Coral Reef Zonation

• Coral reef communities are diverse ecosystems that provide habitat, food, and protection for marine organisms.
• Coral reefs are structured into different zones with distinct characteristics and biological assemblages.

Coral Reef Zonation

• Reef flats extend from 10 m to over 1,000 m from the shore and are subjected to extreme conditions like high temperature, UV exposure, and salinity.
• Reef flats typically have low biodiversity and are composed of back reef coral heads, sand, and seagrass beds.
• Reef crests are located at the seaward edge of the reef flat and are the most wave-exposed zone, resulting in the dominance of calcareous red algae that form high-magnesium calcite limestone.
• Reef crests feature a spur-and-groove system to dissipate wave energy.
• Reef slopes steeply descend from the reef crest and support corals down to depths of 50 m or more in clear water.
• Reef slopes have high biodiversity, competition, and ecological interactions, with zones like the fore reef (buttress zone), escarpment, and deep fore reef (drop-off and wall).
• Caribbean reef slopes have distinct wave zones based on decreasing wave exposure.

Major Reef Organisms

• Gorgonians (soft corals and sea fans) belong to subclass Octocorallia and have skeletons made of organic material and spicules, without significant reef building.
• Caribbean Gorgonians are branching, while Indo-Pacific species are more encrusting or fleshy.
• While many have zooxanthellae, gorgonians rely more on suspension feeding.
• Zoantharians (corallimorphs and zooanthids) are space competitors on reefs, similar in structure to soft corals.
• Zooanthids are more common in the Caribbean, while Corallimorphs dominate Indo-Pacific reefs, and do not occupy significant space but are part of reef communities.
• Sponges are primitive filter-feeders belonging to phylum Porifera, with important functions in water filtration, bioerosion, sediment consolidation, and habitat provision.
• Sponges in the Caribbean are five times more abundant than in the Indo-Pacific, but Indo-Pacific has greater species diversity, with some species like Cliona bioeroding reef limestone significantly.
• Macroalgae are major competitors for space with corals, consisting of three main groups: Chlorophyta, Phaeophyta, and Rhodophyta.
• Chlorophyta (Green algae), including Halimeda, contributes to reef building and sand production.
• Phaeophyta (Brown algae), including Sargassum and Turbinaria.
• Rhodophyta (Red algae) includes crustose coralline algae, crucial for reef structure.
• Healthy reefs have low macroalgal biomass due to herbivore control.
• Overfishing and nutrient pollution lead to macroalgal overgrowth, causing reef degradation.
• Mollusks, echinoderms, crustaceans, and polychaetes play key roles in reef food chains and bioerosion.
• Major bioeroders include parrotfish, sea urchins, and boring sponges.
• Crown-of-thorns starfish (Acanthaster planci) is a significant coral predator species.

Coral Reef Algae and Their Ecological Roles

• Coral reef algae are important in the stabilization of reef structure, primary productivity, and nutrient cycling; as well as major contributors to tropical sand production.
• Coral reef algae diversity includes simple chains of cells to complex thalli up to a meter in length.
• Cyanobacteria (Blue-green algae) are some of the oldest organisms on Earth.
• Green algae contribute significantly to carbonate sediments.
• Brown algae dominate high-energy zones.
• Red algae are the most diverse and play a role in reef consolidation.

Ecological Processes Controlling Algae

• Light availability controls growth and distribution, with species adapting to different depths.
• High nutrient levels favor macroalgae over coral growth.
• Herbivory controls macroalgal proliferation, but overfishing reduces grazing pressure, leading to algal dominance.

Coral-Algal Competition

• There are seven main mechanisms of competition: space limitation, shading, allelopathy (chemical competition), settling attraction to ephemeral algal surfaces, abrasion, basal encroachment, and increased sedimentation.
• Coral recruitment is negatively affected when macroalgae dominate, particularly in disturbed reefs.

Invasive Algal Species

• Invasive macroalgae, such as Caulerpa and Codium, disrupt native ecosystems.
• Macroalgae such as Red algae (Acanthophora spicifera) has invaded many reefs worldwide, forming stable blooms.
• Major vectors of introduction include hull fouling and aquaculture.
• Coral reef ecosystems are complex and dynamic, structured by various biological and environmental factors.
• Zonation patterns define different reef habitats, and the interactions among corals, algae, sponges, and other invertebrates influence the stability of these ecosystems.
• Human activities, such as overfishing and nutrient pollution, are shifting the balance towards macroalgal dominance, threatening reef health.
• Conservation and sustainable management strategies are essential for maintaining the biodiversity and resilience of coral reefs.