Where the warm Kuroshio meets the southern limit of the kelp world — Sargassum forests, agar harvesters, the Longdong diving coast and Keelung's protected reefs.
Northeast Taiwan is the place where the kelp map of the northwest Pacific quietly runs out of country. From Keelung down past the sandstone cliffs of Longdong to the fishing harbours of Yilan County, the coast carries everything a kelp forest needs except the one thing it cannot manage around: heat. The Kuroshio Current, one of the great warm rivers of the ocean, brushes this shoreline with tropical water for much of the year — and true kelps, those cold-water giants, thin out and vanish as it does. What stands in their place is just as interesting: a seasonal brown-algal forest built largely of Sargassum, a coast with a centuries-deep seaweed culture, some of East Asia's most accessible shore diving, and a living lesson in where the kelp world ends and the reef world begins.
Taiwan's northeast coast runs roughly 100 kilometres from the headlands north of Keelung — around Yehliu, with its famous hoodoo rocks — southeast and then south past Bitou Cape and the Longdong (龍洞, "Dragon's Cave") coastline to Yilan's Lanyang Plain and the fishing town of Su'ao. Geology writes the scene: tilted and folded sandstone ridges, fractured by the same tectonic compression that raised Taiwan's mountains, plunge straight into the sea as cliffs, sea caves and shelving platforms of hard rock — ideal substrate for seaweed forests.
Two seasons rule the water. From October to March the northeast monsoon drives long-period swell against these headlands and keeps the surface stirred and turbid; from April to September the sea flattens, the water clears, and typhoons pass through in occasional, forest-pruning fury. Our piece on storms and kelp explains why that disturbance rhythm is not the enemy of a healthy forest but its reset button.
The Kuroshio is the northwest Pacific's western boundary current — the warm twin of the cold Oyashio that feeds the kombu forests far to the north. If you have read our guide to Hokkaido's kombu coast, you have read about the other end of the same system. Up there, cold nutrient-rich water lets true kelps tower. Down here, surface temperatures run roughly 22 °C in late winter to 29 °C in late summer — beyond the comfort zone of most laminarian kelps, as covered in our explainer on the thermal limits of kelp. (The background physics of why boundary currents decide who grows where is in boundary currents.)
So the northeast coast's canopy is built differently. In winter and spring — when the monsoon cools and mixes the water — dense beds of Sargassum species surge up over the shallow platforms: bushy, gas-bladdered fronds that function as a true forest while they last, canopies three-dimensional enough to shelter juvenile fish, crabs, seahorses and swirling baitfish. Come early summer, much of this seasonal canopy sheds or sloughs away, and the seasonal character is the point: the forest here breathes on an annual cycle far more strongly than the perennial forests of colder coasts. Mixed in are turbinaria, colpomenia and a rich understory of red and green algae; on deeper, cooler ledges, warmth-teetering brown forest-formers persist in pockets — the ragged southern fringe of the kelp lineage itself.
Few shores this size carry a seaweed culture this layered. The northeast coast's signature harvest is Gelidium — 石花菜, shihuacai, "stone-flower vegetable" — a red alga cropped by divers from wave-washed rock and dried whole or rendered into agar, the gelling agent of a thousand Taiwanese sweets. Generations of coastal families here have dived for it from baskets and small craft; at harvest time the drying racks line village lanes, and what looks like bales of rust-red wire is tomorrow's marketable agar.
Winter brings the laver growers: Pyropia, close kin of the nori of Japan, cultivated on intertidal racks lashed into the rocky platforms and hand-picked at low water. Abalone divers, sea urchin gatherers and small hook-and-boat inshore fisheries complete a working coastline where seaweed is not an oddity but infrastructure. It is the same deep craft you find down the island arcways of Japan — our Amakusa guide covers the Kyushu version — carried on here on a smaller, rockier stage.
At this biogeographic seam the species mix reads like two field guides opened at once. Over the Sargassum beds cruise tropical visitors that the Kuroshio pumps northward — wrasses, damselfish, lionfish, batfish, turtles — while the cooler months favour temperate-boreal migrants and the resident rock fish and gobies of the sandstone ledges. Green sea turtles now forage regularly off the northeast headlands. Whale sharks once fed the region's most heartbreaking fishery; Taiwan banned their take in 2008, and sightings off this coast have become cautiously celebrated events rather than market headlines.
Hidden in the coves are cuttlefish breeding aggregations in late spring, enormous schools of anchovies working tidelines, and — in the deeper beam of colder water that occasionally wells up around the headlands — flashes of the cooler-temperate reef community that dominates two hundred kilometres of coast further north.
Longdong is the northeast coast's double identity made famous: above water, East Asia's premier sandstone sea-cliff climbing venue; below, Taiwan's most celebrated shore diving. The entry points at Longdong Bay offer protected coves where kelp-like Sargassum beds wave in the shallows before the bottom ramps to 20–30 metres of alga-crusted sandstone with clean, warm water on a calm summer day. Farther up the coast, the bays around the Keelung headlands — including the celebrated Chaojing marine area at Wanghaixiang Bay — offer easy protected shore entries and a volunteer-guarded recovery story: a heavily fished bay turned into a no-take community conservation area, now visibly thicker with macroalgal cover, sea urchins and fish.
Logistics are honest: this is a coast where conditions set the schedule. Summer is diving season; winter is storm-watching season. A hood and 3–5 mm wetsuit cover the annual range here, a world away from the gear logic of our cold-water dive kit list — and the calm bays make this one of Asia's friendliest settings for the fundamentals in our shore diving guide.
The greatest pressure on this coast is the one that defines its ecology: heat. Marine heatwaves — see marine heatwaves 101 — increasingly stack on top of an already warm baseline, periodically bleaching the Sargassum beds and stressing the reef community that depends on them. Typhoons pruning the canopy is ancient; a changing storm regime layered over hotter water is not. Sediment from road cuttings and development smothers the intertidal laver racks in bad years, and shoreline engineering keeps creeping along a coast where every headland originally met clean rock.
The counterweight is a maturing protection ethic: community-managed no-take zones, the slow spread of marine education parks at the harbour towns, and a diving community that has become the reef's most watchful constituency. Taiwan's northeast coast is not a pristine wilderness — it is a lived-in, worked, swum-in piece of sea. That is precisely what makes it instructive: a place where the kelp world ends, where the seaweed economy continues anyway, and where the southern boundary of the forests is an early-warning instrument for everyone watching colder coasts wonder how long their own canopies hold.
Mostly no — and that is the point of this atlas entry. Near-shore Taiwan is warm enough that the classic laminarian kelps of Japan and the northwest Pacific vanish; what fills their niche on the northeast coast is a rich, canopy-forming assemblage of Sargassum and other brown algae, with kelp-lineage species surviving only in cooler pockets. It reads like a kelp forest seasonally, and ecologists watch it as the kelp world's southern thermometer.
Longdong is a sandstone-cliff bay on Taiwan's northeast corner, in New Taipei City near Bitou Cape. Above water it is East Asia's most celebrated sea-cliff rock climbing area; below water it is Taiwan's most dived shore-diving coastline — protected coves, Sargassum beds in the shallows, and sandstone slopes running to 20–30 m under clean warm water in summer.
Broadly April to September. The winter northeast monsoon makes the headlands rough and turbid — spectacular to watch from a teahouse, poor for diving. Inside the summer window, plan around typhoon passages, and expect warm water with occasional brisk cold pockets around the headlands.
Shihuacai ("stone-flower vegetable") is Gelidium, a red alga divers crop from the northeast coast's wave-exposed rock. Dried and boiled, it yields high-grade agar — the natural gelling base of Taiwanese jellied desserts — and the crop has been a coast-defining livelihood here for generations.
Through the same current. The warm Kuroshio that keeps Taiwan's coast kelp-sparse is part of the western-boundary current system whose cold branch — the Oyashio — feeds the kombu forests of Japan's far north. The two coasts are, in oceanographic terms, the two ends of one machine.
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