Chapter 15 – coral reef systems and the complexity of hazards

Chapter 15 – coral reef systems and the complexity of hazards Chapter 15 – coral reef systems and the complexity of hazards Chapter 15 – coral reef systems and the complexity of hazards Chapter 15 – coral reef systems and the complexity of hazards Chapter 15 – coral reef systems and the complexity of hazards

Chapter 15 Coral Reef Systems and the Complexity of Hazards Paul S Kench and Susan D Owen School of Environment, The University of Auckland, Auckland, New Zealand ABSTRACT Coral reefs are unique coastal systems as they represent the balance between ecological and physical processes Known for their high biological diversity, both the ecology and geomorphic structure of reef systems support a range of ecosystem services This chapter explores the complexities of hazards in reef systems, underpinned by an understanding of the dynamic interplay between ecological and physical processes Key drivers of impact in reef systems are examined, which include extreme events and slow-onset changes in the environmental boundary conditions of reefs Natural hazards, incremental environmental change, and anthropogenic stresses can each drive significant impacts on reefs Case studies indicate that the degree of impact is temporally and spatially variable dependent on the antecedent condition of reefs Impacts include the catastrophic loss of living reef cover, erosion of adjacent coastlines, the formation of extensive rubble deposits on reefs, and slow deterioration in reef health, leading to structural collapse of reef systems However, coral reef systems are resilient to natural and anthropogenic perturbations, and the recovery period of reefs to a range of impacts is highlighted This chapter also discusses how the resilience of reefs can be compromised through the compounding effect of natural and anthropogenic stresses on reefs that can force major changes in reef health and structure over decadal timescales A reef system model is used to highlight this complexity and allows for consideration of factors such as impact and recovery timescales in reef systems 15.1 INTRODUCTION Coral reefs are the most biologically diverse marine ecosystems in the world (Wilkinson, 1999) Less well recognized, coral reef systems are also complex three-dimensional geological structures that support the living veneer of biological diversity, which in turn contributes to the ongoing development of reef structure Situated across tropical to temperate latitudes (largely bounded 28 north and south of the Equator), coral reefs provide a suite of Coastal and Marine Hazards, Risks, and Disasters http://dx.doi.org/10.1016/B978-0-12-396483-0.00015-7 Copyright © 2015 Elsevier Inc All rights reserved 431 432 Coastal and Marine Hazards, Risks, and Disasters ecosystem services to coastal communities that include biological and food resources, the physical substrate for island accumulation and human habitation, aggregates for construction, and protection from incident oceanic wave energy Indeed, coral reefs provide the foundation for a number of midocean atoll nations and along continental coastlines millions of people live in close association with reef systems However, at the global scale, coral reefs are considered to be in serious ecological decline as a consequence of anthropogenic impacts, natural stresses, and climate change (e.g., Hughes et al., 2003; Buddemeier et al., 2004) Broad-scale assessments (e.g., Wilkinson, 2004) have argued that 20 percent of the world’s coral reefs have been destroyed and that 25 percent of reefs are under an imminent or long-term risk of collapse Reef systems are subject to a range of natural perturbations from short-term “pulse” events, such as cyclones and tsunamis, to longer-term pressures and shifts in environmental controls on reef state, such as sea-level change and changing ocean water chemistry Added to these natural perturbations are a range of anthropogenic stressors that impact reef systems at event through to long timescales, and include dredging and construction activities, and exploitation of biological resources Natural and anthropogenic stresses on reefs can be categorized into three types, based on the geographic relationship between the stressor and reef system Some impacts are local and direct (e.g., cyclone impact or coral blasting), some are “proximal” (e.g., resort development, harbor construction), and others are distant but are then translated to the reef (e.g., sediment release from catchments) These differences influence the time lag and duration of stress on the reef system The effects of perturbations on reef systems, whether natural or anthropogenic, have the potential to alter the natural functioning of the biophysical system These changes can lead to deteriorations in reef health and reef structure that will compromise the ecosystem services provided by coral reefs and promote exposure to hazards for reef-associated communities The hazards faced by reef systems and communities are frequently a tangle of interconnected stressors, including single events and sustained pressures The magnitude and persistence of these stressors influence the ability of reef systems to respond and recover This chapter presents an overview of the major hazards affecting coral reefs and associated human communities An outline of the unique characteristics of reef systems, as a balance between ecological and physical processes and the ecosystem services afforded by reefs, is first presented as a basis to explore perturbations and impacts to reef systems The chapter then examines the range of hazards affecting reefs with a focus on the influence between temporal (pulse versus slow onset) and spatial (local to distal) perturbations A broad definition of hazards is adopted that encompasses any natural or anthropogenic process that can fundamentally alter the functioning Chapter j 15 Coral Reef Systems and the Complexity of Hazards 433 of reef systems By viewing hazard events in isolation, the connectivity of reef systems is quickly obscured While individual hazard events can shock a reef system, this chapter explores how the interaction of multiple hazards and the cumulative effects of such events can impact reef system resilience (ability to recover) Understanding the complex humaneecosystem dynamics of coral reefs and the implications these relationships have for ecosystem resilience is of high importance The chapter explores such interconnected and complex processes from the perspective of cumulative impacts, thresholds of collapse, and potential for recovery 15.2 STRUCTURE AND FUNCTION OF CORAL REEFS Coral reefs are unique coastal systems formed from the interaction between ecological processes, responsible for the growth of coral and other calcium carbonate producing organisms, and physical processes (waves, currents, and sea-level change) that modulate ecological processes and redistribute carbonate material within reef systems (Figure 15.1; Perry et al., 2012; Kench, 2013; Yap, 2013) Without the presence of carbonate-producing organisms, reef systems would not exist Coral reefs are three-dimensional structures, consisting of veneers of living coral and reef-associated organisms that overlie vast sequences of previously deposited calcium carbonate that can extend thousands of meters beneath midocean reef platforms These structures evolve over geological (millennial) timescales and produce a number of characteristic landform types including atolls, barrier reefs, fringing reefs, and reef platforms (Kench, 2013) These reef structures vary in size from 100 km2 in extent, with some reef networks forming barrier complexes >2400 km in length, such as the Great Barrier Reef Coral reef surfaces also support a range of sedimentary landforms that are coherent accumulations of sediment deposited by wave and current processes on, or adjacent to, a coral reef structure (Kench, 2013) Of interest to the analysis of hazards is the formation of subaerial deposits, such as islands and coastal plains, which are geomorphically important at the human timescale as they form the foundation for coastal communities and provide the only habitable land in a number of midocean atoll nations, such as Tuvalu, Kiribati, and the Maldives While the physical structure of reefs and sedimentary landforms vary (Kench, 2013), they serve similar habitat functions However, their exposure to hazards can vary as a function of their structure and proximity to threats 15.2.1 The Building Blocks of Reef Systems To understand the effect of hazard events, it is necessary to highlight key ecological processes and relationships that underpin reef system health and 434 Coastal and Marine Hazards, Risks, and Disasters FIGURE 15.1 Conceptual diagram of the coral reef system and interaction between ecological and physical processes (central box), the processes driving change in the system, ecosystem services provided by reefs and anthropogenic impacts Chapter j 15 Coral Reef Systems and the Complexity of Hazards 435 complexity (see Kench (2013) for an in-depth review) Central to the formation of reef structure and sedimentary landforms is the generation of calcium carbonate (CaCO3) resulting from ecological processes The major carbonate producers on reefs are typically divided into three groups First, corals are considered the principal building blocks of coral reefs These hermatypic corals are characterized by a symbiotic relationship between a coral animal and single-celled algae, zooxanthellae, which live within coral tissue This relationship enables corals to secrete a rigid skeleton of calcium carbonate through a process known as calcification, a biologically mediated process that converts calcium and carbonate ions in supersaturated seawater to CaCO3 (Kinzie and Buddemeier, 1996) Second, a range of encrusting organisms, such as calcareous algae, assists in the structural development of reefs and also bind loose sediment into the reef framework These first two producers are known as primary producers as their growth can contribute directly to coral reef development The third set of carbonate producers are benthic organisms that dwell on and within the reef Such producers include molluscs, calcareous algae, foraminifera, bryozoans, and echinoderms These organisms are known as secondary producers as they not contribute directly to reef growth However, once they die, their skeletal remains contribute to the detrital sediment reservoir In terms of the structural development of coral reefs, the most important consequence of reef metabolic processes is calcification Rates of calcification on reefs are temporally and spatially variable and are dependent on a range of factors that include the density and growth rates of organisms across reefs Typical rates of carbonate production range from 10 kg mÀ2 yearÀ1 on productive (coral rich) forereef zones to 90 to