Diatom fluxes to the deep sea in the oligotrophic North Pacific gyre at Station ALOHA

Abstract

Planktonic diatoms are important agents of vertical transport of photosynthetically fixed organic carbon to the ocean's interior and seafloor. Diatom fluxes to the deep sea were studied for 2 yr using bottom-moored sequencing sediment traps located in the vicinity of the Hawaii Ocean Timeseries (HOT) program station 'ALOHA ' (22'45'N, 158" W). The average flux of empty diatom frustules was around 2.8 X 10' cells m-' d ' in both years, except in late summer when it increased approximately 30-fold. Flux of cytoplasm-containing diatom cells was much lower (about 8 X 103 cells n1Y2 d") but increased 500-fold in late July 1992 and 1250-fold in August 1994. Mastogloia woodiana Taylor, Hemjaulus hauckii Grunow and Rliizosolenia cf. clevei var. communis Sundstrom were the domlnant diatom species observed during the July 1992 event, with the former 2 species again dominant in August 1994. The 1994 summer flux event occurred about 3 wk after a documented bloom of H. hauckii and M. woodiana in the mixed-layer and a simultaneous increase in vertical flux of these species. This surface flux signal was clearly detectable at 4000 m, suggesting rapid settling rates. A further indication of very high sinking speeds of the diatoms was the much larger proportion of cytoplasm-containing cells in the bottom-moored traps during the 2 summer events. Cells of H. hauckii and R. cf. clevej var. communis frequently contained endosymbiotic cyanobacteria with heterocysts (cf. Richelia), similar to the cells of these specles in the mixed-layer. Our data show for the first time that diatoms containing nitrogenfixing cyanobacteria contribute drectly to the vertical flux of organic matter to the deep sea in the oligotrophic regions. The peak of diatom flux coincided with a significant flux increase of biogenic silica in both years. During periods of rapid sinking, the vertical flux of diatom assemblages out of the uppermost water column seems to be more important than the diatom flux out of the deep chlorophyll maximum layer (DCML). Aggregate formation may be responsible for the fast sinking of the diatoms.