Characterization of colloids before, during, and after a phytoplanktonic bloom in Lake Geneva (Switzerland/France)

Colloids play an important role in trace metal cycling in lakes(Morel & Gschwend 1987) due to their high sorptioncapacity and long residence times. Indeed, because of Brow-nian motion, colloids do not settle individually, even in stillwater; however, they can aggregate into larger particles andstart to settle entraining bound or embedded compounds.Aggregation, and ultimately colloid removal rates, canstrongly be enhanced during a phytoplankton bloom, particu-larly through the production of sticking exopolymers (Pas-sow 2002) released by phytoplankton (Jackson & Burd1998). Sticking exopolymers are also known to promote dia-tom aggregation in both marine (Alldregde & Jackson1995 and other references in this special issue) and freshwa-ter systems (Hoffman et al. 2001).In an international project aiming to assess trace metalremoval in Lake Geneva during a phytoplanktonic bloom,colloids were extensively studied during 3field campaignsbefore, during, and after the 2006 spring bloom. Changes insize, concentration, composition, and aggregation rate of col-loids with time will be extensively presented elsewhere (V.Chanudet, unpubl.). Here we present specifically the resultsof the second campaign (bloom), in which phytoplankton wasexpected to most influence the colloidal particle size distri-bution and play a significant role in the aggregation process.To verify this assumption, we carried out experiments on col-loid aggregation using a single particle counter (SPC), aunique tool allowing measurements of colloids in the sizerange 0.1–2µm at natural concentrations. We were able todemonstrate that during an early phase of bloom in lakes,small centric diatoms settle individually rather than as aggre-gates, possibly because of a low concentration of exopoly-mers.