Understanding the True Nature of Algal Reefs Through Scientific Data

The Editor says: The true nature of the algal reef ecosystem is, in fact, an ecological landscape that emerges from a combination of environmental conditions that are "unfavorable" to the functioning of a typical ecosystem. Where you stand shapes what you see — and scientific data is the best way to substantiate that. Come take a look with the Editor! Originally published in Science Monthly Issue 500, "Understanding the True Nature of Algal Reefs Through Scientific Data" 2023.08.15

Since Taiwan's ecological conservation awareness began to rise in the 1980s, only one case has managed to capture nationwide attention, dominate media coverage, prompt massive government funding, and even trigger a national referendum — the "algal reef controversy" sparked by CPC Corporation's Third Natural Gas Receiving Terminal in the Datan area of Taoyuan. The word "controversy" is not entirely accurate, though: decisions on ecological and environmental issues have never been, nor should they be, a matter of winning or losing. Rather, they should follow the principles of ecological functioning and seek a balanced path forward amid diverse environmental and biological conditions. The real question is: how is that balance to be achieved? That is the truly thorny problem. Making judgments based on science is, in principle, the most rational approach — yet scientific data is always open to interpretation from multiple angles, and holding divergent views or debating them vigorously is fairly normal. In the algal reef case, however, the relentless amplification of one-sided information, combined with the intuitive sympathies for the underdog that pervade news outlets, social media, and public opinion, caused what should have been straightforward scientific commentary to be vilified as complicity with the "tigers" (the government and state-owned enterprises). Now that time has passed and there are no "winners or losers" left to argue over, it is the right moment to calmly revisit the true meaning of the various scientific data points from before.

But even if every individual data point were clarified, would the full picture of the algal reef's role in the ecosystem become clear? Honestly speaking: not necessarily. Yet this is precisely the spirit of science — humble and ever-striving for progress! In pursuing the truth about the algal reefs, the author and research team stood on the shoulders of predecessors to integrate and analyze the data, using publicly verifiable evidence from both supporting and opposing perspectives, along with physical and chemical environmental parameters, and supplemented by ecological survey data from nearby waters that had largely been overlooked. The aim was to sketch out what the algal reef truly looks like — and why it has so captivated so many people.

What we found, however, was utterly unexpected: the reality of the algal reef ecosystem turned out to be the polar opposite of the image portrayed to the public.

The Two Faces of the Algal Reef Ecosystem

What was the algal reef's original image? Social media and various news reports offer plenty of descriptions, but most are fragmented and fail to convey the full picture. The most authoritative accounts — co-authored by several scholars and completed with the efforts of professional editors — are roughly represented by the special feature in Science Monthly Issue 573 and the National Geographic report "Where Are the Algal Reefs Headed?" Both accounts, without exception, likened the algal reef ecosystem formed by crustose coralline algae in the Taoyuan area to the familiar tropical coral reef ecosystem: rich biodiversity, high productivity, stable and complex biological habitat, an important nursery for juvenile fish and shrimp, and a significant contributor to the surrounding marine ecosystem, fisheries output, and even the distribution and dispersal of marine life across East Asia.

This impression may, however, stem from an overextended interpretation of a series of survey data, combined with subjective links and inferences drawn from surface-level observations and domain-specific knowledge. When you finally lift the bridal veil, the name is the same — but the personality is entirely different.

From the author's team's perspective, the Taoyuan algal reef ecosystem can be broadly described as follows:

For thousands of years, Taiwan's western coastline has been a marine ecosystem dominated by sandy shores. The narrow strait topography and the winter and summer northeast and southwest monsoons have created a unique system of ocean currents and tidal flows in these waters. Marine organisms — including those from the local area and from the surrounding South China Sea, East China Sea, and even the Pacific Ocean — have long exploited these currents, releasing their offspring into the flow so that they can "ride the sea's power" to find new territories. Taiwan's western coastal currents thus function as a conveyor belt teeming with larvae of all kinds: fish, shrimp, shellfish, worms, polychaetes, and even land crabs and coconut crabs that have adapted to terrestrial life. Along this journey, the larvae are full of opportunity — but even fuller of danger.

Along the way they are preyed upon by all manner of predators — from zooplankton at the small end to filter-feeding Whale Sharks at the large end. They also cannibalize one another; fish and shrimp larvae that manage to consume coral eggs or embryos are getting truly "high-protein nutrition." So unless they are pelagic organisms built for long-term drifting, any larvae that encounter a suitable place to settle will seize the chance immediately. For mud- and sand-dwelling worms, gastropods, and bivalves — as well as demersal fish such as flatfish, sand smelt, and flatheads — this is no problem, since the western coast is fundamentally a muddy-sandy marine ecosystem with plenty of suitable habitat. But for organisms that require a hard substrate for attachment — such as coral, oysters, barnacles, gobies, blennies, and moray eels — opportunities are far fewer, and no potentially suitable spot can be passed up. Along the long sandy shore, algal reefs are exactly such a place. And they are not found only at Datan in Taoyuan; there are large and small patches distributed throughout Taiwan, including Xinfeng and Danshui. More broadly, the many artificial hard-substrate areas along Taiwan's west coast — harbors and fishing ports — also offer opportunities for these organisms. The question is: can they survive in these environments?

If the habitat they encounter is a tropical coral reef, their chances of survival are high, because the substrate provides a high level of primary productivity — much like a human settlement with natural fruit trees or rice paddies, capable of supplying the environment and food that various organisms need. Primary productivity is a fundamental prerequisite for a thriving ecosystem; similarly, mangroves and seagrass beds can support very high biodiversity. But what about algal reefs? Their productivity is very low — only 65 g/m² per year — which is actually less than the 101 g/m² of intertidal mudflats. For organisms trying to make a living here, it is genuinely a hand-to-mouth existence.

But this is not the fault of the crustose coralline algae that form the reef — they can only grow a little each year to begin with. The entire reef accretes only about 0.1 cm per year, and most of its mass consists of calcareous material that is all but indigestible. People simply have not looked closely enough, assuming it is like ordinary algae — a granary capable of feeding a whole crowd of residents.

At the very least, though, it provides the function of a shelter. The reef structure formed by accumulated crustose algal remains is riddled with pores and cavities — aren't those exactly the kinds of places where worms, polychaetes, small shrimp, and small crabs can live? That is true enough, but this is a thousand-year-old house. It is far less stable than other reef types such as coral reefs or oyster reefs; upkeep is already difficult, and then there are a host of natural disturbances on top of that. The seasonal interplay of winds, waves, and currents continuously causes sediment to accumulate on the algal reef. If the buried algae cannot resurface in time, they truly become a dead "reef," and the organisms living in the crevices perish along with them. Even when lucky — when the accumulated sand is removed in time, usually during seasons of stronger winds and waves or after a major typhoon, and in the Datan area also aided by human-built seawalls that create erosional action — allowing the algae and the hardier small organisms to see daylight again, that can be counted as a blessing. But do not forget: the very harsh weather that saves them is another type of natural disaster that can kill or injure the reef's residents in large numbers. It is precisely this mutually contradictory pattern of intense environmental disturbance that has given rise to the myth of "thriving algal reefs, unique algal reef organisms, and persecuted algal reefs."

The "thriving" comes from the coastal current continuously delivering fresh larvae. Whenever the algal reef is exposed, these larvae rush in to join, striving to grow and desperately trying to fulfill their innate drive to reproduce. An observation made at such a moment will, of course, show life teeming in every direction — at first glance, biodiversity appears high, with all manner of organisms potentially present. The "persecution" comes from the scouring of sediment and powerful waves — two forces that work in opposite directions. Both are merciless killers, yet together they maintain the exact conditions necessary for algal reefs to look the way they do in nature.

How so?

Nature operates according to the principle of ecological succession. In a stable environment, any newly available habitat will attract many species rushing to colonize it. The organisms best adapted to that environment will, by virtue of their competitive advantage, develop into dominant species — abundant and monopolizing resources — while less competitive species gradually dwindle. In ecosystems with high primary productivity, weaker species may still claim a share and survive, sustaining high biodiversity. But in low-productivity areas — and especially in algal reefs, which have almost no productivity to speak of — biodiversity must inevitably decline over time due to competition and food scarcity.

Yet Taoyuan's algal reefs, precisely because of their harsh conditions — sediment accumulation, powerful waves, turbid water, and the intense disturbances brought by frigid cold currents — have their dominant species repeatedly cleared away, their habitat repeatedly swept clean, and fresh larvae repeatedly joining. The adult populations that do grow are kept at a level that is not too large but is dynamically balanced. When this situation is assessed purely from surface-level scientific data, does it not look like a thriving ecosystem with many larvae coming and going and rich biodiversity? Yet its actual biomass per unit area is in fact sparse — it cannot be high. From a theoretical standpoint, this is simply an ecosystem perpetually stuck in an early stage of ecological succession: a child that never grows up.

Rare yet Abundant? The Public's Beautiful Misunderstanding of Algal Reefs

In this kind of fluctuating environment, the dominant species of flourishing ecosystems that the public knows well — such as coral communities and large macroalgae — find it hard to gain a foothold. Instead, it is the broadly adaptable fringe species that thrive: species that lack the competitive edge to succeed in mainstream ecosystems, are habitually pushed to the margins, exist in small numbers, exert limited influence, and are rarely studied. Here, they can finally find their place — and even become prevalent.

Take Polycyathus chaishanensis (Chaishan cup coral), classified as a Category I protected species in Taiwan. Its perceived rarity may be nothing more than a beautiful misunderstanding born of insufficient baseline data. Subsequent targeted surveys have found it at the Yong'an Natural Gas Receiving Port, along the Taitung coastline, and even in corals legally exported from Indonesia to the Netherlands. As for the crustose coralline algae described in multiple algal reef reports as new species discoveries — given their life history and wide geographic distribution, these may simply reflect the previously inadequate scientific investigation of this particular field, rather than being treasures unique to Taiwan.

The outline of how the algal reef ecosystem functions is gradually coming into focus, but one puzzle piece — rarely noticed — is still missing: in a place with such sparse primary productivity, where does the food for residents come from? The answer is that a large portion of the food in the algal reef comes from the turbidity of the surrounding waters. Most of the suspended matter consists of organic detritus with a high caloric content — unpalatable, yet the primary food source for filter feeders, detritivores, and omnivores such as barnacles, oysters, gastropods, horn shells, shrimp, crabs, polychaetes, and worms. This is what makes them the dominant groups in this environment. These organics originate from the influx of nearby rivers, augmented by the organic matter churned up by the strong waves, and are then delivered and deposited into the algal reef by the ebb and flow of tides and the pounding of waves — serving as free, externally supplied food for the reef's residents. And if the season is right and luck is on their side, a passing water mass may bring with it a large quantity of phyto- and zooplankton, pushed into the algal reef zone by rising tides or waves — a true feast for the residents. It is remarkable: the very environmental factors thought to be harmful to organisms turn out to be the algal reef residents' delivered meal package.

From another angle, how many organisms can an algal reef actually support? This is primarily constrained by the limited productivity of crustose coralline algae and the imported suspended matter. Some have argued that algal reefs support periods of lush macroalgal growth at specific times of year, which also contributes significantly to primary productivity. But this description is not entirely accurate, because most of that algal biomass is washed back into the sea to be utilized by the surrounding ecosystem rather than by organisms within the algal reef itself — a comparison with the fate of algae in the tidal pools of the Northeast Coast makes this clear. Moreover, too much macroalgae, covering too large an area and persisting for too long, is actually the greatest competitor of crustose coralline algae. And the sediment accumulation that many conservationists strongly condemn actually functions to suppress the proliferation of macroalgae — serving as a "volunteer militia" defending the crustose coralline algae against invading competitors.

A Simple Sketch of the Algal Reef Ecosystem's True Face

Overall, the true nature of the algal reef ecosystem is an ecological landscape that emerges under a combination of environmental conditions "unfavorable" to the functioning of a normal ecosystem. To put it in plain, accessible terms:

The algal reef itself is a thousand-year-old house. The coastal current delivers wandering children of all kinds. Food within the house is severely scarce; turbid waves bring in barely palatable sustenance; drifting sand mercilessly culls the unfortunate. Only the tenacious and resilient survive. The spaces vacated by the fallen are just right for new young arrivals to keep on joining. Seen through the eyes of an outsider, a thriving nursery — teeming with biodiversity and harboring rare species — appears to have been born.

Once the true face of the algal reef is revealed, its future fate and the strategies for its management become clear thanks to this understanding:

1. Will living algal reefs persist forever? Unlikely. They can only accumulate less than 0.1 cm of thickness per year. Under the increasingly severe trend of global warming, the average annual sea-level rise around Taiwan has already reached 0.34–0.38 cm, and over the next 30 years (projected to 2050), that rate is expected to increase by at least threefold. In other words, given that living algal reefs currently do not exist below a water depth of about 6 m, the remaining living reefs will likely die off relatively soon as the water becomes too deep.

2. Will dead algal reefs (geological reefs) persist forever? Hard to say. If sand burial proceeds quickly enough, they may be preserved. But if coastal wave action intensifies as sea levels rise and erosion increases, the fragile, porous structure of the algal reef will bear the brunt of the damage — and the current situation already looks difficult to sustain for long.

3. Should sediment be removed? It appears unnecessary — unless people wish to alter the natural character of the algal reef — because the advance and retreat of sediment is a key factor in maintaining the algal reef ecosystem, and burial is also a necessary process for geological reef formation.

4. Should construction-related turbidity be controlled? Turbidity with high organic suspended matter is actually an important food source in algal reef areas. Since the turbidity caused by construction is relatively limited compared to the naturally occurring organic suspended matter in local waters, indiscriminately reducing turbidity may actually be an inappropriate action that cuts off the food supply for algal reef residents.

5. Should crustose coralline algae or algal reefs be artificially cultivated?

   • If the goal is to protect the crustose coralline algae at this location: probably not much cause for concern, as they are naturally wide-ranging species — if not welcome here, they will find another place.

   • If the concern is extinction of crustose coralline algae: simple ex-situ preservation is sufficient.

   • If the goal is to cultivate local "algal reefs": at their natural growth rate of only 0.1 cm per year on hard substrate, cultivating 50 cm of reef would take an estimated 500 years. This would simultaneously require stable policy, budget, topography, site conditions, and 500 years of similarly consistent environmental parameters — an enormously high bar given today's rate of climate change and coastal erosion.

Perhaps clearly seeing and facing the truth is itself an act requiring great magnanimity, courage, and wisdom. Yet isn't it precisely this spirit of inquiry, reflection, acceptance, and renewed inquiry — accumulating over time — that drives science, propels civilization forward, and builds a better future? The remarkable story of the algal reef is the finest example of all.

Editor-in-charge: Jenny Tsai

Further reading:

• Science Monthly Issue 500, "Understanding the True Nature of Algal Reefs Through Scientific Data"

• What's Along the Taoyuan Coast? Birdwatching at Xucuogang Wetland, Traditional Stone Fish Weirs at Xinwu, and Citizen Scientist Training

• Taoyuan Coastal Citizen Scientist Biodiversity Database