giving coral reefs a future

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Close-up view of sediment-laden turf formed by algae, growing on a reef in the upper Florida Keys (Alain Duran, Florida International University)

Algal turf is a term that scientists use to describe tiny hairs or filaments of red, green, and brown algae that commonly grow on any hard substrates found on reefs. If the reef is grazed thoroughly by parrotfish, sea urchins and other algae-munching reef inhabitants, those filaments are usually less than a millimeter in length and don’t necessarily prevent coral larvae from finding a place to make a home. However, once a thin sediment layer has formed in the turf, parrotfish and other grazers become powerless to keep the algae in check. The reef dwellers stop grazing on the turf so that the filaments keep growing, while the sediments keep accumulating – thus, forming a persistent, often centimeter-thick mat within a very short period of time.

This mat of sediment-laden turf presents a barrier to the settlement of new corals, scientists report in a new study published in the scientific journal Marine Pollution Bulletin. In this study led by Kelly Speare from the University of California, Santa Barbara, scientists investigated how carpets formed by algal turf and sediments affect larval settlement of endangered elkhorn corals (Acropora palmata) and mountainous star corals (Orbicella faveolata) in the upper Florida Keys.

“Coral larvae are able to settle in algal turfs and can grow amongst them as long as there are no sediments mixed in, because the larvae are still able to find tiny patches of hard substrate to attached to,” Kelly Speare explains. “However, if sediments get trapped in the turf, they form a sediment layer covering the hard substrate that those tiny coral larvae cannot get through, even when it is very thin. Thus, sediment-laden turf, a prevalent substrate type in the Florida Keys that covers up to half of the total reef surface, suppresses settlement for elkhorn and mountainous star corals and may pose a roadblock to recovery of these two important coral species.”

Implications for reef management and restoration

These new insights about the important feedbacks between turf and the natural reef sediments have implications for reef management and restoration work. First of all, reducing sediment loads alone could have a positive effect on coral settlement, especially in areas where human activities increase sedimentation in coral reef habitats. In this study however, most of the sediments observed, seemed to occur naturally.

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Parrotfish are one driver of natural sedimentation on reefs. The fish scrape algae off of rocks and dead corals, grind up the inedible reef material in their guts, and then excrete it as white sand. (Paul Selvaggio)

“No matter where the sediments come from, we need to think about getting rid of them, at least to see if the grazers would be able to get the turf back down to a reasonable level and maintain it there, as we expect,” says co-author and SECORE Research Director Dr. Margaret Miller. So far, scientists don’t really know what to do about this type of substrate. “We don’t have a mitigation method to fix this, because we haven’t recognized this sediment-laden turf as a problem before,” Margaret Miller states.

In terms of reef restoration, the new research results underline the importance of sediment-free hard structures or settlement substrates that offer coral larvae refuge from sediments and sediment-laden turf. “That’s one of the reasons why we at SECORE invest so much time and effort to engineer settlement substrates that provide those microhabitats, where sexually produced coral larvae find favorable conditions to settle and outgrow that mat of turf and sediments. We definitely need to offer this refuge, if we want to increase coral settler survival rates and restore reefs at large scale,” Margaret Miller says. 

One important step was to switch from SECORE settlement substrates made of concrete to tiles that are made of ceramic. “It appears that the ceramic provides a less attractive habitat to turf and its jungle-like community than concrete,” SECORE’s Research Director reports. Unlike many degraded reefs SECORE’s latest settlement substrates also provide more vertical surfaces than horizontal surfaces. “That engineering decision is based on the same idea of trying to create and provide microhabitats for larvae to be able to outcompete the mat formed by sediment-laden turf,” Margaret Miller explains.

The study’s observational data shows that the topography of the reef plays a crucial role in the formation of sediment-laden turf. Margaret Miller: “Reef sites that have more structure and offer more vertical surfaces seem to have less sediment-laden turf and more natural recruitment of corals than reef sites that are flat and offer mostly horizontal surfaces.”

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SECORE’s latest generation of 3D printed settlement substrates (l.). The tiles offer various microhabitats for coral larvae to settle in and find perfect conditions to grow. Larvae seem to favor settlement spots where they can hide from sedimentation like on the underside of our tiles (m.). However, the amount of light reaching the baby corals is limited there. That’s why the new substrates provide a lot of vertical surfaces, where coral recruits can hide but also get enough sunlight (r.) (left: Boston Ceramics/ A. Meyer, A. Jeffery; middle: Valérie Chamberland; right: John Parkinson)

If one plans to restore those flat areas, it could be an idea to outplant coral babies that have stayed in their nurseries for a longer time so that the coral settlers already have reached a certain size. “If you are a tiny coral settler, competing with this turf-sediment jungle could be really stressful. Maybe, once you have grown into a bigger colony you don’t care that much anymore or you are better able to compete,” Kelly Speare says.

From her perspective, reef restoration activities need to focus on sexual reproduced coral larvae. “We know that maintaining genetic diversity is critical for reef conservation, so restoration efforts that help sexually reproduced larvae to get through this first stage of their life are super important. Especially in reef areas such as the Florida Keys, where genotypic diversity of, especially, elkhorn corals is so low. When we tried to collect eggs and sperm for this study, it was really challenging to find at least two genetically different colonies that could fertilize each other”, the scientist says. 

Corals really don’t like it, when the water gets clouded by sediments and sand or any kind of other particles raining down on them. The coral’s surface needs to stay sediment-free to ensure proper feeding and light to reach the algae living in the coral’s tissue. Hence, some corals start cleaning themselves. 

Text: Sina Löschke