BLOOM DYNAMICS AND SEDIMENTATION OF PERIDINIUM GATUNENSE IN LAKE-KINNERET

Temporal changes in the abundance of Peridinium gatunense Nygaard in t he water column of warm monomictic Lake Kinneret were followed during 1990-1994. Sedimentation rates of this dinoflagellate were followed co ncurrently by means of sediment traps with and without a preservative (Formalin), positioned at the base of the epilimnion and within the hy polimnion, for exposure periods of 2-3 weeks. Upper trap catches of to tal P. gatunense (live cells + dead cells + thecae + protoplasts + cys ts) were nearly always higher than lower trap catches, partly due to d ecomposition of the cells as they sank through the water column. Over the 5-year period, total P. gatunense sedimentation rates ranged over 4 orders of magnitude, from values <0.001 to 8.5 g (WW) m(-2) d(-1). A typical seasonal pattern was observed in which sedimentation rates we re relatively low during the bloom increase phase, with thecae (from c ell, division) being the main component, and increased substantially a fter the peak of the bloom, when the relative contribution of senescen t cells, dead cells and protoplasts increased substantially. Cysts wer e trapped in low numbers, usually 1-2 orders of magnitude fewer than l ive cells. Interannual variations in total P. gatunense sedimentation were large and independent of the size of bloom-the proportion of annu al P. gatunense production reaching the hypolimnetic traps ranged from 6% in 1994, the year with the largest bloom, to 68% in 1991, a year w ith an average-size bloom. The high value was exceptional and we specu lated that it resulted from higher resuspension and more severe nutrie nt limitation of microbial decomposition during that low water level, drought year. On average, thecae accounted for 75% of total P. gatunen se sedimentation despite being only 55% of the P. gatunense-produced b iomass, suggesting that thecae were more refractory or less grazed tha n protoplasts. Thecal C:N:P ratio of >3,000:19:1 (vs. 276:51:1 for pro toplasts) indicated that microbial decomposition of thecae is likely t o require N and P inputs from other sources. Ultimately, our study hig hlights for the first time that annual dinoflagellate sedimentation ra tes may vary dramatically as a result of other processes such as decom position, resuspension, and grazing, leading to dramatic variations in the amount of organic matter reaching the bottom sediments.