Bicapitate Nitzschia species: abundant nanoplankton in aggregates during November-December (l992) in the equatorial Pacific

From 12°S to 9°N-140°W on the benthic cruise ofthe US Joint Global Ocean Flux Studies (JGOFS), horizontal near-surfacenet tows (n = 18) and vertical net hauls (n = 15, 200-0 m) yieldedhigh relative abundances of small, bicapitate, intact Nitzschiavalves (average 59.0 and 56.0% of total diatoms, respectively),although >90% of them were too small to have been caughteffectively with the net used. This complex was much less common(average 3.2% of diatoms) in the surface sediment samples (n= 7, 12°S-9°N, below water depths of 4269-4991 m). Examinationunder the light microscope showed concentrations of cells associatedwith organic detritus, planar and cylindrical membranous structures,fecal pellets, and occasionally on other diatoms (e.g. Asterolampra),i.e. on or within particles large enough to be collected inthe net. Although Nitzschio bicapitato Cleve has been consideredto have a characteristic range of shapes, and has often beencited from open-ocean habitats in nano- and microplanktonicstudies, a scanning electron microscopic study revealed severalspecies in our material. Examples of larger and smaller speciesare given, and Nitzschia ikeanae G.Fryx. & H.Lee, sp. nov.,is described here. Ecologically, these observations indicatethat although some are grazed and have been seen aggregatedin fecal pellets, these small pennate diatoms can affix to andgrow on substrates, termed ‘pseudo-benthic habitats’in the open ocean, resulting in unexpected concentrations thathave implications for their life histories and sexual cycles,selection or avoidance by grazers, sinking on substrates, androles in the recycling of nutrients in near-surface waters.