From rigidity to collapse in soft coral slumping within the World Heritage coral gardens in Korean waters.

Coral communities (i.e., soft and hard corals) in Korean waters persist in temperate coastal environments characterized by strong seasonal variability and episodic hydrographic extremes. Within a protected coastal seascape of high ecological and socio-economic value, designated as a UNESCO natural heritage site for its vibrant soft coral gardens, we report the first observation of widespread collapse in soft corals. Based on in situ photographs, we document a previously undescribed loss-of-structural-integrity phenomenon in soft corals. We term this a "slumping event." We define slumping in organisms lacking rigid skeletal support as collapse of the hydrostatic skeleton. The event affected multiple species of non-symbiotic octocorallian soft corals (family Nephtheidae), for which we describe four morphological stages: drooping, dangle, deflation, and disintegration, with a potential additional "inflation" stage. These represent descriptive categories based on cross-sectional observations rather than a confirmed temporal sequence. Environmental analyses indicate that the event coincided with anomalous hydrographic conditions characterized by sustained low salinity and elevated temperature. To quantify prolonged freshwater exposure, we introduce the Degree Freshening Week (DFW) metric, capturing the cumulative duration and intensity of hypo-saline stress, highlighting the importance of stress persistence rather than short-term extremes. This study provides the first field-based documentation and definition of soft coral slumping in situ. The aim is to present initial observations of this phenomenon in a hypothesis-generating context to guide further investigation into the effects of compound environmental stress on temperate coral communities.