Marine Science (0697) | Mangrove forest

Study Notes: 5.6 Mangrove Forests

Welcome to the world of Mangrove Forests! These incredible ecosystems are where the land meets the sea, and they contain some of the toughest, most highly adapted organisms on Earth. In this chapter, we will look at how mangrove trees and the animals that live among them survive the challenging conditions of the intertidal zone.

Studying mangroves is important because they act as vital nurseries for marine life and protect our coasts from erosion and storms!

5.6.1 Structure of a Typical Mangrove Forest

Mangrove forests are defined by their location in one of the most stressful marine environments.

Key Structural Features

• Location: Mangroves grow in the intertidal zone. This means they are regularly covered by the tide (high tide) and exposed to the air (low tide).
• Environment: They are typically found in estuarine environments. An estuary is a partly enclosed body of water where fresh water from a river mixes with salt water from the sea.
• Boundary: The mangrove forest often borders a terrestrial forest (land-based forest) on one side and the open marine water on the other.

Think of a mangrove forest as the tough neighbourhood between the land and the ocean—it gets hit by both salt water and fresh water, and the conditions are always changing!

5.6.2 Abiotic Factor Changes in an Estuary

Because estuaries are tidal and mix fresh and salt water, the non-living (abiotic) factors change constantly over a tidal cycle. These rapid changes make life very difficult for organisms here.

Factors That Fluctuate During the Tidal Cycle:

1. Salinity:
   • When the tide is high, the salinity increases as sea water rushes in.
   • When the tide is low, or after heavy rainfall, the salinity decreases as fresh river water dominates.
   • Mangrove organisms must be osmoregulators—able to handle massive changes in salt concentration.
2. Temperature:
   • When the tide is high, water temperature is relatively stable.
   • When the tide is low, the exposed mud and air temperature can rise or fall dramatically, causing large temperature changes for exposed organisms.
3. Dissolved Oxygen (DO):
   • In the water column, DO levels can vary, but the main issue is the waterlogged soil.
   • The thick mud contains lots of dead organic material. Decomposers use up large amounts of oxygen during aerobic respiration, resulting in very low oxygen (anaerobic) conditions in the soil.

5.6.3 Adaptations of Mangrove Trees (The Masters of Survival)

Mangrove trees have three crucial adaptations to survive the waterlogged, salty environment:

1. Dealing with Low Oxygen in Soil

Adaptation: Aerial Roots (Pneumatophores)

• These roots grow upwards out of the waterlogged mud and soil.
• They act like small snorkels, allowing the roots underground to perform gas exchange (taking in oxygen) from the atmosphere when the tide is low.

Analogy: If you try to breathe underwater (waterlogged soil), you need a snorkel (pneumatophore) to reach the air!

2. Dealing with the Tides and Unstable Mud

Adaptation: Prop Roots

• These roots (also known as stilt roots) grow from the trunk and branches, arching down into the mud.
• They provide fantastic support and stability, anchoring the tree securely against strong tidal currents and wave action in the soft, unstable sediment.

3. Dealing with High Salt Water

Adaptation: Salt Excretion via Leaves

• Mangroves absorb salt water, but they must regulate the salt in their tissues.
• Some species use glands in their leaves to secrete (or excrete) excess salt, which can often be seen as tiny salt crystals on the leaf surface.
• Other species simply concentrate the salt in old leaves and drop them—a neat way to get rid of the problem!

5.6.4 Adaptations of Mangrove Forest Organisms

Many animals have evolved remarkable features to live in the muddy, tidal environment, spending part of their time in the water and part on the land.

A. The Banded Archerfish (Toxotes jaculatrix)

This fish has special features to hunt insects that land on the mangrove leaves and branches:

• Eyes: It has large, forward-set, movable eyes. This gives it binocular vision (like humans), allowing it to judge distance accurately to target prey out of the water.
• Mouth: It possesses a specialised mouth shape that enables it to spit a powerful jet of water up to 150 cm high to knock insects off the branches and into the water.
• Camouflage: Dark bands run across its body, providing excellent camouflage from predators, especially birds (like kingfishers) that hunt from above.

B. The Mudskipper (A truly amphibious fish!)

Mudskippers are fish that spend most of their time out of water, "walking" on the mudflats. They must overcome the challenge of breathing and moving on land.

• Breathing (on land): They can absorb oxygen through their moist skin and the lining of their mouth and throat, provided they remain wet.
• Gill Chambers: They have enlarged gill chambers which can trap a supply of water (like a portable oxygen tank). This allows them to still use their gills to obtain oxygen even when they are walking on land.
• Movement: The fish's pectoral and pelvic fins are strong and adapted into paddle-like or leg-like shapes, which they use for "crutching" or hopping movement across the mud.
• Vision: They have large eyes set high on the top of their head. This allows them to bury their body slightly in the mud while still keeping a lookout for predators or prey above the surface (like a periscope).

Final Summary of Mangrove Ecology

Mangrove forests are vital coastal ecosystems defined by the harsh, fluctuating conditions of the estuary and the intertidal zone. Survival here requires extreme specialisation:

1. Trees use pneumatophores for air and prop roots for stability.
2. Animals like the archerfish and mudskipper have evolved unique ways to hunt and breathe in this challenging land-sea interface.