Intraspecific variation in morphology is widespread among seaweed species in different habitats. We examined the morphological variation in Sargassum thunbergii involving diverse environmental factors. We quantified 16 morphological characteristics on 15 rocky intertidal shores in Korea. A cluster analysis based on morphology identified three groups. Group M1 comprised populations on the northern part of the east coast, where the thalli was short and thick, with large leaf and air-vesicle. Group M3 consisted of populations on the west coast exclusively separated from other populations, with short, slender and sparsely branched thalli. Group M2 comprised populations on the southern part of the east coast and on the south coast (including Jeju Island), with longest thalli and lateral branches. Principal coordinate analyses showed that group M1 and M3 were mostly influenced by strong wave action and large tidal amplitudes, respectively. Group M2 were under the influence of warm temperatures and high irradiance. Biota-environment matching analysis showed that the morphology is affected by combinations of different local environmental factors and also that tidal condition is important as a single variable, suggesting that morphology of S. thunbergii reflects and adapts to local environmental conditions.

This study aimed to investigate the dynamics of the nutrient budget of a macroalgal community exposed to water discharged from aquaculture farms. This study was conducted in the coastal area exposed to water discharged from aquaculture farms located on Jeju Island, Korea, in May and October of 2017. Water-column-dissolved inorganic nutrient concentrations were higher in the intertidal zone than in the subtidal zone. High nutrient concentrations in the intertidal zone resulted in the bloom of Ulva spp., consequently leading to a low species number. This indicates the vulnerability of the intertidal macroalgal community to discharge from land-based aquaculture. Ecklonia cava contributed to 29–53% of the total incorporation of carbon (C), nitrogen (N), and phosphorus (P) in the subtidal zone. In the intertidal zone, the highest tissue N content was found in Ulva spp. due to the high and rapid nutrient uptake rate from the effluent with high nutrient concentrations, thereby indicating high total N incorporation. The estimated total C, N, and P incorporation rates by macroalgae were 181.5, 8.6, and 0.95 tons year⁻¹, respectively. These results suggest that the macroalgal community in this area absorbs or removes large amounts of nutrients from the water column and plays an important role in the budgets and cycling of nutrients in the surrounding coastal areas.

Green tides threaten coastal ecosystems worldwide, yet the physiological and environmental mechanisms driving their dynamics remain poorly resolved. On subtropical islands like Jeju, Korea, where nitrogen inputs are diverse, predicting bloom intensity is particularly challenging. Here we show that water column nutrient levels alone poorly correlated with bloom magnitude, whereas algal tissue nitrogen provides a more reliable indicator of nitrogen status and nutrient exposure history. Seasonal surveys across fourteen sites (2013–2015) revealed annual Ulva blooms exceeding 11,000 t, with stable isotope (δ¹⁵N) analysis indicating that blooms are likely sustained by mixed nitrogen sources rather than a single dominant source, although precise source attribution was not possible. Tissue nitrogen content closely tracked dissolved inorganic nitrogen, while nitrate reductase activity was decoupled from nitrate supply under nutrient-replete conditions and negatively associated with biomass accumulation; suppression of nitrogen reductase activity by ammonium feedback and self-shading within dense mats is inferred from these patterns but was not directly measured. By integrating environmental drivers with physiological traits, we provide mechanistic insights into bloom dynamics and highlight nitrogen management as a critical lever for mitigating green tides in vulnerable coastal ecosystems.