Contamination of sediment trap fluxes by vertically migrating phototrophic micro-organisms 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, d u n n g the spnng bloom and early summer penods 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 spnng bloom in May was dominated by motile dinoflagellates Due to the high abundance and vertical nligration of the cha~n-forming d~noflagellate Peildiniella catenata, the POC fluxes measured by the F-cylinders were on average overestimated by 12 % and at most by 32 % dunng the spring penod 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~Mesodlnium 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 Eutreptlella sp (Euglenophyceae) had significantly higher cell numbers In the Fthan in the C-cylinders They were estimated to contaminate the POC fluxes up to 13 % at 15 m depth (F-cyhnder) The difference in the Eutreptiella cell numbers between the 2 treatments could have been partly due to increased copepod grazing rates inside the C-cyl~nders (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 Eutieptiella sp accounted for 17 and 12'% on average of the total POC sedimentation (F-cylindeis) at 15 and 30 m depths, respectively This study indicates that vertical migration of the phototrophic inicroorganisms can significantly bias sediment trap measurements in shallow coastal areas where sediment traps are often deployed within the vertical range of the migrating microplankton (l e above 30 to 40 m) KEY WORDS Sediment traps Sedimentation Preservation Swimmer contamination Vertical migration Dinoflagellates .