Vertical Flux of Diatoms at Station ALOHA

Renate Scharek, Mikel Latasa, David Karl and Robert Bidigare

School of Ocean and Earth Science and Technology, University of Hawaii, Honolulu, HI 96822

Abstract

Diatoms living near the deep chl a maximum layer (DCML) in the open ocean are hypothesized to control export production due to their ability to respond to stochastic nutrient injections. Variation of diatom abundance in the water column and flux out of the euphotic zone was investigated using light microscopy and HPLC pigment analyses during a series of eleven monthly cruises (June 1994 - July 1995) to Station ALOHA of the Hawaii Ocean Time- series (HOT) Program. Furthermore diatom flux to the deep sea was examined using samples from the deep time-series sediment traps moored close to Station ALOHA.

In the euphotic zone, contribution of diatoms to total chl a was small ( < 5 %). Variation of the diatom pigment marker fucoxanthin was generally low during the course of the year, except in late July when concentrations in the uppermost water column were about three times higher than the annual mean. During the same period, diatom cell numbers peaked in this layer, mainly due to the increase of two species (Hemiaulus hauckii, Mastogloia woodiana); both species were lightly silicified. These same species were responsible for most of the vertical flux of diatoms out of the euphotic zone which likewise peaked in midsummer. Thus, vertical flux of diatom assemblages out of the uppermost water column seems to be more important than the diatom flux out of the DCML. The contribution of diatoms, estimated from cell numbers, to the flux of biogenic silica out of the euphotic zone was approximately 15% during the summer event and considerably lower ( < 5%) during the rest of the year. Estimated diatom contribution to the flux of particulate carbon followed a similar pattern but was generally lower, < 1% during the year and approximately 4% in late July.

The pattern of enhanced diatom flux in late July could be detected in the deep sea. Increased flux of the same two diatom species occurred only two to four weeks later at a depth of 4000 m suggesting rapid settling rates. A further indication for the high sinking speed of the diatoms was the large proportion of cells still containing cytoplasm. During 1992/1993 a similar pattern of diatom sedimentation to the deep sea was observed. In both years, flux of mass, particulate carbon and nitrogen, and biogenic silica peaked likewise in late summer. Estimated contributions of diatoms to biogenic silica flux in 1992/1993 and 1993/1994 were 18 and 30%, respectively, during the high flux events and approximately 2% during the rest of the year.

Although diatoms contribute only a very small fraction to the total particle flux throughout most of the year, they might play an important role in the vertical transport during periods of rapid sinking of material into the deep ocean. It remains to be resolved whether they are mediators of these processes, e.g. due to aggregate formation and subsequent harvesting of other material, or whether the rapid sinking of diatoms and the other particles is a result of some other unknown processes such as aggregate formation due to dust deposition.