Tidal motions of the Red Sea are important to the functioning of the coral reef located along its shores (i.e., Fringing or Marginal reefs). Tidal currents create turbulence, which increases nutrients, phytoplankton, and zooplankton fluxes between corals ecosystem and overlying water column [1]. Changes in water surface elevations due to tidal forces can be important to shallow fringing reef features and occasionally exposing resident corals to wave action and increasing the likelihood of drying out at low tides [2]. Fringing coral reefs have been extensively studied in the Gulf of Aqaba; yet, tidal forces and wind actions and their impacts on fringing coral reefs along the Red Sea are the least studied, particularly in the Gulf of Suez ( [3] [4] and [5]). There is only one tide station located near the entrance of the Suez Canal: approximately 60 km North of Zaki’s Reef, the study site.

Zaki’s Reef is located near the town of Ain Sokhna in the Red Sea’s Gulf of Suez near the most northern latitude for any subtropical coral reefs (29˚32'N and 32˚24'E). The very shallow (50 - 80 m deep) Gulf of Suez lies adjacent to an extremely arid desert where rainfall is minimal, evaporation rates are high, and freshwater inputs are non-existent. The persistent Northeasterly trade winds and extreme temperatures result in high salinity (43 - 45 psu) and large daily and seasonal temperature fluctuations. High temperatures and salinity reduce oxygen solubility, which can stress many species of reef-dwelling organisms. The organisms that thrive here must be able to tolerate these environmental extremes. Of the estimated 335 species of corals found in the Red Sea [6], only 35 species have been identified in the Gulf of Suez, and of these, only six species have been identified to dominate (94% of coral cover) Zaki’s Reef [3]. The fact that only six coral species dominated the study site promoted us to investigate why those six at this location and what might be the reason(s) behind their survival in such an extreme environment?

The main goal of our research in the Gulf of Suez is to address the lack of almost all forms of scientific data in one of the least studied regions of the world’s ocean [3] and [4]. Therefore, the overarching goal for this multi-year study was to establish a knowledge base for fringing coral reefs in the Gulf of Suez and provides basic information for current and future reef management, as reef-related activities generate a significant portion of the country’s tourism revenue, accounting for approximately 11% of Egypt’s GNP and providing 2.2 million jobs.

This research project intends on not only recording a variety of basic scientific data (i.e. air/water temperature, etc.), open and accessible to the public and for future researchers use, but also reveals the correlations, if any, between reef health, local climate and oceanographic conditions in this region. Realizing the reason(s) behind those coral survivals may also provide the critical information needed to protect and enhance the coral reef ecosystems in the Gulf of Suez in particular.

Specific objectives of this research are to apply the least square harmonic method to obtain tidal constants, investigate tidal characteristics and sea level fluctuation at Zaki’s Reef, and the seasonal changes of mean sea level at the study site. We used observed water surface elevation and meteorological data collected at the study site from August 13, 2010 through August 25, 2011, to identify and deduce the major tidal constituents and investigate the physical parameters that control the seasonal changes of sea level. Ain Sokhna has an increasing potential for major future development (20 million by 2050), with two major ports currently in operation. However, no intensive study was reported in literature regarding sea level variabilities or tides (closest tide station is located at Suez City 60 km north of the study site). Therefore, the goals of this study are to 1) Determine tidal constituents and establish a set of constants that predicts water surface elevations in the Gulf of Suez at the northern limits of sub-tropical coral reef ecosystems, 2) Compare our research results to other similar studies along the Red Sea, and 3) Discuss and document tidal impacts on resident biotas on Zaki’s reef (e.g., reef exposure to atmospheric conditions).

There are no long-term studies, particularly for fringing coral reefs, in the Gulf of Suez prior to the research reported here [3]. This multi-year study was set out to establish linkages between fringing coral reefs in the northern Gulf of Suez and prevailing environmental conditions such as local climate, oceanographic conditions, as well as other factors (e.g., oil spells, human encroachment in this region) at Zaki’s Reef. Insight into local meteorological and oceanographic processes may provide a better understanding of the direct or indirect influence of environmental conditions may have on resident biotas. Determining the reasons why coral species and community of organisms found here survive despite all stressors is critical, and such information may hold the key to the preservation of reefs elsewhere.

There are several major reasons behind expanding our data collection to include meteorological data, water temperature and water surface elevation observations. First, it was pure scientific curiosity to discover why corals at Zaki’s Reef survived at extremely high latitude, or are those corals truly heat-adopted? Second, does the local weather create suitable water temperatures and possibly the reason behind Zaki’s Reef survival at this location? Third, do tidal forces have any impacts on corals and their survival at the study site?

All major species of hard and soft corals that survived in the study area and the dominant ones were identified by [3]. They pointed out that of all known coral reef species that exist in the Gulf of Suez, only six dominated Zaki’s Reef; more than 94% of coral coverage. They also examined and discussed in details other environmental factors such as meteorology and oceanography that may have enhanced the survivalship of corals at this location ( [11] and [12]). In this manuscript, the focus is on local tides and its impacts on the resident biotas. Previous researchers [1] reported that tidal motions of the Red Sea are important to the functioning of fringing or marginal reefs located along its shores. Tidal currents create turbulence, which increases nutrients, phytoplankton, and zooplankton fluxes between corals ecosystem and overlying water column. Studying fringing corals in the Gulf of Aqaba, [2] reported that changes in water surface elevations due to tidal forces can be important to shallow fringing reef features, occasionally exposing resident corals to wave action, and increasing the likelihood of drying out at low tides.

The major tidal constituents identified from a tide gauge at Zaki’s Reef (2010-2011) were dominated by the semi-diurnal components (Table 1) and resulted in classifying tidal characteristics as strongly semi-diurnal (Form number = 0.07). Monthly mean sea level variations (non-tidal components) show higher and lower levels in winter and summer, respectively, with a range of 38 cm (Figure 3 and Figure 4). Pre- and post-filtered water surface elevation data analysis (using World Tides software) are identical, which means higher frequency present in those observations (<30 hours) are the main reason to achieve only 87% agreement. Analysis of those high frequency residual data (residual = observed – predicted water surface elevations, i.e., white noise) was dominated by daily frequency, indicating possible wind-dominance.

Results of tidal constituents are similar to results obtained by [13] at Gizan, Saudi Arabia (1800 km south of study site), where tides are defined as strongly semi-diurnal (0.07 vs. 0.07). Tidal ranges were 40 and 37 cm at Gizan and Zaki’s Reef, respectively. Dominant constituents were M₂, N₂, and S₂ at Gizan and Zaki’s Reef. Yet, amplitudes of those constituents at Zaki’s Reef are almost twice as much as the ones found in Gizan. Unlike the similarity in tides between Zaki’s Reef and Gizan, tides at Jeddah (1100 km south of Zaki’s Reef), Saudi Arabia, is mixed, mainly semi-diurnal with large diurnal inequalities, while at Port Sudan (1200 km south of Zaki’s Reef) the tide is diurnal [14]. Yet, co-tidal range as reported by [14] over-predicted, doubled, the observed tidal range at Zaki’s Reef. It should be noted that Jeddah and Gizan are located on the eastern shore, while Port Sudan is located on the western shores of the Red Sea. The fact that the Gulf of Suez is shallow (average water depth is approximately 25 m) may explain the increase in the semi-diurnal amplitudes observed at Zaki’s Reef compared to the ones in Gizan, Jeddah, or Port Sudan. Yet, over tide constituents at Zaki’s Reef were irrelevant as well as daily constituents.

Tides during full moon phase has a profound impact on Zaki’s Reef, particularly during high off-shore wind that causes the reef to be exposed directly to the atmosphere, sun, and wave action (Figure 2). During March, for example, the long-shore wind stress (currents are southeast direction) was high (third highest value of the year) and cross offshore wind stress was the lowest value of the year (Figure 6). The offshore wind tends to push water away from the beach (Zaki’s Reef), and the Super Full Moon (occurred on Saturday March 19, 2011; https://www.timeanddate.com/astronomy/moon/blue-moon.html), have the largest tidal range during the year, which tends to cause the largest changes in water surface elevation. Such combination exposes resident biotas at Zaki’s Reef directly to the atmosphere and the detrimental effect of direct exposure to the hot sun and wave action (Figure 2).

The offshore wind stress (and consequently wind generated currents) is persistent year around (Figure 6), suggesting that moon (weather super or not super moon) may play a major role of exposing corals at Zaki’s Reef to the atmosphere, more hot and cold air temperatures (depending on the season) compared to water temperatures (Figure 4). Exposures of corals at this location to such temperature changes, colder in winter and hotter in summer (Figure 4), may have either detrimental (coral bleaching and die off) or beneficial (resident biotas get “trained” to be more heat tolerant to a wider range of temperatures) effects. For example, it has been reported that exposure to cold water temperature tends to increase coral tolerance to higher water temperature [15], which may be the reason why Zaki’s Reef have shown resilience and survived for many decades at this very high latitude; probably the highest sub-tropical coral reef in the world.

Researchers [15] reported that only coral taxa that survive cold temperature event with limited mortality were those that are also found in other habitats that often experience low temperatures on a regular basis, suggesting species-specific adaptive resistance mechanisms. Corals on Zaki’s Reef experience these extreme seawater temperatures repeatedly, as evident from our seawater temperature observations (see also Figure 3 [12]). Exposure to these high- and low-temperature anomalies on a yearly basis may have caused corals at Zaki’s Reef to develop an adaptive resistance mechanism, as suggested by [15]. The fact that only six (out of 35 identified in the Gulf of Suez) coral species dominate Zaki’s Reef further suggest that their resilience may be attributed to their adaptive resistance mechanisms to extreme temperatures.

Previous comparisons among plants [16] and animals [17] [18] and [19] have shown that some population or species are far more resilient compared to others with marked influences on growth, survival, and disease resistance ( [19] [20] and [21]). Corals at Zaki’s Reef may have been living at the study site for several decades (as coral generally exhibit slow growth, particularly outside their temperature-optimum conditions of 25˚C to 27˚C) and may have the capacity to acclimatize or adapt to external stresses better than others.

If one assumes that Zaki’s Reef existed for several decades, one may conclude then that predominant environmental factors collectively enhanced corals’ survivalship and to exist in such harsh and extreme temperature conditions. This reef may have started out, historically, with the 35 coral species identified by [6] in the Gulf of Suez (i.e., with more than the currently observed and dominated six species at this location) during more favorable and optimum conditions for coral reef to thrive. Over the years, resident biotas at Zaki’s Reef experience frequent exposures to both cold and hot water temperatures and for extended period of times [12], due to the combined effects of offshore and tidal actions, which exposed those 35 species of corals to even hotter and colder temperatures. Yet, more recently, [3] [4] and [12] reported that after four years of studying resident biotas at this location, the same six species (out of 35 species known to exist in the Gulf of Suez) remained dominant. They also reported that, even after the world-wide bleaching event combined with an oil spill in 2005, those six species remained dominant.

Offshore winds at Zaki’s Reef causes the hot nutrient depleted surface water to move offshore, and consequently a return flow supplies the needed nutrients for those corals to survive at this extremely high latitude of sub-tropical reefs. Wind setup, pushing water away from the beach/reef causes an imbalance in the barotropic pressure in the water column. Consequently, a return sub-surface (no current data are available at this location) flow takes place trying to balance this inequality (upwelling). Upwelling are often characterized with enrich nutrients and may provide additional nutrient supply to corals at this location. Upwelling is an oceanographic phenomenon that involves wind-driven motion of dense, cooler, and usually nutrient-rich water towards the ocean surface, replacing the warmer, usually nutrient-depleted surface water. It is also no surprise to see all fishermen boats (personal observations) are parked/fishing at and near this reef, no other fishing ports are near this area for many kilometers.

Current coral conditions of only six dominant species at Zaki’s Reef are the results of many years and cycles of exposures to those extreme conditions; training, adaptation, or one might say “survival of the most adaptive/tolerant.” It is important to recognize that the final six and dominant species at Zaki’s Reef, not only manage to survive, but also are more adoptable to these harsh/extreme conditions, which make them an ideal specimen for relocation to increase the chances of reviving impacted reefs elsewhere.

The current study was driven mainly by scientific curiosity regarding the discovery of a fringing coral reef ecosystem that thrived at extremely high latitude (one of only few coral reefs that exist at such high latitudes). We quickly learned that there are no significant databases for this region, previous published or un-published research around the study site, except for the vastly different Gulf of Aqaba environment. We therefore integrated other environmental data collection to investigate, why this fringing coral reef is flourishing at such high latitude? Such

information may hold the key to the preservation of reefs elsewhere.

We concluded that it is not a single reason behind Zaki’s Reef survival at this location. We believe as our data indicated that a combined effect of several factors may have contributed to the survival and resilience of corals at Zaki’s Reef to thrive at one of the most northern latitudes for sub-tropical reefs. The combination of tidal motions, particularly Super Moon, offshore and longshore currents generated by the prevailing local winds, may have played a major role in selecting those six species out of the 35 species known to exist in the Gulf of Suez, and over the decades “trained” those six species to become heat and cold temperature adopted. Air temperature played a key role at this location, causing water temperature to be remarkably similar to air temperature. Furthermore, when the reef is exposed during super moon (Figure 7), or when offshore current is strong (offshore wind stress is persistent year around at this location), air temperature enhances the ability of those six selected species to become more heat and cold tolerant. This mechanism, through the combined forces of wind and tides, may be responsible of “training” or causing the entire community in the Gulf of Suez to survive such extreme conditions. Those “trained” corals can be used/relocated to areas experiencing deteriorating conditions due to overuse by divers, such as Hurghada (400 km south of the study site and a heaven for divers).

Special thanks to all family members, Zaki and Zachariah Moustafa for providing field assistance and reviewing an earlier version of this manuscript. It is with great appreciations to receive PADI and North Carolina research consortium financial support of this research, which was used to purchase the weather station.

The authors declare no conflicts of interest regarding the publication of this paper.

Moustafa, M.Z. and Moustafa, M.S. (2020) Fringing Red Sea Corals Survival: Is It Tide or Local Wind? Open Journal of Ecology, 10, 225-242. https://doi.org/10.4236/oje.2020.105015