The ecology and dynamics of coral reef communities in an extreme reef environment: the southern Persian Gulf

Coral reefs are among the most threatened ecosystem, partly due to the sustained and ongoing effects of global climate change. An important goal of a contemporary coral reef ecologist therefore, is to understand the dynamics of reefs corals subject to extremes of environmental conditions, and to determine limits to the capacity of coral populations to adapt to ongoing changes. A prerequisite for understanding reef dynamics in a rapidly warming ocean is to quantify population processes (i.e., reproduction, settlement, recruitment, growth, mortality) that regulate population densities through time. Changes to any one process may alter demographic rates, in turn affecting overall population dynamics and reef resilience. However, we still know little about sensitivities of coral at different life-stages to environment extremes. The purpose of this research was to quantify key ecological processes in the Persian Gulf, considered one of the most extreme environments for coral reefs, in order to understand the likely effects of climate change on the structure and dynamics of coral populations and communities. The first step towards understanding possible effects of climate change on coral reefs is determining how the structure of coral assemblages (usually containing similar genus and/or species) vary along steep environmental gradients within a single geographical location. Chapter 2 provides a comprehensive regional examination of coral communities around the northeastern Arabian Peninsula (within the southern Persian Gulf, Strait of Hormuz, and Gulf of Oman), and a comparison of environmental variables considered important in structuring these communities. Multivariate analysis revealed distinct sub-regional coral communities among the southern Persian Gulf, Strait of Hormuz and Gulf of Oman, associated with considerable spatial heterogeneity in oceanic conditions. While temperature and salinity accounted for major differences in the structure of coral assemblages among different locations, additional environmental parameters, including chlorophyll-a, surface currents and winds were also found to be important factors in structuring these assemblages. Aside from the strong spatial patterns, there have also been considerable temporal shifts in coral reef community structure throughout the northeastern Arabian Peninsula over the past two decades. Most dramatic are the apparent changes from Acropora dominated to poritid and faviid dominated communities, particularly in the southern Persian Gulf and the Strait of Hormuz, which is linked to taxonomic differences in susceptibility to extreme temperatures. Extreme temperatures have significant direct effects on coral populations and communities, which are most apparent during multi-specific large-scale bleaching events. However, climate change is also likely to impact coral reproductive processes through modified thermal environments that may disrupt tightly regulated reproductive cycles or highly synchronized temporal spawning events. The aim of Chapter 3 was to: (1) determine whether corals in southern Persian Gulf (Dubai, UAE) are reproductively active, (2) determine the month(s) of spawning of six common coral species within this region, and (3) examine the reproductive biology of the three most abundant species: Acropora downingi, Cyphastrea microphthalma and Platygyra daedalea. Direct measures of the reproductive condition of coral colonies showed that coral reproduction was seasonal, with peak reproductive activity around the full moon in April. These results demonstrate that coral reproduction patterns within the region are highly seasonal and that multi-species spawning synchrony is highly probable. Moreover, Acropora downingi, Cyphastrea microphthalma and Platygyra daedalea were all hermaphroditic broadcast spawners with a single annual gametogenic cycle of between 7 and 9 months. Fecundity and mature oocyte sizes for all species were comparable to those in other regions. Collectively these results demonstrate that the reproductive biology of corals in the southern Persian Gulf is similar to other regions, indicating that these species have adapted to the extreme environmental conditions in the southern Persian Gulf. Coral settlement, a key ecological process in the maintenance, recovery and resilience of coral reef ecosystems, is also highly susceptible to increasing climate change. Elevated temperatures can affect coral larvae during early stages of development reducing rates of settlement, which in turn can have profound affects on overall reef dynamics. Chapter 4 investigated spatial and temporal patterns of coral settlement at six sites over two years using coral settlement tiles. Results from this thesis showed there is clear evidence of ongoing coral settlement in the southern Gulf, despite very high temperatures and high frequency disturbance events (i.e., bleachings). Coral settlement in the southern Gulf was low (≈ 113 spat m-2 year-1), but highly seasonal with peak settlement during the summer (June and August), when daily-mean temperatures approached 35°C, well above the critical thermal limit for coral larvae reported from other experimental studies. However, results also showed that peak settlement was recorded up to 4 months after the major spawning season in April and May suggesting either delayed larval settlement or spatial and/or taxonomic disparity between studies of reproduction and versus settlement. Furthermore, the composition of coral spat in the southern Gulf differed considerably from those reported on most reefs in the Indo- Pacific because there were few Acroporidae coral spat (11%), and no Pocilliporidae. Importantly, data from this thesis indicates that the observed long-term shifts in community structure of adult corals are likely being reinforced at the level of settlement, such that there is limited scope for recovery of former Acropora-dominated coral assemblages in the Gulf. Fluctuations in a range of environmental variables (i.e. physical and chemical), mediated by changes in global climate, are predicated to directly affect the growth rates of corals and thus the overall size structure of coral populations. Chapter 5 tested for consistent differences in the size-structure of four different coral populations along established gradients of increasingly severe environmental conditions from the southern Persian Gulf (Dubai and Abu Dhabi) to the eastern Persian Gulf (western Musandam). Significant and consistent differences were observed in mean colony sizes and size distributions for all four coral species between locations. All corals in the southern Gulf, where sea surface temperatures, salinity, and the recent frequency of mass bleaching are all significantly higher, were smaller, and their size structure positively skewed. Differences in size structure between locations were more pronounced than differences between species at each location, suggesting that extreme differences in environmental conditions. Importantly, results suggest that even within regions, such as the Persian Gulf, where corals are considered particularly tolerant of environmental extremes, increased frequency of bleaching events will lead to persistent declines in the size of coral colonies. Overall, the fate of different corals will depend on the recurrence of major disturbance events relative to their capacity for recovery, which in turn depends on their capacity for individual and population growth. This research shows at least some species of scleractinian corals can survive prolonged exposure to temperatures well above those (e.g., 32-34°C) considered to be above the critical thermal limits of corals throughout much of the Indo-Pacific. Moreover, corals within the Persian Gulf (where temperatures range from 35°C in summer) are continuing to spawn, settle and grow. These results have far reaching implications, suggesting that there is capacity for corals to withstand sustained and ongoing climate change. However, corals within the Persian Gulf have become adapted to extreme temperature ranges and specific local conditions over thousands of years, and it is questionable whether corals in locations that have until now experienced much milder conditions posses (or can develop) similar capacity to withstand extreme environments. This largely depends upon the physiological or genotypic basis of increased temperature tolerance in corals from the Gulf, which requires considerable further research.