RAPID FORMATION AND SEDIMENTATION OF LARGE AGGREGATES IS PREDICTABLE FROM COAGULATION RATES (HALF-LIVES) OF TRANSPARENT EXOPOLYMER PARTICLES (TEP)

Two hypotheses have been proposed to account for the precipitous forma tion of large, rapidly settling aggregates at the termination of phyto plankton blooms in nature; aggregation due primarily to cell-cell coll isions, and aggregation resulting from the presence of abundant transp arent exopolymer particles (TEP), a recently discovered class of parti cles formed from polysaccharides excreted by phytoplankton. The hypoth esis of TEP-driven coagulation in three disparate systems, a freshwate r lake, a coastal ocean, and a saltwater mesocosm was evaluated, by co mparing TEP abundance to several related factors including phytoplankt on concentrations, measured sediment fluxes, and abundances of large a ggregates. The timing of large aggregate formation and sedimentation e vents was related to coagulation rates expressed in terms of particle half-lives, t(1/2), calculated as the time for TEP or phytoplankton to decrease to half their concentration through shear coagulation. While TEP have been previously investigated only in marine systems, it is r eported here that TEP also can be present in high concentrations (860 ml(-1)) in freshwater lakes (Lake Constance, Germany) and that high fl uxes of particulate organic matter at depth coincide with the disappea rance of abundant TEP from overlying waters. The half-lives of TEP in the three different systems indicate that large aggregate formation an d massive sedimentation events following diatom blooms occur when the TEP half-life decreases to less than a few days. By comparing TEP and phytoplankton half-lives in these systems, it is concluded that the fo rmation of rapidly sinking aggregates following blooms of mucous-produ cing diatoms is primarily controlled by concentrations of TEP, not phy toplankton.