Bloom dynamics and sedimentation of Peridinium gatunense in Lake Kinneret
Limnol. Oceanogr., 43(2), 1998, 175-186 | DOI: 10.4319/lo.1998.43.2.0175
ABSTRACT: Temporal changes in the abundance of Peridinium gatunense Nygaard in the water column of warm monomictic Lake Kinneret were followed during 1990-1994. Sedimentation rates of this dinoflagellate were followed concurrently by means of sediment traps with and without a preservative (Formalin), positioned at the base of the epilimnion and within the hypolimnion, for exposure periods of 2-3 weeks. Upper trap catches of total P. gatunense (live cells + dead cells + thecae + protoplasts + cysts) were nearly always higher than lower trap catches, partly due to decomposition of the cells as they sank through the water column. Over the S-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-l. A typical seasonal pattern was observed in which sedimentation rates were relatively low during the bloom increase phase, with thecae (from cell division) being the main component, and increased substantially after the peak of the bloom, when the relative contribution of senescent cells, dead cells and protoplasts increased substantially. Cysts were trapped in low numbers, usually l-2 orders of magnitude fewer than live cells. Interannual variations in total P. gafunense sedimentation were large and independent of the size of bloom-the proportion of annual P. gatunense production reaching the hypolimnetic traps ranged from 6% in 1994, the year with the largest bloom, to 68% in 1991, a year with an average-size bloom. The high value was exceptional and we speculated that it resulted from higher resuspension and more severe nutrient limitation of microbial decomposition during that low water level, drought year. On average, thecae accounted for 75% of total P. gafunense sedimentation despite being only 55% of the P. gatunense-produced biomass, suggesting that thecae were more refractory or less grazed than protoplasts. Thecal C : N : P ratio of >3,000 : 19 : 1 (vs. 276 : 51 : 1 for protoplasts) indicated that microbial decomposition of thecae is likely to require N and P inputs from other sources. Ultimately, our study highlights for the first time that annual dinoflagellate sedimentation rates may vary dramatically as a result of other processes such as decomposition, resuspension, and grazing, leading to dramatic variations in the amount of organic matter reaching the bottom sediments.