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Seaweed can mislead scientists about reef health

For decades, scientists have relied on the amount of seaweed at the ocean’s surface as an indicator for estimating the health of the coral reefs below. However, according to a new global study led by the University of British Columbia (UBC), such an approach has been misleading, and may have even hidden signs of reef stress. 

The findings suggest that scientists should find new ways to determine whether and to what extent human activity is harming coral reefs.

Seaweed belongs to a group of organisms known as macroalgae that usually float on the ocean’s surface, covering coral reefs. The extent and features of macroalgae are relatively easy and quick to measure, and have long served as a proxy to assess reef health. Since the 1970s, experts have assumed that local human impacts increase macroalgae, while damaging the reefs below.

However, by examining data from over 1,200 sites in the Indian and Pacific Oceans during a 16- year period, the UBC researchers found that the previous assumptions may have been faulty. For instance, macroalgae covering depends heavily on the species growing in particular regions. While Sargassum is unlikely to grow in water contaminated by agricultural runoff, a species such as Halimeda will thrive.

Thus, using macroalgae cover as an indicator of local human impacts can actually obscure how much anthropogenic actions are damaging coral reefs, and cause scientist to misidentify the reefs that are most in need of intervention.

“Despite evidence that macroalgae respond to local stressors in diverse ways, there have been few efforts to evaluate relationships between specific macroalgae taxa and local human-driven disturbance. The convention to use percent cover of macroalgae as an indication of local human disturbance therefore likely obscures signatures of local anthropogenic threats to reefs,” said the researchers.

“Our limited understanding of relationships between human disturbance, macroalgae taxa, and their responses to human disturbances impedes the ability to diagnose and respond appropriately to these threats.”

The study is published in the journal Global Change Biology.

Human impacts on coral reefs

Human activities have profound impacts on coral reefs. These activities largely exacerbate the natural threats that reefs face, causing more frequent and severe damages. Here are the key ways humans impact coral reefs:

Climate Change

Human-induced climate change is one of the most significant threats to coral reefs. The burning of fossil fuels and deforestation have led to an increase in greenhouse gases in the atmosphere, causing global warming and ocean acidification. This results in coral bleaching events and impedes coral growth, respectively.


Coastal development, industrial activities, and agricultural runoff can increase the amount of sediment, nutrients, and pollutants in the water, which can smother coral reefs or lead to destructive algal blooms.

Overfishing and Destructive Fishing Practices

Overfishing can disrupt the balance of reef ecosystems, removing key species that help control algal growth. Destructive fishing practices, like blast fishing or cyanide fishing, can directly destroy coral reefs.

Unsustainable Tourism

While eco-tourism can bring attention and resources to the conservation of coral reefs, irresponsible practices can harm them. Damage can be done through direct physical damage from anchors or divers and indirect impacts like increased waste and pollution.

Coastal Development

Construction along coastlines can lead to increased sediment in the water, which can smother corals. Coastal development can also result in the destruction of mangroves and sea grass beds, which serve as important buffer zones that reduce the amount of sediment and pollutants reaching coral reefs.

Coral Mining

Coral reefs are often mined for limestone, used in construction, and for coral rock, used in souvenir industry. This practice directly removes the reef.

Introduction of Invasive Species

Human activities can unintentionally introduce non-native species to reef environments. These invasive species can outcompete native species for resources, disrupt local ecosystems, and cause significant harm to the reefs.

Can these human impacts be reduced?

The impact of human activities on coral reefs can be mitigated with various strategies and efforts. Here are some key ones:

Policy Changes

Government regulation can play a critical role in coral reef conservation. Policies can regulate fishing to prevent overfishing and destructive practices, enforce pollution controls to maintain water quality, manage coastal development to reduce habitat destruction, and encourage renewable energy and other climate change mitigation strategies.

Education and Public Awareness

Education can help the general public understand the value of coral reefs and the threats they face, inspiring individuals to adopt practices that support reef health. This can include understanding safe and sustainable tourism practices, supporting sustainable seafood choices, and reducing carbon footprints.

Sustainable Practices

Businesses and industries can adopt sustainable practices to reduce their impact on coral reefs. This can include responsible waste management, sustainable sourcing and supply chain management, and transition to renewable energy sources.

Direct Action and Restoration

Conservation groups and researchers are working on reef restoration initiatives. This includes projects that actively restore damaged reefs, such as coral gardening and the use of artificial reefs.

Scientific Research

Continued research is critical to understanding coral reef ecosystems, the threats they face, and the most effective methods for their conservation.

Climate Change Mitigation

Given the impact of climate change on coral reefs, efforts to mitigate climate change are critical. This includes reducing greenhouse gas emissions, adopting renewable energy sources, and promoting carbon sequestration efforts.

Marine Protected Areas

Establishing and enforcing marine protected areas (MPAs) can provide a refuge for coral reefs and the species that inhabit them. MPAs can help maintain biodiversity, prevent overfishing, and provide areas for reefs to recover from disturbances.

These strategies together can significantly reduce the human impact on coral reefs. The challenge is substantial but so too is the importance of these unique and invaluable ecosystems.


By Andrei Ionescu, Staff Writer

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