Evaluation of the collection efficiency of upper ocean sub-photic-layer sediment traps: A 24-month in situ calibration in the open Baltic Sea using 234Th

Orjan Gustafsson, Per Andersson, Per Roos, Zofia Kukulska, Dag Broman, Ulf Larsson, Susanna Hajdu, Johan Ingri

Limnol. Oceanogr. Methods 2:62-74 (2004) | DOI: 10.4319/lom.2004.2.62

ABSTRACT: The collection efficiency of cylindrical sediment traps of common design were assessed in situ for over two years in an offshore shelf regime using the 234Th proxy and simultaneously collected hydrological and biogeochemical parameters. The traps were found to systematically record an under trapping bias of 234Th by on average a factor 3 (range 0.8 to 10). Seasonal variations in trapping efficiency were related to the seasonally varying properties of the settling particle pool. Sediment trap 234Th fluxes agreed within a factor of two of the estimated 234Th export from the overlying surface waters in the winter to spring periods in both 1999 and 2000 and appeared related to a particle pool that included, presumably rapidly sinking, detrital abiogenic particles and diatom tests. In contrast, discrimination against slowly settling organic-rich aggregates of apparent exudate origin resulted in under trapping of 234Th by factors 3 to 10 throughout the summer to fall seasons. These data are consistent with hydrodynamic predictions that the collection efficiency of sinking particles is inversely related to their intrinsic settling velocities. Recognition of changing particle composition along a settling velocity spectrum combine with carbon mass balance restrictions to suggest that these 234Th-based collection efficiencies, ranging from 0.1-1, may not be directly applicable to correct sediment trap fluxes of other components such as organic carbon. However, the 234Th-derived insights of settling-velocity related under trapping may beneficially be taken into account when interpreting trap records both in studies of biogeochemical element fluxes and in phytoplankton ecology.