FORMATION OF RECALCITRANT ORGANIC-MATTER - HUMIFICATION DYNAMICS OF ALGAL DERIVED DISSOLVED ORGANIC-CARBON AND ITS HYDROPHOBIC FRACTIONS

The incorporation of radiolabeled carbon into XAD2-fractions of dissol ved organic matter was followed during the growth, stationary and degr adation phases of a batch culture of the Antarctic diatom Thalassiosir a tumida. Changes in the fractions over the 267 day experiment were co rrelated with the production and decomposition of particulate organic matter. The bulk of the acid and neutral hydrophobic XAD-fractions, tr aditionally considered to be humic, were produced mainly during the di atom growth phase: 70% of this carbon was present in a relatively stab le form by the end of the first month, representing 5% of the maximum carbon biomass. There was a progressive accumulation of a hydrophobic DOC fraction tightly bound to the XAD resin. This fraction is normally not considered following XAD2 extraction of aquatic humic material. H owever, it is evidently an important component of a resistant DOC pool , increasing during the experiment to levels between 40 and 50% of tot al DOC after 7-8 months. This fraction is thought to be mainly a produ ct of particulate material degradation. At the end of the experiment t he sum of all hydrophobic DOC fractions was 14% of the maximum particu late carbon.