NEW MEASUREMENTS OF PHYTOPLANKTON AGGREGATION IN A FLOCCULATOR USING VIDEOGRAPHY AND IMAGE-ANALYSIS

Diatom aggregation can increase the sedimentation rate of the intense pulses of carbon formed during diatom blooms. Laboratory studies of ag gregation dynamics have been troubled by the unavoidable disruption of aggregates in laboratory flocculators due to subsampling and electron ic particle counting. Aggregate disruption prevents accurate measureme nt of aggregation rate and makes observation of particle structure imp ossible. This led us to develop a new non-disruptive method to quantif y diatom aggregation using high-resolution video and image analysis. T he system easily resolved cells of Chaetoceros neogracile 6 to 8 mu m in diameter, and facilitated observations of aggregate morphology. C. neogracile made either ball-like or net-like aggregates depending on c ulture density and physiological state. Only the net like aggregates r eached a substantial size (>1 mm). A Si-limited batch culture experime nt indicated that stickiness of C. neogracile increased significantly with increasing severity of nutrient limitation. This increase was not detectable by the subsampling and electronic particle counting method . It is likely that spine formation, cell-surface bound sugars, and tr ansparent exopolymer particles all played a role in determining aggreg ate structure and changes in stickiness. The stickiness calculation mo del assumes cultures are initially suspensions of single cells which f orm doublets as they aggregate. This assumption was tested by comparin g mean particle size in a culture with the percentage of tells in aggr egates as determined using nearest neighbor distances, allowing us to assess the mean number of cells per aggregate. C. neogracile commonly formed aggregates containing anywhere between 2 and 10 cells showing t hat model assumptions can be violated under normal experimental condit ions. A larger field of view was necessary to estimate the larger part icle sizes typical of coastal diatom blooms.