Underwater Weed(s) - Macroalgae of Koh Tao

Macroalgae have been painted with a broad brush as being inherently bad for the beautiful, vibrant, yet rapidly declining coral reef ecosystems across our planet. While the reputations that many of these underwater weeds have earned are well deserved and grounded in scientific understanding, the story is always more complex and nuanced than it first appears. Algae have taken quite the PR hit in recent decades, as tropical reefs, particularly those in the Caribbean and the Bahamas, have become overrun with these dense forests of brown, green and black marine plants. Not offering the same colourful beauty and habitat diversity of a thriving coral reef, it’s no wonder why they’ve come under such heavy fire from marine biologists and dive enthusiast. But there’s so much more to the story here.

Dominant Species Mango 2The Story So Far

Macroalgae have coevolved on reef ecosystems since the first ancestral reefs appeared, sharing the shallow tidal zones as each of them try to find their own particular niche for survival. The interaction of all of these different organisms living attached to the shallow sea floor (coral, tunicates, sponges, macroalgae, bryozoans, etc.) is called epibiosis, and in stable conditions with only minor infrequent disturbances, the interactions between them all is rather harmonious. While it is of course competitive, these organisms have coevolved in one another’s presence for such long periods of time that many of them could not exist if it weren’t for the others. Macroalgae are one such organism, and on a healthy reef ecosystem, we can find a wide-ranging diversity of these amazing marine plants. Recenttly, a researcher named Johanna Gijsbers Alejandre, of the University of Amsterdam, joined us here at the NHRCP, and in early 2019 published her findings from one of the first ecological studies of macroalgal abundance and diversity around Koh Tao. A big congratulations is owed to her, and thanks as well for shinning a light on a woefully underexplored area of reef ecology here on the island.

Around the world coral reefs are collapsing under anthropogenic stresses, and are being replaced by macroalgae dominated ecosystems. Even here on Koh Tao we're watching the slow and painful decline of many of our favourite reefs. But it’s not just the biodiversity of the corals that suffers, but all of the organisms that depended on corals for their habitat, food, and survival that will disappear as well. When a species of marine plants become dominate, many more marginalized species of macroalgae won’t be able to thrive in the newly changed landscape. Here we will examine some of the most dominant genera of marine macroalgae on Koh Tao to familiarize ourselves with the flora that are flourishing around our waters.

Phaeophyceae: The Brown Algae

The Phaeophytes, also known as brown algae is the name given to them thanks to an accessory pigment called fucoxanthin, which is brown. The brown algae are amongst the most dominant types of macroalgae on Koh Tao, and when examining areas that are highly stressed, it is normally this class of marine plants that has established itself.


IMG 7782Lobophora variegata 2

              Lobophora is a species rich genus of Phaophytes. These marine plants grow broad leaves known as thalli, which encrust over the reef, adhering to any surfaces it can make a home on. This genus is visually the most ubiquitous group of macroalgae around Koh Tao, which was confirmed in Johanna’s study. It can often be seen interacting with coral colonies, and growing in such close proximity that it can often smother a struggling, stressed coral. It can be overlooked in surveys, due to the fact that it grows tightly to the substrate, which make it inconspicuous even though it is so widely distributed across Koh Tao's reefs. It can range from dark, inconspicuous brownish-greens (pictured on the right) to orange rust colours (pictured on the left). The physical removal of many genera of macroalgae, including Lobophora, will normally only aid in its dispersal, further spreading the plant around the reef, while the small root-like rhizoids that are left behind will simply regrow the macroalgae that was thought to be taken care of.                                                                     


               This macroalgae has a rather confusing name, given there is a particularly beautiful genus of coral that goes by the very same name. The algae, unfortunately, is far more abundant than the coral around Koh Tao, and can be seen in high abundance in shallow waters all around the island. It uses pneumatocysts (gas filled sacks) to float itself upright in the water column, maximizing its exposure to the sun for photosynthesis. Interestingly, recent studies on the effects of macroalgal-coral interactions have shown that Turbinaria might not be as immediately harmful to coral growing in proximity to them as other types of macroalgae. Despite these studies, coral stress is still readily observable when the tough tissues of Turbinaria come into contact and abrade that coral through wave action.

Turbinaria ornata


               One of only two genera of calcifying brown macroalgae, this alga has been seen in particularly large abundances in the deep reefs of Tanote Bay. It forms pale, disk shaped thalli much like Lobophora, but these structures don’t cling to the reef topography in the same way. Instead, Padina will form dense clusters that shades any coral unfortunate enough to grow in proximity to it. While the majority of its populations seem to disappear during colder seasons, it is still present in the sands as rhizoids, waiting for temperatures to rise so it can grow once more.

Padina 2


               Perhaps one of the most consistently misused macroalgal names, many people lump all brown marine flora under the category of Sargassum. While it is an incredibly species rich genus, we don’t observe large Sargassum beds around Koh Tao like we do elsewhere in the Gulf of Thailand. Sargassum needs relatively undisturbed shallow waters to thrive, and has difficulty finding those on the island. Much of the Sargassum that we do see here on Koh Tao will appear as detritus in large wash ups following storms, and is actually coming from neighboring islands that provide the necessary conditions for its growth.

Sargassum oligocystum

Chlorophyceae: The Green Algae

               The chlorophytes are referred to more commonly as the green algae, thanks to the vibrant green colours they can express through chlorophyll a and b. This class of macroalgae are the most closely related to terrestrial plants, so next time you see a green alga underwater you might want to thank it for colonizing land and making this planet a much nicer place to live than it would otherwise be.


               This genus is a complicated group of species of green algae. It includes one of the largest single-celled organisms on the planet, Caulerpa taxifolia, which is found on Koh Tao in small, isolated populations. It has been discovered that certain species within this genus can actually change their appearance based on the growing conditions they are exposed to. While they aren’t particularly abundant on healthy reefs around Koh Tao, they can be seen thriving in certain bays that have experienced land-based stresses such as water run-off, sewage, and nutrient inputs, and in the hot seasons large blooms can be found overgrowing corals.

IMG 7749


               Perhaps one of the most frequently noticed macroalgae, Valonia ventricosa has received a great deal of attention from curious divers and confused snorkelers. It is an incredibly large unicellular algae, and has earned itself the name the Sailor’s Eyeball due its unique, metallic appearance, which can vary depending on the cell’s concentration of chloroplasts. Unlike the other types of algae discussed, Valonia rarely competes with coral growth since it is often found growing alone or in small clusters in the reef rubble, and rarely forming large colonies.

Valonia ventricosa


               These calcareous green algae can be seen in calm, sandy, shallow bays. The algae require access to lots of light and nutrients to help it produce enough energy so that it can afford the resources needed to lay down a calcium carbonate structure, which serves as a base on which the living plant tissue grows. Halimeda forms a dense system of roots (rhizomes) that trap sediments to add weight to the plants base, which forms an anchor point for the rest of the plant to grow off of, and also helps control sand erosion. In some regions, it is the dominant calcifier, contributing to carbon sequestration and supplying vital nutrients to the shallow reef ecosystems.

Halimeda macroloba

Rhodophyceae: The Red Algae

               The red algae are the last class of algae to be discussed, and are also some of the most alien, given the large differences that they have when compared with terrestrial plants and the chlorophytes. They can range in colour from vibrant reds, to blues and purples, all the way to black, which is determined by the expression of their accessory pigments, phycocyanin and phycoerythrin, the proportions of which change with depth. This class of algae includes the all-important Crustose Coralline Algae, a calcium carbonate secreting flora that plays a vital role on reefs, especially in larval recruitment as you can read about [on our coral spawning page]


               These tufts of red algae can be seen all throughout areas such as Mango Bay, and are one of the three co-dominant species that are occupying the bay. The algae appear almost fuzzy, which is due to the filamentous structures that grow off of the plant’s branching calcareous stalks.

Actinotrichia fragilis


               Another genus of algae that is generally abundantly around Koh Tao is the coralline Amphiroa. These ancient calcium carbonate secreting species can form dense, bushy mats at the bases of Acropora branching corals. These coralline algae sacrifice fast growth for a more resilient structure, which means that they normally can’t be competitive with other species of marine flora in well lit areas. Instead, these algae thrive in shaded regions where light levels are too low for other, more temperamental, species of marine flora.

IMG 7751

What Do We Do With This Information?

To be a good gardener, an important thing to know is the different types of plants growing in your garden and how each of them interact with one another. Weeds earn their title through the harmful impacts they can have on a garden’s diversity. Many genera of macroalgae covered in the article are posing threats to Koh Tao’s reefs in a similar way. Many species of macroalgae are actually used by scientists as a bioindicator, meaning that they can help us better understand our own role in these fragile, beautiful habitats. The rise of these underwater weeds is a direct product of our own impact on the balances being struck on coral reefs, and quite frequently when an outbreak of marine flora occurs, we need look no further than the shores we’re occupying to find the answer to our problem.

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