SEDIMENT TRAP STUDIES IN SOUTHEASTERN LAKE MICHIGAN - FECAL PELLET EXPRESS OR THE MORE TRAVELED ROUTE

This study investigated the composition of particulate matter in a ser ies of sediment traps set in the offshore waters of southeastern Lake Michigan, April to October 1982. Of particular interest was the role o f zooplankton fecal pellets in rapidly transporting matter from the eu photic zone to lake sediments. Particulate pur: was examined microscop ically. Dry weight and settling velocities were then estimated based o n literature conversion factors and predictive equations. There was an excellent correspondence between estimated flux and direct measuremen ts of flux. Flux was dominated by biological particles of autochthonou s origin: fecal matter, fecal pellets, organic aggregates, phytoplankt on, and zooplankton exoskeletons. Particles were small with slow (gene rally < IO m/day) settling velocities. Flux varied seasonally, being h ighest in spring and autumn, during vertical mixing, and lowest in mid -summer during strong thermal stratification. High trapping rates duri ng isothermal conditions previously have been associated with resuspen sion. However free minerals were a relatively small component of flux. This suggests that isothermal events are associated with the resuspen sion of previously-settled mineral particles with these particles and slowly-settling biological particles being kept in suspension until th e onset of thermal stratification. Because zooplankton fecal pellets a nd fecal matter are subject to significant degradation and reingestion during the days to weeks that are required for them to reach the lake poor, these egestion products may play a greater role in the recyclin g of matter in the water column than in the direct transport of matter to the sediments. During summer, when the abundant zooplankton popula tion is food-limited and cladocerans, which produce diffuse egestion p roducts are numerous, relatively little of materials produced in the e pilimnion settles immediately to the lake floor.