EFFECT OF SAMPLING FREQUENCY ON DETECTION OF NATURAL VARIABILITY IN PHYTOPLANKTON - UNATTENDED HIGH-FREQUENCY MEASUREMENTS ON BOARD FERRIESIN THE BALTIC SEA

This paper demonstrates the importance of adequate sampling frequency in both time and space when collecting data for the evaluation of regi onal differences and long-term trends in phytoplankton estimates. Vari ability in phytoplankton in the Baltic Sea has been recorded using aut omated flow-through analysers on board ferries with an average spatial and temporal resolution of 200 m and one day, respectively. Data coll ected in the Arkona Sea and in the Western Gulf of Finland during 1994 are used as examples. The analysis shows that changes in phytoplankto n biomass may not be reliably detected when temporal and spatial sampl ing resolution is low. Because a fixed sampling station may not be rep resentative of a sea area, a network of stations covering the entire b asin with an interval of a few kilometres, or at least a transect over the area, has to be sampled. The appropriate resolution depends on th e variability in Chl a concentrations. In the Arkona Sea, one weekly t ransect with a 1-km sampling interval is required during bloom periods when variability is highest, especially in spring, to reveal a 5% dif ference in the average Chl a concentrations between two different year s at the 0.05 significance level. In summer, the resolution needed to detect a similar difference is one weekly transect with a 2.5 km sampl ing interval. Such high spatial and temporal sampling frequencies are difficult to obtain by traditional methods and the use of unattended r ecordings on board ''ships-of-opportunity'' may solve this problem. (C ) 1998 International Council for the Exploration of the Sea.