Exploring the fine scale spatial distributions of carbon and biomass using shadowgraphy and direct collections of marine snow

Based on our shadowgraph data, gels numerically represent more than 85% of all particles in the size range from ~20 to several hundred micrometers in the ocean, and their relative contribution further increases below the euphotic zone. Gels also act as the “glue” in the formation of marine snow, resulting in small-scale heterogeneity that remains under-explored by conventional oceanographic sampling methods.

Here I present data on the relative enrichment of organisms and organic carbon using direct collections of individual marine snow particles combined with simultaneous imaging. After detailed image analysis and conversion of particle outlines to volumes, we found that both organic carbon and biomass (based on ATP) are substantially more enriched on particles relative to ambient seawater than previously assumed. Organic carbon is more strongly enriched than biomass in marine snow, likely reflecting two factors: (1) marine snow contains a higher proportion of refractory material such as detritus, dead cells, and fecal matter, and (2) gels themselves constitute a large fraction of the particle matrix.

Across all depths, marine snow exhibits a pronounced shift in community composition, with higher relative enrichment of eukaryotes compared to prokaryotes, including elevated contributions from eukaryotic microbes and even metazoans. In the bathypelagic ocean, the relative importance of fungi and the functionally related labyrinthulomycetes also increases on marine snow. These patterns on the spatial scale of particles mirror large-scale biogeographic trends, with increasing relative eukaryotic abundance from oligotrophic to eutrophic systems. However, in extremely eutrophic systems such as estuaries, biomass as a proportion of total carbon decreases again, likely due to enhanced allochthonous inputs of organic carbon, detritus, and gels.

The close spatial coupling of organisms and organic substrates on particles may allow biogeochemical processes to occur that would otherwise fall below functional thresholds. Collectively, our results indicate that particle-associated processes play a disproportionate role in regulating oceanic biogeochemical cycles.