Vertical fluxes of particulate organic matter were measured with sediment traps above and below the chcmocline of Big Soda Lake to dcfinc the seasonality of sinking losses from the mixolimnion and dctcrmine the effectiveness of the chcmocline (pycnocline) as a barrier to the sinking of biogenic particles. Seasonality of sedimentation rates reflected seasonal changes in the community of au-totrophs. During summer-autumn, when production is dominated by autotrophic bacteria, vertical fluxes were small: z 100 mg C m- * d-l and ~0.5 mg Chl a m- * d-r. Following the winter diatom bloom, vertical fluxes increased markedly: ~570 mg C m- * d-l and 23 mg Chl a m- * d-r. The bulk of the seston (> 80%) and particulate carbon ( = 65%) sinking to the chemocline passed through it, showing that this very sharp density discontinuity dots not effectively retard the sinking of particulate matter. However sinking losses of particulate carbon were generally small ( = 10%) relative to previous measures of primary productivity, indicating that the mixolimnion is a zone of efficient carbon cycling. Exceptions occurred following the winter bloom when sinking losses were a larger fraction ( = 40%) of productivity. Because density discontinuities inhibi
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