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D~atorn aggregation can increase the sedimentdtion rate of the Intense pulses of carbon formed durlng d ~ a t o m blooms. Laboratory studies of aggregation dynarn~cs have been troubled by the unavoidable d~sruption of aggregates in laboratory flocculators due to subsampl~ng and electronic par t~cle countlng Aggregate disruption prcvcnts accurate measurement of aggregation rate and makes observat~on of part~cle structure impossible. This led us to develop a new non-disruptive method to quantify diatom aggregation using high-resolut~on video and image analysis. The system easily resol\~ed cells of Chaetoceros neogracile 6 to 8 pm in diameter, and facilitated observations of aggregate morphology. C. neogracile made either ball-like or net-like aggregates depending on culture denslty and physiological state. Only the net-llke aggregates reached a substantial slze ( > l mm). A Silimlted batch culture expenment indicated that stickiness of C. neogracile increased significantly with increas~ng severity of nutrient limitation. T h ~ s increase was not detectable by the subsampling and electronic particle counting method. It is likely that spine formation, cell-surface bound sugars, and transparent exopolymer particles all played d role in determining aggregate structure and changes in stickiness. The stickiness calculation model assumes cultures are initially suspensions of single cells which form doublets as they aggregate. This assumption was tested by comparing mean particle size in a culture with the percentage of cells in aggregates a s determined using nearest neighbor distances, allowing us to assess the mean number of cells per aggregate. C. neograc~le commonly formed aggregates contaming anywhere between 2 and 10 cells showing that model assumptions can be violated under normal experimental conditions. A larger field of view was necessary to estimate the larger particle sizes typlcal of coastal diatom blooms. K E Y WORDS Aggregation Adhesion . Video Image analysis Dlatom Chaetoceros neogracile

New measurements of phytoplankton aggregation in a flocculator using videography and image analysis