In spring, Xiamen Bay is shrouded in unpredictable mist and clear skies. Construction on Xiang'an Airport, China's planned gateway for cross-strait exchanges, continues unabated, while the Xiamen-Kinmen Bridge capitalizes on the good weather to accelerate progress. Across the sea, on Kinmen Island, Taiwanese citizen scientist Hong Qingzhang diligently surveys the habitat of juvenile horseshoe crabs. The ebb and flow of the tides seem to have deepened the mud, making the horseshoe crabs' presence increasingly difficult to detect.
"I spent more than two hours this morning and only recorded a small number of small horseshoe crabs in an important juvenile horseshoe crab habitat. No molting was found on the tide line." Hong Qingzhang wrote down his observation notes for the day on the "Kinmen Intertidal Zone" Facebook page. The last sentence of the conclusion, "The habitat has obviously deteriorated," reveals a feeling of loss between the lines.
The three-spined horseshoe crab, also known as the Chinese horseshoe crab, is also known as the "living fossil of the intertidal zone." It was listed as endangered on the International Union for Conservation of Nature (IUCN) Red List in 2019. Previously found from the Yangtze River estuary south to the Beibu Gulf in Guangxi, its population has plummeted due to the devastating habitat destruction caused by coastal reclamation and sea reclamation projects. Today, Xiamen's natural coastline is sparsely populated, leaving juvenile horseshoe crabs with virtually no habitat. Military control along the Kinmen coastline in the early years maintained a stable horseshoe crab population structure and intertidal zones. However, with the increasing encroachment of coastal development, the rampant Chinese sand dredging vessels, and the reclamation of land for the Xiang'an Airport, habitat loss has been gradual.
In recent years, the topography of Kinmen's northern and western coasts has changed significantly. Some local beaches have become severely muddied, and even near-shore sandbars have gradually formed. The marine ecosystem is quietly collapsing.
Li Zhongpan, professor emeritus of the Department of Marine Environment and Engineering at National Sun Yat-sen University in Taiwan, said that the original habitat has been covered by sand and mud, which is a catastrophe for the original ecology, because the change of the beach is the change of ecology. From mudflat bottom to sand beach, the entire ecosystem will change, so the horseshoe crab population in Kinmen will gradually change.
Hong Qingzhang pointed out that when the intertidal zone's sediment ratio changes, the entire environment, ecology, and food chain are affected, as different sand-to-sand ratios are ideal for different organisms. Juvenile horseshoe crabs, for example, primarily inhabit the north and west sides of Kinmen, where oyster fields are located. The mid- to high-tide seabed in these areas is muddy, with a ratio of approximately 60% mud to 40% sand. Walking on it creates a slight sinking sensation, but not too deep, making it ideal for juvenile horseshoe crabs. However, the current sediment-to-sand ratio is disrupted.
In terms of the horseshoe crab's life history, adult horseshoe crabs live in the ocean. During the breeding season, pairs emerge in the intertidal zone to lay and fertilize their eggs. After hatching, the juveniles live in mudflats, lurking in the sand during high tide and emerging to forage after low tide. Hong Qingzhang explains that in sandy or completely sandy environments, horseshoe crabs have difficulty burrowing into the sand, and with fewer benthic organisms, their food sources are also limited. Horseshoe crabs also cannot survive in excessively muddy conditions, as they breathe through their gills. The fine pores of mud make oxygen exchange difficult, making them very demanding in terms of substrate quality.
The warning signs of a juvenile horseshoe crab population collapse in Kinmen came quickly and dramatically. "Five or six years ago, we could still find many habitats suitable for horseshoe crabs to live and reproduce. However, the coastline has changed dramatically in the past year or two, and there are only about two or three spots left in the entire Kinmen area." Hong Qingzhang has noticed the rapid decline in the juvenile horseshoe crab population. "Now, juvenile horseshoe crabs are only easily observed in the Xiongshibao area of Kinmen and the Shanglin intertidal zone of Little Kinmen (Lieyu Island), and these areas are not large. They may be found elsewhere, but their numbers are very small."
Jiangong Island and Xiashu Island on Kinmen's west coast were once important habitats for juvenile horseshoe crabs. A few years ago, Hong Qingzhang led students to the area for ecological observations, where they could easily spot horseshoe crabs. However, since the year before last, they have been almost completely absent, going from a hotspot to a declining area in just a few years. He estimates that the number of juvenile horseshoe crabs in Kinmen has decreased by over 90%, indicating that the next generation of horseshoe crabs is disappearing.
From Xiamen to Kinmen, the "horse crab generation" has vanished, and both sides of the strait have actively engaged in restoration and stocking efforts. Over the past decade, Fujian's fisheries authorities have artificially bred approximately two million three-spined horseshoe crabs, all of which have been used for stocking and stocking. The Kinmen County Fisheries Research Institute has also been engaged in long-term artificial breeding of horseshoe crabs, releasing fry annually for over 20 years in an effort to increase the abundance of wild populations. In March of this year, the Kinmen County government further announced a year-round ban on all harvesting of three-spined horseshoe crabs.
"Although Kinmen releases over 200,000 to 300,000 juvenile horseshoe crabs annually, theoretically the population should be increasing with such a large release, but the opposite is true," Hong Qingzhang warned. "Unlike fish, which can swim to a suitable location once released, horseshoe crabs won't stray more than 100 to 20 meters from the release point. They hide in the mud and sand, but if the habitat isn't suitable, they die."
Without the intertidal zone with the right sediment ratio, juvenile horseshoe crabs simply cannot survive. Hong Qingzhang pessimistically stated that habitat degradation will become increasingly severe. Because large-scale land reclamation near Xiang'an Airport has blocked the flow of water in the surrounding waters, significant environmental changes will only be seen in a few years. Furthermore, the ongoing construction of a cross-sea bridge from Xiamen to Xiang'an Airport will further impact the area.
Dadeng Island, where Xiang'an Airport is located, is only 2 or 3 kilometers from Kinmen. To coincide with the launch of flights at the end of next year, airport transportation development has been accelerated. Construction of the Xiamen-Kinmen Bridge (Xiamen section) began at the end of October 2023. The 17.34-kilometer bridge, connecting Xiamen Island and Dadeng Island, features 85 piers and is expected to open to traffic by the end of next year. Furthermore, the Xiang'an Bridge, spanning the eastern waters of Xiamen Island, also opened in January 2023. Currently, there are eight major access roads, with bridge piers dotting Xiamen Bay.
From the airport's artificial island to the layers of cross-sea bridges, Kinmen's intertidal zone has been caught up in environmental turmoil, and the living space of horseshoe crabs has been continuously compressed.
The piers of a cross-sea bridge are columnar structures embedded in the sea. Li Zhongpan uses the example of a relatively small, six-meter-diameter pier. If there were 100 of these, they would occupy approximately 600 meters of waterway. Consequently, the waterway area would be reduced, the flow rate would increase, and waves and currents would cause scouring of the seabed around the piers. For river bridges, aside from tidal sections, upstream water accelerates as it flows between the piers. This vortex forms after passing through the piers, stirring up sediment beneath. Therefore, downstream protection is necessary to prevent scouring.
Li Zhongpan believes that it is currently impossible to determine the impact of the Xiamen-Kinmen Bridge on the Kinmen coast, but the flow conditions will definitely change, and it may also bring mud and sand. The sea current speed in the ditch near the Kinmen-Xiamen waterway is very fast, so it will probably be carried to places with low flow speed and small waves and deposited, such as the area around the Beishan Oyster Field in Guningtou.
In recent years, the coast of Guningtou on Kinmen's north shore has been severely covered in silt, causing ecological and livelihood problems. "Is there a way to first implement localized sand stabilization, trapping the sand within a certain area to preserve a certain area of the mudflats, thereby maintaining the ecology and the livelihoods of the local oyster farmers?" Li Zhongpan proposed a solution. "The beach itself is a resource. We need to first predict and infer where the sand will end up, and then plan for overall changes."
It's not just horseshoe crabs that are facing a desperate situation; the entire marine ecosystem is also facing a crisis. The intertidal benthic ecosystem is quietly collapsing, even impacting higher-level food chains like waterfowl and otters. "When this species disappears, many more species disappear, so it's a mutually reinforcing process," Hong Qingzhang said, reflecting deeply on the battlegrounds of Kinmen, which once protected the coastal ecosystem. But now, biodiversity is declining year by year. "Compared to the intertidal sites five or six years ago, the biodiversity I've observed in the past two years has deteriorated significantly."
Coastal development has profound environmental impacts. Hong Qingzhang cites the environmental impacts of the Jiugong Wharf expansion and the Golden Gate Bridge as examples. He has conducted ecological surveys in the intertidal zone of Lieyu Island for many years, and the back is one of the best locations. "Now, when I walk to the mid- and low-tide areas, the surface is almost entirely mud. I used to record many organisms here, including Kinmen bell snails, many rare sea slugs, and snails and shellfish. But now, there are almost no special organisms. Many algae and cnidarians have disappeared. Many organisms have disappeared because the mud makes it impossible for them to survive."
In contrast, the Xiamen Bay city-building project on the other side of the strait has a development scale and sea disturbance intensity far exceeding Kinmen. Moreover, this is a semi-closed inner bay, and the environmental damage caused by a large number of offshore projects, large-scale sand pumping, and land reclamation will be more prominent.
"The impact will undoubtedly be more significant, because the entire wave and current fields will change, and so will the topography, benthic ecosystems, and ecosystems," said Li Zhongpan. "If we were undertaking these projects, we would definitely conduct environmental impact assessments to assess the ecological impacts and whether there are any compensation measures. If they decide to undertake a project, the environmental impact assessment will likely only give a favorable rating, stating that there will be no impact. The Three Gorges Dam is a typical example."
"Because of their project, Kinmen has changed. These changes are not something we have assessed, but they are already happening," he added. "So we must monitor and understand the speed and scope of the changes, including how the original ecosystem is being altered. We should conduct an investigation as soon as possible."
However, Li Zhongpan is well aware of the particular difficulties of conducting research in the waters off Kinmen and Xiamen. His research team once attempted to study changes in sediment load along the Kinmen coast, with Fujian's Jiulong River being a major source of sediment. Recalling their experience with cross-strait academic collaboration, he said, "The Jiulong River flow figures provided by Xiamen University were 10 times greater than those of the Yangtze River. That's impossible. We reminded them of this, and they immediately changed the figures."
Later, the study was abandoned due to difficulties in obtaining data and questions about its reliability. Currently, the Kinmen coast is plagued by siltation. "Unless we can collect sand from the beach near Xiang'an Airport, then continuously collect sand from various sections in the Kinmen-Xiamen waters, analyze its mineral composition, and compare it with the sand on Kinmen beaches, we can probably accurately determine whether the sand came from there," said Li Zhongpan, a sand table simulation research method. "Unless we can obtain this data, we will probably have to use mathematical simulations (of wave and current field changes) to make our judgment."
