KINETICS OF PHYTOPLANKTON DECAY DURING SIMULATED SEDIMENTATION - CHANGES IN BIOCHEMICAL-COMPOSITION AND MICROBIAL ACTIVITY UNDER OXIC AND ANOXIC CONDITIONS

A series of oxic and anoxic incubations examined the decay of two mari ne phytoplankton, the diatom Thalassiosira weissflogii and the coccoid cyanobacterium Synechococcus sp, in flow-through systems without macr ozooplankton grazers. The major biochemical fractions of algal carbon (protein, carbohydrates, and lipid) were quantified over time together with bacterial abundance and activity. Oxic decay constants of bulk a nd individual biochemical fractions showed good agreement between both phytoplankters, suggesting that composition at the molecular level wi thin a particular biochemical class does not influence decay rate as m uch as differences among the major biochemical fractions. Large differ ences in decay rates did exist among biochemical classes, with carbohy drates utilized most rapidly under oxic conditions, followed by protei n and then lipid. Turnover times among the particulate pools ranged fr om 10.7 days for diatom and cyanobacterial carbohydrates under oxic co nditions to over 160 days for cyanobacterial lipids under anoxia, with oxygen having a substantial effect on overall rates of algal carbon d ecomposition. PON values tracked POC with an average POC:PON ratio of 4.99 +/- 0.52 for diatoms and 4.48 +/- 0.66 for cyanobacteria througho ut the experiments. Bacterial abundances and activity varied substanti ally over the course of the incubations with greatest activity during periods of greatest particulate loss. Bacterial abundances and metabol ism were comparable under oxic and anoxic conditions even though the a mount of material degraded under anoxic conditions was significantly l ess than when oxygen was present, suggesting that oxygen increased rat es of particulate material degradation.