Dinoflagellate bloom development and collapse in Lake Kinneret: A sediment trap study

Warm monomictic Lake Kinneret, Israel, is characterized by a winter - spring water bloom of the large (∼50 μm diameter) dinoflagellate Peridinium gatunense NYGAARD. Usually the P. gatunense bloom declines in May-June and a less prominent bloom of smaller dinoflagellates (mostly Peridiniopsis spp. of ∼20-30 μm diameter) develops. Water column abundances and sedimentation losses to those dinoflagellates were followed throughout 1994 and 1995. The objective was to quantify the variables that describe population dynamics, that in turn will shed more light on the seasonal patterns of bloom dynamics. Sedimentation losses were measured by means of sediment traps with and without a preservative (formaldehyde) that were exposed for 24 h once every 2-3 weeks. Annual sedimentation losses of Peridinium (hypolimnetic trap catches) were 209 g wet wt m^-2 year^-1 in 1994 and 187 g wet wt m^-2 year^-1 in 1995, which constituted 16 and 23% of Peridinium production in those years, respectively. This study revealed that increased death rates preceded a mass sedimentation flux of Peridinium and caused the decline of the bloom in Lake Kinneret. Annual sedimentation losses of Peridiniopsis were 55 g wet wt m^-2 year^-1 in 1994 and 34 g wet wt m^-2 year^-1 in 1995. In contrast to live Peridinium cells, Peridiniopsis cells continued to swim to the lower trap after the onset of thermal stratification, possibly taking advantage of the higher nutrient concentrations below the thermocline, at a time when the lake is already stratified and the epilimnion is nutrient depleted. This could be an important factor allowing Peridiniopsis spp. to peak after the decline of Peridinium.