CONTAMINATION OF SEDIMENT TRAP FLUXES BY VERTICALLY MIGRATING PHOTOTROPHIC MICROORGANISMS IN THE COASTAL BALTIC SEA

Sedimentation rates of phytoplankton and particulate organic carbon (POC) were studied in the northern Baltic Sea, SW coast of Finland, during the spring bloom and early summer periods in May and June 1988. Sediment traps were moored at 15 and 30 m depths, and the cylinders were either preserved with formaldehyde (F) or remained as unpreserved control cylinders (C). The phytoplankton spring bloom in May was dominated by motile dinoflagellates. Due to the high abundance and vertical migration of the chain-forming dinoflagellate Peridiniella catenata, the POC fluxes measured by the F-cylinders were, on average overestimated by 12%, and at most by 32% during the spring period. Between 3 and 12% of the P. catenata population was estimated to migrate daily below 15 m depth; part of the population also reached 30 m depth. Another migrating phototrophic organism present in the water column was the ciliate Mesodinium rubrum. Generally, a larger fraction of the M. rubrum population (2 to 26%) migrated, but they did not contaminate the measurements to the same degree as P. catenata (< 3% of total POC fluxes) due to their lesser abundance in the water column. In June, Eutreptiella sp. (Euglenophyceae) had significantly higher cell numbers in the F- than in the C-cylinders. They were estimated to contaminate the POC fluxes up to 13% at 15 m depth (F-cylinder). The difference in the Eutreptiella cell numbers between the 2 treatments could have been partly due to increased copepod grazing rates inside the C-cylinders (up to 90% of the total difference if they fed on phytoflagellates only). Contamination caused by the total apparent vertical migration of P. catenata, M. rubrum and Eutreptiella sp, accounted for 17 and 12%, on average, of the total POC sedimentation (F-cylinders) at 15 and 30 m depths, respectively. This study indicates that vertical migration of the phototrophic microorganisms can significantly bias sediment trap measurements in shallow coastal areas where sediment traps are often deployed within the vertical range of the migrating microplankton (i.e. above 30 to 40 m).