Visualizing the barely detectable: Particle characteristics of the elusive aquatic gel phase in the northeastern subtropical Atlantic

Marine gels constitute a large yet poorly explored ocean carbon pool. Shadow imaging was tuned to detect particles at two sensitivity levels: high–optical density particles (HODP), resembling the particles captured by most imaging systems, and low–optical density particles (LODP), representing a vastly more abundant and gel-dominated pool. This method enabled us to map and characterize a portion of oceanic gels without filtration and staining. In the subtropical northeast Atlantic, LODP outnumbered HODP of the same size by one or two orders of magnitude. Morphology and particle number spectra differed greatly between the two pools, with LODP showing greater morphological homogeneity and diel pulses in abundance. The inclusion of LODP is critical for the parameterization and quantitative interpretation of biogeochemical models that directly affect global carbon flux estimates. Due to a paucity of in situ observations, these particles are largely absent from even the most advanced ocean models.