Comparison of automatic and conventional physical and chemical analyses in Lake Saimaa, Eastern Finland

The Saimaa lake complex (4460 km²) is a mosaic of interconnected basins draining into the Gulf of Finland via Lake Ladoga. Limnologically, most of the basins are dysmesotrophic or oligotrophic, while some areas are markedly affected by nutrient loading. Aquatic research has been conducted in the Lake Saimaa area with the 27-metre research vessel R/V Muikku since 1989, and the laboratory and research facilities on the vessel have been built up and enhanced over this time to include temporally and spatially reliable automatic measuring systems such as CTD (ConductivityTemperatureDepth) probes and a continuous-flow in vivo fluorometer. This paper uses linear regression to compare vertical CTD measurements with standardised laboratory analyses of water temperature, O₂ concentration, conductivity and pH in different basins of the Lake Saimaa complex from data sets collected on board R/V Muikku in 2000–2001, and tests in vivo automatic fluorometer measurements of filtered and unfiltered discrete water samples from the Haukivesi basin of Lake Saimaa against in vitro chlorophyll-a concentrations. All the linear regressions showed a significant overall consistency between the CTD and fluorometer results and the laboratory analyses, but the pH comparisons showed some instability between methods. Clear spatial and vertical variations in conductivity and oxygen concentrations were apparent in the CTD results for three basins (Lakes Pyhäselkä, Haukivesi and Etelä-Saimaa), and there was also a significant linear relationship of unfiltered and filtered in vivo fluorescence to in vitro chlorophyll-a concentrations. The unfiltered fluorometer measurements were nevertheless markedly higher than those obtained from filtered water samples. Thus humic and other dissolved substances in lake water may be said to yield enhanced fluorometer readings independent of the chlorophyll-a concentrations, as has been noted earlier in similar studies.