Coral Reef Science: What is a Reef/Parts of a Reef

Reef

A term reserved to describe just the mineral structure.

Reef Community

The organisms that live on a reef.

Coral

• They are an animal, “plant,” and mineral.

• Animal: small anthozoans (polyps)

• Plant: Contain photosynthetic zooxanthellae (algae)

• Mineral: Polyp takes up Ca and C atoms and secrete a hard mineral skeleton.

Defining Reef Traits

• Topographic Relief

• Rigidity

• Form in-place

• Framework of Skeletons

• High Biodiversity

Reef Definition: Reef Tract (Ecosystem)

• The suite of interconnected ecological zones around and including the structural reef and the associated high diversity biological community.

• Includes the shore, back-reef lagoon, reef crest, and fore-reef.

Reef Definition: Structural Reef

The complex rigid structure built from the skeletons of organisms that forms in-place and rises above the sea floor.

Reef Definition: Classic Reef

The bright, vibrant, lively underwater ecosystem made of coral which is often used for snorkeling and diving.

Types of Reefs: Atolls

• Donut-shaped

• Often without a high island in the water

• Out in the deep ocean

Types of Reefs: Fringing Reefs

Reefs located a short distance offshore and going around it.

Types of Reefs: Barrier Reefs

Reefs that follow the coastline of the shore, separated by a moderate distance by a deep lagoon

Types of Reefs: Bank Reefs

• Categorizes all of the other reefs that don’t easily fall into other categories.

• May or may not be associated with a coastline, islands, or a lagoon/barrier.

Types of Reefs: Patch Reefs

• Small reefs

• Many different scales

• Some are just a few meters across while others are 1km across

• Usually located within the lagoon of a bigger reef system

Reef Characteristics: Physical Environment

Includes:

• Warm water (greater than 18C)

• Supersaturated Water (CaCO₃)

• Low Turbidity

• Nutrient Desert

• Hardbottoms

Physical Characteristics: Warm Water

• Reefs require a minimum average temperature of 18C

• The average reef is greater than 20C

• Water has different properties at different temperatures—it holds more CaCO₃ and less CO₂ at higher temps.

Physical Characteristics: Supersaturated in CaCO₃

• Corals, clams, and other organisms use CaCO₃ to make skeletons.

• Tropics are 3 to 4 times supersaturated in CaCO₃.

• Adding CO₂ makes the water more acidic, lowering the CaCO₃ saturation, making it harder to form skeletons.

• Burning fossil fuels adds CO₂ to oceans, decreasing saturation, and increasing ocean acidification.

Physical Characteristics: Low Turbidity

• Low turbidity results from low photosynthetic plankton productivity.

• Nutrients accumulate in the lower cold areas of the thermocline.

• In tropics, there is no change in the thermocline, so nutrients and the plankton that eat them stay low under the water.

Physical Characteristics: Nutrient Desert

• Too many nutrients allow algae to smother all the coral.

• Low nutrients allow for a bustling area of biodiversity because of the large amount of competition (no algae or plankton smothering the whole area).

Physical Characteristics: Hardbottoms

• Lots of CaCO₃ precipitation leading to a very different sedimentation.

• Much more 3D, vertical topography for organisms to inhabit.

• Softbottoms don’t allow organisms to anchor, hardbottoms do.

• Organisms rapidly build up structures forming inorganic cements.

• Bioeroders erode cavities into structures.

• Topography is built by organisms rather than gravity.

• Results in a complex 3D structure that varies from place to place based on the present organisms.

Reef Characteristics: Biological

• Mineralized Skeletons

• Modular Organsims

• Functional Plants

• High Diversity

• Area Zonation

Biological Characteristics: Mineralized Skeletons

• Organisms create carbonate skeletons.

• Making carbonate skeletons is very efficient for energy use in many organisms.

Biological Characteristics: Modular Organisms

• Modular organisms have parts/modules that can replace each other.

• Partial mortality, one part dying doesn’t mean the entire organism ceases to exist.

• Quick replacement and building of each part from the rest of the organism.

• Each module is a cloned individual (true corals, soft corals, anemones, sponges, algae).

Biological Characteristics: Functional Plants

• Corals are endosymbionts, they contain zooxanthellae (single-celled algaae).

• Algae is located within the cells of the coral, containing pigments that give corals their color.

• Corals get most of their energy from their endosymbiont zooxanthellae (95 to 98 percent of their calories).

Biological Characteristics: High Diversity

• “High” diversity is relative. High diversity compared to similar/proximate areas.

• Large differences compared to non-reef areas.

• Reefs support many different and unique ecological niches.

Biological Characteristics: Area Zonation.

• Corals form differently based on depth and light exposure.

• Low light leads to slow growth plates, high light leads to rapid branching.

• Though small and fragile, branching corals grow the fastest, allowing them to recover quickly from storms and heavy waves destroying them at the surface.

Coldwater Reefs

• Not a true reef zone, no access to light. Dead reefs.

• Topographic relief

• Rigidity to waves

• Forms in-place

• Made of biological skeletons

• High diversity (versus surrounding area)

Worm Reefs

• Not true reefs, don’t fit all characteristics.

• Not modular, gregarious settlement.

• Annelids secrete a calcareous tube creating rigidity and a 3D structure.

• Reef framework is the biological skeletons, built on place on old skeletons.

• High diversity

Reef Sections: Fore Reef

• Aphotic (not a true reef zone)

• Disphotic/Rariphotic

• Deep Fore-Reef (Red Algae Zone)

• Mesophotic Reef

• Euphotic Fore-Reef

• Wall/Drop Off

• The Cryptos

• Spur and Groove

• Barren Zone

Reef Sections: Back-Reef/Lagoon

• Reef Flat

• Carbonate Shelf

• Sand and Mud Bottoms

• Seagrass and Algae Beds

• Hardgrounds

• Patchreef/Fringing Reefs

• Mangroves

• Cays (not a true reef zone)

• Salt Turns (not a true reef zone)

Reef Sections: Reef Crest

• Algal Pavement

• Reef Crest/Algal Rim

Reef Sections: Aphotic

• Hard to Research, requires a submersible

• Very dark, zero light.

Reef Sections: Disphotic (Rariphotic)

• Very little light, not enough for growth

• Lower bounds of true reef zones.

Reef Sections: Deep Fore-Reef (Red Algae Zone)

• Lowest photic zone

• Some autotrophs can live deeper than others, depends on clarity.

• The algae is red because red light is absorbed first, blue and green wavelengths reach the deepest, and the algae needs as much light as it can get.

Reef Sections: Mesophotic

• Lowest level to support minimal coral/plant growth.

• Too deep for divers, requires subs and rebreathers.

• Contains tons of cavities and crevices for species to take refuge, supporting biodiversity.

• Important for conservation of reefs—it is shielded from sedimentation and overfishing.

Reef Sections: Euphotic Fore-Reef

• Platy Coral Zone

• Massive Coral Zone

• High Diversity Zone

• Branching Coral Zone

• Receives the most light.

• Most diverse and active zone.

Reef Sections: Wall/Drop Off

• Creates areas for species to take refuge and breed.

• Absorbs many large waves that would otherwise damage the inner lagoons.

• Prevents coastal erosion.

Reef Sections: Cryptos

• All of the microhabitats hidden below coral (dark crevices, tunnels, and coral undersurfaces).

• Created by accretion and bioerosion.

• Extremely important and diverse as it provides tons of shelter for organisms.

• Up to 8X the surface area hidden under in this zone compared to the visible surface.

• Contains 40% of species and 60% of biomass contributed to higher trophic levels.

Reef Sections: Spur and Groove

• Prominent in fairly shallow water.

• Spurs shoot up from coral growth, grooves dip down between.

• Creates tons of space for species to hide.

Reef Sections: Barren Zone

• Absorbs full energy of waves.

• Dominated by strong branching corals.

• Relatively close to the reef crest.

• Waves from storms will break in the barren zone before they can reach the reef crest.

Reef Sections: Algal Pavement

• Contains tons of encrusting algae towards the shallow surface leading to the reef crest.

• Made of a relatively flat, hard rock surface that prevents and absorbs heavy wave breaks.

Reef Sections: Algal Rim (Reef Crest)

• Reef crest is often exposed at low tide.

• Algal ridge surrounds reef crest and holds in water for the corals.

Reef Sections: Reef Flat

• Landward of the crest.

• Flat area, surface close to sea-level.

• Can be a productive reef environment, best developed in fringing reefs.

Reef Sections: Carbonate Shelf

• Lagoon, wide and shallow

• Protected by barrier reef from surf, calm area.

• Variety of islands and patch reefs

• Produces a lot of carbonate

Reef Sections: Sand and Mud Bottoms

• Loose carbonate sand, mud, and silt.

• Home to many burrowing organisms.

• Can be home to seagrass beds hosting many small fish.

• Hunting grounds for larger marine predators.

Reef Sections: Seagrass and Algae Beds

• Algae produce tons of carbonate.

• Seagrass and associated invertebrates are a major energy source for the reef.

• Fish will hide in the reef during the day and forage over grass beds at night.

Reef Sections: Hardgrounds

• Allows organisms to anchor and grow relatively high.

• Transition zone to the main coral zones.

• Provides moderate shelter for many different organisms.

Reef Sections: Patch Reefs/Fringing Reefs

• Fringing reefs occur around cays

• Patch reefs occur on ancient high points

Reef Sections: Mangroves

• Patches of trees, tiny shrubs, and plants that have adapted to grow directly in seawater.

• Not a ton of diversity in the trees themselves, but they support a high diversity of aquatic life and nutrient production.

• Protect islands from storms.

• Provide tons of shelter and habitats for many different organisms.

• Act as a nursery for many fish.

Reef Sections: Cays and Salt Turns

• Help absorb waves.

• Filter nutrients and salt.

• Support biodiversity, serve as nursery grounds for fish.