Mangroves and Coral Reefs Lecture Notes

Comparison of Tropical Coastal Ecosystems: Mangroves and Coral Reefs

• Unifying Themes:

  • Both mangroves and coral reefs are typically considered tropical ecosystems.

  • They are often found adjacent to each other in the landscape.

• Habitats and Substrates:

  • Mangroves: Characterized by mud and soft sediments; they are intertidal, meaning they exist in the area between high and low tide.

  • Coral Reefs: Characterized by hard substrates; they are typically subtidal, existing primarily below the low tide line.

• Energy Environments:

  • Mangroves: Prefer low-energy environments (calm water).

  • Coral Reefs: Prefer high-energy environments with significant wave action.

• Environmental Preferences:

  • Coral Reefs: Do not perform well in the intertidal zone; they are restricted to rocky parts and are not found in temperate systems.

  • Mangroves: Thriive in sediments and are often located around estuaries. They are terrestrial trees confined to a very narrow intertidal vein.

Mangrove Biology and Adaptations

• Definition: Terrestrial, salt-tolerant trees that have moved into the marine environment.

• Species Diversity: There are approximately $60\text{--}70$ different tree species, depending on the taxonomist consulted. Like other terrestrial-to-marine transitions (e.g., sea grasses, salt marshes), the number of species is relatively low compared to purely terrestrial forests.

• Temperature Restrictions: Mangroves cannot tolerate frost; they are killed by freezing temperatures, which restricts them to tropical and sub-tropical regions.

• Salt Management:

  • Mangroves take in salt water and must eliminate the excess salt.

  • Excretion: Many species have specialized glands to excrete salt, which can be seen as literal salt crystals on the leaves.

• Root Morphologies:

  • Stilt Roots (Prop Roots): These extend out to the side of the trunk to anchor the tree in loose, unconsolidated sediments, helping them withstand storms and hurricanes.

  • Pneumatophores (Snorkel Roots): These grow vertically out of the sediment to breathe. They contain pores that allow the tree to take in oxygen.

• The Importance of Breathing:

  • Terrestrial soils usually have oxygen available through the root system.

  • In the marine environment, the roots are inundated with water, which contains very little oxygen. Pneumatophores allow the plant to "breathe" when the tide is out to sustain the root system in anoxic mud.

Ecological Stressors and Zonation in Mangroves

• The Inverse Gradient:

  • In seaweed-dominated intertidal systems, the primary stressor is desiccation (drying out) because the organism is marine.

  • In mangroves, the trees are terrestrial, so the primary stressor is waterlogging (drowning).

• Zonation Pattern:

  • Mangroves cannot go further down than a certain point because if they spend too much time submerged, they cannot perform enough photosynthesis or get enough oxygen.

  • The Red Mangrove is the most tolerant of drowning and is found furthest down toward the sea.

  • Sea grasses, being truly marine, take over in the deeper subtidal areas.

• Refugium Strategy: Mangroves are not fast-growing or strong competitors against faster-growing terrestrial trees and bushes. They live in the intertidal zone as a refugium, occupying a space where other terrestrial plants cannot tolerate the salt and drowning stress.

Productivity and Carbon Sequestration

• Productivity Values: Despite looking lush and green, mangroves have relatively low benthic productivity, often less than $500\,\text{g carbon/m}^2/\text{year}$.

• Causes for Low Productivity:

  • High Maintenance Costs: Unlike phytoplankton (which are essentially "packets of chlorophyll"), trees must spend significant energy building and maintaining non-photosynthetic structures like trunks, stilt roots, and complex root systems.

  • Environmental Stress: Constant energy is expended to manage salt and avoid drowning.

• Blue Carbon Sinks:

  • Mangrove leaves are full of salt and fall into anoxic (low oxygen) sediments where they do not decompose well.

  • This organic material gets buried in layers.

  • Net Carbon Sinks: Mangroves remove $CO_2$ from the atmosphere and deposit it into the sediment. Over millions of years, this material can turn into oils or gases.

Biodiversity in Mangrove Forests

• Shifting Perceptions: Historically viewed as "mosquito-infested" low-diversity areas, they are now recognized as biodiversity hotspots with two functioning systems (terrestrial and marine) occurring simultaneously.

• Terrestrial Component: At low tide, truly terrestrial organisms move in. The tree crowns host terrestrial insects, birds, and epiphytic plants (mosses, lichens).

• Epiphytes: Hundreds of epiphytic species can live on the trunks, which in turn provides habitat for arthropods and insects.

• Marine Component and Nursery Functions:

  • The prop roots provide complex habitat for small and juvenile fish.

  • Nursery Habitat: Mangroves allow small fish to hide from large predators (sharks, crocodiles, larger fish). Many of these species move to coral reefs or sea grass beds once they grow larger.

  • Sessile Organisms: Root structures provide hard surfaces in otherwise muddy environments, allowing sponges, hydrozoans, anthozoans, bryozoans, and barnacles to settle. These organisms further create habitat for worms and small crustaceans.

Human Impact and New Zealand Mangroves

• Global Loss: Mangroves are lost due to infrastructure development, tourism, urban expansion along the coast, and conversion into agriculture (shrimp and prawn farms).

• Climate Change: Sea level rise is an increasing stress factor for mangrove survival.

• The New Zealand Case (Avicennia):

  • NZ has only one species: Avicennia marina.

  • Distribution: Most northern regions (can grow up to $10\,\text{m}$ tall). As you move south (around $38\,^\circ S$), the trees become much smaller due to the cold.

  • Trend: Unlike the rest of the world, mangroves are spreading in NZ due to warming temperatures and high sediment runoff from land (which they enjoy).

  • Conservation Status: Clearance was historically common for boating access or urban views, but there is now a shift toward protection as people recognize their ecological value.

Coral Reef Biology: The Building Blocks

• Definition: Corals are colonial sea anemones belonging to the order Scleractinia (stony or hard corals).

• Composition: There are approximately $800$ different species of hard corals.

• Calcium Carbonate Skeleton: Unlike standard sea anemones, these produce a calcium carbonate skeleton that remains after the polyp dies, forming the physical structure of the reef.

• Global Distribution:

  • Limited by temperature: They prefer warm waters between $20\,^\circ C$ and $30\,^\circ C$.

  • Oceanic Gyres: Corals extend further from the equator on the eastern sides of continents (e.g., Australia) because warm water currents (boundary currents) move toward the poles. Conversely, cold currents on the western sides of continents (e.g., South America/Africa) restrict corals to the equator.

• Productivity: Coral reefs are the most productive marine ecosystems, reaching levels of $3,700\text{--}4,000\,\text{g carbon/m}^2/\text{year}$.

Coral Symbiosis and Feeding

• Zooxanthellae Symbiosis:

  • Corals host microalgae, primarily dinoflagellates of the genus Symbiodinium.

  • Mutualism: The algae perform photosynthesis, providing the coral with organic matter and oxygen. The coral provides the algae with protection, $CO_2$, and inorganic nutrients from respiration/waste.

  • Coloration: The white calcium carbonate skeleton is covered by colorful tissue provided by these brown/green symbionts.

• Coral Bleaching: Under extreme heat stress (heatwaves), corals expel their Zooxanthellae. The coral turns white (the skeleton shows through). If the stress is brief, they can regain symbionts; if it persists, the coral dies.

• Four Feeding Modes:

  1. Zooplankton Capture: Using stinging cells in tentacles to catch copepods.

  2. Photosynthesis: Via the Zooxanthellae.

  3. Mesenterial Filaments: Corals can expel their gut filaments to digest organic material outside their body.

  4. Mucus Trapping: Corals produce slime that captures organic particles falling through the water column, which is then ingested.

Coral Morphology and Reef Growth

• Form and Function:

  • Branching (e.g., Acropora): Fast-growing, reaches for light, but fragile and easily broken by storms/snorkeling.

  • Massive/Brain Corals: Slow-growing but have low drag; they survive hurricanes and storms much better than branching forms.

  • Plates/Foliose: Provide large surface areas for light capture.

• Reef Construction:

  • Individual polyps grow upward, leaving dead skeleton below. A single polyp can build $10\text{--}15\,\text{cm}$ of reef structure.

  • Broken rubble and skeletons consolidate over time into a hard, fixed substrate.

  • Sea Level Rise: Reefs can grow at a rate that stays pace with sea level rise, provided the rise is not faster than the reef's accretion capacity.

Types of Coral Reefs and Darwin’s Theory

1. Fringing Reefs: Directly adjacent to land; features a shallow reef flat and a steep reef crest where large fish are found.

2. Barrier Reefs: Separated from the mainland by a large, deep lagoon (e.g., the Great Barrier Reef lagoon can be $15\text{--}20\,\text{km}$ wide).

3. Atolls: Ring-shaped reefs surrounding a lagoon with no central land mass. They exist in the middle of deep oceans.

• Darwin's Theory of Atoll Formation:

  • A volcanic island forms with a fringing reef around it.

  • The island begins to erode and subside over millions of years.

  • As the island shrinks, the reef keeps growing upward, creating a lagoon between the land and the reef (barrier reef).

  • Eventually, the island disappears entirely underwater, leaving only the ring of coral (atoll).

  • Geological drilling has confirmed this theory by finding volcanic rock deep beneath meters of carbonate limestone.

Coral Reef Ecology: Interactions and Seaweeds

• Seaweed Functional Groups:

  • Turf Algae: Very fast-growing; important food for herbivores.

  • Halimeda: A green algae that produces calcium carbonate; when it dies, it breaks into segments that form much of the white sand on tropical beaches.

  • Sargassum (Fukuids): Large brown algae that form underwater forests in hot water (unlike kelps, which prefer cold).

  • Crustose Coralline Algae: Slow-growing; acts as "cement" that holds the reef rubble together.

• Trophic Interactions:

  • Competition: Sessile organisms compete fiercely for space. Corals use mesenterial filaments for "warfare" to kill neighbors or overgrow them.

  • Grazing: Parrotfish have beaks to crunch coral and algae; they poop out undigested calcium carbonate, contributing to sand production. Surgeonfish graze on turf algae.

  • Predation: Examples include Crown-of-thorns sea stars (which eat coral), Moray eels, and Lionfish (venomous predators).

• Mutualisms:

  • Clownfish/Anemones: Protection for the fish.

  • Cleaner Stations: Small fish (wrasses) pick parasites off larger fish. The larger fish signal they will not eat the cleaners, creating a co-evolved cleaning station.

  • Camouflage Smells: The filefish nips at coral to take on its chemical scent, hiding its smell from predators.

Case Study: The Great Barrier Reef

• Scale: World’s largest reef system; $3,000$ individual reefs and $900$ islands.

• Age: Modern colorful structures are relatively young, only about $10,000$ years old.

• Biodiversity: Over $13,000$ species described ($450$ species of hard corals).

• Status and Threats:

  • Ecosystem Services: Huge value from tourism, fisheries, and coastal protection.

  • Stressors: Ocean acidification, warming temperatures (bleaching), sediment runoff, and overfishing.

  • Trophic Cascades: Overfishing of predators and herbivores allows seaweeds to outcompete corals.

• Restoration: The "Decade of Restoration" involves putting out artificial substrates and advanced research into capturing coral larvae during spawning events to re-seed damaged reefs.

Questions & Discussion

• Q: Has anyone tried snorkeling in mangroves?

  • A: It can be very interesting in the tropics at high tide to see the fish among the roots, though New Zealand mangroves are less colorful and the water is generally not as clear as a coral reef.

• Comment on Seaweed Restoration: The lecturer noted that significantly more money goes into coral restoration compared to seaweed restoration in temperate systems.

• Closing Remarks: The lecturer reminded students to work on their long reports and mentioned that tomorrow's session would cover data analysis, statistics, and report requirements.