Phytoplankton monitoring: the effect of sampling methods used during different stratification and bloom conditions in the Baltic Sea

Investigation of the phytoplankton community is an important part of the overall water quality monitoring. Different countries and research institutes have different methods and traditions in sampling methods. In this study, the applicability of five different types of samplers - a hose, rosette, small and large bottle sampler and ship-of-opportunity-like pump sampling - were investigated in phytoplankton monitoring. Also, the effect of arithmetic integration of samples was studied. Sampling took place at three stations at the entrance to the Gulf of Finland and in the northern Baltic Proper during weak stratification and phytoplankton minimum, and strong stratification and a cyanobacterial bloom in the summer of 2005. Chlorophyll a and the number of selected species were analysed. There were some significant differences between the applied samplers but they were not consistent and - although there was a weak indication of the largest bottle sampler being the most reliable - the differences between the samplers were not large if they were operated correctly. However, ship-of-opportunity samples may underestimate the biomass of phytoplankton during cyanobacterial blooms. Chlorophyll a concentration estimated from arithmetically-integrated samples did not differ from that estimated from pooled samples.