Along with coral, seaweed is one of the proxies frequently used by marine biologists to assess the effect human activity is having on a reef. But a new study questions this, suggesting that seaweed’s behaviour isn’t uniform enough to use it as a simple measure.
“This is especially critical today, given that reefs globally are threatened by climate-driven stressors,” says Dr Sara Cannon, a postdoctoral fellow at the Institute for the Oceans and Fisheries, University of British Columbia, Canada, lead author of a paper describing the research in Global Change Biology.
Macroalgae (seaweed) cover is easy to measure when assessing a reef, and scientists have generally assumed that more macroalgae indicates a poorer-health reef, while more coral indicates better health.
But this isn’t always the case – studies have been questioning whether “coral dominance” always indicates a healthy reef for decades.
Plus, different species of macroalgae can respond to stressors in different ways.
In this study, an international team of researchers examined data from 1205 reef sites, over 16 years, in the Indian and Pacific Oceans.
They looked at seaweed cover in each site, along with indicators of human disturbance like fishery management and agriculture runoff, and other overall reef health indicators.
They found that a simple percentage cover of seaweed was not a “robust” indicator of human disturbance.
No genus of seaweed correlated with all indicators of human disturbance, and lots of other factors that were unrelated to human disturbance also affected seaweed cover.
The researchers also found that assessing reef health via macroalgae cover could conceal reef stress.
But they did note that looking at seaweed at the genus level could be a better way to track reef health.
For instance, seaweed in the Sargassum genus tends not to grow in places with high agricultural runoff. A genus called Halimeda grows more.
In their paper, the researchers call for a reconsideration of how reef health is defined.
“Evaluating how coral reefs are being affected by disturbance is an indispensable part of research and management, but the most common metrics used in that work are based on an oversimplified and poorly tested paradigm,” they write in their paper.
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