Effect of temperature on the aggregation of Skeletonema costatum (Bacillariophyceae) and the implication for carbon flux in coastal waters

Abstract

Skeletonema costatum (Greville) Cleve was isolated from a mucilaginous algal bloom in the Northern Adriatic Sea during April 1993. In nitrate-limited continuous culture the formation of aggregations of cells was positively correlated with temperature. Raising the temperature from 10 to 20C caused cells to aggregate without a change in culture biomass. Reducing the temperature to 10C caused washout of the aggregates, again with no change in biomass. In batch culture there was a positive relationship between aggregate concentration and temperature at 10, 15, 20 and 25C. Aggregates and the cell surface of S. costatum adsorbed Alcian blue, a stain for acid and sulphated polysaccharides. Disruption of the aggregates on addition of 0 2 M Na2EDTA indicated that cross-linking between polysaccharides by divalent cations (probably Ca2+ in seawater) bound the aggregates together. Aggregates may form in situ under conditions of temperature increase and high phytoplankton biomass. Surface water temperature increased by 0.2"C d-' in the second half of July 1992 in the Northern Adriatic, coinciding with a chlorophyll a increase of 10 to 50 pg I-'. A temperature difference of 1.2OC was observed at 2 stations 4 m apart in May 1992 on either side of a plume front associated with the River PO; transfer of phytoplankton in eddies from cold to warm waters may lead to the formation of aggregates. Aggregate &sruption may occur with the sinking of aggregates from surface waters to the relatively cool waters below the thermocline. The temperature decrease through the thermocline vaned between 2.7 and 4.1C in May 1992. The relationship between aggregation and temperature may be an important factor in determining the flux rate of fixed carbon and nutrients from the photic zone to deeper waters and the sea bed. Aggregated cultures had signlf~cantly higher sinking rates than unaggregated cultures.