Oral B5 Coastal and Marine Ecosystems, Blue Carbon and Regional Ocean

3:15 PM - 3:30 PM JST(6:15 AM - 6:30 AM UTC)

[TPZePaqP-06] Growth, physical loss, and detrital-release timing in Saccharina longissima kelp beds of eastern Hokkaido, Japan, from long-term validation to future climate projection

*Ahmed Eladawy¹ (1. Japan Agency for Marine-Earth Science and Technology (JAMSTEC) (Japan))

Kelp forests are highly productive, yet their contribution to coastal carbon cycling depends not only on standing crop but also on when biomass is detached and where it may be transported. We developed a process-based, station-scale model for the long-blade kelp Saccharina longissima across its confirmed range in eastern Hokkaido (Kushiro-Nemuro), Japan. The model extends a biomass-internal quota formulation with carbon-reserve bookkeeping and a current-driven erosion term used as a detrital-release proxy. It is driven by daily temperature, currents, nitrate, and surface shortwave irradiance from the high-resolution FORA-JPN60 reanalysis and the FORP-NP10 nutrient product over 1991-2020, then extended to RCP2.6 and RCP8.5 scenario forcing for 2006-2100. Validation used 963 Habomai blade wet-weight measurements collected at 52 monitoring sites over 14 years (May-June, 2001-2014), summarized as annual May and June medians. The model reproduces the annual median biomass signal more clearly in June (annual-median r = 0.48, RMSE ≈ 25 g DW) than in May (r = 0.39, RMSE ≈ 27 g DW). Regionally, group-median peak biomass was high in the late 1990s (~149-150 g DW), fell to ~55-72 g DW at the 2014 minimum, and recovered to ~123-133 g DW after 2016. These shifts tracked Jun15-Jun15 light and temperature anomalies, while current-driven loss mainly affected years when strong flow coincided with high standing biomass. In the future ensemble, late-century peak biomass remains high under RCP2.6 (about 145 g DW per thallus) but falls sharply under RCP8.5, from roughly 47 g DW in the eastern group to 11 g DW in the western Kushiro group. Late-century standing biomass totals ~3.3×10⁴ t DW under RCP2.6 against ~4.3×10³ t DW under RCP8.5. We therefore provide the source strength, timing, and location of modeled detrital release needed to couple kelp production to transport-and-fate models that can test carbon export and sequestration.