Seamount effects and shallow hydrothermal plumes in the Mariana Arc: impacts on chlorophyll concentrations and phytoplankton communities

Abstract

Seamount effects describe phenomena associated with bio-physical coupling at seamounts, often localized chlorophyll anomalies or shifts in community composition. Shallow seamount-associated hydrothermal plumes can also influence the euphotic zone, and metal requirements and toxicity thresholds vary across taxa. This study characterized the interactions between hydrography, chlorophyll, nutrients, hydrothermal plumes, and phytoplankton community composition at the deep chlorophyll maximum (DCM) across four seamounts (Ruby, Esmeralda, NW Rota-1, and Tracey) in the Mariana arc during a 10-day transect in late March 2026 aboard the R/V Thompson. Depth-integrated chlorophyll at Ruby was 300% higher than at all other locations (Tukey HSD, p < 0.0005) alongside faster outward-radiating currents, despite an absence of isopycnal doming or hydrothermal activity. Hydrothermal plumes were identified at NW Rota (400-450 m depth) through concurrent chemical and optical anomalies, including elevated dFe (5-17nM) and dMn (2nm) above background values (dFe: <2nm, dMn < 0.05nM). Very high dFe (15-35nM) was also measured at 200m at Esmeralda. Si(OH)4 was significantly elevated at seamounts with hydrothermal plumes within the mixed layer, while NH4 was elevated beneath it. Surface Crocosphaera biomass was higher at active than at inactive seamounts (Tukey HSD, p = 0.035). Microscopic identification of net tow samples at the DCM observed the same four most abundant taxa (Radiozoa, Bacillariophyceae, Dinophyceae, and Ciliophora) at all stations regardless of seamount proximity or plume influence (ANOSIM with Bray-Curtis dissimilarity R-statistic: 0.333, p = 0.1431). These findings indicate strong spatial patchiness in chlorophyll driven by the dynamic topography of the Mariana arc region, but no detectable restructuring of the dominant phytoplankton taxa at the DCM, establishing a baseline in the region for future work; these findings especially indicate a need to understand the synergistic impacts of shallow plume Fe and Si(OH)4 fertilization on silicious phytoplankton and the role of diazotrophs in a hydrothermal plume-influenced oligotrophic region.