REDEPOSITION OF POLLEN GRAINS IN LAKE SEDIMENT

Evidence from sediment traps shows that in dimictic Frains Lake pollen grains and the bottom mud in which they are contained are rcsuspendcd during spring and autumn mixing, while in Saylcs Lake, a nonstratificd lake nearby, similar quantities of pollen-bearing sediment are resuspended at irregular intervals throughout the year. In the nonstratified lake resuspended material is poorly mixed in the lake water, while in the dimictic lake, during the seasons of water mixing, resuspended sediment is similar in amount, perccntagc a5 .h weight, and pollen composition at various lcvcls throughout the water column. Rcsuspension occurs without sorting or diffcrcntial movement of individual pollen grains. The pollen content of rcdepositcd scdimcnt serves as a tracer, showing that sediment is moved from the littoral zone to the deeper basin of the lake. In the littoral zone annual stirring may involve the uppermost 6-12 mm of sediment; even in the dccpcr part of the basin, the uppermost millimeter at least is stirred by this process every year. Redeposition of sediment and pollen has been rcportcd from experiments with scdimcnt traps in Frains Lake, a thermally stratified lake in Michigan (Davis 1968); circulating water stirs and rcsuspends scdimcnt during the spring and fall seasons of water mixing. This process is capable of redistributing scdimcnt within the lake basin (Tutin 1955) and thus affecting the Final distribution of pollen grains in sediment, so it has obvious importance for the interpretation of fossil pollen. Seasonal measurcmcnt of redcposi tion has been cxtcnded to a shallow, nonstratified lake for comparison with Frains Lake. The constrast in timing demonstrates that absence of thermal stratification is csscntial for extcnsivc rcdcposition. To evaluate movcmcnt of the scdimcnt and its effect on pollen distribution, scdimcnt traps wcrc set at various depths in various parts of both lakes. With pollen grains serving as a tracer, thcsc traps dcmonstratc the movcmcnt of scdimcnt from one part of the lake basin to another. They serve further to detect any sorting due to differential movement that may occur within the pollen component of the scdimcnt, a phenomenon that had been suspected from the uneven disL Contribution No. 147 from the Great Lakes Restarch Division, The University of Michigan. tribution of different kinds of pollen within lake basins (Davis et al. 1971). The total amount of pollen deposited in traps at diffcrcnt stations scrvcs to assess the thoroughness of water mixing, In addition to information on the dircction of sediment movement, the amount of movcmcnt, and the distribution of pollen within the suspcndcd sediment, the results of the study provide an estimate of the depth to which sediment in lakes is stirred by the action of water currents. Throughout this paper rcferencc is made to “pollen grains.” In fact the refcrcncc is to the waxy exine of the grains, which is identifiable taxonomically and which is rcsistant to decay. Pollen grains filtered from lake water during the flowering season still retain protoplasm, but a few weeks or months later, only the exine remains. Ccrtdn cxccptional types of pollen arc complctcly destroyed soon after entering the lake by bacteria or fungi (Havinga 1967) or benthic organisms (R. Davis 1969). For example poplar pollen was foulld in traps during the flowering season, but was virtually absent from the sediment on the lake floor. However, pollen is not destroyed after it has been incorporated in sediment; this is cstablishcd by the constancy through time of the ratio of pollen grains to silt and clay in the sediment (Davis 1968). It is for LTMNOLOGY AND OCEANOGRAPIIY 44 JANUARY 1973, V. 18( 1) REDEPOSITION OF POLLEN GRAINS 45 this reason that pollen serves as a useful tracer for suspension, movement, and rcdeposition of the scdimcnt as a whole. I gratefully acknowlcdgc the assistance of J, IM. Bciswengcr and L. B. Brubakcr in the design and cxccution of thcsc cxpcrimcnts. W, R. Solomon kindly made available his data on pollen content in the air at Ann Arbor. The George Rcscrvc, The University of Michigan, pcrmittcd the USC of sediment traps on Sayles Lake in 19651966. The work was supported by National Science Foundation Grants G,B 2377, GB 5320, and GB 7727. DESCRIPTION OF SITES Frains Lake (42”20’N, 83”37’W), 12 km northeast of Ann Arbor, Michigan, is about 200 m wide and 500 m long, sloping gradually to a maximum depth of 10 m in the center. There arc no inflowing or outflowing streams. The surrounding topography is nearly flat, (2uitc unprotcctcd from wind, the lake is surrounded by meadows with a relief of only 3-5 m. ICC about 30 cm thick persists for 3-4 months each winter. In summer the water is strongly stratified, with the top of the thermocline at about 4 m. (For more dctailed information on topography, see Davis ct al. 1971.) Saylcs Lake (42”26’N, 84”4’W), near Pinckncy, Michigan, and 35 km west of Frains Lake, is similar in area and shape to Frains Lake and, like Frains Lake, has no major inflowing or outflowing s trcams. Howcvcr, the lakes do differ in two major ways: 1) Saylcs Lake is less than 1 m deep over most of its area and only rcachcs a maximum depth of 3 m in an area just south of the basin’s ccntcr, and 2) Sayles Lake is surrounded by woods on three sides with a steep ridge 20 m high along the south short. ICC cover is similar to Frains Lake, but Saylcs Lake ncvcr develops thermal stratification.