Estimation of in situ rates of heterotrophy using diurnal changes in dissolved organic matter and growth rates of picoplankton in diffusion culture

A scheme has been developed for observing diurnal changes in dissolved organic matter in the photic zone and correlating the with specific microbial fractions and their rates of growth and uptake. Particulate ATP for procaryote and protist size fractions were augmented by pigment analyses to differentiate phototroph from phagotroph dominated accumulations. A temporary daytime increase in carbohydrates of some 32% above pre-dawn threshold values accounted for 41% of the labile DOC. Polysaccharide and monosaccharide maxima were mainly associated with phagotrophic protists, the monosaccharide maxima occurring during the daytime. Apparent maximum in situ heterotrophic uptake rates of this released DOC of 9.8 g C L^–1 hr^–1 agree well with the growth rates of natural populations of the bacterial size fraction (picoplankton) on in situ water in diffusion culture of 5.1 g C L^–1 hr^–1. This growth was associated with phototroph maxima but occurred only during the afternoon and evening hours and not during the early morning and intense daylight hours. Proposed follow-up studies are outlined.