Abstract
Particles are the major vector for the transfer of carbon from the upper ocean to the deep sea. However, little is known about their abundance, composition and role at depths greater than 2000 m. We present the first number-size spectrum of bathy- and abyssopelagic particles to a depth of 5500 m based on surveys performed with a custom-made holographic microscope. The particle spectrum was unusual in that particles of several millimetres in length were almost 100 times more abundant than expected from the number spectrum of smaller particles, thereby meeting the definition of “dragon kings.” Marine snow particles overwhelmingly contributed to the total particle volume (95–98%). Approximately 1/3 of the particles in the dragon-king size domain contained large amounts of transparent exopolymers with little ballast, which likely either make them neutrally buoyant or cause them to sink slowly. Dragon-king particles thus provide large volumes of unique microenvironments that may help to explain discrepancies in deep-sea biogeochemical budgets.
Highlights
Particles are the major vector for the transfer of carbon from the upper ocean to the deep sea
While most information about bathypelagic particles has come primarily from analyses of the contents of sediment traps[3], a few surveys have explored particle numbers via optical means in their undisturbed state[4,5,6], with the deepest bathypelagic number spectra reported to date from 1200–1400 m7 and 2500 m8,9
In contrast to lens-based systems, digital inline holographic microscope (DIHM) allows a focal depth of 7 cm, yielding relatively large volumes per image (1.8 mL) at high resolution
Summary
We deployed a custom-made digital inline holographic microscope (DIHM)[11] at 16 stations in the subtropical and subarctic Atlantic and one station in the Arctic, with a maximum deployment depth of 5500 m (Supplementary Fig. 1). Coagulation by shear and differential settlement with the contribution of prokaryotes[31] may slowly build larger particles from smaller ones at depth, and it has been suggested that the deep sea environment is conducive to the self-assembly of gels[32] Their accumulation may be facilitated by the fact that organisms known to significantly consume and fragment similar-sized particles in the mesopelagic layer[1,33] are generally absent from bathy- and abyssopelagic environments[34]. It has been suggested that the solubilization of particles is faster than their remineralization[38], resulting in an abundance of dissolved matter in the pore water of marine snow This enrichment in dissolved organic material is important because thresholds for nutrient uptake in particle pore water and their plumes most likely exceed the minimum concentration required for the growth of deep-sea prokaryotes[39]. Dragon-king particles may have been overlooked with traditional sampling methods such as sediment traps, but the dominance of this size class suggests that particle-associated small-scale heterogeneity needs to be recognized in an environment that represents the largest oceanic subsystem in terms of volume
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