Lipid composition and flux of sinking and size-fractionated particles in Chesapeake Bay

The distributions of major lipid classes and of fatty acids, sterols and phytol were determined for bulk and size-fractionated (< 10, 10–64, 64–202, and 202 μm) particles from surface and near bottom waters of the mesohaline region of Chesapeake Bay. Particle samples were collected in early spring and fall by in situ pumping as well as by sediment traps deployed at the same site. Lipids, POC, and dry mass of suspended particles were highest in the smallest (< 10 μm) particles at both depths and sampling periods. Polar lipids comprised > 60% of total lipids except for the largest particles in spring which had high levels (> 50%) of triacylglycerols originating from grazing copepods. Detailed analysis of size-fractionated particles revealed substantial variations in the lipid composition of individual particulate pools, reflecting the multiple origins of organic matter. The presence of phytoplankton sterols and microscopically observed diatom cells and aggregates in large (> 202 μm) particle sizes during fall suggests that large particles may play an important role even in these shallow systems by rapidly delivering material to underlying sediments. Elevated amounts of fatty acids diagnostic of bacteria, the absence of polyunsaturated acids and the low POC values in material collected in sediment traps compared to particles in the surrounding water suggest the input of bottom sediments and/or significant decomposition in traps during deployment. Although surface water traps represented material originating principally from in situ production, traps deployed near the bottom contained substantial amounts of resuspended bottom sediments. Trap depolyments in the shallow, dynamic environment of Chesapeake Bay appear to be a better indicator of episoidc resuspension events than integrators of net downward flux.