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An in situ method of simultaneously measuring the settling velocity of three size ranges of particulate organic carbon (POC) particles and 10 species of phytoplankton is described. The measurements took 2 h and were done at four different periods during the diurnal cycle. The POC fluxes of particles in the size ranges l-10 p, lo-64 p, and ~64 p were about equal for each size range and showed very little diurnal variation. In contrast, the phytoplankton showed considerable response to the varying light intensities of a day. The flagellates migrated downward at sunset; a species of diatom settled more slowly during daylight than at night; a blue-green species showed maximum buoyance at midnight and some green species changed from downward sinking on a bright day to upward movement on a dull day.The settling velocity of phytoplankton was considered as an important variable in one of the earliest quantitative phytoplankton system models (Riley et al. 1949). Since then, numerous estimates of settling velocities of phytoplankton and particulate carbon have been used in different models; however, relatively few in situ measurements have been made of the settling velocities of different phytoplankton species (Eppley et al. 1968; Reynolds 1975 Reynolds , 1976 Happey-Wood 1976; Kamykowski and Zentara 1977) or particulate organic carbon (POC) (Burns and Pashley 1974; Spencer et al. 1978; Linnegren 1979). In situ measurements are essential because the settling velocity of phytoplankton can vary with season, nutrient concentrations, age of the population, time of day, and relative brightness of the day. More such measurements are needed if we are to reach an understanding of the factors controlling the settling of living and detrital organic particles.We here compare the POC settling velocities and fluxes in three size ranges: l-10 p, lo-64 p, and ~64 p, as defined by the mesh sizes of the screens used. We also compare the settling velocities of phytoplankton with those of POC particles of similar size. Finally, we estimate the contribution of phytoplankton carbon to the settling flux of POC from the epilimnion. The effect of the brightness and time of day on the settling velocity of 10 different phytoplankton species is described.We thank L. Janus for counting the phytoplankton and P. J. Wade for removing the zooplankton from POC samples. We also thank J. Leslie for his help. MethodsThe settling experiments were carried out at Lake St. George, a small, eutrophic lake (area = 5.8 ha, maximum depth = 15 m), about 40 km north of Toronto, Ontario. Mooring systems were installed in the middle of the lake so that cables ran from submersible floats through pulleys attached to anchor weights and then to winches mounted ashore. The floats could be submerged by winching the cables in, thereby providing taut-wire moorings. The floats were allowed to surface and the settling chambers retrieved by releasing the winched cables.During the experiment the epilimnion was 4 m thick and isothermal at about 20°C and the the...

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