Vertical and horizontal particle transport in the coastal waters of a large lake: An assessment by sediment trap and thorium‐234 measurements

[1] The horizontal and vertical flux of particulate material in the nearshore of southern Lake Michigan (0–40 m) was estimated with the naturally occurring radionuclide 234Th. Horizontal fluxes of 234Th supplemented apparent vertical fluxes of 234Th in the water column (based on local 234Th/238U disequilibria) by a factor of 7–14, reinforcing the importance of lateral transport in coastal environments. Calculated onshore transport of particulate material across the 40 m isobath was as high as 1.1 × 106 kg km−1 d−1, and exceeds estimates of terrigenous (riverine and bluff erosion) loading. Estimates of onshore flux of organic carbon exceeded areal primary productivity by as much as ∼300%, and should be considered in nearshore carbon budgets. Bottom-tethered sediment traps (placed 5 m above the bottom) measured sedimentation rates that were ∼1 order of magnitude lower than 234Th derived mass fluxes from the water column and ∼2 orders of magnitude lower than 234Th derived mass fluxes to the lakebed. We ascribe this difference to under collecting by the sediment trap either because of trap hydrodynamics or flux occurring below the trap capture plane. Cross-shore fluxes showed a periodicity of ∼4 days and correlated strongly with a topographic vorticity wave that is present throughout the year in southern Lake Michigan. The impact of this wave (as a driver of bidirectional cross-shore flux) on biogeochemical cycling and both nearshore and offshore food webs has not yet been explicitly considered.