, U.K. 3 Department of Zoology, Suez Canal University, Ismailia, Egypt Received 5 January 1991, accepted 11 July 1991

A quantitative survey of reef community structure was carried out within the Ras Mohammed Marine National Park in the southern Sinai. Ten transects were surveyed to a depth of 40 m around the southern tip of the Ras Mohammed Peninsula. Percentage cover of designated taxa was recorded within 9m 2 quadrats located at 5 m depth intervals. Sedimentation rates were recorded. Different taxa showed clear patterns of distribution related to depth and to site. These are related to variations in slope, water movement, sedimentation, and diving pressure. The implications of the results for the Marine Park management are summarised. KEY WORDS:

coral reefs, depth, sediment, Red Sea.

Introduction Study area and methods Results . . Quadrat survey data Variation in abundance with depth Variation in abundance with site Sediment trap results Discussion Effect of depth on distribution Effect of site on distribution . . Implications for the Marine National Park Acknowledgements References Appendix

269 270 273 273 273 273 277

279 279 281 282 282 283 285

INTRODUCTION

Ras Mohammed is located at the southern tip of the Sinai Peninsula in the northern Red Sea (Fig. 1). It possesses diverse reef formations, and its location at the junction of the Gulf of Suez, the Gulf of Aqaba, and the Red Sea proper subjects it to diverse hydrographic regimes. In 1990 the region around Ras Mohammed was created


270

M.M.A. Kotb, R.G. Hartnell and· A.-F. Ghobashy

the first Marine National Park in Egypt, and steps to preserve and manage the area have been implemented. The southern Sinai has a rapidly developing tourist industry, based predominantly upon sport divers, for whom Ras Mohammed is the major attraction. The reefs of Ras Mohammed are at risk from several potential impacts, the chief being damage from increasing numbers of divers, and oil pollution from the offshore industry in the Gulf of Suez. Previous work on coral reefs of the northern Red Sea has concentrated on the Gulf of Aqaba, particularly its northern part (e.g. LOYA & SLOBODKIN 1971; LoYA 1972, 1975; MERGNER & ScHUHMACHER 1974; MERGNER 1979; BouCHON 1980; HusTON 1985b). A few studies have included some limited mention of the Ras Mohammed area. ScHEER (1971) commented on the geographical relationships of the coral genera, GVIRTZMAN & FRIEDMAN (1977) made geomorphological observations, FISHELSON (1980) discussed the potential for marine reserves, FRICKE & SHUHMACHER (1982) investigated the depth limits of corals, and BENAYAHU (1985) studied the soft corals. However, in view of the lack of detailed information on Ras Mohammed reefs, this study was made to characterise the distribution and abundance of the major benthic cover components of the reef community. Quantitative records were taken from a sequence of transects to a depth of 40 m. The transects were selected to represent reefs of varying aspect and profile.

STUDY AREA AND METHODS

Study area The Ras Mohammed Marine National Park covers an extensive area including both the Ras Mohammed Peninsula and regions of the adjacent mainland. The 10 transects were concentrated on the southern part of the peninsula (Fig. 1), the region of greatest environmental interest, and were grouped in pairs at five study sites. The peninsula is surrounded by reefs, but there is considerable variation in the width of the reef flat and the reef slope profile. The eastern and southeastern parts are characterised by a very narrow reef flat with an abrupt drop-off into deep water. Around the southern tip of the headland there is a wide reef flat, bordered by a terrace at about 15 m depth with several patch reefs. The western side has a large lagoon, a shallow mangrove channel, and generally wider reef flats and a less steep reef face. Rainfall is sparse, so there is very little freshwater input, and the water is generally clear and of constant salinity (ca 40%o). However, particularly during winter the prevailing northerly winds tend to bring more turbid water down the western side of the peninsula from the shallow Gulf of Suez. In contrast water coming down the eastern side from the direction of the Gulf of Aqaba is generally clearer, with little sediment. Strong tidal currents are found around the southern tip of the peninsula.

Transect descriptions The profiles of the 10 transects are shown in Fig. 2, with reef flats and reef faces drawn to various horizontal scales. The reef flats vary enormously in width from 15 to over 400 m. Most are dominated by corals, but the wider flats include areas of sand and sea grasses. The reef faces show three basic profile types: vertical or overhanging cliffs descending to over 70 m (transects 2a and 2b); an initial steep face to 10-20 m, followed by a more gentle slope (transects 1a, 1b); a gently sloping reef down to about 20 m, then becoming increasingly steep (transect 3a). On the slopes the substrate was generally hard, but the gentler slopes had some areas of sand. The general situation of each study site is as follows.

271


Coral reef at Ras Mohammed

2

Ras Mohammed

5km 100 km

Fig. 1. - Maps of the study area. The inset shows the general location of Ras Mohammed, the main map the position of the five study sites around the Ras Mohammed Peninsula.

Site 1. Lying on the east side of the peninsula exposed to the strong prevailing northerly winds. Regularly exposed to wave action, so that the reef flat is well scoured. Wind-driven surface currents flow from the direction of the Gulf of Aqaba. Site 2. Situated at the base of high cliffs on the SE side of the peninsula, where the bottom drops off very rapidly to deep water. The site is open to the SE and south, and occasional strong southerly winds blowing up the length of the Red Sea from the south create strong wave action. Site 3. On the southern tip of the peninsula, facing directly down the Red Sea. The site is backed by a 400 m wide reef flat, and is in the area of two patch reefs which are very popular with dive parties. There are strong tidal currents. Site 4. On the SW side of the peninsula at the entrance to the Gulf of Suez, and subject to wave action from prevailing winds blowing down the Gulf of Suez from the north. The reef flat is quite wide (100-150 m). The site is also exposed to any wave action from the south. Site 5. On the western side, facing the Gulf of Suez. A reef flat of moderate width (50-100m). Some pollution from oil blown down the Gulf of Suez by prevailing winds from the north. Transect survey methods The survey was based upon interrupted belt transects. The depth profile of each transect was initially surveyed, the width of the reef flat was measured, and on the reef face the depth was recorded at 5 m horizontal intervals seawards from the reef edge. Sampling commenced just below

M.M.A. Kotb, R.G. Hartnoll and A.-F. Ghobashy

272 44m

a

2

40


.

25m

113m

,___450m

110m

15m

60m

0

~

;; E

4a

3b

3a

.!: 20 .t::

..

Q. Q

40 120m

40m

80m

40m

45m

65m

0

20

4b

5b

40

Fig. 2. - Depth profiles of the transects. Note that the horizontal scales vary, and the full width of the reef flat is not shown to scale in all cases. The numbers above each profile are the widths in metres of the reef flat, and of the reef face down to 40 m.

the seaward edge of the reef flat (depth 0 m). Quadrats were stationed at depths of 1, 5, 10, 15, 20, 25, 30, 35 and 40 m. The quadrat at 1 m had dimensions of 1 by 9 m, with the long axis parallel to the reef edge. This was to minimise its depth variation because the community tends to change very rapidly with depth over this critical zone. The deeper quadrats covered the same area, but measured 3 by 3 m. The top of each quadrat was positioned at the depth indicated. Within each quadrat the abundance of the designated taxa (see Appendix) was recorded. Hard and soft corals were identified to genus, but most other groups were recorded as broader taxa. Abundance was recorded as percentage cover, and to facilitate estimation the margins of the quadrats were marked at 0.25 m intervals. Percentage cover was recorded as falling into one of seven ranges: < 1, 1-5, 6-10, 11-25, 26-50, 51-75, and > 75% cover. This system is quicker and more robust than direct percentage cover estimates (BAK 1976). For colonies lying partly outside the quadrat, only the included portion was assessed. Colonies under overhangs or other colonies were estimated, so it was quite possible for total quadrat cover to exceed 100%.

Sedimentation measurements Sedimentation rates in each transect were measured during February-March 1989. Plastic bottles were used as sediment traps, with a mouth of 8 em diameter and a depth of 25 em,


273

proportions found to be most efficient by LoYA (1976). Each trap was fixed to a wooden rod with insulating tape, and tied to the reef in a vertical position. On each transect traps were located at depths of 2, 10 and 20 m. They were left in position for 34 to 38 days. On collection the lid was screwed onto the bottle in situ, and the trap removed to the laboratory. The contents were filtered through a preweighed filter paper, dried for 24 hr at 80 °C, then reweighed (BAK 1976).

RESULTS


Quadrat survey data The detailed raw data have been presented in full as an appendix in KoTB (1989). The data was analysed in two ways. First to see how the abundance of taxa varies with depth, and secondly to examine differences between sites to see if differences can be correlated with other environmental variables.

Variation in abundance with depth The mean percentage cover for selected taxa at each depth for the combined transects is presented in Table 1. The data is presented not only for the selected taxa, but also for some aggregations such as 'hard' and 'soft' corals, and the various coral growth forms. For certain taxa the overall distribution with depth is shown in Fig. 3. Different taxa show different patterns of depth-related distribution. Thus hard coral have maximum cover near the surface, and decrease steadily in abundance with depth. Soft corals show little effect of depth, whilst hydroids increase in abundance with increasing depth. Coral growth forms also vary in response to depth. Branched corals are most abundant near the surface, whilst flat coral do not appear in any number until a depth of 10 m. Such differences also occur between genera of hard corals: Pocillopora decreases rapidly from the surface down, whilst Montipora reaches maximum cover at 20-30 m. The effect of depth on abundance was analysed more critically by ANOVA. Initially a 2-factor ANOVA was calculated (factors: site, depth), and where depth was found to be a significant factor, the effect of depth was further analysed by a 1-way ANOVA with multiple pair tests (Fisher PLSD; SNEDECOR & CocHRAN 1980). The outcome of this analysis is presented in Table 2, which indicates the level of significance of the depth factor, and where significant, the groupings revealed by the Fisher PLSD. An examination of these groupings shows little evidence of consistent patterns. The 1 m depth shows up as a discrete group in a number of the analyses, which is as might be expected, the environmental factors change most rapidly in the shallow water. Otherwise the picture is of a steady change with depth, with no sharp horizons demarcating one zone from another.

Variation in abundance with site To compare the abundance of taxa between sites mean percentage cover values were calculated for all depths combined (Table 3). Some clear inter-site differences are to be seen. Hard corals are most abundant at site 1, soft corals at site 3, and sponges and algae at site 2. There are likewise differences in abundance of coral growth forms (branched and massive coral at site 1), and of the major coral genera (Acropora at site 1, Montipora at site 2).

72.0±4.3 16.2±9.0 26.6±7.6 0.4±0.30 0.7±0.39 0 0.1±0.05 1.5±0.46

32.7±4.2 14.7 ±2.4 24.4 ±2.4 0.3 ±0.30 0.1±0.05

14.4±4.2 4.8±1.6 5.9±0.88 7.5 ±2.3 2.8±1.8 9.1±1.5 3.9±1.1 4.6±0.84

0.7 ±0.40 0.9±0.46

Branched coral Encrusting coral Massive coral Flat coral Solitary coral

Acropora Favia Favites Millepora Montipora Pocillopora Porites Stylophora

Dendronephthea Xenia

1

Hard coral Soft coral Algae Sponges Hydroids Black coral Gorgonians Molluscs

Taxon

5.2±2.2 10.1 ±2.6

3.9±1.1 2.1±0.46 3.6±0.71 8.2±1.7 8.1±1.1 2.0±0.76 4.4 ± 1.1 3.0 ±0.87

7.2±2.0 2.6±0.71 4.4±1.1 11.4±2.2 6.6± 1.6 3.6±0.71 6.2±2.2 2.7±0.92

4.9±2.2 8.1±3.4

15.1±2.8 20.4±2.3 19.5 ±2.6 3.0±1.7 1.0±0.44

58.9±6.2 24.4±3.5 4.4± 1.8 14.9±6.0 1.2±0.49 0 2.9±1.8 0.4±0.30

10

18.9±2.2 21.1±2.9 21.1±2.9 0 2.2±0.97

63.2 ±6.4 22.7 ±4.4 4.5±2.2 5.9±2.1 1.2±0.49 0.4±0.30 1.4±0.78 0.4±0.30

5

3.4±1.7 15.8 ±4.0

4.8± 1.6 2.9±1.1 1.7±0.78 4.8±0.74 8.1 ± 1.1 1.3 ± 0.48 9.2±3.2 1.9±0.45

13.4±2.4 16.2 ± 1.8 21.4±4.3 2.7 ± 1.8 0.8 ±0.75

54.4±6.2 30.7±5.6 2.6± 1.1 4.9±0.70 0.3±0.30 0.1 ±0.05 1.2±0.76 0.3 ±0.11

15

12.5 ±4.0 12.8± 1.7 13.5 ±2.0 1.1±0.77 0.1±0.050

14.0±2.5 17.0±1.1 19.6±4.8 2.8±1.8 0.7±0.40

2.9± 1.1 11.8±2.0

3.3 ± 1.2 15.1±3.1

5.9±3.5 2.9±0.65 2.5±0.37 2.7±0.67 7.2± 1.3 0 3.1±0.85 4.4 ± 1.6

39.9±6.3 24.3 ±4.2 3.7 ± 1.1 6.6± 1.5 2.1 ±0.75 0.4±0.30 2.3±0.74 0.1±0.067

54.0±7.6 22.7±3.1 2.5± 1.0 5.3 ±0.75 1.7 ±0.78 0.4±0.30 2.0±0.78 0.1 ±0.067

5.6±2.1 4.0± 1.7 3.7±1.6 4.3±0.92 10.5 ± 1.5 0.9±0.46 3.8±0.90 3.8±0.90

25

20

Depth (m)

2.2±0.97 13.1±1.9

2.9±0.63 3.2±0.54 2.2±0.44 5.3±1.6 9.5 ± 1.3 0 4.7±1.0 4.2±0.97

8.4 ± 1.3 17.4±2.7 15.8±2.8 1.2±0.66 0.1±0.067

42.8±4.5 23.2 ±2.8 3.6±0.95 8.7±2.1 3.2 ±0.57 1.2±0.49 3.0±0.87 0.1±0.05

30

1.3 ±0.44 8.4± 1.7

0.7 ±0.30 1.5±0.50 1.3±0.48 2.4±0.40 6.7±1.4 0 4.1±0.80 2.6±0.74

3.1±1.0 11.7±1.9 9.2± 1.4 0.7±0.40 0.1 ±0.050

24.6±3.1 13.8±2.2 3.8±1.0 6.3± 1.4 2.1±0.74 0.7±0.40 2.8±0.90 0.1±0.05

35

Table 1. The mean percentage cover, ± SE, at each depth for the taxa indicated, averaged for the 10 transects (n= 10).


2.8±0.90 9.7±3.5

0.7 ±0.40 2.6±0.46 2.1 ±0.47 2.5 ±0.37 6.3±0.45 0 3.6±0.71 1.7 ±0.74

5.4±1.8 12.1±1.1 11.8±1.4 3.0±1.7 0.4±0.30

32.6±3.7 16.2±3.8 2.9±0.96 7.4±2.0 6.2±2.0 1.9±0.99 3.5 ±0.98 0.3 ±0.30

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Coral reef at Ras Mohammed 100

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10 20 30 40 15 25 35

5

10 20 30 40 15 25 35

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15

20 30 40 25 35

Depth in metres Fig. 3. -The abundance of selected taxa with depth, showing mean percentage cover for the 10 quadrats at each depth. Major coelenterate taxa in the first column, hard coral morphotypes in the second, and selected hard coral genera in the third.

However, the large differences between depths makes direct comparisons difficult, and the inter-site differences were further examined by a 2-factor ANOVA (factors: site, depth). Where site was shown to have a significant effect, a 1-way ANOVA was calculated with site as the factor, and individual sites compared by the Fisher PLSD test. The results of this analysis are presented in Table 4. Interestingly the interaction effect between site and depth was not significant in any of the analyses, suggesting that the response of the various taxa to depth was similar at all sites. Table 4 shows that the effect of site was significant for fewer taxa than was the effect of depth, especially at the generic level. An examination of the linkages shown by the Fisher PLSD test do not show any sites as being consistently different from the others for a range of taxa.

Table 2. Significance of depth as a factor in 2-way ANOVA (factors: site, depth), and where depth is significant, relationships as shown by the Fisher PLSD test (linked depths not significantly different) applied to a 1-way ANOVA (factor: depth).


Taxon

Significance of depth effect

Hard coral

0.0001

Soft coral

0.11

Algae

0.0001

Sponges

Linkages

5

10

15

20

30

25

40

35

1

40

5

10

35

25

30

15

20

0.038

10

30

40

25

35

5

20

15

Hydroids

0.0004

40

30

35

25

20

5

10

1

Black coral

0.026

40

30

35

5

20

25

15

Gorgonians

0.205

NS

Molluscs

0.004

1

10

5

35

40

15

20

25

30

Branched coral

0.0001

1

5

10

20

15

25

30

40

35

Encrusting coral

0.037

5

10

30

20

15

25

40

35

Massive coral

0.0007

1

15

5

20

10

30

25

40

35

Flat coral

0.47

NS

Solitary coral

0.046

5

10

15

20

40

30

35

Acropora

0.0002

1

5

25

20

15

10

30

35

40

Favia

0.32

Favites

0.002

1

5

20

10

25

30

40

15

35

Millepora

0.0007

5

10

30

15

20

25

40

35

Montipora

0.017

20

30

10

15

25

35

5

40

Pocillopora

0.0001

1

5

10

15

20

40

25

30

35

Porites

0.039

15

5

30

10

35

1

20

40

25

Stylophora

0.29

NS

Dendronephthea

0.37

NS

Xenia

0.006

15

25

30

20

10

40

35

5

1

NS

15 10

25

NS


277

Table 3. Mean percentage cover, ± SE, at each of the five sites for the taxa indicated (mean values for all depths in the two transects, n = 18). Site


Taxon 2

3

4

5

Hard coral Soft coral Algae Sponges Hydroids Black coral Gorgonians Molluscs

63.2 ±6.2 17.8±2.5 3.8± 1.3 7.2±1.4 0.7 ±0.30 0.5 ±0.27 1.2 ±0.97 0.2 ±0.057

40.6±3.4 14.0 ± 1.9 8.4±3.4 10.1±3.5 3.4±0.99 0 3.8±0.78 0.6±0.27

48.4±5.3 29.1±3.7 3.6±2.1 7.6±1.6 1.6±0.59 0.4±0.42 2.5±0.71 0.3±0.17

51.4±4.6 21.4±2.7 6.9±2.3 4.3±0.66 1.8±0.47 0.4±0.23 2.1 ±0.47 0.3 ±0.17

42.1±5.1 25.5±5.2 7.5±3.5 4.1±0.99 2.9±0.98 1.3±0.48 0.9±0.31 0.4±0.25

Branched coral Encrusting coral Massive coral Flat coral Solitary coral

22.3±3.4 15.5±2.2 22.3±3.4 2.4± 1.0 0.6±0.44

10.4±2.1 15.6±1.2 11.6±1.2 2.9±1.3 0.1 ±0.038

13.6±2.6 16.5 ± 1.7 17.2±2.2 0.3 ±0.23 0.8±0.46

13.7±2.4 17.1±1.3 19.3 ± 1.6 0.5 ±0.27 0.8±0.45

8.4 ± 1.8 14.8±1.8 16.3 ±2.3 2.0 ± 1.0 0.6±0.27

Acropora Favia Favites Millepora Montipora Pocillopora Porites Stylophora

10.2±2.8 3.0±1.0 4.9±1.8 6.0± 1.6 8.4 ± 1.5 2.8±1.1 7.0± 1.9 4.5 ±0.69

2.7±0.70 1.9±0.48 2.3±0.80 4.6±0.59 10.0± 1.2 1.7 ± 1.0 4.8±0.71 3.1 ± 1.1

5.7±2.1 3.0±0.91 3.3±0.66 5.5± 1.1 6.3±0.96 1.1 ±0.48 3.6±0.66 3.8±0.41

5.1 ± 1.2 3.4±0.50 3.2± 1.1 6.7 ± 1.3 5.9±0.65 1.4±0.59 4.2±0.61 3.7±0.52

1.7 ±0.48 3.1±0.55 3.4±0.84 4.5 ± 1.3 6.2±0.63 2.4±0.64 3.8±0.78 0.8±0.33

Dendronephthea Xenia

1.1 ±0.97 12.3±2.3

5.0±1.5 6.3 ± 1.4

1.4 ±0.35 12.0±2.1

2.7±0.84 8.5 ±2.2

4.2± 1.0 12.4±2.5

Sediment trap results The weight of sediment collected over a 30 day period is given in Table 5. There was little difference with depth. The mean weights (with standard errors) collected per trap were 4.21 ± 3.29 gat 2 m, 3.68 ± 1.80 gat 10m, and 3.48 ± 1.96 gat 20 m. A 1-way ANOVA confirmed that the effect of depth was not significant (P = 0.976). However, differences between the sites were quite striking. The mean weight of sediment per trap at each site was as follows: site 1- 2.04 ± 0.95 g; site 2- 1.82 ± 0.52 g; site 3-0.89 ± 0.46 g; site 4- 1.36 ± 0.33 g; site 5- 12.86 ± 5.62 g. A 1-way ANOVA showed that site was a significant factor affecting sedimentation (P = 0.013). The Fisher PLSD test showed the following pattern:

5

1

2

4

3

Site 5, subjected to the full influence of the more turbid Gulf of Suez, had significantly higher sedimentation rates than the other four sites, which did not differ significantly between themselves.

Table 4. Significance of site as a factor in 2-way ANOVA (factors: site, depth), and where depth is significant, relationships as shown by the Fisher PLSD test (linked sites not significant difference) applied to a 1-way ANOVA (factor: site). Significance of site effect


Taxon

Hard coral

0.003

Linkages

1

4

3

5

2

3

5

4

1

2

Soft coral

0.0068

Algae

0.054

NS

Sponges

0.15

NS

Hydroids

0.029

NS

Black coral

0.048

5

1

3

4

2

Gorgonians

0.034

2

3

4

1

5

Molluscs

0.45

Branched coral

0.0001

1

4

3

2

5

Encrusting coral

0.89

Massive coral

0.003

1

4

3

5

2

Flat coral

0.21

NS

Solitary coral

0.55

NS

Acropora

0.0011

3

4

2

5

Favia

0.60

NS

3

5

4

4

2

5

NS

NS

Favites

0.089

NS

Millepora

0.059

NS

Montipora

0.018

Pocillopora

0.056

NS NS

2

Porites

0.087

Stylophora

0.007

Dendronephthea

0.055

NS

Xenia

0.083

NS

3

Table 5. Mean weight of sediment (g dry weight) collected per trap at each site. Depth (m) Site

1 2 3 4• 5

2

10

20

0.05 0.54 0.70 1.92 17.31

4.12 2.70 0.00 1.53 10.09

1.45 2.17 1.97 0.64 11.19


279 DISCUSSION


Effect of depth on distribution The general trend at Ras Mohammed is for the overall abundance of hard corals to decrease with depth. This is a feature common to most coral reef surveys, both in other areas of the world (e.g. GoREAU 1959, TuRSH & TuRSH 1982, DINESEN 1983), and in the northern Red Sea (e.g. LoYA & SLOBODKIN 1971, SoHN 1977). However, there are exceptions to this trend. BouCHON (1981) found maximum coral cover at 10 m at Reunion Island, decreasing only beneath that depth, and a similar pattern at the north of the Gulf of Aqaba (BoucHON 1980). In the Netherlands Antilles, BAK (1975, 1976) found that hard corals only began to decrease in density below 35 m. Although there is a steady decrease in overall hard coral cover with depth, the pattern does not extend to all growth forms. Massive and branched corals decrease steadily, but encrusting corals show little change, and flat corals do not appear in any number above 10 m. A similar shift from branched and massive forms near the surface to encrusting and flat forms in deeper water is described by other workers (BAK 1976; BoucHON 1980, 1981). At Ras Mohammed the branched corals are the most abundant growth form in the shallowest areas, where wave action is at its strongest. Where wave action is severe, the shallow reefs tend to be dominated by massive or encrusting forms rather than branching ones (SHINN 1963; Roos 1964, 1971; BAK 1975, 1976; BouCHON 1981; SHEPPARD 1982). However, the more wave-exposed sites at Ras Mohammed are really relatively sheltered compared to exposed sites in many areas, and the severity of water movement is not such as to inhibit the abundance of branched corals. At the generic level differences in depth response are even more striking. Pocillopora is most abundant near the surface, decreasing rapidly with depth. Acropora is also most abundant near the surface, but decreases less rapidly and less regularly with depth. This preference of Acropora for shallow water is widely reported (MERGNER 1971, FISHELSON 1973, BAK 1976, BOUCHON 1980, RYLAARSDAM 1983). Millepora, on the other hand, reached maximum density at 5 m, declining below this depth. There is general agreement that Millepora is a dominant genus on the upper reef edge, in the Red Sea (LoYA & SLOBODKIN 1971; MERGNER 1971, 1984; BoucHON 1980), Aldabra (BARNES et al. 1966), and the Caribbean (GRAus & MACINTYRE 1989). Finally there are genera which reach their maxima at greater depths, such as Porites at 15m, as already noted in the Red Sea by LoYA & SLOBODKIN (1971), and Montipora at 20-30 m. Although the total cover of hard corals decreased with depth (Figs 3 and 4), the same trend was not apparent in genus richness. Whether plotted as total genera recorded in the survey, or as mean genera per transect, the maximum occurred at 1015 m (Fig. 4). Fewer genera occurred in shallow water, and the number also decreased with greater depth. Similar trends in hard coral species richness are reported from several areas by HusToN (1985b), though maximum species richness can occur slightly deeper in the 20-30 m range. In contrast to the hard corals, the soft corals (alcyonaceans) are most abundant at intermediate depths. In the Gulf of Aqaba, BouCHoN (1980) found that soft coral abundance increased down to 20 m, and then decreased. A general pattern has been observed that soft corals tend to replace hard corals with increasing depth, along the Sinai coast (FRICKE & ScHUHMACHER 1982), at Eilat in the Gulf of Aqaba (BENAYAHU

280


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& LoYA 1981), in the central Great Barrier Reef (DINESEN 1983), and in Papua New Guinea (TuRSCH & TuRSCH 1982). Xenia is the commonest of the soft corals, and it reaches maximum abundance from 15-30 m. It is similarly the commonest soft coral on other Red Sea reefs (FrsHELSON 1970, BENAYAHU & LoYA 1981, MERGNER & ScHUHMACHER 1981, BENAYAHU 1985), in the Great Barrier Reef (DrNESEN 1983) and in the Caribbean (STODDART 1969). In all of these areas it is commoner in deeper waters, and less common near the reef crest. Among the other taxa, some show a preference for shallow water, some for the depths. Algae are most common in the topmost level, where their growth is favoured by high light intensity (see also STODDART 1969, FISHELSON 1973, BENAYAHU & LOYA 1977, SHEPPARD 1982). The common algae in the study area, Colpomenia Derb. & Sol. 1856, Hypnea, ]ania Lamouroux 1816, Padina and Turbinaria, are also common on the reefs in the Gulf of Aqaba (FrsHELSON 1973, BENAYAHU & LoYA 1977). There the abundance of algae varies through the year, but this was not examined in the present study. Molluscs are similarly most abundant near the surface, due to the preference of Tridacna and vermetids for shallow depths. Sponges, in contrast, were very scarce near the surface, but from 5 m downwards were fairly evenly distributed. In the Netherlands Antilles sponges were more abundant on the reef below 10 m (BAK 1975), and on the Barrier Reef showed maximum abundance at 15-20 m (WILKINSON & EVANS 1989). The most marked preference for deep water was shown by gorgonians, hydroids and black corals, which all increased to maximum abundance at 40 m. This is a widespread feature of these groups (FISHELSON 1970, BAK 1975, BENAYAHU & LoYA 1981, HusToN 1985a). It is uncertain to what extent this distribution is due to environmental preferences (GRIGG 1984), or to competition with other groups such as algae (HusTON 1985a), or hard and soft corals (BENAYAHU & LoYA 1981).


281

It is possible to characterise a general pattern of zonation with depth at Ras Mohammed, though the 'zones' do not have sharp boundaries. An Acropora zone occurred from the reef crest down to 5 m, followed by a Millepora zone from 5-10 m. From 10m down to the bottom of the survey was a Xenia zone, though between 2025 m dominance was shared with Montipora. Studies in other areas tend to agree on an upper Acropora zone, in the Netherlands Antilles (BAK 1976), the Seychelles (RosEN 1971), and East Africa (HAMILTON & BRAKEL 1984). However there is no general agreement over the patterns of zonation on deeper reef areas.


Effect of site on distribution Site 1 supported the highest cover of hard corals. This can be attributed to a combination of favourable factors: strong water movement, clear water, and a steep but not over-steep profile. It also had a low cover of soft corals, the inverse relationship between these two taxa extending to site as well as to depth. Strong water movement is generally reported to encourage hard coral growth (STODDART 1969, MERGNER 1971, SHEPPARD 1982, HUSTON 1985a) but to inhibit soft corals (FISHELSON 1970, SCHUHMACHER 1975, SHEPPARD 1982, DINESEN 1983, BENAYAHU 1985). Site 1 has the highest cover of Acropora, reflecting the preference of this genus for more turbulent conditions (BROWN & DUNNE 1980, HAMILTON & BRAKEL 1984). Site 2, although experiencing very similar conditions to site 1 in terms of water movement and clarity, showed the lowest hard coral cover of any site. The difference from site 1 is in the steep and overhanging reef profile, which will inhibit hard coral growth because of the great drop in illumination (KuHLMANN 1983, HusTON 1985a, GRAus & MACINTYRE 1989). The overhanging walls may also make settlement difficult for the larvae. By contrast the site has the highest cover of ascidians, hydroids, sponges and gorgonians. This may be partly due to the abundance of holes and crevices in the reef face which favour these taxa (BAK 1975, SHEPPARD 1982). However, the low abundance of both hard and soft corals is probably more significant, since these are the major competitors for space with other sessile forms (BAK et al. 1982, TuRSCH & TuRSCH 1982, RYLAARSDAM 1983, HusTON 1985a). Although site 2 has the lowest cover of soft coral, it does have the highest cover of Dendronephthya, which is not dependant on light, and is intolerant of competition (FRICKE & SHUHMACHER 1982, BENAYAHU 1985). Site 3 has a moderate cover of hard corals, and the highest cover of soft corals. This is the area which has now been most intensively dived on for a number of years, and it is possible that diving has affected the coral distribution. Divers damage hard coral by breakage, and by the resuspension of sediment. Dive boats cause damage when anchoring, and by fishing activities from the boats. In addition divers feed the fish, and coral-eating fish are particularly abundant at site 3: parrotfish, pufferfish, triggerfish and butterflyfish. The effect of heavy fish grazing on reef composition has been discussed (BAKUS 1966, 1967; BROCK 1979; FRYDL 1979; HusTON 1985a, 1985b), but there is no clear consensus on its effect on coral abundance. Unfortunately it is not possible to determine the effects of past diving on site 3, since there are no baseline data from before it became popular. Site 4 is located on the corner at the entrance to the Gulf of Suez. It has similar levels of water movement to site 3, and probably more turbid water. Yet it has a higher cover of hard coral than site 3, and a lower soft coral cover. This reinforces the


282


idea that the community structure of site 3 has been affected by diving activity. Site 5 has the lowest cover of hard coral, except for site 2 with its atypical overhanging profile. It receives turbid water from the Gulf of Suez, and had the highest sedimentation rates, both factors to inhibit hard corals (FISHELSON 1973, BENAYAHU 1985). Further up the Gulf of Suez coral distribution becomes very restricted, whilst no such inhibition is apparent in the much clearer Gulf of Aqaba. A number of studies record the effect of turbidity in reducing hard coral cover (LoYA & SLOBODKIN 1971, SMITH et al. 1971, MARAGOS 1972, BAK 1976, ANTONIUS 1980, SHEPPARD 1982). The commonest hard coral growth form at site 5 was massive, with branched forms considerably reduced. This is surprising, since both STODDART (1969) and SHEPPARD (1982) consider branched corals as most resistant to sedimentation. In contrast to hard corals, soft coral cover was not reduced. FISHELSON (1970) and SHUHMACHER (1975) both consider soft corals as more tolerant of sedimentation. Of hard coral genera, Porites, a genus well known as tolerant of turbidity (STODDART 1969, PoTTS et al. 1985, EDMUNDS & DAVIES 1989), was common in shallow water at site 5. Millepora did not form its usual distinct zone as it did at the other sites, and on other turbid reefs it is less successful than elsewhere (GRAus & MACINTYRE 1989). Sponges and gorgonians were both in low abundance. Sedimentation generally results in reduced abundance of sponges (WILKINSON & EvANS 1989). Site 5 was the only one showing any overt signs of oil pollution. This was limited to the reef flat, which had a thin layer, and the strand line covered with a thick layer of hardened oil. The reef flat had little coral, but plenty of algae. Although oil is known to decrease coral growth (FISHELSON 1973, LoYA 1975), in this case it is impossible to discriminate the effects of oil pollution and turbidity. There was no obvious presence of oil on the reef face, which was to be expected as the oil had blown on the surface down from the Gulf of Suez.

Implications for the Marine National Park The present survey does not enable firm conclusions to be reached regarding whether the anthropogenic impacts of tourist diving and oil pollution have had adverse effects on the Ras Mohammed reefs. The survey may, however, form a baseline for deciding whether adverse changes occur in the future. There are indications that the most heavily used areas have been affected by diving, and that the changes include a reduction in the abundance of hard coral. The damaging impact of diving activities on reefs has been demonstrated clearly elsewhere in the southern Sinai (HAWKINS 1991). The extent of such damage is a serious concern of the Marine Park management authorities, and will require further study. The management plans aim to regulate and limit the level of diving activity at specified sites. Oil pollution currently appears to have limited ecological effects on Ras Mohammed, though it presents an aesthetic problem. Further monitoring of oil effects will be necessary. ACKNOWLEDGEMENTS This work was carried out as part of the Suez Canal University /Liverpool University Joint Marine Biology Project. This Project was funded by the European Economic Community, and we are grateful for this support. The data on which this paper is based is from the M.Sc. Thesis of M.M.A. Kotb.


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Algae Blue greens Caulerpa Lamoureux 1809 Cladophora Kiitzing 1843 Halimeda Lamoureux 1812 Hypnea Lamoureux 1813 Padina Adanson 1763 Turbinaria Lamoureux 1828 Turf algae Sea grasses Halodule uninervis (Forsk.) Aschers 1882 Halophila ova/is (R.Br.) Hook 1858 Halophila stipulacea (Forsk.) Aschers 186 7 Other taxa Finger sponges Encrusting sponges Irregular sponges Tubular sponges Mussels Pinctada Bolten 1798 Pteria Scopoli 1777 Spondylus Linnaeus 1758 Tridacna Bruguiere 1797 Vermetids Ascidians Dead substrata Coral rock Coral rubble Dead coral Sand