Mangroves and Coral Reefs Lecture Notes

Course and Professional Information:
- Course Code: 212 Marine Biology.
- Instructor: Mads S. Thomsen (Room 233).
- Reference Material: Chapters 14 and pages $287-291$ in Castro and Huber.
General Comparison of Systems:
- Mangroves (Estuaries): Characterized as hot, intertidal, sedimentary, and low-energy environments.
- Coral Reefs: Characterized as hot, subtidal, hard-rock, and high-energy environments.

General Characteristics:
- Defined as "terrestrial" salt-tolerant trees.
- Consists of approximately $60-70$ intertidal species.
- Vulnerable to climate extremes: they are killed by frost.
- Exist in low wave-energy environments with sedimentary systems.
Root Specializations:
- Prop-roots (Stilts): Specialized roots that provide structural support in soft sediment.
- Pneumatophores (Snorkels): Upward-growing roots that allow the plant to obtain oxygen in anaerobic sediment conditions.
Ecosystem Dynamics:
- Relatively low primary productivity compared to other systems, yet supports surprisingly high biodiversity.
- Zonation: Species are distributed across the intertidal zone based on their specific tolerances.
- Biodiversity Changes: Marine diversity fluctuates between low tide (exposed roots) and high tide (submerged habitat).
- Ecosystem Services:
  1. Provisioning: Supplies food such as fish.
  2. Regulating: Assists in climate regulation and storm protection (coastal buffering).
  3. Supporting: Provides critical habitats and acts as a nursery for various species.
  4. Cultural: Offers educational and recreational value.
  5. Carbon Sequestration: Noted for "blue carbon" storage capabilities.
Food Web Structure:
- Producers: Mangrove trees and associated algae.
- Consumers: Various marine and terrestrial organisms that interact with the system at different tidal stages.

Species: Only one species is present, Avicennia marina var. resinifera.
Latitudinal Gradient: Displays significant variation in stature based on location:
- North: Grows up to $10\,m$ tall.
- South (at $38^{\circ}\,S$ ): Stunted growth, reaching only $0.5\,m$ .
Biodiversity: Characteristics include relatively low(er) diversity compared to tropical mangals.
Current Trend: The species is currently spreading, potentially due to factors such as high sedimentation, increased nutrient availability, and global warming.

Biological Foundation:
- Built primarily by members of the order Scleractinia (stony or hard corals).
- Corals are hermatypic, meaning they are reef-building organisms.
Coral Polyp Anatomy:
- Composed of interconnected polyps that secrete a calcium carbonate ( $CaCO_3$ ) skeleton.
- Possess nematocysts (stinging cells) for defense and prey capture.
- Utilize mucus and mesenterial filaments.
Symbiosis with Zooxanthellae:
- Hermatypic corals contain Zooxanthellae, which are photosynthetic dinoflagellates living within the coral tissue in a mutualistic relationship.
Feeding Strategies:
- Catching zooplankton and detritus using tentacles and mucus.
- Photosynthesis (energy provided by Zooxanthellae).
- Mesenterial Filaments: Used to digest organic material outside the body.
- Mucus Nets: Used to absorb dissolved organic matter (DOM).
Morphological Variation: Coral growth forms include:
- Acropora (branching/table modes).
- Diploria (brain coral).
- Fungia (mushroom coral).
- Montipora.
- Turbinaria.
- Dendrogyra.
- Lobophyllia.

Temperature: Optimal range is between $20-30\,^{\circ}C$ , with an upper limit around $35\,^{\circ}C$ .
Light: High light intensity is required for Zooxanthellae photosynthesis.
Water Clarity: High clarity (low turbidity) to allow light penetration.
Salinity: Specifically stenohaline (narrow tolerance); typically requires $35$ psu.
Oxygen: Requirements are high.
Food Availability: Requires high concentrations of zooplankton.
Hydrodynamics: High water flow is beneficial for nutrient delivery and waste removal.
Substrate: Requires hard substrates for attachment.
Negative Stressors: Sensitive to high sediments (which smother polyps) and various biological enemies.

Reef Types:
- Fringing Reefs: Grow directly from the shore; the most common type.
- Barrier Reefs: Separated from the shore by a lagoon.
- Atolls: Circular reef systems surrounding a central lagoon, usually found in the open ocean.
Darwin's Theory of Atoll Formation:
- Proposes a geological evolution: Fringing reef $\rightarrow$ Barrier reef $\rightarrow$ Atoll.
- This occurs as a volcanic island subsides or sea levels rise, while the coral continues to grow upward toward the light.

Primary Productivity: Extremely high primary productivity compared to surrounding oceanic waters.
Alternative Producers:
- Seaweeds including Halimeda (calcareous green algae).
- Turf algae.
- Sargassum.
- Encrusting algae.
Competitive Mechanisms:
- Use of mesenteric filaments to digest neighbors.
- Feeding tentacles.
- Sweeper Tentacles: Specialized attack tentacles for stinging competitors.
- Deployment of toxins.
- Shading out competitors for light.
Consumer Roles:
- Grazers: For example, Parrot fish, which help control algal growth.
- Predators: Animals that eat coral polyps or the mucus secreted by corals.
Symbiotic Relationships:
- Mutualism and commensalism are common (e.g., File fish like Oxymonacanthus longirostris).

System Overview:
- A World Heritage Site and the world's largest reef system.
- Includes approximately $3,000$ individual reefs and $900$ islands.
- Extends length-wise for $2,600\,km$ covering an area of $348,700\,km^2$ .
- The living reef is approximately $6,000-8,000$ years old.
- Shallow reefs cover $20,679\,km^2$ ; $80\%$ grow on Pleistocene limestone platforms.
- Vertical growth rate: $1-25\,cm/y$ (to a maximum depth of $150\,m$ ).
GBR Biodiversity Statistics (Total Species: $12,806$ ):
- Sponges: $2,500$
- Molluscs: $3,000$
- Bony Fishes: $1,625$
- Crustaceans: $1,300$
- Bryozoans: $950$
- Tunicates: $720$
- Echinoderms: $630$
- Worms: $500$
- Hard Corals: $450$
- Red Algae: $323$
- Green Algae: $195$
- Soft Corals/Sea Pens: $150$
- Sharks and Rays: $136$
- Brown Algae: $111$
- Jellyfish: $100$
- Anemones: $40$
- Whales and Dolphins: $30$
- Insects/Arachnids: $25$
- Breeding Sea Snakes: $14$
- Marine Turtles: $6$
- Dugong: $1$

Major Environmental Threats:
- Warming Seas and Heatwaves: Leads to thermal stress.
- Sediment Runoff: Smothers the coral polyps.
- Nutrient Pollution: Causes algal blooms that outcompete corals.
- Overfishing: Removes top predators and essential grazers.
Coral Bleaching:
- Occurs when stressed corals expel their mutualistic Zooxanthellae.
- Repeated bleaching events lead to coral mortality.
Restoration Efforts:
- Significant research is currently focused on coral reef restoration.
- Scientists are working to increase temperature tolerances in coral species to combat climate change impacts.