Lithogenic to Organic Particle Transition Around V olcanic Saipan

Abstract

Suspended particle composition in coastal waters surrounding volcanic islands is expected to shift from lithogenic to organic dominance with increasing distance from shore, driven by rapid gravitational settling of terrigenous sediment on steep bathymetric slopes. This study examines the spatial extent of this transition around Saipan, a volcanic island in the oligotrophic Western Pacific, using in-situ turbidity and chlorophyll-a fluorescence measurements along two transects: a 25 km east transect originating from the largest river mouth on the island, and a 15 km west transect across the shallow insular shelf. The normalized fluorescence-to-turbidity ratio (F:T) was used as the primary indicator of particle composition. Results revealed contrasting patterns between the two transects. On the east, turbidity unexpectedly increased from 0.17 NTU in the inner shelf to 0.18 NTU in the offshore (p << 0.01), which is small but statistically significant, while fluorescence declined sharply within the first 3 km and remained near zero offshore, with the F:T ratio reaching near zero by the 4-8 km section - much closer to shore than the predicted 12 km transition. In the west, turbidity decreased with distance (p << 0.01), and the F:T ratio increased offshore, driven by turbidity decline rather than fluorescence increase. Mass concentration data supported the interpretation that the east is more lithogenically influenced, while the west is more organically influenced nearshore, as the east showed no significant turbidity-mass correlation while the west showed a stronger positive relationship (R2 = 0.852). These results suggest that particle dynamics around Saipan are more spatially complex than a simple nearshore-to-offshore gradient, and that bathymetric differences between island sides play a fundamental role in controlling particle composition in oligotrophic island systems.