Minipellets: A new and abundant size class of marine fecal pellets

Minipellets, fecal pellets from 3 to 50 μm in diameter, were found on detritus collected by a particle interceptor trap array in the upper 2000 m of the eastern tropical Pacific. The fluxes of minipellets reached 5 × 106 m−2 day−1, and exceeded fluxes of larger (\u3e50 μm diameter) fecal pellets by 3 orders of magnitude. Carbon flux of minipellets was 11–49% that of larger pellets; however, carbon flux of ultrastructurally intact cells (microalgae and bacteria) in minipellets was equal to that of intact cells in the larger pellets. Minipellets also occurred in water samples from similar depths, where they numbered up to 105 m−3, and were usually not associated with particles. Minipellets appear ubiquitous; we have found them in all our samples of particulates from other cruises from surface waters to bathypelagic depths. Minipellet morphologies ranged from Type A, which contained intact, picoplankton-sized cells (cyanobacteria, nitrifying bacteria, morphologically non-descript, Gram-negative bacteria, Chiarella-like cells) in an amorphous matrix surrounded by a boundary, to Type D minipellets, which were identical to previously described olive-green cells. Minipellets are probably wastes of protozoans and small invertebrates that consume marine snow and larger fecal pellets throughout the water column, thereby maintaining the high numbers of minipellets from the surface to 2000 m. We found several sources of minipellets: two groups of sarcodine protozoans (phaeodarian and spumellarian radiolarians) and small hydromedusae. The minipellet producers reprocess a major portion of surface-derived detritus, and represent important biological intermediates that transform particulate matter settling through the ocean

Cryptic Zooplankton Swimmers in Upper Ocean Sediment Traps

Sediment traps are the major oceanographic tool for collecting passively sinking particulate material (the “particle flux”) in the ocean. Sediment traps in the upper ocean also collect actively sinking zooplankton that are usually manually removed prior to analysis. Microscospic analysis of sediment trap samples collected over a 19-month period in the eastern North Pacific reveals that zooplankton “swimmers” are a larger problem than previously recognized. Zooplankton that are cryptic (i.e. difficult to see or distinguish from the detrital material) and difficult to remove (principally gelatinous zooplankton) may have contributed up to 20 mg C m−2 day−1 to the “particulate flux”, with the highest values in the upper 150 m. This swimmer problem is in addition to the previously recognized presence of crustaceans and other large metazoans in traps. Additionally, the detritus-laden, mucous-feeding structures (houses)of larvaceans probably enter the traps with the larvaceans and would be impossible to remove. We estimate that the contribution of the cryptic swimmers and larvacean houses could be as much as 96% of the measured carbon flux. The contribution is greatest in the euphotic zone and drops sharply below 200 m. Subtracting out this potential artifact at the VERTEX station results in vertical profiles of organic carbon flux that differ dramatically from the standard flux profile for carbon in the upper ocean: specifically, the implied “regeneration” rate is greatly reduced. Screened traps (300 μm screens below the baffles) contained numerous metazoans smaller than the screen mesh size. These traps also contained lower levels of other types of sinking particles, and it is unclear to what extent the screens reduced the relative contribution of swimmers to the trap-collected carbon. Although the expanded swimmer problem presented here is now documented at just the VERTEX site, we expect it exists elsewhere. The extent of this swimmer problem requires resolution before sediment traps, especially those deployed in the upper few 100 m, can be used to measure the “flux of particulate material.