Macroalgae in the coral reefs of Eilat (Gulf of Aqaba, Red Sea) as a possible indicator of reef degradation

Abstract

The current state of health of the coral reefs in the northern Gulf of Aqaba (Red Sea), notably the Eilat reefs, is under debate regarding both their exact condition and the causes of degradation. A dearth of earlier data and unequivocal reliable indices are the major problems hinder a clear understanding of the reef state. Our research objective was to examine coral–algal dynamics as a potential cause and an indication of reef degradation. The community structure of stony corals and algae along the northern Gulf of Aqaba reveal non-seasonal turf algae dominancy in the shallow Eilat reefs (up to 72%), while the proximate Aqaba reefs present negligible turf cover (<6%). We believe that turf dominancy can indicate degradation in these reefs, based on the reduction in essential reef components followed by proliferation of perennial turf algae. Our findings provide further evidence for the severe state of the Eilat coral reefs.

Introduction

Coral reefs have deteriorated extensively during the past decades as a result of environmental disturbances of both anthropogenic and natural sources (Graham et al., 2006, Rogers and Miller, 2006, Wilkinson, 2004). According to Wilkinson (2004) 20% of the world’s coral reefs have been effectively destroyed and approximately 24% of the remaining reefs in the world are under imminent risk of collapse through human pressure. One way of rehabilitating damaged reefs is via reef management. A major impediment to management and prevention of further deterioration of the reefs lies in our inability to predict such decline by identifying the early warning signs of potentially dramatic community changes. Scientists are still unable to fully recognize the conditions that lead to the collapse of reef communities, even on reefs that are relatively well studied (Bellwood et al., 2004). For example, the coral reefs of the Caribbean have been thoroughly studied in relation to various human and natural effects, such as overfishing, hurricanes, sea urchin disease outbreak, and bleaching events (Hughes, 1994, Ostrander et al., 2000), providing quantitative data regarding reef degradation (Hughes et al., 1999). Although such features of reef degradation as loss of macro-fauna, reduced fish stocks, a shift from fish-dominated to echinoid-dominated herbivory, and reduced coral recruitment, have been well documented, the trajectory to ecosystem collapse remained unrecognized. Therefore, the ecosystem’s failure to regenerate remained unforeseen (Bellwood et al., 2004).

Degraded reefs often experience a transition in the structure of their benthic communities from dominant reef-building corals to overtaking marine macroalgae, a phenomenon termed ‘phase shift’ (Done, 1992, Hughes, 1994). Such shift may have significant ecological implications among them loss of structural complexity (Done, 1999, Spencer and Viles, 2002) that can affect coral recruitment (McCook et al., 2001), fish recruitment, competition, and predation (Öhman et al., 1998). These changes may eventually lead to a decline in species richness and biodiversity and, therefore, to an unstable ecosystem (Graham et al., 2006, Kiessling, 2005). Interactions among the coral reef occupants have been demonstrated in various models, in which stony corals, algae, ‘bottom-up’ (i.e. eutrophication) and ‘top-down’ (i.e. presence of grazers) factors are considered to be the main forces controlling the coral reef equilibrium (Done, 1992, Knowlton, 1992, Mumby, 2006). According to these models, the balance of scleractinian corals and algae is not expected to be interrupted when driven by nutrient enrichment alone. However, the combined effects of reduced herbivory together with enhanced nutrient supply (eutrophication) can shift the equilibrium in favor of algal dominancy (McCook, 1999, Miller, 1998, Thacker et al., 2001).

The coral reefs of the Red Sea are considered to be among the “healthiest” in the world (Hodgson, 1999), or, at least, in “relatively good health” (Wilkinson, 2004). Defining coral reefs based on their state of health, especially when they have been classified as being in a “good condition”, can lead to a subsequent lack of concern by decision-makers and, consequently, no action being taken when a situation arises that requires countermeasures. A small-scale evaluation of the status of local reefs along the Red Sea can thus be beneficial in providing a tool to obtain a reliable focal assessment for any given reef.

The coral reefs of Eilat (northern tip of the Gulf of Aqaba, Red Sea) are among the most intensively studied, spatially-limited, coral reefs in the world. Despite the intensive research, the data related to the reef state is relatively poor, and therefore researchers have conflicting opinions regarding the reef’s health. These conflicts are well expressed in two contradictory reports by an International Expert Team (IET), which aimed at providing the Israeli Government with a reliable synopsis of the northern Gulf of Eilat’s ecological state. According to the first IET report “The unique coral reef in Eilat has been deteriorating at an alarming rate over the last three decades…the Israeli coral reefs apparently suffer from lost diversity, decrease in coral cover, low rates of coral–larval settlement and recruitment” (Atkinson et al., 2001). However, in the second report, the IET states that “there was more evidence presented of reef improvement than of reef decline” (Atkinson et al., 2004). The problem in determining the reef state was further reflected in other publications, which also demonstrate contradictory conclusions regarding the reef state (Ben-Tzvi et al., 2004, Fishelson, 1995, Loya, 2007, Rinkevich, 2005), and the potential factors responsible for reef decline (Abelson et al., 2005, Glassom et al., 2005, Loya et al., 2004). Most of the degradation-related research that was carried out in the coral reefs of Eilat was focused on changes in several reef community indices associated with corals while neglecting the accompanied changes in benthic algae and their potential role which have often been overlooked.

The present study was aimed at examining the structure and dynamics of benthic-algal assemblages, notably turf algae, as a tool to indicate a reef’s state of health. Turf algae dominancy may have significant ecological implications for coral reefs, including its adverse effects on recruits (Birrell et al., 2005). Moreover, turf algae may compete for space with coralline algae, which are known for their essential role in coral reefs in inducing larval settlement and metamorphosis (Heyward and Negri, 1999, Kitamura et al., 2007), and for their significant contribution to reef construction by calcification (Fabricius and De’ath, 2001). Therefore, examination of the spatial distribution and effects of turf algae in the coral reefs of Eilat is expected to shed light on the actual state of health of the reefs.

In view of the fact that algae spread and occupy vacant substrates that were formerly occupied by reef-building corals and coralline algae (Jompa and McCook, 2003, Miller, 1998, Miller and Hay, 1998), the assessment of changes in algal assemblages can function as an additional index that may contribute to filling in the gap in the accumulated data regarding the state of health of Eilat’s reefs.