The dependence of growth, electron transport system activity and chemical composition on the size of diatoms was examined during the exponential phase of growth. The six different marine centric species compared ranged in volume from 7.7 μm3 to 62 × 105μm3. A size dependence was observed for growth, 14C uptake, respiration and the productivity index (14C/chl a). Although the size dependence of all parameters was similar, the results indicate that on a carbon basis, growth efficiency decreases with increasing size. The C/N and C/chl a ratios were not size dependent. The importance of the surface area to cell volume ratio, and the importance of carbon per unit volume in determining the observed size dependence are discussed.

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The supply and utilization of organic carbon in the deep western Mediterranean Sea was investigated based on measured electron transport system (ETS) activities of the nanoplankton and microplankton. The total carbon oxidation rate between 200 and 3000 m, as calculated from ETS activity, was 15.0 g C m−2 yr−1. This represents 21% of the primary production and is similar to published estimates of the annual new production. A vertical advection ‐ diffusion ‐ reaction model based on profiles of salinity, oxygen, and the carbon oxidation rate converted to oxygen consumption yielded a deepwater residence time of about 7 years, in close agreement with a published estimate based on bomb‐produced tritium profiles. This suggested that the ETS‐based rates in the deep waters were reasonably accurate. These deep rates were much greater than ETS‐based rates from the same depths in the Atlantic and equatorial Pacific Oceans. In the western Mediterranean, ETS‐based rates also greatly exceeded the rate predicted from the primary production rate and sediment trap relationships. The rapid rates observed in the deep western Mediterranean are not consistent with the supply of organic matter via rapidly sinking particulate material. Instead, rates may be supported by dissolved organic carbon (DOC) transported to depth by wintertime deepwater convection. In order to account for the portion of the ETS‐based rate which was not explained by the sediment trap flux, DOC concentrations in the surface waters entrained during deepwater formation would need to be only 11 μmol C L−1 greater than those in the deep waters exiting the basin. ETS activities from the equatorial Pacific (Packard et al., 1988) may also implicate DOC in supporting deep‐sea metabolism. There, ETS‐based carbon oxidation rates between 200 and 5000 m greatly exceeded rates calculated from sediment trap data in the same region. The source of the organic matter being respired may ultimately be the high rates of new production in the equatorial Pacific region, but the mechanism by which this material is transported to depth cannot be determined from these data. The ETS data from both the Mediterranean and the Pacific indicate much greater rates of carbon oxidation in the deep sea than expected from existing sediment trap results. Globally, transport of DOC into the deep sea possibly could rival the sinking particulate flux in importance for deep‐sea metabolism.

Profiles of light transmission, dissolved oxygen, dissolved nutrients, electron transport system (ETS) activity, temperature and salinity were made in the northeastern tropical Pacific Ocean. A particle maximum at 150-300 m within the oxygen minimum and secondary nitrite maximum was associated with the salinity maximum of Subtropical Subsurface Water. A subsurface maximum in ETS activity was also found to be associated with the secondary nitrite maximum and the particle maximum. Persistence of these features at a constant depth and their location within a minimum in vertical static stability suggest an advective and/or in situ origin for the particles and an in situ development of the associated chemical and biochemical extremes.

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