Coral Reef Degradation

Coral reefs, among the Earth’s oldest ecosystems, face increasing vulnerability from both natural and human-driven stress factors. These threats infringe on their health across the globe, as evidenced by the loss of roughly 14% of coral on reefs between 2009 and 2018. To ensure their protection and preservation, a tool known as rubble stabilisation has emerged. This is a reef restoration technique that secures loose rubble to minimise its movement along the seafloor. The field of rubble stabilisation continues to evolve, with ongoing innovation and research shaping restoration science.

Rubble Formation and Subsequent Problems

Rubble is the result of fragments, particularly coral and reef rock, that has been broken off from their original structure. Some of the biological and chemical processes that play a part in its formation include cyclones, acidification and bioerosion. Anthropogenic impacts include ship grounding, blast fishing and damage through ship anchors. This process becomes problematic when the substrate instability is prolonged by hydrodynamic forces, small fragment sizes, and the absence of natural binders.

Interventions Required

Since coral larvae require hard, secure surfaces for furthering growth and development, the constant motion of loose rubble makes proper attachment difficult. Even if they do attach, survival is low due to abrasion or burial. Restoration intervention is recommended for sites classified as ‘disturbed persistent rubble beds’. To address these conditions, several stabilisation methods have emerged.

Rubble Stabilisation Methods

According to the Rubble Stabilisation Guidelines, four main methods are used for rubble stabilisation. The first is known as ‘direct manipulation of the substrate’. Commonly used for ship grounding incidents, it focuses on altering or removing existing material to limit the movement of rubble. A common, first-step initiative is to remove loose rubble since it prepares the site for subsequent restoration activities. Exposing a hard under layer and/or securing loose fragments to the seafloor allows for a more suitable environment for coral settlement since the substrate is less mobile.

Restricting Rubble Movement Through Physical Structures

The ‘addition of structures to restrict rubble movement’ is another technique used for rubble stabilisation. With the purpose stated in the name, it is accomplished through the use of either flat structures or barrier fences. Due to their affordability and relative ease of installation, flat structures like meshes and grids are commonly used to hold rubble in place. To hinder movement and stabilise loose rubble, barrier fences are set up to limit downslope movement. As of now, this technique has solely been tested in Indonesia. This so far has limited the technique’s proven effectiveness and broader applicability to regions with different environmental conditions. It is scientifically recommended that this strategy be coupled with others, such as flat structures.

Alternative Substrates and Artificial Reef Structures

If the reef structure has been compromised, the ‘addition of alternative substrates’, like artificial reefs, can be a promising solution. An intentional focus on sustainability and functionality has been reflected in these creations. Diverse structures that mimic reefs can enhance structural complexity and promote the recruitment of diverse reef fauna, including coral larvae. Materials used to achieve these methods are either natural or manufactured: an often cost-effective approach is to use locally sourced materials, such as sand and shells. Other common materials include concrete and even plastic. Each material can impact coral settlement differently, depending on its durability, sustainability, and biodegradability.

Biological Approaches: Corals Propagation

‘Propagation of corals and sponges’ is another key stabilisation technique, primarily carried out through coral transplantation and gardening initiatives. If fragments are too small for transplantation, then they are placed into nurseries where they can grow to a suitable size through asexual reproduction. The introduction of new corals to a stable area can diversify the reef and subsequently attract more larvae.

Biological Approaches: Sponge Propagation

The other form of propagation is through sponge seeding. By reattaching harvested sponge fragments to rubble, this technique can accelerate the restoration process. Within months, studies have observed that erect, branching sponges can spread across multiple pieces of rubble. Their growth effectively binds these fragments together, strengthening substrate stability and functioning as a natural preliminary binder during the early phases of restoration. However, there currently is a lack of evidence on the effects of sponge seeding due to the absence of recent studies.

Case Study: The Great Barrier Reef

Falling into the category of the ‘addition of structures as an alternative substrate’, so-called Reef Stars developed by the Australian organisation MARS were installed in 2020 at the Reef Magic Marine World Pontoon. Those are steel-based, hexagonal structures coated in gritty sands to provide a sufficient texture for attachment. They also have the ability to interlock with one another. The project was led by several marine biologists, James Cook University researchers and Australian Indigenous rangers. The work aims to boost coral resilience by restoring colonies destroyed after Cyclone Yasi in 2011. The study intended to add nearly fifty additional Reef Stars every six months. Only eighteen months after installation, coral cover had increased by an astonishing 50%.

Assessing Effectiveness

New, innovative techniques to restore coral reefs are continually being developing, with rubble stabilisation being one of them. It appears the most sustainable outcomes result from a combination of both physical and biological techniques. However, given the nascency of the field, there remains a limited amount of research on the subject. By gaining a greater understanding of a study site’s condition -including wave energy, local disturbances, climate change impacts, and more- informed decisions can be made for the betterment of the reefs in question and for the development of functioning coral colonies. As pressures from natural and human-driven stressors continue to rise, rubble stabilisation appears to become an increasingly important tool for coral recovery. These restoration efforts help maintain ecosystems that support roughly 25% of marine life and the livelihoods of 1 billion people worldwide.